Effect of cardiotonic lactones on calcium metabolism of dog cardiac microsomes.

The effects of several lactones were studied in a microsomal fraction of dog myocardium thought to be sarcoplasmic reticulum. The lactones increased the steady-state accumulation and turnover of calcium only in the presence of ATP, and augmented the calcium-stimulated ATPase activity. When the effective concentrations of the lactones were exceeded, there were no further alterations in calcium accumulation or turnover. A correlation between the capacity of these lactones to increase calcium accumulation and turnover and their relative cardiotonic activity, as reported in the literature, was noted. The potency of the lactones in relation to calcium metabolism in the microsomes is influenced by ring saturation, position of the double bond, and presence of a steroid ring system to the lactone moiety. ADDITIONAL