Effect of twitch interval duration on the contractile function of subsequent twitches in isolated rat, rabbit, and dog myocardium under physiological conditions.

Many studies have shown that a change in stimulation frequency leads to altered contractility of the myocardium. However, it remains unclear what changes occur directly after a change in frequency and which ones are a result of the slow processes that lead to the altered homeostasis, which develops after a change in stimulation frequency. To distinguish the immediate from the slow responses, we assessed contractile function in two species that have distinctively different calcium (Ca(2+))-handling properties using a recently developed, randomized pacing protocol. In isolated dog and rat right ventricular trabeculae, twitch contractions at five different cycle lengths within the physiologic range of each species were randomized around a steady-state frequency. We found, in both species, that the duration of the cycle length just prior to the analyzed twitch (primary) positively correlated with the increased force of the analyzed twitch. In sharp contrast, the cycle lengths, one and two more removed from the analyzed twitch ("secondary" and "tertiary"), displayed a negative correlation with force of the analyzed twitch. In additional experiments, assessment of intracellular Ca(2+) transients in rabbit trabeculae revealed that diastolic Ca(2+) levels were closely correlated to contractile function outcome. The relative contribution of the primary cycle length was different between dog (51%) and rat (71%), whereas in neither species was a significant effect on relaxation time observed. With the use of randomized cycle lengths, we have distinguished the intrinsic response from the signaling-mediated effects of frequency-dependent activation on myofilament properties and Ca(2+) handling.