Effects of clonidine on cardiovascular function

Abstraction

It is known that the antihypertensive drug Catapres acts on ?2- adrenoceptors. This survey was designed to see whether the alpha 2-agonist, Catapres, binds to its ?2- adrenoceptor receptors within the stray bosom and impacting the cardiovascular system, the contractility of the bosom and the bosom rate through its consequence on the pacesetter. In the stray bosom, the highest concentration of Catapres used ( 1×10-5M ) had produced the maximal consequence. The bosom of a rat and guinea hog ( both males ) had been isolated and perfused utilizing the Langendorff setup maintaining the bosom alive. The consequence of Catapres was compared to the effects of acetylcholine which decreased the bosom rate and contractility and norepinephrine which increased the bosom rate and contractility. The dosage of drug administered to the stray bosom was of import. An addition in drug concentration resulted in an addition in consequence besides. Upon the add-on of 0.1ml of 1×10-8M Catapres the mean beats per minute ( beats per minute ) came to 183 beats per minute. An EC50 value of 59 % at the concentration of 1×10-7M of Catapres was obtained. Besides there was a lessening of 58 % in the force of contraction when Catapres was administered to the stray bosom ( from the control to the highest concentration of 0.1ml of 1×10-5M ) which is about half. In decision, it is apparent that the add-on of Catapres to the stray bosom produced a bradycardiac consequence take downing both the bosom rate and the force of contraction by triping ?2- adrenoceptors.

Recognitions

A particular thanks to my supervisor Dr M. Hussain who guided me through my undertaking. I would besides wish to thank my parents for all the support they have given me.

1.0 Introduction

Cardiac musculus is myogenic, as it has the ability to contract itself via the Sino-Atrial Node and can bring forth action potencies on its ain. The parasympathetic and sympathetic activity of the autonomic system has an influence on the bosom rate and the force of contraction ( Girgis I, et al 1998 ) . The nerve cells that connect these tracts are the preganglionic and postganglionic.

Acetylcholine and norepinephrine are the two chief senders that operate in the autonomic system. Acetylcholine acts on the muscarinic receptors, whereas noradrenaline Acts of the Apostless on both alpha- or beta- adrenoceptors ( Brodde and Michel 1999 ) .

The ?2- adrenoceptors are distributed extensively in the cardiovascular system. There are three types of adrenoceptors that are present within the cells ( myocytes ) of the bosom, which are alpha 1- , alpha 2- and beta 1- adrenoceptors ( Brodde and Michel 1999 ) .

There are two types of alpha receptors on vascular smooth musculus, alpha 1 ( ?1 ) and alpha 2 ( ?2 ) . The ?1-adrenoceptors are located in the vascular smooth musculus whereas the ?2- adrenoceptors are located both on the sympathetic nervus terminuss and on the vascular smooth musculus.

Alpha-adrenoceptor agonists ( ?-agonist ) such as Catapres bind to their complementary alpha receptors on vascular smooth musculus. When ?-agonists bind to their receptors they produce sympathetic effects such as contraction of smooth musculus and vasoconstriction ( Molin and Bendhack 2004 ) .

Clonidine is known for its chief usage as an antihypertensive medicine belonging to the category of imidazoline ? -receptor agonists used to handle high blood force per unit area ( high blood pressure ) , with its vasoconstrictive belongingss ( Mukaddam-Daher, Lambert et Al. 1997 ) .

Clonidine ( antihypertensive and antisympathetic drug agent ) is an alpha 2-agonist ( Knaus, Zong et Al. 2007 ) which acts centrally and stimulates receptors in the encephalon that are sensitive to degrees of catecholamines ( endocrines such as epinephrine and norepinephrine released due to emphasize ) in the blood.

The mechanism of clonidine action is non to the full understood but it involves the stimulation of adrenoceptors and or imidazoline receptors in the cardinal nervous system bring forthing peripheral effects including pre- and post-junctional alpha 2- sympathomimetic stimulation ( Azevedo, Newton et al. 1999 ) .

It is known that the action of Catapres is regulated by a negative feedback cringle. The encephalon controls the production of catecholamines via the falling sympathetic nervousnesss in the adrenal myelin. Clonidine causes the encephalon to believe that the degrees of catecholamines are really high ; the encephalon so reduces its signals that are sent to the adrenal myelin which cause a lessening in the degrees of catecholamines in the blood. This consequences in decrease in blood force per unit area and a lower bosom rate ( Ruffolo 1985 ) .

