Beta adrenoceptor control of the microvascular reserve in rabbit myocardium

This study was performed to determine if the unperfused microvascular reserve in the rabbit heart can be controlled by beta adrenoceptors. Anesthetized, open chest rabbits (N = 54) were subjected to saline i.v., 0.7 mg/kg of atenolol i.v., 1 mg/kg of practolol, 0.1 μg/kg of salbutanol, 1 μg/kg of salbutamol or 0.7 mg/kg of atenolol + 1 μg/kg of salbutamol treatments. In half these animals coronary flows were determined before and after treatment using radioactive microspheres. The others were given 100 mg/kg of fluorescein isothiocyanate-dextran and the hearts were removed and analyzed for perfused and total microvascular morphology. The fluorescence marked the perfused microvessels and the slides were stained to show all vessels. Control group blood flow was 213 ± 25 ml/min/100 g. Flows decreased 25% with atenolol and practolol whereas no change was seen with salbutamol or atenolol + salbutamol. Total capillary volume fraction ranged from 0.15 to 0.20 mm³/mm³ with 60% of this perfused in controls. Atenolol significantly reduced this 47% whereas practolol (90%), high-dose salbutamol (97%) and salbutamol + atenolol (99%) resulted in a significant mobilization. Low-dose salbutamol resulted in a mobilization of capillaries to a degree intermediate (79%) between controls and high-dose salbutamol. Total arteriolar volume fraction ranged from 0.002 to 0.004 mm³/mm³ and in controls 65% of this was perfused. This percentage was reduced with atenolol and significantly increased with all other treatments except practolol. Thus, blockade of beta-1 adrenoceptors results in decreases in the percentage of perfused microvessels whereas stimulation of beta-2 adrenoceptors results in an increase. These changes in the percentage of microvessels perfused were relatively independent of coronary flow.