Chronic K depletion stimulates rat renal brush-border membrane Na-citrate cotransporter.
Chronic K depletion (KD) causes hypocitraturia. In the present studies, the effect of KD, induced by a low-K diet for 14 days (serum [K] 4.1 +- 0.1, control vs. 2.2 +/- 0.1 meq/l, KD, P less than 0.01), on renal cortical brush-border membrane (BBM) Na-citrate cotransporter activity was examined. KD significantly decreased fractional citrate excretion (3.90 +/- 0.68 vs. 0.53 +/- 0.10%, P less than 0.01). This was paralleled by a significant increase in the initial linear rate of BBM Na-dependent citrate transport (81 +/- 4 vs. 141 +/- 6 pmol citrate.5 s-1.mg protein-1, P less than 0.01). Kinetic studies varying extravesicular citrate concentration demonstrated that KD increased the maximal activity (Vmax) (152 +/- 17 vs. 296 +/- 14 pmol citrate.5 s-1.mg protein-1, P less than 0.01) with no difference in citrate affinity (118 +/- 23, control vs. 135 +/- 22 microM citrate, KD). Similarly, when extravesicular Na concentration was varied, KD increased the Vmax (132 +/- 9 vs. 206 +/- 6 pmol citrate.5 s-1.mg protein-1, P less than 0.01) with no difference in Na affinity (29 +/- 3, control vs. 28 +/- 1 mM Na, KD). The effect of KD on Na-citrate cotransport was specific in that KD did not alter Na-glucose (84 +/- 12, control vs. 89 +/- 5 pmol glucose.5 s-1.mg protein-1, KD) or Na-proline (87 +/- 3, control vs. 84 +/- 5 pmol proline.5 s-1.mg protein-1, KD) cotransport. In conclusion, KD increases the Vmax of the proximal tubule apical membrane Na-citrate cotransporter.(ABSTRACT TRUNCATED AT 250 WORDS)
Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas 75216.
M
M
Levi
L A
LA
McDonald
P A
PA
Preisig
R J
RJ
Alpern
D000469:Alkaline Phosphatase / Q000378:metabolism; D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism*; D007668:Kidney / Q000201:enzymology / Q000378:metabolism / Q000503:physiopathology*; D007700:Kinetics; D007931:Leucyl Aminopeptidase / Q000378:metabolism; D008297:Male; D008871:Microvilli / Q000378:metabolism*; D011191:Potassium Deficiency / Q000201:enzymology / Q000378:metabolism / Q000503:physiopathology*; D051381:Rats; D011919:Rats, Inbred Strains; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism; D000520:alpha-Glucosidases / Q000378:metabolism; D005723:gamma-Glutamyltransferase / Q000378:metabolism
D002352:Carrier Proteins; D005723:gamma-Glutamyltransferase; D000469:Alkaline Phosphatase; D000520:alpha-Glucosidases; D007931:Leucyl Aminopeptidase; D000254:Sodium-Potassium-Exchanging ATPase