Stimulation of P (i) transport in opossum kidney cells by hyposmotic media
Loghman-Adham, M.; Motock, G.T.
American Journal of Physiology 264(4 Pt 2): F585-F592
1993
ISSN/ISBN: 0002-9513 PMID: 7682788 Document Number: 409550
Exposure of various cells to hyposmotic media (Hypo) results in a rapid inhibition of both receptor-mediated and fluid-phase endocytosis. We used this maneuver to investigate the role of endocytosis in regulation of P-i transport in opossum kidney (OK) cells. Following exposure to Hypo, Na+-dependent P-i uptake increased rapidly, reaching a maximum within 5 min, and remained elevated up to 30 min. This was associated with a simultaneous reduction of horseradish peroxidase uptake. Kinetic studies showed increased apparent V-max for P-i (9.38 +- 0.93 vs. 13.08 +- 1.04 nmol cntdot mg-1 cntdot 5 min-1 for control and Hypo, respectively; P lt 0.05, n = 6) with no change in apparent K-m. The effect was specific for P-i with no change in the Na+-dependent or -independent uptake of L-proline, L-glutamine, or methyl-alpha-D-glucopyranoside. Stimulation of P-i transport persisted when control and Hypo had identical ionic compositions. Stimulation of P-i transport was rapidly reversed when cells were returned to an isosmotic medium. Preincubation with Hypo at 4 degree C had no effect on P-i transport. Addition of cycloheximide or actinomycin D did not prevent the increased P-i uptake after exposure to Hypo. The effect also persisted after protein kinase C downregulation. Stimulation of P-i transport by Hypo is consistent with reduced endocytic retrieval of Na+-P-i cotransporters from brush-border membrane (BBM), resulting in an increase in their number on the BBM.