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 Ca-activated TMEM16A Cl channels is thought to cause an increase in cyst volume. Recent work shows the pro-proliferative role of the Ca 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 Ca ([Ca]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 Ca levels and enhanced purinergic Ca 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 [Ca]i. Ca signals, proliferation, and Cl secretion were largely reduced by knockdown or blockade of TMEM16A. TMEM16A may be therefore important for enhanced Ca release from IP-sensitive Ca stores in polycystic kidney disease. KEY MESSAGES: • ADPKD leads to continuous decline of renal function by growth of renal cysts. • Knockdown of PKD1 or PKD2 increases TMEM16A expression. • TMEM16A enhanced intracellular Ca signals, Cl secretion, and proliferation. • TMEM16A contributes to cyst growth in ADPKD.