Mechanisms of heterogeneity of Na(+)-Pi cotransport in superficial and juxtamedullary renal cortex.
To explore the mechanisms of axial heterogeneity of proximal tubular Pi transport, brush border membrane vesicles (BBMV) were prepared from superficial (BBMV-SC) and juxtamedullary (BBMV-JM) cortex of rat kidneys. Na+ gradient-dependent Pi transport was measured after the imposition of an inside-negative electrical potential created by either a K+ gradient ([K+]i greater than [K+]o) or a proton gradient ([H+]i greater than [H+]o) in the presence of ionophores. The initial Na(+)-dependent Pi 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 Pi transport was taken into account. The stoichiometry of Na(+)-Pi cotransport was 2:1 in BBMV-SC and BBMV-JM. Phosphonoformic acid (PFA) competitively inhibited Pi transport. The inhibitory constant (Ki) for PFA was lower in BBMV-SC (237 +/- 1.7 microM) than in BBMV-JM (409 +/- 53 microM) (P less than 0.05). Arrhenius plots showed a higher rate of Pi uptake in BBMV-SC compared to BBMV-JM at all temperatures. However, the transition temperatures did not differ. We conclude that axial heterogeneity of Pi transport is not due to differences in electrogenicity or stoichiometry of transport.
Loghman-Adham
M
M
Department of Pediatrics, University of Utah School of Medicine, Salt Lake City.
D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002352:Carrier Proteins / Q000378:metabolism*; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D004563:Electrochemistry; D017245:Foscarnet; D007672:Kidney Cortex / Q000187:drug effects / Q000378:metabolism*; D007700:Kinetics; D008871:Microvilli / Q000187:drug effects / Q000378:metabolism; D010710:Phosphates / Q000037:antagonists & inhibitors / Q000378:metabolism; D010746:Phosphonoacetic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D011188:Potassium / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D012964:Sodium / Q000378:metabolism*; D050608:Sodium-Phosphate Cotransporter Proteins; D027981:Symporters; D013696:Temperature
D002352:Carrier Proteins; D010710:Phosphates; D050608:Sodium-Phosphate Cotransporter Proteins; D027981:Symporters; D017245:Foscarnet; D012964:Sodium; D010746:Phosphonoacetic Acid; D011188:Potassium