Effect of thyroxine administration on phosphate transport across renal cortical brush border membrane

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Abstract
Previous studies indicate that in hyperthyroid states the renal tubular reabsorption of phosphate is enhanced. To determine the cellular basis of this phenomenon, the effect of L-thyroxine (T4) administration on Pi transport across brush border membrane vesicles (BBMV) from rat renal cortex was investigated. Rats were thyroparathyroidectomized, fed a diet containing 1.2% phosphate, and treated i.p. for 6 days with 200 .mu.g T4 .cntdot. 100 g body wt-1 .cntdot. day-1. At the end of the treatment period, the rats had a significantly (+.DELTA. 25%) elevated plasma Pi and a slightly decreased plasma Ca compared with controls. The renal clearance of Pi was not different between the 2 groups. Na+ gradient-dependent uptake of 32Pi by BBMV from renal cortex was significantly enhanced in T4-treated rats. BBMV uptake of 32Pi in the absence of Na+-gradient and Na+ gradient-dependent uptake of D-[3H]glucose and L-[3H]proline did not differ between BBMV from T4-treated and control rats. Kinetic analysis showed that the Na+ gradient-dependent 32Pi transport system in BBMV from T4-treated rats had a significantly increased Vmax compared with controls (5.2 .+-. 0.4 vs. 4.1 .+-. 0.4 nmol Pi .cntdot. 30 s-1 .cntdot. mg protein-1) and also a slightly higher affinity for Pi (apparent Km in controls, 95 .+-. 9; in T4-treated, 78 .+-. 8 .mu.M). Gluconeogenesis in cortical slices was not significantly different between T4-treated rats and controls. Specific activities of alkaline phosphatase and .gamma.-glutamyltransferase were significantly lower in BBMV from the T4-treated group compared with controls. T4 administration apparently increases specifically the Na+ gradient-dependent Pi uptake in BBMV; this phenomenon may be, at least in part, the cellular basis for the enhanced proximal tubular reabsorption of Pi elicited by T4 administration or observed in hyperthyroid states.