Barriers in cardiac substrate supply.

The capillary wall, due to its diffusional resistance, causes concentration differences between the vascular space and the interstitial space for substances which are released or taken up by the heart. Estimation of capillary transfer and interstitial concentration in isolated hearts, however, indicates a variable diffusional resistance, which in the case of glucose results from an insulin dependent transfer mechanism and in the case of lactate from a dependence of lactate transfer on lactate concentration or direction of transfer. Due to the unpredictable interstitial concentration investigation of sarcolemmal transfer appears to be possible at present only with isolated cardiac myocytes. Sarcolemmal transfer was studied for glucose and lactate. Recent investigations of lactate transfer revealed saturation kinetics, dependence on pyruvate (inhibition at low lactate concentration and enhancement at high lactate concentration) and dependence on pH (linear increase with lowered pH [8.0 to 6.7]).