Chemical sympathectomy and utilization of coronary capillary reserve in rabbits

The effect of chemical sympathectomy with 6-hydroxydopamine (60 mg/kg, 5 days prior to the experiment) on coronary blood flow and the percentage of perfused myocardial arterioles and capillaries was investigated in anesthetized open-chest rabbits and compared to a control group. In half of the animals, coronary flow was determined using radioactive microspheres. The others were given fluorescein isothiocyanate-dextran to mark the perfused microvessels. Alkaline phosphatase stain was employed to locate the total microvasculature. A group of control and sympathectomized rabbits were administered 2 mg/kg of propranolol. Myocardial norepinephrine content was significantly decreased from 1108 +/- 161 (mean +/- SEM) in the control group to 162 +/- 31 ng/g wet weight in the denervated group as determined by HPLC technique and electrochemical analysis. However, the decrease in arterial blood pressure, heart rate, and coronary blood flow of the denervated group as compared to the control was not significant. Chemical sympathectomy significantly increased the number (Na) of the perfused capillaries from 57 +/- 3 to 66 +/- 3% and arterioles from 57 +/- 5 to 76 +/- 9%. A similar increase in the percentage of microvessels perfused with denervation was observed after propranolol. It is concluded that although denervation had no significant effect on the hemodynamic conditions or average coronary blood flow, it significantly increased the utilization of the myocardial microvessels. We suggest that the sympathetic nervous system and alpha adrenoceptors exert significant control of the utilization of coronary microvascular reserve.