title: Effect of thyroxine administration on phosphate transport across renal cortical brush border membrane.
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)
mesh_terms: D000818:Animals; D001692:Biological Transport; D002118:Calcium / Q000652:urine; D005947:Glucose / Q000378:metabolism; D007672:Kidney Cortex / Q000378:metabolism*; D008297:Male; D008871:Microvilli / Q000201:enzymology / Q000378:metabolism*; D010710:Phosphates / Q000378:metabolism* / Q000652:urine; D011392:Proline / Q000378:metabolism; D051381:Rats; D013974:Thyroxine / Q000097:blood / Q000494:pharmacology*; D013997:Time Factors
chemical_list: D010710:Phosphates; D011392:Proline; D005947:Glucose; D013974:Thyroxine; D002118:Calcium