Involvement of G protein-coupled receptor kinase 4 and 6 in rapid desensitization of dopamine D1 receptor in rat IEC-6 intestinal epithelial cells

Dopamine-induced inhibition of Na+-K+-ATPase has been suggested to play a role in the regulation of Na+ absorption at the intestinal level, and these effects were mediated by dopamine D1-like receptors. The aim of this work was to evaluate the effect of the activation of the D1-like receptors on the activity of the Na+/H+ exchanger (NHE) in the rat intestinal epithelial cell line IEC-6. The presence of D1 receptors was confirmed by immunoblotting. The dopamine D1-like receptor agonist SKF-38393 produced a concentration-dependent inhibition of NHE activity and stimulation of adenylyl cyclase (AC), this being antagonized by the D1 selective antagonist SKF-83566. Effects of SKF-38393 on NHE and AC activities were maximal at 5 min of exposure to the agonist and rapidly diminished with no effect at 25 min. Exposure of cells for 25 min to dibutyryl-cAMP (0.5 mM) or to the AC activator forskolin (3 μM) effectively inhibited NHE activity. Pretreatment of cells with heparin (1 μM), a nonselective G protein-coupled receptor kinase (GRK) inhibitor, prevented the loss of effects on NHE activity after 25 min exposure to SKF-38393. The presence of GRK4, GRK6A, and GRK6B was confirmed by immunoblotting. Overnight treatment with the anti-GRK4–6 antibody complexed with Lipofectin was also effective in preventing loss of the effects of SKF-38393 on NHE and AC activities. It is concluded that dopamine D1 receptors in IEC-6 rapidly desensitize to D1-like agonist stimulation and GRK4 and 6 appear to be involved in agonist-mediated responsiveness and desensitization.