Ovariectomy enhances SR Ca²⁺ release and increases Ca²⁺ spark amplitudes in isolated ventricular myocytes

This study compared Ca(2+) homeostasis in ventricular myocytes from 8-month-old female C57BL/6J mice that had either a bilateral ovariectomy (OVX) or a sham surgery at 3 weeks of age. Cells were loaded with fura-2 and field-stimulated or voltage-clamped with steps to membrane potentials between -40 and +80 mV (37°C). Peak Ca(2+) transients increased by two-fold in OVX myocytes when compared to sham, and Ca(2+) transient rates of rise and decay were faster in OVX cells. In contrast, Ca(2+) current densities were similar in sham and OVX cells. Sarcoplasmic reticulum (SR) Ca(2+) content, assessed by caffeine, also was higher in OVX compared to sham cells (111.7 ± 11.9 vs. 61.2 ± 10.4 nM; p<0.05). Furthermore, the gain of Ca(2+) release (Ca(2+) release/Ca(2+) current) was significantly greater in OVX than in sham cells (16.3 ± 2.5 vs. 7.7 ± 2.0 nM/pApF(-1) at 0 mV; p<0.05). As changes in unitary Ca(2+) release might account for the increased gain in OVX cells, spontaneous Ca(2+) sparks were compared in fluo-4-loaded myocytes (37°C). Spark frequency was higher in OVX cells than in sham cells. In addition, spark amplitudes were greater in OVX than in sham myocytes (ΔF/F(0)=0.379 ± 0.006 vs. 0.342 ± 0.006; p<0.05). However, spark widths and time courses were similar in the two groups. These data suggest that the size of individual SR Ca(2+) release units is larger and the SR Ca(2+) content is greater in myocytes of OVX mice, producing augmented gain and SR Ca(2+) release. These observations show that OVX disrupts intracellular Ca(2+) homeostasis and suggest that sex steroid hormones modulate unitary Ca(2+) release in individual cardiac myocytes.