In a survey carried out on neurosurgical patients, Catapres had been given as a pre-anaesthetic medicine, where the blood force per unit area and the bosom rate were closely monitored. It was found that Catapres had caused a lessening in the blood force per unit area and bosom rate, but the disposal of a higher dosage of Catapres significantly reduced the blood force per unit area further including the bosom rate ( Gaumann, Tassonyi et Al. 1991 ) .

When utilizing Catapres there can be side effects on the cardiovascular system, such as bradycardia, tachycardia, congestive bosom failure, palpitations, faint, orthostatic symptoms, Raynaud ‘s phenomenon, besides electrocardiographic abnormalcies including auriculoventricular block, fistula node apprehension and arrhythmias ( Girgis I, et al 1998 ) . This will be taken note of when working with the stray bosom.

In this experiment Catapres and other agonist drugs such as acetylcholine and norepinephrine will be used to measure the effects and the mechanisms of action on the bosom rate and force of contraction. Isolated rat bosom tissue readyings will be perfused with Catapres, utilizing the Langendorff setup.

This langendorff isolated perfused bosom allows the scrutiny of the bosom in a alimentary rich fluid with O but without the hormonal and neural effects. This allows the contractility of the bosom to be measured, along with the bosom rate. The bosom so can be observed for many hours ( Skrzypiec-Spring, Grotthus et Al. 2007 ) .

The stray bosom in this experiment is denervated, so this can be an advantage when detecting merely the cardiac map, while the other sympathetic and pneumogastric stimulation is ceased. The denervation of the bosom may be compensated by the perfusates with add-on of the neurotransmitters itself or by the add-on of other peripheral factors.

There are non many surveies that show the direct effects of Catapres on the bosom, but surveies show that Catapres is able to move non merely on its mark receptors, ?2- adrenoceptors but it is besides able to work without moving on its specific receptors, as it can besides move on ?1-adrenoceptors ( Jie, new wave Brummelen et Al. 1984 ) . It has besides been found that Clonidine has a partial agonist consequence on the ?1-adrenoceptors ( Timmermans and van Zwieten 1981 ) .

The activation of the alpha-2 adrenoceptors by Catapres may hold other proteins that contribute to this consequence of Catapres, but this has yet non been apparent or proved. Clonidine can move on two different sites to take down blood force per unit area, such as one illustration, the activation of alpha-2 adrenoceptors in encephalon root nuclei leads to a lessening in the sympathetic tone. In another illustration, it can besides trip repressive alpha-2 adrenoceptors on postganglionic fibres cut downing the release of norepinephrine.

A bradycardiac consequence of Catapres has besides been observed in mice in stray atria. This consequence occurred by the suppression of the cardiac hyperpolarisation-activated pacesetter current. The cardiac hyperpolarisation pacesetter current plays an of import portion in bring forthing pacesetter potencies in the sino-atrial node cells. The figure of pacesetter potencies was besides lowered in the sino-atrial node cells of the bosom ( Knaus, Zong et Al. 2007 ) .

In add-on to this bradycardiac consequence, mice injected with Catapres after being injected with anesthesia, the bosom rate and arterial force per unit area was significantly reduced. The same consequence was besides seen in mice that were awake, every bit good as mice that had their alpha-2 adrenoceptor removed.

In a different survey it has been noticed that, the consequence of the selective drug Catapres on the alpha-2 adrenoceptor, can excite contractions, but this is wholly dependent on the extracellular Ca ( Janssens and Verhaeghe 1984 ) .

In another survey, it has been seen that if other drugs are absent ( agonists and adversaries ) , Catapres is able to contract the denuded aortal cells, this consequence of Catapres occurs in the alpha-1 adrenoceptors merely ( Molin and Bendhack 2004 ) .

The beta 1 – adrenoceptors within the bosom have a membrane edge enzyme known as adenylate cyclise which leads to the intracellular accretion of cyclic adenosine 3 ‘ , 5’-monophosphate ( camp ) a 2nd courier which is present in all cells and tissues ( Hayashi and Maze 1993 ) .

