V asopressin (AVP) plays a key role in maximal urine concentration by stimulating NaCl reabsorption in the medullary thick ascending limbs of Henle (MAL) and by increasing water permeability in the medullary collecting tubules (MCT). These effects of AVP in MAL and MCT are mediated by cAMP. Alpha 2-adrenergic stimulation in MCT, and high ambient Ca2+ and PGE2 in MAL inhibit AVP-dependent cAMP production and thereby modulate urine concentration. The present study was undertaken to clarify the mechanisms underlying the inhibition of AVP-dependent cAMP production by these agents using microdissected mouse MAL and MCT. Preincubation of MCT and MAL with 1 microgram/ml pertussis toxin for 3 and 6 h, respectively, resulted in ADP-ribosylation of an approximately 41-kD protein, which was presumably an alpha subunit of the inhibitory GTP-binding protein Gi. Epinephrine, 10(-6) M, via alpha 2-adrenergic stimulation, inhibited AVP-dependent cAMP production in MCT. Preincubation of MCT for 3 h with pertussis toxin abolished the inhibition of AVP-dependent cAMP production by epinephrine. High ambient Ca2+ and PGE2 both inhibited AVP-dependent cAMP production in MAL. Preincubation of MAL for 6 h with pertussis toxin abolished the inhibition by high ambient Ca2+ and attenuated the inhibition by PGE2. Preincubation of MCT or MAL with pertussis toxin for 1 h was ineffective in ADP-ribosylation and did not modify the inhibition of AVP-dependent cAMP production by these agents in both nephron segments. Our data suggest that the inhibition of AVP-dependent cAMP production by alpha 2-adrenergic stimulation in MCT, and by high ambient Ca2+ and adrenergic stimulation in MCT, and by high ambient Ca2+ and PGE2 in MAL, is mediated, at least in part, through activation of Gi.