Pharmacology Prac - Chapter 5: CARDIOVASCULAR INTEGRATION II DRUGS & REFLEXES PDF

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Chapter 5: CARDIOVASCULAR INTEGRATION II
DRUGS & REFLEXES...

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Chapter 5: CARDIOVASCULAR INTEGRATION II

DRUGS & REFLEXES In this practical you will 1. Use a computer simulation program of an anaesthetised rat to: a. Observe the effects of some common classes of drugs that act on the cardiovascular system through different mechanisms b. Observe the relationship between dose and effect. c. Investigate the role of peripheral resistance, heart rate and cardiac contractile function in regulation of mean arterial blood pressure. d. Observe the effects of some agonist / antagonist interactions. e. Examine the role of cardiovascular reflexes. Introduction This computer simulation: The Rat Cardiovascular System V3.2.2 (Sept 2001)  was designed by John Dempster, University of Strathclyde 1996-2001. The Virtual Rat is a simulation of an anaesthetised rat experiment - a whole animal preparation that is widely used as a tool for investigating the physiology of the heart and vascular system and their interactions. It is widely used for screening the actions of new pharmaceutical compounds on the cardiovascular system. Compared to studies on isolated cells or tissues, whole animal preparations such as this have the capacity to reveal the multiplicity of effects that a drug can have on the different organ systems. The simulation allows you to observe the recordings of arterial blood pressure (ABP), left ventricular pressure (LVP), venous blood pressure (VBP), myocardial contractile force (HF, heart force) and heart rate (HR) on the screen. It allows you to administer a variety of different drugs, electrically stimulate components of the autonomic nervous system and to observe their effects. The Anaesthetised Rat Preparation This program simulates the preparation of an adult rat, anaesthetised and artificially ventilated via tracheal intubation. Three cannulae are inserted into: the femoral artery; the femoral vein and; the right carotid artery, with the latter passed down into the left ventricle of the heart. The femoral arterial cannula is connected to a pressure transducer to continuously measure arterial blood pressure. The arterial blood pressure (ABP) and heart rate (HR) (calculated from a count of the ABP pulse rate), are recorded on the chart recorder. The left ventricular cannula is connected to a second pressure transducer to produce a continuous record of left ventricular pressure (LVP). A measure of the contractile force of the heart (HF) is derived from the LVP. The venous cannula is connected to a third pressure transducer and used to produce a trace of central venous blood pressure (VBP). Drugs are also injected into the animal via the venous cannula. A specially designed pithing rod can be passed down the spinal cord of an anaesthetised animal, destroying all nerve connections with the brain, and hence disabling the central blood pressure

reflexes associated with the carotid artery and aortic arch baroreceptors. The pithing rod has a stimulating electrode near its tip and by careful positioning can be used to selectively stimulate parts of the sympathetic outflow. This program simulates the selective stimulation of parts of the sympathetic nervous system. The vagus nerve is isolated in the neck adjacent to the carotid artery and it too can be electrically stimulated.

To start the program, double click the Virtual Rat icon. Enter a student name in the dialogue box You will be presented with a 5-channel screen displaying 1. arterial blood pressure wave (ABP), 2. left ventricular pressure (LVP), 3. venous blood pressure (VBP), 4. myocardial contractile force (HF, heart force) and 5. heart rate (HR). Using the simulation: 1) Select New Rat from the File menu, to clear the chart. 2) Click the Start button to start the chart recorder running. 3) To inject a drug into the rat’s circulation: a) Select a drug from the Standard Drugs menu. b) Select the required dose from the list of doses. c) Click the Inject Drug button to add the drug. 4) You can make quantitative measurements from the traces by moving the mouse cursor over the trace and noting the value in the readout section at the bottom of the screen. If at any time you would like better resolution and more accurate readings you can select options from the menu and select only the traces of the channels that you are interested in. The display of fewer channels will still occupy full screen. 5) You can add as many doses and/or drugs as necessary. When you have finished your experiment or at any time during the experiment, click the Stop button to stop the chart (and put your rat on hold). 6) When you have completed an experiment you can save it to a storage file by selecting Save Rat ... from the File menu. (To re-load an experiment, select Load Rat ...). You can load a new rat at any time (data from the old rat will be lost) by selecting New Rat from the File menu. 7) To exit from the simulation program, select Exit from the File menu.

Drugs available for use in this simulation Adrenaline : α + β-adrenoceptor agonist (α = β). Noradrenaline : α + β-adrenoceptor agonist (α >β) Phenylephrine : α-adrenoceptor agonist. Isoprenaline : β-adrenoceptor agonist. Milrinone : Phosphodiesterase inhibitor Acetylcholine : Cholinoceptor agonist (Muscarinic + nicotinic). Angiotensin II : Vasoconstrictor

Glyceryl Trinitrate : Nitrovasodilator. Adenosine : Adenosine receptor agonist. Cromakalim : Potassium channel opener. Digoxin : Na+/K+ pump inhibitor. Phentolamine : α-adrenoceptor antagonist.

