[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--cou-pled burst of vagal stilmulation on the heart rate were investigated in 27 turtles. Thenegative chronotropic response depended on timing of the stimulus burst within the car-dinc cycle. At first, the A-A interval was lengthened progressively and then showed arapid decrease. The amplitude (AT) of the mean phase--response curve to right vagalstimulus in turtles treated by propranolol at normal temperature was 1484.10±213.10 ms. The minimum--to--maximum phase difference of the response curves was804.00±210.90 ms. Both the AT and (St--A)_(max) of the response curve varied directlywith AA. In contrast, (St-A)_(min) was not influenced by AA. Moreover, it was alsofound that the TFR, AT, AA and (St--A)_(min) in hibernant animals were all higher thanthose in normal ones. However, in three of the six hibernant turtles maximum positivechronotropic response could be elicited at the time when (A--A)_(min) was elicited by vagalstimulation. The results indicated that the phase--dependent response to right vagus inturtles was similar to that in homothermal animals, but significantly different both inthe 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 suggestedthat the occurrence of phase-response was due to changes of responsiveness of automaticcells to ACh released from the vagus nerve.