Adrenergic regulation of salt and fluid secretion in human medullary collecting duct cells

Transepithelial salt and fluid secretion mediated by cAMP in initial inner medullary collecting ducts (IMCDi) may be important for making final adjustments to urine composition. We examined in primary cultures of human IMCDicells the effects of adrenergic receptor (AR) agonists and antagonists on intracellular cAMP levels, short-circuit current ( ISC), and fluid secretion. Epinephrine (1 μM), norepinephrine (1 μM), and isoproterenol (10 nM) individually increased intracellular cAMP levels 57-, 2-, and 25-fold, respectively, and stimulated ISC3.3-, 2.9-, and 3.4-fold, respectively. β-AR activation increased net fluid secretion by cultured human IMCDicell monolayers from 0.09 ± 0.04 to 0.26 ± 0.05 μl·h−1·cm−2and freshly isolated rat IMCDifrom 0.02 ± 0.01 to 0.09 ± 0.02 nl·h−1·mm−1. In monolayers, these effects were eliminated by blocking β2-AR, but not β1-AR. Activation of α2-AR with guanabenz inhibited isoproterenol-induced ISCby 37% in human IMCDimonolayers and fluid secretion by 91% in rat IMCDi. Immunohistochemistry of human medullary tissue sections revealed greater expression of β2-AR than β1-AR; β2-AR was localized to the basolateral membranes of human IMCDi. Immunoblots identified α2A-AR and α2B-AR in cultured human IMCDicell monolayers. We conclude that 1) catecholamines stimulate cAMP-dependent anion and fluid secretion by IMCDicells primarily through β2-AR activation and 2) α2-AR activation attenuates cAMP-dependent anion secretion.