Characterization of primary rabbit kidney cultures that express proximal tubule functions in a hormonally defined medium

Primary cultures of rabbit-kidney epithelial cells derived from purified proximal tubules were maintained without fibroblast overgrowth in a hormone-supplemented serumfree medium (Medium RK-1). A hormone-deletion study indicated that the primary cultures derived from purified rabbit proximal tubules required all of the three supplements in Medium RK-1 (insulin, transferrin, and hydrocortisone) for optimal growth but did not grow in response to EGF and T₃. In contrast, the epithelial cells in primary cultures derived from an unpurified preparation of rabbit kidney tubules and glomeruli grew in response to EGF and T₃, as well as insulin, transferrin, and hydrocortisone. These observations suggest that kidney epithelial cells derived from different segments of the nephron grow differently in response to hormones and growth factors. Differentiated functions of the primary cultures derived from proximal tubules were examined. Multicellular domes were observed, indicative of transepithelial solute transport by the monolayers. The proximal tubule cultures also accumulated α-methylglucoside (α-MG) against a concentration gradient. However, little or no α-MG accumulation was observed in the absence of Na⁺. Metabolic inhibitor studies also indicated that α-MG uptake by the primaries is an energy-dependent process, and depends upon the activity of the Na⁺/K⁺ ATPase. Phlorizin at 0.1 mM significantly inhibited 1 mM α-MG uptake whereas 0.1 mM phloretin did not have a significant inhibitory effect. Similar observations have been made concerning the Na⁺-dependent sugar-transport system located on the lumenal side of the proximal tubule, whereas the Na⁺-independent sugar transporter on the peritubular side is more sensitive to inhibition by phloretin than phlorizin. The cultures also exhibited PTH-sensitive cyclic AMP synthesis and brush-border enzymes typical of proximal cells. However, the activities of the enzymes leucine aminopeptidase, alkaline phosphatase, and γ-glutamyl-transpeptidase were lower in the cultures than in purified proximal-tubule preparations from which they are derived.