Mechanisms of heterogeneity of Na +-P i cotransport in superficial and juxtamedullary renal cortex

Abstract

To explore the mechanisms of axial heterogeneity of proximal tubular P i transport, brush border membrane vesicles (BBMV) were prepared from superficial (BBMV-SC) and juxtamedullary (BBMV-JM) cortex of rat kidneys. Na + gradient-dependent P i transport was measured after the imposition of an inside-negative electrical potential created by either a K + gradient ([K +] i > [K +] o) or a proton gradient ([H +] i > [H +] o) in the presence of ionophores. The initial Na +-dependent P i uptake was higher in BBMV-SC than in BBMV-JM, both in the presence and absence of ionophores. Na +-dependent d-glucose uptake remained unchanged. We did not find a significant electrogenic transport component in either BBMV population when the non-specific effect of ionophores on P i transport was taken into account. The stoichiometry of Na +-P i cotransport was 2:1 in BBMV-SC and BBMV-JM. Phosphonoformic acid (PFA) competitively inhibited P i transport. The inhibitory constant ( K i) for PFA was lower in BBMV-SC (237 ± 1.7 μM) than in BBMV-JM (409 ± 53 μM) ( P < 0.05). Arrhenius plots showed a higher rate of P i uptake in BBMV-SC compared to BBMV-JM at all temperatures. However, the transition temperatures did not differ. We conclude that axial heterogeneity of P i transport is not due to differences in electrogenicity or stoichiometry of transport.