CFTR and Complex Cyst of Kidney: Taken all together, we propose that as in kidney cysts, in KV cells the drop of the intracellular Ca2+ levels caused by the knockdown of Polycystin-2, leads to the activation of AC5 and AC6 and to the inhibition of PDE1A, raising the levels of cAMP and, thus, activating CFTR (Fig. 5D).