Abstract
1. The regulation of cardiac Cl- current (ICl) by tyrosine and serine/threonine phosphorylation was examined in guinea-pig and rat ventricular myocytes. The protein tyrosine kinase (PTK) inhibitor genistein (GST) and phosphotyrosine phosphatase (PTP) inhibitor sodium orthovanadate (VO4) were used to modify tyrosine phosphorylation, whereas forskolin (FSK), cAMP, and other agents were used to modify cytoplasmic cAMP concentration and protein kinase A (PKA) phosphorylation. 2. Low concentrations (0.1 microM) of FSK did not activate the PKA-regulated cystic fibrosis transmembrane regulator (CFTR) ICl in guinea-pig ventricular myocytes, but strongly potentiated activation of an ICl by 20-100 microM GST. The potentiation did not occur when GST was replaced by PTK-inactive daidzein, and it was strongly inhibited by 1 mM VO4. 3. Potentiation by 0.1 microM FSK was linked to a small stimulation of the adenylate cyclase-cAMP-PKA pathway. The potentiation was not mimicked by inactive 1,9-dideoxyforskolin, and was inhibited by muscarinic stimulation (ACh) and by a PKA inhibitor. Internal application of a cAMP solution that alone was too weak to activate CFTR ICl strongly potentiated the activation of ICl by 50 microM GST and occluded potentiation by 0.1 microM FSK. 4. The foregoing suggests that potentiated ICl flows through cAMP-dependent CFTR channels. In agreement with this interpretation, GST did not increase ICl when CFTR was maximally activated by a high concentration (5 microM) of FSK and okadaic acid, and neither GST nor GST plus FSK activated an ICl in CFTR-deficient rat myocytes. The lack of effect in rat myocytes was not due to the absence of functional, channel-relevant PKA and PTK-PTP systems, because (as in guinea-pig myocytes) L-type Ca2+ current (ICa,L) was stimulated by FSK and inhibited in a VO4-reversible manner by GST. 5. The synergistic activation of CFTR by low concentrations of FSK and GST cannot be explained by either a GST-induced elevation of cAMP concentration or inhibition of serine/threonine phosphatase. Rather, it appears to be due to tyrosine dephosphorylation that facilitates PKA-mediated phosphorylation of the channels.