RÉSUMÉ
Picrotoxin is an anatagninst of gamma-aminobutyric acid, which is an internal inhibition-transmitter in the central nervous system, Picrotoxin exerts a biphasic action on the blood pressure and heart rate in rats and cats in vivo. That is to say, in the initial stage, picrotoxin can lower the blood pressure and heart rate, and then an elevation of these two even above the original level can be observed, up to the present, from the authors limited literature, there has been no report dealing with the problem whether picrotoxin can act on an isolated heart directly.In this study, the heart of a frog was isolated and routine intubation of the heart was done for its perfusion. Physiological polyconduction instrument was inserted through a mechanical transducer to record the heart rale and myocardial contractility. A suspending glass microelectrode coupling with a microamplifier is used to record the action potential of the ventricular myocardium. Real time analysis of all the data was accomplished with a microcomputer. The dosages of picrotoxin used were 1,5, 3.0, 6.0, and 12.0 mg per kilogram of body weight.It was found that picrotcxin can directly act on the isolated frog heart. The results were as follows.1 ) Picrotoxin exerts inhibition on the special conduction system of the heart,and the A-V node and venous sinus are very sensitive. Complete or partial transmission block can be induced.2 ) It can elicit clearly a fall of the heart rate but no biphasic action can berevealed. 3) It can reduce the myocardial contractility, suggesting that the calciuminflow during the functioning period of the action potential is effected. 4 ) It can reduce the amplitude of the action potential but no effect on themaximal depolarization speed is observed, suggesting that picrotoxin islikely to affect the level of resting potential but not the action potentialin the depolarized period.