CARDIAC AND CORONARY VASCULAR EFFECTS OF CHRONICALLY ADMINISTERED ESTROGEN IN THE DOG

The acute administration of conjugated equine estrogen (CEE) to dogs s ignificantly attenuated the severity and incidence of ventricular arrh ythmias during ischemia and reperfusion. We hypothesized that one of t he cardioprotective mechanisms of estrogen might be the ability to mai ntain electrical stability of the heart during ischemia. The current s tudy was conducted to determine the effect of chronic administration o f estrogen, simulating hormone replacement therapy, on the ventricular arrhythmias of ischemia and reperfusion. Chronically-treated (100 mu g/kg/week CEE, or vehicle) male beagles were anesthetized and subjecte d to regional ischemia (20 min) and reperfusion. Although there was a trend towards a lower incidence of arrhythmias during ischemia in estr ogen-treated dogs, values did not achieve significance at P < 0.05. Ba seline coronary vascular resistance was significantly higher in estrog en-treated dogs (2.3 vs 1.5 mmHg/ml/min/100 g, P < 0.05) indicating an increase in vasomotor tone. There was also an increase in the time it took hyperemic coronary blood flow to reach a peak value upon reperfu sion (71 sec in estrogen-treated dogs vs 12 sec in vehicle-treated dog s, P < 0.05). This slower reflow is consistent with increased coronary vascular resistance upon reflow in estrogen-treated dogs. We conclude that the chronic administration of CEE to male dogs increased coronar y vascular tone, and impaired the rate of reperfusion, but did not dec rease the incidence of ventricular arrhythmias caused by ischemia.