Staircase in lizard ventricle. II. Time-course analysis in the hypodynamic state.

The analysis of the staircases in lizard ventricle, that was the object of the preceding paper, has been extended to the hypodynamic condition, induced by the prolonged perfusion of the ventricular preparation at high flow rate. For comparison, the effects of lowered [Ca]0 on the normal ventricle as well as of raised [Ca]0 on the hypodynamic ventricle have been also investigated. The results show that the low rate (less than or equal to 2 min-1) staircase in hypodynamic condition is formally described by the same algebraic sum of exponentials as in normal hearts: Pt - Ps - - phi 1 - phi 2 + phi 5, although with altered values of both their extrapolations P' at the time t0 and their time constants tau. On the contrary, t high beat rates the staircase profiles obey to the the eqn.: Pt - Ps = - phi 2 + phi 3 - phi 4 + phi 5, which is characterized by the absence of the phi 1 component, as well as by the occurrence of the phi 4 component, that fits to an early phase of rapid build-up of the isometric twitch tension from the reduced values of the hypodynamic "rested state contraction'. A two-fold increase of the [Ca]0 approaches the time-course of the hypodynamic staircases, whether at low or at high beat rate, to that displayed by the ventricles in normal inotropic state. The results are discussed on the assumption of the double-source hypothesis for the Ca that activates the contraction in the lizard ventricular cells.