Does dopamine use several signal pathways to inhibit Na-Pi transport in OK cells?

Abstract

In opossum kidney (OK) cells, L-dihydroxyphenylalanine (10 microM) raised dopamine to 10 nM and inhibited Na-inorganic phosphate (Pi) uptake 20% (P = 0.001). Inhibition was completely blocked by carbidopa or SCH23390. Dopamine (1 microM) inhibited uptake 55% (half-maximal inhibition, 0.03 microM). Fenoldopam (0.1 microM, DA1 agonist) inhibited uptake 45 +/- 2%. DA1 antagonists (SKF83566 and SCH23390), but not DA2-antagonist (sulpiride), blocked dopamine inhibition. Quinpirole (DA2 agonist) did not modify Pi uptake. Bisindolylmaleimide (10 microM), a protein kinase C inhibitor, blocked inhibition of Pi uptake by phorbol ester but had no effect on the response to dopamine. Dopamine inhibited Pi uptake in cells that had been exposed to phorbol ester for 18 to 24 h. Dopamine inhibition was not reduced by 1 microM U73,122 but was reduced 20% by 10 microM, which is 10 times the concentration reported to completely inhibit phospholipase C in OK cells. Adenylate cyclase inhibitors SQ 22536 (100 microM) and 2,5-dideoxyadenosine (100 microM) reduced dopamine-stimulated cAMP production, but not dopamine inhibition of Pi uptake. Rp-cAMPS counteracted the inhibition of Pi uptake by Sp-cAMPS but had no effect on the dopamine response. H-89 inhibited dopamine-stimulated protein kinase A activity, but neither H-89 nor H-9 alone or with bisindolylmaleimide altered dopamine inhibition of Pi uptake. Genistein and herbimycin A (tyrosine kinase inhibitors) reduced Pi uptake. However, dopamine, a benzoquinone like several tyrosine kinase inhibitors, did not inhibit tyrosine kinase activity. Thus, dopamine inhibited Pi uptake in this OK cell clone by activating a G protein-linked pathway that operates independently from adenylyl cyclase, protein kinase A, protein kinase C, and protein tyrosine kinase.