Prazozin : α1 adrenoceptor antagonist.

Captopril : Angiotensin converting enzyme (ACE) inhibitor.

Propranolol : β-adrenoceptor antagonist. Atenolol : β1 adrenoceptor antagonist.

L-NOARG : Nitric oxide synthase inhibitor.

Atropine : Muscarinic cholinoceptor antagonist.

8-SPT (8-parasulphophenyltheophylline): Adenosine receptor antagonist.

Losartan : Angiotension II receptor blocker

Glibenclamide : Potassium channel blocker. Verapamil : Calcium channel blocker.

Exercise 1. Simple observation: Using the mouse-controlled cursor, click Start and record for two chart divisions (~20 s). Stop the recording and write the average values in the table below. These are your baseline control values. *** Note – round values to nearest whole number.(e.g. 122.3 = 122 mmHg) *** Systolic BP mmHg

Diastolic BP mmH g

Pulse Pressure mmHg

Mean BP (estimate) mmHg

LVP mmHg

Venous pressure mmHg

Contractile HR force (HF) B/min g

Start the recording again, select the standard drugs menu and administer test doses of each of the following drugs allowing time for recovery to control values in between. In the table below, describe the effects you see and tabulate the measurements of main responses on ABP and HR. Note: The simulation computer system is old and the screen will freeze once the tracing reaches the end of the computer screen. You have enough room to take a baseline reading (~20sec) and two additional 20sec readings for the next 2 drugs below before the screen freezes. Once you have this data, hit stop and then start again to start the screen over and repeat this format (baseline and 2 drugs) until you have completed all of the drug and stimulation scenarios.

Drug Noradrenaline

Dose 10µg/kg

Adrenaline

5.0µg/kg

Isoprenaline

1.0µg/kg

Effects

↓ BP, (SBP –40mmHg, DBP –50mmHg); ↑Pulse P; ↓LVP (-40mmHg); ↑Heart Force ↑HR (+80bpm); ↑VBP (+1.5mmHg)

Phenylephrine

10µg/kg

Acetylcholine

5.0µg/kg

Glyceryl Trinitrate

10mg/kg

Exercise 2. Explore the effects of sympathetic nerve stimulation Nerve stimulation Sympathetic nerves (adrenal only)

Describe effects

Sympathetic nerves (excluding adrenal)

Sympathetic nerves (heart only)

Exercise 3. Renin angiotensin system Describe effects Angiotensin II (1.0 µg/kg)

Renal nerve stimulation

Inject Captopril (ACE inhibitor) 5mg/kg Wait ~30 sec then re-test renal nerve stimulation and Angiotensin II Compare effects on renal Angiotensin II Renal Nerve Stimulation nerve stimulation and angiotensin II

Allow renal nerve stimulation response to recover, then: Inject Losartan (a2 receptor antagonist) 10mg/kg Wait ~30 sec then re-test renal nerve stimulation and Angiotensin II Compare effects on renal Angiotensin II Renal Nerve Stimulation nerve stimulation and angiotensin II

Consult your demonstrator before proceeding with the next part: Exercise 4. Effects of antagonist drugs on the sympathetic nervous system You will use either: 1. Prazosin (1mg/kg) (α1 adrenoceptor antagonist) 2. Propranolol (10mg/kg) (β - adrenoceptor antagonist.) 3. Atenolol (100mg/kg) (β1 adrenoceptor antagonist.) 4. Atropine ( 2 mg/kg) (muscarinic antagonist, blocks parasympathetic nervous system effects) 5. Pithed rat After injecting the antagonist drug explore the effects of adrenaline, noradrenaline and nerve stimulation

Questions: 1.

Isoprenaline (sympathomimetic, β-adrenoceptor agonist), acetylcholine (parasympathetic transmitter, muscarinic agonist) and glyceryl trinitrate (vasodilator) all reduce blood pressure in anaesthetised rat. Explain their different effects on HR. a. Isoprenaline

b. Acetylcholine

c. Glyceryl Trinitrate

2.

Isoprenaline (β-adrenoceptor agonist) has powerful effects on the heart and blood vessels where it mimics adrenaline effects on β-adrenoceptors but does not activate αadrenoceptors. Despite isoprenaline stimulating an increase in contractility (heart force), the left ventricle developed pressure falls. What is your explanation for this?

3.

On the basis of your observations of the effects of sympathomimetic drugs and your knowledge of the sympathetic nervous system, a. Explain the different effects of sympathetic nerve stimulation on blood pressure and heart rate. (compare: “excluding adrenal” with “adrenal only”)

b. Which sympathetic nerve stimulation had the greatest effect on venous BP and what is your explanation?

4.

Renal nerve stimulation causes renal artery vasoconstriction. a. How does this cause a rise in blood pressure?

b. What is the basis for the effect on heart rate?

5.

The following drugs can both be used to treat hypertension. What is the basis for the antihypertensive action of: a. β1-adrenoceptor antagonist Atenolol:

b. α1 adrenoceptor antagonist Prazosin...