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

The adrenergic system is important in regulating proximal tubule sodiu m reabsorption. Although alpha-adrenergic receptors have been identifi ed in proximal tubules, the presence and function of -adrenergic recep tors (BAR) in proximal tubules is less certain. The purpose of our stu dy was to determine whether functional BAR are present on apical or ba solateral surfaces of proximal tubule epithelial cells (PTEC) of rat k idney. We specifically focused on BAR coupling to adenylate cyclase an d on differences between requirements for apical and basolateral recep tor coupling to adenylate cyclase. To determine BAR expression and fun ction, 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. Api cal 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 medi ated 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 anta gonists. Apical B(1)AR required endocytosis prior to adenylate cyclase activation, since increases in cAMP were prevented by phenylarsine ox ide or colchicine. B(1)AR-adenylate cyclase coupling was independent o f intra- or extracellular calcium, cyclooxygenase metabolites, and pro tein kinase C (PKC) and dependent on G(s) guanine nucleotide regulator y protein. Prolonged exposure to ISO resulted in time- and concentrati on-dependent homologous desensitization of cAMP responses. Desensitiza tion was independent of receptor sequestration, PKA, or PKC. We conclu de the following: B(1)AR are present on both apical and basolateral su rfaces 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.