Acetylcholine by and large causes vasodilation, an indirect consequence, which acts on the vascular endothelial cells let go ofing azotic oxide doing relaxation of the smooth musculuss. The muscarinic receptors which are G-protein conjugate receptors ( Felder 1995 ; Wess 1996 ) have five sub-types ( M1-M5 ) , where the M2 subtype is located in the bosom.

Analyzing the effects of Catapres could be good, as Catapres is used by some patients to handle high blood pressure, and so the sums given to the human patients can be either increased or decreased depending on the sum already taken ( RM Boyar et Al 1980 ) .

1.1 Purposes and Aims

To look into the effects of Catapres on stray rat bosom readyings and besides set up whether clonidine affects the bosom rate and force of contraction through its consequence on the pacesetter current.

Different drugs ( agonist such as acetylcholine and norepinephrine ) will be used against Catapres to detect and measure the effects produced. The experimental technique used to prove this consequence of Catapres, will be a Langendorff setup utilizing an stray perfused bosom where the bosom will be kept alive.

2.0 MATERIALS AND METHODS:

2.1 Preparation of stray Guinea hog and rat Black Marias

Male Dunkin-Hartley Guinea hogs were kept on a Harlan 2040 diet and Wistar rats were kept on a Harlan 2018 diet along with Ad lib filtered tap H2O ( both weighing at 250g+ ) . Animals came from Harlan UK carnal research research lab. The animate beings were housed in a group, with grade 6 woodchip and sizzle nest bedclothes for the rat and grade 6 woodchip and hay for the guinea hog, along with enrichments including plastic iglu, gnawing blocks, plastic and composition board merriment tunnels. They were exposed to 12 hours of visible radiation and 12 hours of dark, kept at a temperature between 19-23 oC with room humidness of 45-65 % .

Animals were killed by cervical disruption under the Scientific Procedures Act ( 1986 ) and the bosom was so isolated, taking its environing tissue. The set-up used in this experiment for perfusing the bosom, was the Langendorff Apparatus ( Havard Apparatus UK ) . This is so that the bosom can be kept alive leting alimentary rich, oxygenated fluid to be perfused through the coronary vasculature via a catheter that is inserted into the go uping aorta. Drugs to be used were prepared with Krebs salt solution gassed with 95 % O2 and 5 % CO2, kept at changeless force per unit area, pH and temperature of 37 oC ( see below for composing ) .

The bosom was kept in a beaker of Krebs on ice to hold its activity, merely before the cannula was inserted into the aorta of the bosom. Electrodes were besides attached to the bosom, to enable the bosom to be field stimulated. After the cannula was inserted into the aorta and secured utilizing a cotton tie, the bosom was perfused with normal Krebs salt solution. The vertex of the bosom was clipped utilizing a bosom cartridge holder connected to a force transducer to mensurate the tenseness ( Havard Apparatus UK and ADInstruments ) .

Contractile activity of the bosom was allowed to brace ( for 5 proceedingss with Krebs salt solution ) before the add-on of drugs. Drugs used were taken from a stock solution of 10mls and diluted to acquire the lower concentrations.

100µl of 1×10-5 M ( 0.1ml/10ml ) of Catapres was pipetted into the first 100ml bath giving a concluding bath concentration ( FBC ) of 1×10-8 M, and was so allowed to run, until a response was observed. Following from this the following higher concentration was so added cumulatively, up until the add-on of 1×10-5 M as the concluding bath concentration. Before add-on of the following concentration the stray bosom was washed out with the control solution ( KSS ) to see if the consequence was reversible.

Following 100 µl ( 0.1ml/10ml ) of 1×10-5 M of acetylcholine, was added to the 2nd 100ml bath and allow to run for 1-2 proceedingss ) and observe if there was any response. The stray bosom was washed out with KSS followed by the add-on of a 100 µl ( 0.1ml/10ml ) of 1×10-5 M of norepinephrine to the 3rd 100ml bath, therefore exchanging immediately between physiological solutions to see consequence. Drugs were added cumulatively to accomplish the declared concluding bath concentration ( see consequences ) .

Drugs were tested once more by increasing concentration of each drug, in a dose-dependent manner utilizing noradrenaline and acetylcholine. The concentrations of drugs used ranged from 1×10-8M to the highest of 1×10-5M, and each clip 0.1ml of each drug was added to the stray bosom. Washing out of the old drug followed the process described before, utilizing control Krebs Salt Solution. The bosom was allowed to brace.

