Influence of Temperature and Rate on the Contractility of Isolated Turtle Myocardium

¹ From the Department of Pharmacology, State University of New York, Upstate Medical Center, Syracuse, New York

The effects of changes in rate of stimulation on contractile force of turtle myocardium were studied at various temperatures. At 38°C an increase in rate produced an increase in the contractile force (positive treppe). At 14°C increasing the rate produced a decrease in the contractile force (negative treppe). At intermediate temperatures an increase in rate produced first a positive followed by a negative treppe. Under anaerobic conditions the results were qualitatively the same although contractility was reduced by 50–75%. The increase in contractile force seen after an episode of rapid stimulation (poststimulation contraction) was studied at different temperatures. An increase in the rapid stimulation rate produced an increase in poststimulation potentiation at all temperatures studied except at 14°C where an increase in rate produced a decrease in potentiation. Some of these results can be explained on the basis of energy production which is probably depressed at low temperatures. On the other hand some other factor related to rate of stimulation must influence contractile force. This factor may be related to the active state of A. V. Hill or may be related to ionic movements postulated by Hajdu and Szent-Gyorgyi.