Aim: To study the Effect of Drugs on Normal And Hypodynamic Perfused Frog Heart.
Principle: The myocardial contraction of a normal and hyperdynamic frog heart takes place according to starling’s law of the heart. According to this law force of systolic contraction is directly proportional to the fiber length in diastole. Since systolic contraction represents cardiac output and the fiber length in diastole indicates venous pressure, the law indicates that cardiac output (i.e.stroke volume) is directly related to the venous return or venous pressure during diastole.
When the cardiac musculature fails to obey this relationship as in a failing heart ( i.e.congestive heart failure ) there will be a decrease in stroke volume(cardiac output), incomplete emptying of the ventricles during systole, and enlargement of heart size due to residual blood in the heart at the end of the systolic contraction. When the heart is in this state,i.e. inability to contract to physiological normal it is said to be a hyperdynamic heart.
An experimentally hyperdynamic heart can be produced by perfusing the heart with a ringer containing less quantity of Calcium as this bivalent ion is essential for myocardial contraction.
Drugs: Digitalis (Digoxin stock solution 50µg/ml) Calcium chloride (stock solution 100 µg/ml)
Physiological solution Normal frog ringer and frog ringer containing ¼ CaCl2.
- Set up the perfusion of the frog heart with a normal frog ringer solution as described in the earlier experiment.
- Record the effects of (0.1,0.2,0.4, and 0.5 ml) digoxin and CaCl2 (0.1,0.2,0.4, and 0.5 ml). Note the dose that gives an adequate response.
- Replace the perfusion fluid with a modified Ringer containing only ¼ that the Calcium chloride as compared to that of the normal ringer. Note the change in the pattern of the recording of the heart.
- When the heart is depressed markedly in presence of modified Ringer, administer digoxin (0.1,0.2,0.4 and 0.5 ml) and CaCl2 (0.1,0.2,0.4 and 0.5 ml). Note the change in contractility.
- Fix the tracing and compare the responses of these drugs in the normal and hyperdynamic hearts.
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