2.2 Solutions and drugs:

The drugs used were clonidine, noradrenaline, acetylcholine, and ascorbic acid. Ascorbic acid was added merely to catecholamines to forestall oxidization. All drugs were ordered from Sigma-Aldrich UK reference. Drugs were dissolved in distilled H2O to fix stock solutions and so diluted in Krebs Salt Solution ( accurate to ±0.2ml ) . Krebs salt solution and drugs dilutions were all made up fresh everyday.

The highest concentration of drugs used was at 1×10-2 M giving a FBC of 1×10-2 M and the lowest concentration at 1×10-5 M giving a FBC of 1×10-8 M. All drugs were kept on ice and had ascorbic acid added to them to forestall oxidization. The composing of Krebs salt solution ( in mmol/L ) was NaCl 118, NaHCO3 25, glucose 11, K chloride ( 10 % solution ) 4.7, CaCl2 10 % solution 2.5, K phosphate 10 % solution 1.18, Mg sulphate 10 % solution 1.18, gassed with 95 % O2 and 5 % CO2.

2.3 Statistical analysis:

Consequences are displayed as average S.E.M ( Labchart 7.0 Reader ) . Responses measured by taking away the lower limit ( baseline ) from the upper limit and ciphering a average value. The natural information was analysed by utilizing the one sample t-test utilizing IBM SPSS PASW Statistics 18 package, ( n=8 ) . P values less than 0.05 are taken to be statistically important.

3.0 Consequences

The complete bosom of both guinea hog and rat was perfused with oxygenated, alimentary rich fluid via the Langendorff setup, and the consequence of Catapres, acetylcholine and norepinephrine every bit good as the control ( Krebs salt solution ) was measured. Effect of drug was observed for one minute after the disposal of the drug used and the higher concentration was so added.

The hints that follow ( Figure 1 ) compare the effects of each drug used on the stray bosom. Clonidine was compared to acetylcholine and noradrenaline at concentrations get downing from 1×10-8M to higher concentrations of 1×10-5M, and besides normal Krebs salt solution. Addition of acetylcholine to the stray bosom had shown that there is a lessening in the bosom rate and contractility.

Figure 1a illustrates a bradycardiac consequence that was observed with the add-on of Catapres to the stray bosom. The add-on of Catapres to the stray bosom had ever shown a lessening in the force of contraction and the bosom rate, which had either taken consequence within one minute or a twosome of proceedingss after stabilization, depending on the tissue itself as both guinea hog and rat bosom was used to prove.

After running the stray bosom on Krebs salt solution until the contractions had become stabilized Catapres was added. Consequences in the hint show that one time clonidine is added to the stray bosom there is a rapid lessening in the bosom rate and force of contraction within one minute of administrating the drug.

Acetylcholine was besides added to the stray bosom so that the consequences could be compared to the consequence of Catapres, as acetylcholine besides decreases the contractility and bosom rate ( bradycardiac consequence ) . Results show that acetylcholine decreased the bosom rate more compared to clonidine. Clonidine and acetylcholine show an illustration of negative inotropic and chronotropic consequence shown in Figures 1a and B.

The last drug to be tested on the bosom was 0.1ml of ( of each concentration ) norepinephrine which increases the bosom rate and contractility an consequence described as a positive chronotropic and inotropic consequence as seen in Figure 1c.

When the lowest concentration of Catapres ( 1×10-8M ) was added to the bosom, there was non much consequence of the drug recorded, but as the concentration was increased, there was a gradual lessening in the bosom rate and contractility as seen on the hint in one minute ( Figure 1d ) . The following concentration of Catapres was added cumulatively every one minute.

a. Clonidine 1×10-5M

1 minute

b. Acetylcholine 1×10-5M

1 minute

c. Noradrenaline 1×10-5M

1 minute

d. Clonidine-dose response

Figure 1: ( a ) The consequence of add-on of clonidine 1×10-5M on the stray bosom at the pointer. ( B ) The consequence of acetylcholine 1×10-5M on the stray bosom. ( degree Celsius ) The consequence of noradrenaline 1×10-5M on the stray bosom, and ( vitamin D ) The add-on of Catapres to the stray bosom in a dose-response mode, get downing from adding 100ul of 1×10-8M Catapres and a contractile response to clonidine ( 1×10-5M ) , demoing the tenseness in gms ( g ) vs. clip in proceedingss ( n= 8 experiments-of each drug used ) .

