INCREASE IN CALCIUM RESPONSIVENESS OF CARDIAC MYOFILAMENT ACTIVATION IN OVARIECTOMIZED RATS

To evaluate a possible role of ovarian sex hormones in the Ca2+ respon siveness of cardiac myofilament activation, the relations of pCa (-log Ca2+ molar concentration) to actomyosin adenosine triphosphatase (ATP ase) activity of isolated myofibrillar preparations from 8-10 week ova riectomized (Ovx) rat hearts were compared with those from sham-operat ed hearts. Deficiency of ovarian sex hormones in plasma of ovariectomi zed rats was indirectly verified by a significant reduction in uterine weights. Body weights of the ovariectomized rats were significantly g reater than those of sham-operated controls. Despite a significant inc rease in heart weight of 10 week ovariectomized animals, the percent o f heart weight-to-body weight ratio was not different from control gro up. The maximum myofibrillar ATPase activity at pH 7.0 was significant ly suppressed after ovariectomy in both eight and ten week groups. How ever, the maximum ATPase activity at pH 6.5 was significantly suppress ed only in 10 week ovariectomized hearts. Surprisingly, in every condi tion with depressed maximum myofibrillar ATPase activity, the pCa-acto myosin ATPase relationships of ovariectomized cardiac myofilaments dem onstrated a significant leftward shift in pCa(50) (-log half-maximally Ca2+ activation) from those of sham-operated controls. There was, how ever, no change in the Hill-coefficient of these cardiac myofilaments after ovariectomy. Analysis of myofilament proteins using gel electrop horesis demonstrated neither change nor loss of any thin filament prot eins. These results indicate a possible modulating effect of ovarian s ex hormone deficiency on the Ca2+ responsiveness of cardiac myofilamen t activation by induction of myofilament Ca2+ hypersensitivity but sup pression of maximum myofibrillar ATPase activity.