Tyrosine kinase and phosphatase regulation of slow delayed-rectifier K⁺ current in guinea-pig ventricular myocytes

The objective of this study was to investigate the involvement of tyrosine phosphorylation in the regulation of the cardiac slowly activating delayed-rectifier K⁺ current (I_Ks) that is important for action potential repolarization. Constitutive I_Ks recorded from guinea-pig ventricular myocytes was suppressed by broad-spectrum tyrosine kinase (TK) inhibitors tyrphostin A23 (IC₅₀, 4.1 ± 0.6 μM), tyrphostin A25 (IC₅₀, 12.1 ± 2.1 μM) and genistein (IC₅₀ , 64 ± 4 μM), but was relatively insensitive to the inactive analogues tyrphostin A1, tyrphostin A63, daidzein and genistin. I_Ks was unaffected by AG1478 (10 μM), an inhibitor of epidermal growth factor receptor TK, and was strongly suppressed by the Src TK inhibitor PP2 (10 μM) but not by the inactive analogue PP3 (10 μM). The results of experiments with forskolin, H89 and bisindolylmaleimide I indicate that the suppression of I_Ks by TK inhibitors was not mediated via inhibition of (I_Ks -stimulatory) protein kinases A and C. To evaluate whether the suppression was related to lowered tyrosine phosphorylation, myocytes were pretreated with TK inhibitors and then exposed to the phosphotyrosyl phosphatase inhibitor orthovanadate (1 mM)Orthovanadate almost completely reversed the suppression of I_Ks induced by broad-spectrum TK inhibitors at concentrations around their IC₅₀ values. We conclude that basal I_Ks is strongly dependent on tyrosine phosphorylation of Ks channel (or channel-regulatory) protein.