[The phase-dependent response of the heart to vagal stimulation in soft-shelled turtles]

Using cardiac cycle length (A-A interval) as an index, the effect of phase-coupled burst of vagal stimulation on the heart rate were investigated in 27 turtles. The negative chronotropic response depended on timing of the stimulus burst within the cardiac cycle. At first, the A-A interval was lengthened progressively and then showed a rapid decrease. The amplitude (AT) of the mean phase-response curve to right vagal stimulus in turtles treated by propranolol at normal temperature was 1484.10 +/- 213.10 ms. The minimum-to-maximum phase difference of the response curves was 804.00 +/- 210.90 ms. Both the AT and (St-A)max of the response curve varied directly with AA. In contrast, (St-A)min was not influenced by AA. Moreover, it was also found that the tau FR, AT, AA and (St-A)min in hibernant animals were all higher than those in normal ones. However, in three of the six hibernant turtles maximum positive chronotropic response could be elicited at the time when (A-A)min was elicited by vagal stimulation. The results indicated that the phase-dependent response to right vagus in turtles was similar to that in homothermal animals, but significantly different both in the amplitude of the phase-response and the latency of negative chronotropic response. Since the phase-response was abolished by atropine, not by propranolol, it was suggested that the occurrence of phase-response was due to changes of responsiveness of automatic cells to ACh released from the vagus nerve.