AJP - Renal Physiology, Vol 258, Issue 6 1616-F1624, Copyright © 1990 by American Physiological Society
Heterogeneity of Pi transport by BBM from superficial and juxtamedullary cortex of rat
M. Levi
Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas.
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)
