The impact of long term testosterone withdrawal on cardiac excitation-contraction coupling in the mouse
Ayaz, Mohammad Omar
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The impact of chronic testosterone withdrawal on the structure and function of the heart was investigated in male C57BL/6 mice following bilateral gonadectomy (GDX) or a sham-operation at 4 weeks of age. In vivo echocardiography showed few changes, although hearts were smaller and relaxation was slowed in GDX compared to sham controls. Peak contractions and Ca2+ transients were similar in sham and GDX field-stimulated ventricular myocytes, although both were prolonged in GDX cells. However, when the duration of depolarization was controlled by voltage clamp, GDX suppressed peak contractions and Ca2+ transients. Underlying cellular mechanisms were investigated by examining cardiac excitation-contraction coupling mechanisms. Peak Ca2+ current was smaller and Ca2+ current decay was prolonged in GDX myocytes, but there was no effect on net Ca2+ flux across the sarcolemma. This change in L-channel function was not due to effects of GDX on the underlying protein (e.g. Cav1.2) expression. Both the sarcoplasmic reticulum (SR) Ca2+ content and fractional Ca2+ release from the SR were reduced by GDX. GDX had no effect on the expression of the major cardiac SR Ca2+ release channel isoform (e.g. RyR2). However, spontaneous Ca2+ sparks, which represent fundamental Ca2+ release units from the SR, were smaller, less frequent and decayed more slowly in GDX myocytes. There was no change in expression of the cardiac sarco/endoplasmic reticulum Ca2+-ATPase (e.g. SERCA2), however, the expression of its endogenous inhibitor, phospholamban, was increased by GDX. Microelectrode studies showed that action potential duration (APD) was prolonged in GDX cells compared to sham controls. This was not due to effects on K+ channel expression (e.g. Kv1.5), but possibly due to the prolonged Ca2+ current decay, which would be expected to prolong repolarization. These longer action potentials disrupted electrical activity in the myocytes by increasing the number of early afterdepolarizations and spontaneous contractions. These findings suggest that GDX disrupts Ca2+ handling and may promote diastolic dysfunction and fatal arrhythmias, such as “torsade de pointes”, in older men with low testosterone.