The consequence of alpha-2 agonist Catapres on the bosom round was assessed. The following hint shows Catapres administered in a dose-response mode, from 1×10-8M to 1×10-5M concluding bath concentration.

The normal bosom rate of a rat and a guinea hog is about 350 to 450 beats per minute ( beats per minute ) , whereas in worlds, the resting bosom rate is about 60 to 80 beats per minute. In this survey the bosom rate is quantified by mensurating the figure of beats in one minute. It was observed that upon the add-on of 0.1ml of 1×10-8M Catapres the mean figure of beats per minute came to 183 beats per minute utilizing LabChart 7 by foregrounding the hint and ciphering the figure of beats ( Figure 2 ) . The maximal consequence seen after the add-on of the highest concentration of 1×10-5M of Catapres which was 99 beats per minute seen in Figure 2 below.

To farther trial and verify the consequence of Catapres on the stray bosom, the concentration of Catapres had been increased by 10 crease. This produced a hint that showed the rate of contraction and the force of the bosom lessening significantly. Thereafter, the concentrations of all drugs were increased and the consequence of dosage ( of Catapres ) administered to the stray bosom was taken into history ( see Figure 3 ) .

Concentration-effect curves in Figure 3, represents the effects of Catapres, acetylcholine and norepinephrine. Accumulative add-on of 0.1ml of increasing concentrations of Catapres produced a maximum response but non greater than the response produced by acetylcholine and norepinephrine. It was besides observed that, superimposed contractions were often produced by norepinephrine but was ne’er associated with Catapres.

Figure 2 above: The figure of bosom beats per minute against concentration of Catapres administered to the stray bosom.

Figure 3: Concentration-response curves demoing Catapres, acetylcholine and norepinephrine in comparing to each other. Points represent average + S.E.M. of readyings from different animate beings expressed as gms ( g ) .

The maximal value minus the minimal value of the response produced by the drugs on the stray bosom was measured, taking the mean. Figure 4a shows the mean tenseness produced by the bosom when the different concentrations of Catapres were added. The S.E.M is besides shown on the saloon graph.

The highest response recorded is the control ( Krebs salt solution ) ; thenceforth the response to clonidine on the stray bosom is greatly reduced, demoing that Catapres is impacting the ?2- adrenoceptors within the bosom musculus. It is apparent that an addition in concentration of Catapres decreases the bosom rate and force of contraction as the tenseness is decreased ( Figure 4a ) .

To exemplify the mean tenseness for each concentration administered, the values obtained were expressed as a per centum ( Figure 4b ) . The Krebs salt solution ( control ) was the 100 % value and the different concentrations of Catapres was so calculated as per centum of the control.

The concentration of the alpha 2- agonist Catapres that produces a response halfway between the upper limit and the baseline is 1×10-7M, known as the EC50 value. The lower limit ( baseline ) value being of 41 per centum and the maximal being 100 per centum, these values were subtracted from each other giving an EC50 value of 59 % which come to a concentration of 1×10-7M of Catapres ( Figure 4b ) .

A lessening in force of contraction by about 18 % from the control to the add-on of Catapres at a concentration of 1×10-8M whereas the add-on of a higher concentration of 1×10-5M compared to the control decreased by 58 % which is about half ( Figure 4b ) .

a.

B.

Figure 4: ( a ) Illustrates the consequence of Catapres in comparing with the control ( Krebs salt solution ) on the bosom rate and force of contraction. The points represent average + S.E.M. of readyings from different animate beings expressed as gms ( g ) . ( B ) : Datas are average + SEM and are expressed as per centums of the control, bosom obtained fro different animate beings. The concentration of Catapres ( alpha 2-agonist ) bring forthing a half-maximal response ( EC50 ) .

The T-Test ( Figure 5 ) carried out, showed the Mean, Standard Deviation, and Standard Error Mean of the mean of all the experiments carried out with Catapres, acetylcholine, norepinephrine and besides the control. The P value obtained was less than 0.05 therefore it is known to be statistically important.

