THE INFLUENCE OF CALCIUM ON THE CARDIOTROPIC ACTIONS OF RHODOCHLORIN AND OUABAIN

¹ Department of Physiology and Pharmacology, University of Nebraska College of Medicine, Omaha, Nebraska

Spontaneously beating hearts of Rana pipiens perfused by a modified Straub technique were arranged to record ventricular and atrial contractions by separate photoelectric mechanotransducers connected to a physiograph. Electrograms were simultaneously recorded. Ringer's solution was used, and the effects of lowering the calcium from 1.08 mM to zero were observed with and without the addition of rhodochlorin (1.7 X 10^-6 M) and ouabain (6.7 X 10^-7 M). Calcium deficiency alone resulted in impaired contractions without significant effect upon heart rate as determined electrically. Both rhodochlorin and ouabain in the above concentrations, when added to calcium-deficient solutions, restored contractility to normal or supernormal levels. The maximum positive inotropic effect with rhodochlorin when added to Ringer's solutions containing 0.54 mM and 0.27 mM of calcium was seen after 1169 and 1784 contractions respectively; the corresponding figures for ouabain when added to similar calcium-deficient solutions were 586 and 1073 contractions. In the complete absence of calcium, neither rhodochlorin nor ouabain was capable of restoring ventricular contractions. Cardiac standstill produced by rhodochlorin occurred in systole, whereas standstill with ouabain occurred in diastole. It is concluded that rhodochlorin possesses inotropic actions upon calcium-deficient frog hearts similar to those of ouabain and has approximately the same potency. These experiments suggest that the inotropic action of these drugs is linked with the facilitation of calcium influx normally associated with systole.