Endocytosis and Na+/solute cotransport in renal epithelial cells.

Endocytic uptake of [3H]sucrose and lucifer yellow, markers for fluid-phase endocytosis, was studied in cultures of the renal epithelial cell lines LLC-PK1 and OK. Endocytosis in LLC-PK1 cells was inhibited when the cells were grown in the presence of gentamicin (1 mg/ml) for 4 days or when the cells were treated with concanavalin A (1 mg/ml) for 5 h. These changes occurred without perturbation of intracellular Na+ and K+ content, indicating that the cells maintained normal ion gradients. The inhibition of endocytosis was accompanied by marked increases in the apparent Vmax for Na+-dependent cell uptake of solutes such as Pi and L-alanine. The apparent Km was unchanged. In contrast, treatment of OK cells with concanavalin A produced marked stimulation of endocytosis and inhibition of the Na+-dependent uptake of Pi and L-glutamate. These changes occurred in the absence of changes in intracellular Na+ and K+ content. Neither gentamicin nor concanavalin A had a direct effect on Na+/solute cotransport in these cell lines. The changes in Na+/Pi cotransport induced by concanavalin A in both LLC-PK1 and OK cells were blocked by keeping the cells at 4 degrees C during exposure to the lectin, suggesting that endocytosis may be part of the mechanism which mediates the changes in solute uptake. The reciprocal relationship between the changes in endocytosis and the changes in Na+/solute cotransport is consistent with the possibility that the number of Na+/solute cotransporters present in the plasma membrane may be altered by an increase or decrease in the rate of membrane internalization by endocytosis. The Vmax changes in Na+/solute cotransport provide indirect support for this conclusion.

Colchicine blocks the action of parathyroid hormone but not nicotinamide on renal phosphate transport.

Nicotinamide, like parathyroid hormone, is a rapidly acting specific inhibitor of Na+-dependent transport of phosphate (Pi) across the brush-border membrane of the proximal tubule of the mammalian kidney. Pretreatment of rats with colchicine (0.7 mg/kg body weight) for 1 h led to a significantly diminished phosphaturic response to parathyroid hormone (synthetic 1-34 fragment, 4 micrograms/kg). In contrast, the same dose of colchicine had no effect on the renal response to nicotinamide (1.0 g/kg), measured both as the change in urinary Pi excretion and as Na+-dependent Pi uptake by isolated brush-border membrane vesicles. These data suggest indirectly that the intracellular mechanism that mediates the inhibitory effects of nicotinamide on renal Pi transport does not require intact microtubules.