Figure 5 below: Showing the one sample t-test statistics including the Mean, Standard Deviation, and Standard Error Mean of the mean of all the experiments carried out with Catapres, acetylcholine, norepinephrine and besides the control. The one sample t-test, demoing the significance value which is less that p=0.05 turn outing that there is a important difference in the drugs used.

One-Sample Statisticss

Drug

Nitrogen

Mean

Std. Deviation

Std. Error Mean

Control

Tension

8

4.773988

.3866564

.1367037

Acetylcholine

Tension

32

2.955781

1.6253850

.2873302

Noradrenaline

Tension

32

4.362122

1.9355059

.3421523

Clonidine

Tension

32

2.401338

1.5938132

.2817490

The difference of agencies of both control and Catapres ( 4.773968 – 2.401338 ) came to a value of 2.37263. The same was carried out with the other two drugs ( acetylcholine and norepinephrine ) giving values of 1.81616 and 0.411846. Therefore, a important alteration has been observed which allows the alternate hypothesis to be accepted and the void hypothesis to be rejected, as there is a important difference between the drugs used.

4.0 Discussion

This survey was designed to measure the consequence of the alpha 2- agonist Catapres on cardiovascular map, mensurating the bosom rate and force of contraction utilizing an stray rat/guinea pig bosom. The purpose of this survey has been achieved, as there has been an consequence on the cardiovascular system. The consequences obtained have been supported by old research.

The consequence of all the drugs used on the stray bosom was on self-generated bosom round ( without stimulation ) , where Catapres had decreased pacesetter potencies. However, when the bosom was non crushing as strongly, the stray bosom was field stimulated for about 10 seconds so that it can bring forth normal beats and so the drug was added.

It has been demonstrated in a old survey that Catapres has a cardinal function in the cardinal nervous system bring forthing bradycardiac effects. However when Catapres is administered to an stray bosom which is wholly denervated, there is no bradycardiac consequence shown, which indicated that the drug has no direct action on the bosom ( Laverty 1973 ) , but it is still able to bring forth a response.

The difference in cardinal actions of Catapres and norepinephrine and acetylcholine may reflect the different manner of action, as seen in old research by ( Farsang, Varga et Al. 1984 ) . The consequences show that Catapres, peculiarly administered in a higher concentration has a direct consequence on the stray bosom. The mechanism involved in the lessening of bosom rate and force of contraction may be due to the activation of alpha -2 adrenoceptors or imidazoline binding sites that are present within the bosom.

Alpha 2-adrenoceptors act in a presynaptic feedback cringle suppressing the release of norepinephrine from sympathomimetic nervousnesss. This may be the ground for Catapres cut downing the rate of contraction and the bosom round by the suppression of neurotransmitter within the bosom ( Knaus, Zong et Al. 2007 ) . The activation of the pre-synaptic ?2- adrenoceptors by Catapres did bring forth a lessening in sympathetic activity ( Molin and Bendhack 2004 ) .

Depending on the concentration of Catapres used and the species the drug was used on, the stimulation of ?2- adrenoceptors produced at first, a little addition in bosom rate, but so there was a rapid lessening in the bosom rate ( Young, Asahina et Al. 2006 ) . It has been observed that even little doses administered to the bosom produce a lessening in bosom rate ( Armstrong and Boura 1973 ) .

The sum of clip and the concentration of the drug is exposed to the stray bosom is of import as, the higher the concentration the greater the consequence seen in a short sum of clip as seen in Figure 1d. The sum of clip it took for the drug to acquire to bosom via the Langendorff setup was one minute, before the add-on of the following drug concentration ( Gaumann DM, et al 1991 ) .

It has been observed that there is a lessening in the figure of contractions induced by the activation of ?2- adrenoceptors produced by the bosom ( Boyar, Fixler et Al. 1980 ) . In this survey, Catapres had induced relaxation of the cardiac musculus in comparing to another survey where the add-on of Catapres has besides reduced vascular smooth musculus contraction cut downing the bosom rate and force.

