Vasopressin Regulates the Phosphorylation State of AMP-activated Protein Kinase (AMPK) in MDCK-C7 Cells
AMP-activated protein kinase (AMPK) is a regulatory kinase coupling cellular metabolism with ion transport. Madin-Darby Canine Kidney-Clone 7 (MDCK-C7) cells possess characteristics of the renal principal cell type, express the cystic fibrosis transmembrane regulator and the epithelial Na+ channel, and display NPPB and amiloride-sensitive transepithelial transport when stimulated with [Arg8]-vasopressin. [Arg8]-vasopressin binding to its receptor on the basolateral membrane of MDCK-C7 results in cAMP production, activation of cAMP-dependent protein kinase A (PKA), and increases in Cl- and Na+ transport. Ussing-style electrophysiology showed that the PKA inhibitor, H89, blocked Cl- and Na+ transport. Unexpectedly, [Arg8]-vasopressin stimulation resulted in the dephosphorylation of pAMPKthr172. H89 did not prevent this, suggesting that the dephosphorylation is independent of PKA. 24 hour, but not 15 minute, incubation with the AMPK activator, AICAR, also blocked [Arg8]-vasopressin-stimulated currents. Contrary to previous studies, immunoblotting revealed that AICAR did not increase abundance of the active, phosphorylated form of AMPK (pAMPKthr172); although, AICAR treatment significantly blocked [Arg8]-vasopressin -stimulated cAMP production. [Arg8]-vasopressin still caused pAMPKthr172 dephosphorylation in the presence of AICAR, suggesting that this effect is also independent of cAMP. In summary, these data suggest [Arg8]-vasopressin regulates AMPK phosphorylation and that AICAR inhibits ion transport independently of AMPK in MDCK-C7 cells.