BETA-ADRENERGIC-RECEPTOR FUNCTION IN RAT PROXIMAL TUBULE EPITHELIAL-CELLS IN CULTURE

The adrenergic system is important in regulating proximal tubule sodium reabsorption. Although alpha-adrenergic receptors have been identified in proximal tubules, the presence and function of -adrenergic receptors (BAR) in proximal tubules is less certain. The purpose of our study was to determine whether functional BAR are present on apical or basolateral surfaces of proximal tubule epithelial cells (PTEC) of rat kidney. We specifically focused on BAR coupling to adenylate cyclase and on differences between requirements for apical and basolateral receptor coupling to adenylate cyclase. To determine BAR expression and function, primary cultures of rat PTECs were grown on permeable supports. Scatchard analysis of I-129-labeled cyanopindolol binding revealed a single class of receptors on both apical and basolateral surfaces. Apical isoproterenol (ISO) resulted in time- and concentration-dependent increases in adenosine 3',5'-cyclic monophosphate (cAMP) that were 50% of responses after basolateral ISO. Apical BAR-cAMP coupling was mediated by B-1-adrenergic receptors (B(1)AR), since apical cAMP responses were abrogated with apical (but not basolateral) B-1 but not B-2 antagonists. Apical B(1)AR required endocytosis prior to adenylate cyclase activation, since increases in cAMP were prevented by phenylarsine oxide or colchicine. B(1)AR-adenylate cyclase coupling was independent of intra- or extracellular calcium, cyclooxygenase metabolites, and protein kinase C (PKC) and dependent on G(s) guanine nucleotide regulatory protein. Prolonged exposure to ISO resulted in time- and concentration-dependent homologous desensitization of cAMP responses. Desensitization was independent of receptor sequestration, PKA, or PKC. We conclude the following: B(1)AR are present on both apical and basolateral surfaces of rat PTECs. Apical B(1)AR 1) are coupled to adenylate cyclase by G(s), 2) require endocytosis to activate adenylate cyclase, and 3) undergo homologous desensitization.