Electrical and mechanical responses of the intact rabbit heart in relation to the excitation interval. A comparison with the isolated papillary muscle preparation.

Experiments were performed on anesthetized open thorax rabbits with complete heart block and on isolated papillary muscles from rabbit hearts. Both preparations were basically paced at 1.0 Hz. Two consecutive test stimuli (denoted 1 and 2, respectively) were applied at various intervals. Action potentials were recorded; using suction electrodes in the intact heart and microelectrodes in the isolated tissue. Peak rate of rise of left ventricular pressure (DP) and of isometric force (DF, in vitro) were taken as measures of the contractile response. Action potential duration (AP), at 50% repolarization, were longer in vivo (175 ms) than in vitro (130 ms). AP and DP (DF) of the test contractions were similarly related to the test pulse intervals in the two preparations. DP2 was significantly correlated with DP1 and AP1 (r: 0.92-0.96). The regression coefficient for the dependence of DP2 on DP1 was significantly greater in vivo (0.61) than in vitro (0.21). This was interpreted to mean that recirculation of activator calcium from one heart beat to another was greater in vivo than in vitro. The isolated tissue would therefore be more dependent on calcium entering during the action potential. This could explain the greater postextrasystolic potentiation in vitro than in vivo (255% and 141% of steady state-responses, respectively).