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  • Title: Effect of thyroxine administration on phosphate transport across renal cortical brush border membrane.
    Author: Espinosa RE, Keller MJ, Yusufi AN, Dousa TP.
    Journal: Am J Physiol; 1984 Feb; 246(2 Pt 2):F133-9. PubMed ID: 6696115.
    Abstract:
    Previous studies indicate that in hyperthyroid states the renal tubular reabsorption of phosphate is enhanced. To determine the cellular basis of this phenomenon, we investigated the effect of L-thyroxine (T4) administration on Pi transport across brush border membrane vesicles (BBMV) from rat renal cortex. Rats were thyroparathyroidectomized, fed a diet containing 1.2% phosphate, and treated intraperitoneally for 6 days with 200 micrograms T4 X 100 g body wt-1 X 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 two 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 as well as 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 X 30 s-1 X mg protein-1) and also a slightly higher affinity for Pi (apparent Km in controls, 95 +/- 9; in T4-treated, 78 +/- 8 microM). 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.(ABSTRACT TRUNCATED AT 250 WORDS)
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