Absence of capillary recruitment during increased coronary blood flow in the working dog heart

Myocardial plasma flow (MPF) and capillary permeability--surface area product (PS) were determined simultaneously on a regional basis in subendocardial and subepicardial layers of the left ventricular free wall in the working dog heart at control flow rate and during coronary vasodilation. The value for MPF was determined by the radioactive microsphere method and PS was assessed by an integral tissue-uptake method using [51Cr]-EDTA as test molecule. During coronary vasodilation achieved by intra-coronary infusion of adenosine, MPF increased markedly from 101.5 +/- 14.2 to 778.1 +/- 135.2 ml X min-1 X 100 g-1 in subendocardial layers and from 77.7 +/- 10.3 to 635.3 +/- 85.9 ml X min-1 X 100 g-1 in subepicardial layers of the left ventricular wall. Raising MPF by 800% was accompanied by a modest but statistically significant increase in PS[51Cr]-EDTA. In subendocardial layers PS increased from 49.3 +/- 5.0 to 60.0 +/- 3.3 ml X min-1 X 100 g-1 and in subepicardial layers from 43.8 +/- 4.1 to 52.1 +/- 2.4 ml X min-1 X 100 g-1, respectively. Taking into account back-diffusion of the tracer from the tissue to capillary blood which was higher at low flows than at high flows, PS[51Cr]-EDTA showed no increase with an eight-fold increment in flow. This observation rebuts the concept of additional perfusion of capillaries which are devoid of plasma flow under resting conditions during coronary vasodilation.