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.