Vitamin D deficiency reduces the inotropic effect of ouabain

The inotropic effect of ouabain (10(-6)M) was studied "in vitro" on rat ventricle strips electrically driven at 1 Hz. The mx% increase in the force of contraction was 64.8 +/- 4.7 in the case of controls, 25.4 +/- 3.7 in the case of vitamin D-deficient rats, and 55.8 +/- 5.9 in the case of vitamin D-deficient rats pretreated with cholecalciferol (1.000 I.U./Kg/day s.c. for 5 days before heart removal). A decrease in the Ca2+ concentration from 3.1 to 1.8 mM had no effect on the basal isometric tension developed by control ventricles, it greatly reduced that developed by ventricles from rachitic rats. These results indicate that vitamin D is important for heart contractility and for the activity of cardiac glycosides, and that the hearts of rachitic animals may have difficulty in increasing their performance when solicited to do so.