Polycystic kidney disease (PKD) leads to continuous decline of renal function by growth of renal cysts. Enhanced proliferation and transepithelial chloride secretion through cystic fibrosis transmembrane conductance regulator (CFTR) and Ca2+-activated TMEM16A Cl- channels is thought to cause an increase in cyst volume. Recent work shows the pro-proliferative role of the Ca2+ activated Cl- channel TMEM16A (anoctamin 1), and demonstrates the essential contribution of TMEM16A to CFTR-dependent Cl- secretion. The present data demonstrate an increase in intracellular Ca2+ ([Ca2+]i) signals and Cl- secretion by TMEM16A, in renal collecting duct principle cells from dog (MDCK) and mouse (M1) as well as primary tubular epithelial cells from PKD1-/- knockout mice. M1 organoids proliferated, increased expression of TMEM16A, and secreted Cl- upon knockdown of endogenous polycystin 1 or 2 (PKD1,2), by retroviral transfection with shPKD1 and shPKD2, respectively. Knockdown of PKD1 or PKD2 increased basal intracellular Ca2+ levels and enhanced purinergic Ca2+ release from endoplasmic reticulum. In contrast, ryanodine receptors were found not to be expressed in mouse renal epithelial cells and caffeine had no effects on [Ca2+]i. Ca2+ signals, proliferation, and Cl- secretion were largely reduced by knockdown or blockade of TMEM16A. TMEM16A may be therefore important for enhanced Ca2+ release from IP3-sensitive Ca2+ stores in polycystic kidney disease. Key messages center dot ADPKD leads to continuous decline of renal function by growth of renal cysts. center dot Knockdown of PKD1 or PKD2 increases TMEM16A expression. center dot TMEM16A enhanced intracellular Ca2+ signals, Cl- secretion, and proliferation. center dot TMEM16A contributes to cyst growth in ADPKD.