Cardiac and coronary vascular effects of chronically administered estrogen in the dog

The acute administration of conjugated equine estrogen (CEE) to dogs significantly attenuated the severity and incidence of ventricular arrhythmias during ischemia and reperfusion. We hypothesized that one of the cardioprotective mechanisms of estrogen might be the ability to maintain electrical stability of the heart during ischemia. The current study was conducted to determine the effect of chronic administration of estrogen, simulating hormone replacement therapy, on the ventricular arrhythmias of ischemia and reperfusion. Chronically-treated (100 micrograms/kg/week CEE, or vehicle) male beagles were anesthetized and subjected to regional ischemia (20 min) and reperfusion. Although there was a trend towards a lower incidence of arrhythmias during ischemia in estrogen-treated dogs, values did not achieve significance at P < 0.05. Baseline coronary vascular resistance was significantly higher in estrogen-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 reperfusion (71 sec in estrogen-treated dogs vs 12 sec in vehicle-treated dogs, 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 coronary vascular tone, and impaired the rate of reperfusion, but did not decrease the incidence of ventricular arrhythmias caused by ischemia.