Stimulation of Na+-K+-2Cl(-) cotransport in rat medullary thick ascending limb by dopamine

Dopamine receptors are present in the medullary thick ascending limb (mTAL) of Henle, but their effect on ion transport in this nephron segment has not been tested. Therefore, we studied the short-term effects of dopamine on Na+-K+-2Cl(-) cotransport (assessed by 100 mu M bumetanide-sensitive Rb-86 uptake) in rat mTAL tubular suspensions. Dopamine (1 mu M) stimulated bumetanide-sensitive Rb-86 uptake (72.1 +/- 10.6% vs. control, n = 5) by increasing total Rb-86 uptake and by decreasing bumetanide-insensitive Rb-86 uptake; this effect was concentration dependent. The dopamine-induced stimulation of Na+-K+-2Cl(-) cotransport activity was mimicked by calyculin A, a protein phosphatase (PP) inhibitor, and Sp isomer of adenosine 3',5'-cyclic monophosphothioate (Sp-cAMP[S]), a protein kinase A (PKA) agonist, and blocked by Rp isomer of 8-(4-chlorophenylthio)-cAMP[S] (Rp-8-CPT-cAMP[S]), a PKA inhibitor (n = 5). Dopamine did not increase the stimulatory effect of the PP inhibitor. However, the stimulatory effect of the PP inhibitor and PKA agonist was additive and approached the stimulatory effect of dopamine. The stimulatory effects of dopamine, PP inhibitor, and PKA agonist persisted even when intracellular sodium was clamped by 5 mu M monensin. When K+ channels were blocked by 1 mM BaCl2, the effects of dopamine and calyculin A on the cotransport were no longer apparent, although the stimulatory effect of the PKA agonist was attenuated. We conclude that dopamine stimulates Na+-K+-2Cl(-) cotransport activity. This action is mediated mainly by PKA-dependent phosphorylation/dephosphorylation processes and modulated by dopamine actions on K+ channels.