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Abstract
PURPOSE: To determine the effects of dopamine on Na+,K+,Cl- cotransport in human ciliary nonpigmented epithelial (NPE) cells. METHODS: The authors used 86Rb+ as a marker for K+ to study ouabain-insensitive, bumetanide-sensitive 86Rb+ uptake in cultured fetal human NPE monolayers. RESULTS: Na+,K+,Cl- cotransport was stimulated 1.63-fold by 10 microM dopamine. Stimulation was dose dependent, with a maximum stimulation occurring at 10 microM dopamine and an EC50 of 0.5 microM. NaK-ATPase (measured as ouabain-sensitive, bumetanide-insensitive 86Rb+ uptake) and bumetanide-insensitive, ouabain-insensitive 86Rb+ uptake were not affected by dopamine. The D1-receptor-specific antagonist, SCH23390, inhibited stimulation by 10 microM dopamine more than 90% at 1 microM, with an IC50 of 4 nM, whereas the D2-receptor-specific antagonist, sulpiride, was over 250 times less effective. Similarly, a D1 agonist, SKF81297, was more potent than the D2 agonist bromocriptine in stimulation of Na+,K+,Cl- cotransport. The beta-adrenergic antagonists timolol and propranolol did not significantly inhibit stimulation of Na+,K+,Cl- cotransport by dopamine. Conversely, SCH23390, showed minimal inhibition of isoproterenol stimulation of Na+,K+,Cl- cotransport. Stimulation by maximally stimulating concentrations of isoproterenol and dopamine were not additive, but were similar to stimulation by 1 microM forskolin, suggesting that adenylyl cyclase may be close to maximally activated by either catecholamine. In vivo concentrations (stimulation approximately 25% over control) of dopamine, isoproterenol, and norepinephrine added together stimulated Na+,K+,Cl- cotransport 80% to 89% of stimulation by maximal concentrations of these drugs. The protein kinase A inhibitor N-[2-p-bromocinnamylaminoethyl]-5-isoquinolinesulfonamide (H-89) blocked dopamine stimulation of Na+,K+,Cl- cotransport by more than 75%, whereas phorbol 12-myristate 13-acetate (PMA), a protein kinase C activator, given with 10 microM dopamine inhibited Na+,K+,Cl- cotransport by 75% to 80%, similarly to inhibition by PMA given alone. CONCLUSIONS: Dopamine stimulates Na+,K+,Cl- cotransport in NPE through dopamine-D1-type receptors and activation of protein kinase A. Beta-adrenergic receptors do not appear to play a role. Inhibition of Na+,K+,Cl- cotransport by protein kinase C is dominant over stimulation of Na+,K+,Cl- cotransport through the cyclic adenosine monophosphate pathway.