VARIABLE EFFECTS OF ISOPROTERENOL ON ACTION-POTENTIAL DURATION IN GUINEA-PIG PAPILLARY-MUSCLE - DIFFERENCES BETWEEN NON-STEADY AND STEADY-STATE - ROLE OF EXTRACELLULAR CALCIUM-CONCENTRATION

Abstract

The effect of 3 different isoproterenol (ISP) concentrations (10-6, 10-7 and 10-8 M) on action potential duration (APD) was studied in guinea pig papillary muscles at low (0.6-0.9 mM), intermediate (1.8 mM) and high (3.6 mM) Ca concentrations (Ca2+)0. All 3 ISP concentrations produced (Ca2+)0-dependent changes during both nonsteady (NSS) and steady state (SS). During NSS, initial prolongation of APD occurred in 22 of 22 experiments at low, in 16 of 36 experiments at intermediate and 0 of 15 experiments at high (Ca2+)0. In the remaining experiments, APD because progressively shorter. The NSS-APD changes were associated with increase in plateau amplitude attributed to slow inward current (isi). No correlation was found between the amplitude of plateau and APD change. Apparently, the (Ca2+)0-dependent differences were due more likely to differences in the rate of isi inactivation than to differences in the absolute isi magnitude. The SS-APD was also dependent on (Ca2+)0 but, during progression from NSS to SS, plateau amplitude did not change, while the force of contraction increased. This supports the hypothesis that the dependence of SS-APD on (Ca2+)0 is mediated by the time-independent K conductance controlled by intracellular Ca concentration. The early and the late ISP effects on ADP appeared to differ in their mechanism, but both depended on (Ca2+)0.