Transport of hexoses, potassium and neutral amino acids into capillaries isolated from bovine retina

Capillaries were isolated from bovine retina by homogenization and glass bead filtration in order to study their ability to transport certain solutes. Viability of the microvessels was demonstrated by their ability to maintain linear rates of substrate oxidation for more than two hours. Hexose uptake (measured using 3-o-methyl-d-glucose) could be inhibited by cytochalasin B, phloretin and phlorizin, but not by 2,4-dinitrophenol or ouabain. d-Glucose, 2-deoxy-d-glucose, d-mannose, d-galactose and d-xylose inhibited 3-o-methyl-d-glucose uptake, while l-glucose, d-ribose and d-fructose did not. When incubated at 37°C with 5 mm-d-glucose, the microvessels contained much more free d-glucose than d-glucose metabolites. Thus, transport was not rate limiting for metabolism. l-Glucose entered capillaries more slowly than other hexoses and served as a marker for simple diffusion of sugars into the cells. ⁸⁶Rb⁺ was used as an indicator of K⁺ transport activity. Uptake of ⁸⁶Rb⁺ was temperature sensitive and markedly inhibited by I mm ouabain, thus indicating the presence of an active K⁺ transport system. Na⁺-dependent amino acid transport was demonstrated using α-(methylamino)isobutyric acid as a model substrate. Capillary uptake of this neutral amino acid analogue was inhibited after abolishing the Na⁺ gradient with 1 mm-ouabain. Uptake of the organic anion p-aminohippuric acid (PAH) was slightly greater than uptake of the extracellular marker sucrose. There was a small inhibition of PAH uptake by fluorescein and penicillin but not by probenicid. Our results indicate the presence of several transport processes in retinal capillary endothelial cells which may be important for the maintenance of homeostasis within the retina.