Heterogeneity of Pi transport by BBM from superficial and juxtamedullary cortex of rat.

There is an axial heterogeneity for proximal tubular phosphate (Pi) transport. Sodium gradient-dependent Pi transport (Na-Pi cotransport) is greater in the proximal convoluted tubule (PCT) than in the proximal straight tubule (PST). In brush-border membrane vesicles (BBMV) isolated from the superficial cortex (SC-BBM) and the juxtamedullary cortex (JMC-BBM), we have also found a heterogeneity in BBM Na-Pi cotransport activity. The greater Na-Pi cotransport activity in SC-BBM was not caused by an alteration in the stoichiometry (2 Na+:1 HPO42-) or the activation of the Na-Pi cotransporter by Na+ (KNa = 37 in SC-BBM vs. 44 mM Na in JMC-BBM, P = NS, not significant). Despite a higher affinity for Pi in JMC-BBM (KPi = 156 in SC-BBM vs. 105 microM Pi in JMC-BBM, P less than 0.001), the Vmax for Na-Pi cotransport was higher in the SC-BBM (Vmax = 2,939 in SC-BBM vs. 1,421 pmol Pi.5 s-1.mg BBM protein-1 in JMC-BBM, P less than 0.001). Na gradient-dependent Pi-protectable phosphonoformic acid (Na-PFA) equilibrium binding studies revealed that the higher Vmax for Na-Pi cotransport in SC-BBM was in part due to a higher number of Na-Pi cotransport units (Bmax = 4.24 in SC-BBM vs. 2.67 nmol PFA.30 min-1.mg BBM protein-1 in JMC-BBM, P less than 0.001). In addition, the fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (rDPH), which is inversely related to fluidity, was lower in SC-BBM (rDPH = 0.247 in SC-BBM vs. 0.254 in JMC-BBM, P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)