Regional coronary blood flow and coronary vascular reserve in unanesthetized calves at rest and during pharmacologic stress

The present study was carried out: (a) To determine whether the uniformity of transmural left ventricular (LV) myocardial blood flow (MBF) is due to a gradient of vasoactive tone or to a gradient of vascularity favoring the deeper layers. (b) To examine the changes in right ventricular (RV) MBF and coronary vascular reserve of conscious calves brought about by various kinds of stress. Hemodynamics and MBF (15-μm tracer microspheres) were studied in six awake calves before and during maximal coronary vasodilatation induced by adenosine infusion (4 μM/kg/min). Although heart rate increased by 46% and mean aortic blood pressure decreased by 29% with adenosine infusion, the RV and LV MBF had increased by 710 and 394%, respectively. Thus under basal conditions we found RV coronary vascular reserve to be greater than that in the LV. The endo:epi ratio during maximal coronary vasodilatation did not decrease below 1.00 in both ventricles. This suggested that normally higher LV subendocardial perfusion (i.e., endo:epi > 1.0) of resting awake calves is most likely not due to a out → in diminishing gradient of vasomotor tone. Minimal coronary vascular resistance was identical for both ventricles. MBF was also studied in six additional calves at control, during isoproterenol infusion (0.2 μg/kg/min), pacing, and acute volume overload before and after propranolol pretreatment. The percentage increase in RV MBF per unit weight was higher than that in the LV MBF with each stress. With isoproterenol stress, LV endo:epi had decreased significantly below 1.0 while RV endo:epi was well maintained. Propranolol pretreatment significantly blunted the increase in RV and LV MBF brought about by acute volume overload.