Clonidine, the centrally moving vasodilative Acts of the Apostless on the alpha 2-adrenoceptors. This activation of receptors causes a lessening in the bosom rate and contractility ( You-hua, You-cheng et Al. 1997 ) . Tension detected by the transducer decreased bring forthing fewer contractions and bosom beats. In Figure 1a and 2, it can be seen that Catapres produces a bradycardiac consequence on the bosom, cut downing the frequence of self-generated SAN pacesetter potencies, where the self-generated oscillations produced were decreased significantly ( Elghozi, Laude et Al. 1991 ) .

The activity of the bosom ( bosom rate and force of contraction ) can be influenced via the parasympathetic and sympathetic activity of the autonomic system, although it has the ability to contract itself via the Sino-atrial node. Clonidine is known to diminish sympathetic activity, the degrees of antidiuretic hormone and renin but it increases sensitiveness of baroreceptors and besides increases parasympathetic tone ( Girgis, Chakko et Al. 1998 ) .

Muscarinic receptors are preponderantly of the M2 subtype within the bosom. Stimulation of these muscarinic receptors produces a direct negative inotropic and chronotropic effects by suppression of adenylyl cyclase via a Gi-protein ( Giessler, Dhein et al. 1999 ) . Stimulation of the postganglionic vagal fibres causes the release of acetylcholine decelerating the bosom rate by moving on the pacesetter cells. It has besides been observed, that cardiac deceleration occurs when acetylcholine is applied straight to the bosom by moving on the muscarinic receptors ( Campbell, Edwards et al. 1989 ) .

The efficaciousness of all drugs used was high ( Figure 3 ) . When Catapres was administered to the stray bosom, the consequence was moderate compared to noradrenaline and acetylcholine. Here the drug norepinephrine showed the highest efficaciousness, followed by acetylcholine and so Catapres ; this is due to the abundant figure receptors with high affinity for norepinephrine and acetylcholine as they are produced of course within the bosom.

Clonidine can merely excite a contraction in presence of voluminous sums of Ca solution. In order for Catapres to excite contractions ( bring forth a response ) it is dependent on extracellular Ca, whereas norepinephrine can excite contractions but is partially dependent on both intracellular and extracellular Ca shops.

Noradrenaline ( an agonist ) which is a adrenergic drug can move on both the alpha and beta receptors in the bosom. In Figure 3, it can be clearly seen that norepinephrine is a really powerful drug that has taken consequence at its highest concentration, demoing that the consequence is does-dependent.

The receptors for norepinephrine are G-protein-coupled receptors, associated with a specific 2nd courier system. The ?1-adrenoceptors are found in the bosom, here the receptors act by exciting adenylyl cyclase ( Limbird 1988 ) . Therefore adding norepinephrine to the bosom produces positive inotropic and chronotropic effects, which is an addition in this instance. This is due to these blood vass ( viz. the coronary arterias ) being innervated by the sympathetic sympathomimetic nervousnesss.

Due to the nature of the bosom and the sum of clip that can be spent on one tissue is of import, as they can be other factors that are act uponing the response produced by the bosom. Further work that can be carried out is to see whether Catapres has an consequence on other receptors such as the alpha 1- and or beta adrenoceptors. Here Catapres will be tested with its agonist ( such as phenylephrine, and norepinephrine ) and antagonist ( such as phenoxybenzamine ) to see the effects that take topographic point.

The consumption of Catapres can mensurate in an stray rat bosom, to see the sum taken up by the adrenoceptor ( a step of the affinity of sympathimimetic drugs ) with methods similar to the one seen in ( Iversen 1963 ) . In this alternate method, the animate being to be tested will be a rat but the one thing to take into history is the high bosom rate with short action possible continuance of the animate being when utilizing anti-arrhythmic drugs, and so this can non be compared to worlds.

Following this, the action of Catapres on alpha 1 -adrenoceptors can be investigated, as in old research it has been noticed that Catapres has a partial consequence on alpha 1 -adrenoceptors ( Timmermans and van Zwieten 1981 ) and by barricading both alpha 1 and alpha 2 adrenoceptors does clonidine still bring forth an consequence in the bosom.

In decision, this survey has emphasized the fact that ?2- adrenoceptors are sensitive to the adrenergic drug Catapres doing a decrease in the bosom rate and in the force of contraction. Besides the myocardial contractility and the bosom rate of the stray bosom are greater when the concentration of Catapres is increased. Clonidine bring oning ?2- adrenoceptors besides shows a reversible consequence when tested with control Krebs salt solution.