Plasma protein concentration and control of coronary vascular resistance in isolated rat heart

Isolated externally paced (300 beats/min) rat hearts were perfused at constant pressure (70 mmHg) using a modified Krebs-Henseleit solution with (n = 52) and without (n = 15) washed bovine red cells. Albumin concentration varied from 1 to 10 g/dl. With increasing albumin concentration in blood-perfused hearts, wet-to-dry weight ratio, peak reactive hyperemic flow, and control geometric resistance were decreased, whereas oxygen consumption, control flow, and minimal geometric resistance remained constant. For plasma-perfused hearts, there was a decrease in both control and peak flow, and the other results were similar to the blood-perfused hearts. These results indicate the following. 1) Increase in interstitial fluid volume is not sufficient to cause a significant increase in minimal vascular resistance. 2) Increase in blood viscosity is compensated by vasodilation maintaining steady flow constant. 3) Minimal vascular resistance is determined by physical geometry of the vascular bed. 4) Regulation of coronary flow is postulated to be achieved by smooth muscle response to interstitial osmolarity, with a negative feedback signal coming from variation of capillary arterial pressure to variations in flow.