The Effect of Ovariectomy on Calcium (Ca2+) Handling in Guinea Pig Cardiomyocytes

Abstract

This study addressed the hypothesis that long term absence of ovarian hormones alters calcium handling in ventricular myocytes. Female guinea pigs were randomly assigned to have either a bilateral ovariectomy (Ovx) or a sham operation. Pellets containing 17β-estradiol (E) (1mg, 60-day release), were placed subcutaneously in selected Ovx animals. Cardiac myocytes were enzymatically isolated and action potential durations and L-type Ca2+ currents (ICa,L) were measured under current- or voltage-clamp respectively with a switch clamp system. Action potential morphology was investigated and, while action potential duration at 90% repolarisation (APD90) remained unchanged, APD10 was 1.46 times longer and the rate of repolarisation was slower in the Ovx group. In Fluo-4 loaded cells, Ca2+ transients were 1.34 fold larger in Ovx group compared with sham along with a greater fractional release. Sarcoplasmic reticulum Ca2+ stores were greater by 20% in the Ovx group and these cells also showed higher frequency of Ca2+ sparks and waves during a quiescent period following 2 Hz field-stimulation. The changes occurring in the Ovx group did not take place in the Ovx+E group. Cardiac myocytes isolated from the Ovx group showed increased peak ICa,L with the voltage dependence of activation and inactivation shifting to more positive voltages which will increase the likelihood of channel opening. The protein kinase A inhibitor H-89, resulted in less reduction in peak ICa,L in the Ovx group compared with sham. These findings suggest long term absence of ovarian hormones lead to potentially detrimental changes in Ca2+ handling mechanisms that may cause the formation of a more proarrhythmic substrate. Estradiol replacement prevented these adverse effects.