Uptake and release of carnitine by vascular endothelium in culture; effects of protons and oxygen free radicals.

The present paper shows that cultured bovine endothelial cells can be labeled with 3H-carnitine by incubation. This process is slow and is uphill, requiring Na+/K+ ATPase activity. After 3 days incubation isotopic equilibrium is reached, when the cells contain about 0.5 mM (total) carnitine at a medium concentration of about 3 microM. The plasmamembrane barrier is rather resistant to acidosis and oxygen free radicals (OFR). The rate of carnitine release increases significantly only at pH below 5.8. At pH 6.0 the release of stored carnitine can be initiated by the addition of D- or L-lactate. OFR, generated by the addition of xanthine and xanthine oxidase, did not affect carnitine release. Both mild acidosis and OFR left plasmamembranes of endothelial cells intact as judged by the absence of lactate dehydrogenase loss from the cells. Therefore, the known increase of capillary permeability during ischemia and reperfusion may not be due to plasmalemmal disruption of individual endothelial cells, but to increase of inter-endothelial spaces.