Mechanical behavior of rest contractions in cardiac muscle.

The effects of increasing rest periods on rest contractions and rest potentiation were analysed on isometric contractions of guinea pig papillary muscles at room temperature. Rest contraction amplitude is dependent on pause duration and potentiated for short pauses, which decreases as the rest period increases. their inotropic state is also dependent on the inotropic state of steady state control contractions. This statement is validated by the interdependence between control contractions and rest contractions amplitude and also because positive (5.0 mM CaCl2) and negative (0.5 mM NiCl2)inotropic interventions do not change the time course of dF/dt pause duration curves. They also show a slow activation characterized by a prolonged TTP that depends on pause duration being observed even in contractions occurring after short pauses. Rest contractions can be elicited under depolarized conditions produced by TKBa (Tyrode with high potassium and barium) and TKAdr (Tyrode with high potassium and adrenaline), showing that they are dependent on calcium influx occurring during the slow response of the cardiac action potential. The present study, carried out under depolarized conditions and in low extracellular Na+, shows that Na+ entry during the Na-dependent fast component of the action potential is necessary for the occurrence of rest potentiation. These findings suggest that the intracellular Na+, that can be exchanged by external calcium with the Na/Ca exchange mechanism, is necessary for the occurrence of the rest potentiation and to avoid the accelerated depression of the rest contractions with increasing pause duration.