The Impact of Ovariectomy on Calcium Homeostasis and Myofilament Calcium Sensitivity in the Aging Mouse Heart

This study determined whether deficiency of ovarian estrogen starting very early in life promoted age-associated Ca²⁺ dysregulation and contractile dysfunction in isolated ventricular myocytes. Myocytes were isolated from anesthetized C57BL/6 female mice. Animals received an ovariectomy or sham-operation at one month and were aged to ~24 months. Excitation-contraction coupling parameters were compared in fura-2 loaded myocytes (37°C). While Ca²⁺ transients were larger and faster in field-stimulated myocytes from ovariectomized mice, ovariectomy had no effect on peak fractional shortening. Similarly, ovariectomy had no effect on fractional shortening measured in vivo by echocardiography (values were 60.5 ± 2.9 vs. 60.3 ± 2.5% in sham and ovariectomized, respectively; n=5 mice/group). Ovariectomy did decrease myofilament Ca²⁺ sensitivity, as evidenced by a 26% increase in the Ca²⁺ required to activate actomyosin MgATPase in ovariectomized hearts. Larger Ca²⁺ transients were attributable to a 48% increase in peak Ca²⁺ current, along with an increase in the amplitude, width and frequency of Ca²⁺ sparks measured in fluo-4 loaded myocytes. These changes in Ca²⁺ handling were not due to increased expression of Ca²⁺ channels (Ca_v1.2), sarcoplasmic reticulum Ca²⁺ ATPase (SERCA2) or Na⁺-Ca²⁺ exchanger in ovariectomized hearts. However, ovariectomy increased sarcoplasmic reticulum Ca²⁺ stores by ~90% and promoted spontaneous Ca²⁺ release from the sarcoplasmic reticulum when compared to sham controls. These observations demonstrate that long-term ovariectomy promotes intracellular Ca²⁺ dysregulation, reduces myofilament Ca²⁺ sensitivity and increases spontaneous Ca²⁺ release in the aging female heart.