D1-like dopamine receptors downregulate Na+-K+-ATPase activity and increase cAMP production in the posterior gills of the blue crab Callinectes sapidus

Dopamine-mediated regulation of Na+-K+-ATPase activity in the posterior gills of some crustaceans has been reported to be involved in osmoregulation. The dopamine receptors of invertebrates are classified into three groups based on their structure and pharmacology: D-1- and D-2-like receptors and a distinct invertebrate receptor subtype (INDR). We tested the hypothesis that a D-1-like receptor is expressed in the blue crab Callinectes sapidus and regulates Na+-K+-ATPase activity. RT-PCR, using degenerate primers, showed the presence of D1 beta R mRNA in the posterior gill. The blue crab posterior gills showed positive immunostaining for a dopamine D-5 receptor (D5R or D1 beta R) antibody in the basolateral membrane and cytoplasm. Confocal microscopy showed colocalization of Na+-K+-ATPase and D1 beta R in the basolateral membrane. To determine the effect of D-1-like receptor stimulation on Na+-K+-ATPase activity, intact crabs acclimated to low salinity for 6 days were given an intracardiac infusion of the D-1-like receptor agonist fenoldopam, with or without the D-1-like receptor antagonist SCH23390. Fenoldopam increased cAMP production twofold and decreased Na+-K+-ATPase activity by 50% in the posterior gills. This effect was blocked by coinfusion with SCH23390, which had no effect on Na+-K+-ATPase activity by itself. Fenoldopam minimally decreased D1 beta R protein expression (10%) but did not affect Na+-K+-ATPase alpha-subunit protein expression. This study shows the presence of functional D1 beta R in the posterior gills of euryhaline crabs chronically exposed to low salinity and highlights the evolutionarily conserved function of the dopamine receptors on sodium homeostasis.