The relation of pericardial pressure to chronotropic response of isolated frog hearts.

Pericardial distensibility curves and chronotropic changes accompanying pericardial distension were investigated in 24 isolated perfused frog hearts. The distensibility curve had a slow phase and a steep phase attributable to elastic and collagen components of the pericardium. Changes in pericardial pressure were accompained with reversible changes in the heart rate. The mean control rate rose from 33/min to a peak of 44/min (33% acceleration) as the pericardial pressure was raised from 0 to 4mmHg. Further increase in pericardial pressure resulted in progressive bradycardia, the mean heart rate declining to 13/min (60% declination) at 20mmHg. The acceleration response (p<.01) was observed in 8 out of 14 (57%) hearts but bradycardia (p<.001) occurred in all hearts. Atropine and propranolol did not influence these chronotropic changes. With increase in temperature and in summer the initial heart rate was high and bradycardia was a dominant feature, The chronotropic effects could only be attributed to alterations in the mechanical stretch of the pacemaker cells. The positive and negative chronotropic changes corresponded with slow and steep phases of the pericardial distensibility curve and may be due to increased stretch and relaxation of pacemaker, respectively.