HORMONE AND AUTACOID REGULATION OF CAMP PRODUCTION IN RAT IMCD SUBSEGMENTS

The inner medullary collecting duct (IMCD) of the rat consists of two structurally and functionally distinct segments, i.e., the initial and the terminal IMCD. To identify factors that may regulate the transport function in the IMCD segments, we assessed whether catecholamines, carbachol, prostaglandin E2 (PGE2), bradykinin, glucagon, calcitonin, parathyroid hormone, or epidermal growth factor affects adenosine 3',5'-cyclic monophosphate (cAMP) production in microdissected tubules in the presence and absence of arginine vasopressin (AVP, 0.1 nM). All experiments were performed in the presence of 3-isobutyl-1-methylxanthine, and cAMP was measured by radioimmunoassay. Epinephrine (greater-than-or-equal-to 50 nM) and clonidine (greater-than-or-equal-to 1-mu-M) markedly decreased AVP-induced cAMP levels in both IMCD segments. However, phenylephrine did not show an effect. The inhibitory effect of epinephrine was blocked by yohimbine (50 nM) but not by prazosin (50 nM). In isolated perfused terminal IMCDs, epinephrine inhibited AVP-stimulated urea permeability. Isoproterenol (1-mu-M), in the absence of AVP, caused a significant increase in cAMP level only in the initial IMCD. Propranolol (1-mu-M) inhibited this isoproterenol effect, but atenolol did not. Dopamine (less-than-or-equal-to 1-mu-M) had no effect on cAMP levels in either IMCD segment. Carbachol, PGE2, and the various peptide hormones had no effect on cAMP levels (+/-AVP) in either IMCD segment. We conclude that an adrenergic beta-2-receptor is present only in the initial IMCD, where its occupation increases cAMP production. We conclude also that an adrenergic alpha-2-receptor is present in both IMCD segments, where its occupation inhibits AVP-induced cAMP production. The study also showed that carbachol, PGE2, and several peptide hormones had no effect on cAMP production in both IMCD segments.