A Comparative Study of Effects of Isoproterenol and Dihydroouabain on Calcium transients and Contraction in Cultured Rat Ventricular Cells

We investigated the effects of isoproterenol, a β-adrenergic agonist, and dihydroouabain, a Na+,K+-pump inhibitor, on Ca2+ transients and contraction of cultured rat ventricular cells and compared the effects with those of altered external ion concentrations, with special reference to the changes in diastolic intracellular free calcium concentration ([Ca2+]i). We measured [Ca2+]i of cultured cell aggregates, stimulated at 1.0 Hz, with the use of dual-wavelength microflourometry of fura-2, at room temperature (24-26°C). The contraction was measured as a shortening of the aggregates using a photodiode array placed on a video monitor. Isoproterenol increased the magnitude of contraction and the peak amplitude of the Ca2+ transient, in a concentration (10 9-10 6 M)-dependent manner, but did not change the diastolic Ca2+ level. Isoproterenol at 10-7 M or higher significantly shortened the duration of contraction and half decay time of a Ca2+ transient yet it did not change the time to peak. Dihydroouabain (10 7 -10-3 M) increased the contraction and elevated both systolic and diastolic calcium levels but it did not alter the duration of contraction, the time to peak and the half decay time. The effects of dihydroouabain on Ca2+ transients were mimicked by lowering [K+]o (0.4 mM), by lowering [Na+]o (74 mM) or by elevating [Ca2+]o (3.6 or 5.4 mM). Ryanodine (10 3 M), by itself, decreased systolic Ca2+ transient amplitude, increased diastolic Ca2+ levels and prolonged the time to peak and the half decay time. In the presence of ryanodine, isoproterenol increased both systolic and diastolic [Ca2+]i. Thus, most procedures that increased the systolic Ca2+ transient amplitude increased the diastolic Ca2+ levels as well, and enhanced the contraction. The only exception was isoproterenol that markedly increased the systolic Ca2+ transient amplitude without affecting the diastolic Ca2+ level, a finding in keeping with the observation that isoproterenol stimulates Ca2+ uptake by the sarcoplasmic reticulum.