Effect of AC3, AC4, and AC6 small interfering RNA (siRNA) on AVP-stimulated cAMP accumulation in primary cultures of mouse IMCD. Cells were treated with AC3, AC4, or AC6 siRNA as described in methods. For cAMP studies, control cells (treated with irrelevant random sequence RNA) or cells treated with AC isoform-specific siRNA were preincubated with 1 mM IBMX for 30 min, followed by addition of 1 nM AVP for 5 min (n = 16–18 each data point). *P < 0.01 vs. control; **P < 0.001 vs. control.