Na+ alters the affinity for glucose and phosphate in rat renal brush-border membranes: a study of NMR relaxation rates

Abstract

Transport of D-glucose (Glc) and Pi across renal microvilli brush-border membrane vesicles (BBMV) is coupled to Na+.Na+ may alter substrate binding and dissociation or the mobility of the translocator and its complexes. To evaluate substrate binding in NaCl or KCl solutions containing BBMV, we measured nuclear magnetic resonance transverse relaxation rates (1/T2) of [1-13C]Glc and 31P(i), as well as their temperature, magnetic field, and pH dependencies. The data indicate the following: 1) the alpha-anomer, but not the beta-anomer, of Glc binds to BBMV, more in the presence of Na+ than of K+; 2) interactions of P(i) with BBMV that increase its 1/T2 are significant in the presence of K+ but not of Na+; 3) temperature and magnetic field dependencies of the 1/T2 of P(i) bound to BBMV are consistent with more rapid exchange with free P(i) in the presence of Na+ than of K+; and 4) in the presence of Na+, [H+] promoted interactions of P(i) with BBMV that increase its 1/T2, probably by interfering with formation of rapidly dissociating Na(+)-P(i)-carrier complexes. In conclusion, Na+ promotes association of alpha-Glc to BBMV but promotes rapid exchange and dissociation of P(i) from BBMV. In both cases, Na+ effects coupling by altering the affinity of the cotransporters for the substrate.