Aims: To examine collecting duct adenylyl cyclase (AC) isoforms and their regulation by AVP in the mouse. Methods: We performed RT-PCR for AC isoforms 1-9 in microdissected cortical, outer medullary and inner medullary collecting duct (IMCD). AVP-induced cAMP accumulation was examined in IMCD using pharmacologic agents and siRNA. Results: All collecting duct regions contained AC3, 4 and 6 mRNA, while cortical and outer medullary CD, but not IMCD, contained AC5 mRNA. IMCD expressed AC3 and 6 proteins by Western analysis; insufficient cortical and outer medullary collecting ducts were obtained for protein studies. M1, mIMCD3 and mpkCCD cell lines contained AC2, 3, 4 and 6 mRNA; mIMCD3 also contained AC5 mRNA. AVP increased cAMP in acutely isolated mouse IMCD; this was not affected by blockade of nitric oxide, reactive oxygen species, cyclooxygenase, PI3 kinase, Akt or protein kinase C. Chelation of extracellular (EDTA) or intracellular (BAPTA) Ca, and blockade of calmodulin (W7) inhibited AVP-stimulated cAMP, suggesting that Ca/calmodulin-stimulated AC3 is involved in mediating AVP action. Blockade of calmodulin kinase (CaMKII, KN93) or endoplasmic reticulum Ca ATPase (thapsigargin) markedly inhibited the AVP response. CaMKII normally inhibits AC3 activity, but can stimulate AC6; similarly thapsigargin should either not change or increase AC3 activity, but has been reported to reduce AC6 (presumably due to stimulation of capacitative Ca entry). Transfection of AC6 or AC3 siRNA into primary cultured mouse IMCD caused a 42% and 24% decrease in AVP-stimulated cAMP accumulation, respectively. AC4 siRNA had no effect on AVP signaling. Conclusion: These data indicate that AC6 and AC3, but not AC4, mediate AVP-stimulated cAMP accumulation in collecting duct. This raises the possibility that AVP regulation of solute and water transport, as well as cell proliferation, may be partly mediated by differential activation of at least two AC isoforms.