Insensitivity of cardiac delayed-rectifier I _Kr to tyrosine phosphorylation inhibitors and stimulators

1 The rapidly activating delayed-rectifying K ⁺ current (I _Kr) in heart cells is an important determinant of repolarisation, and decreases in its density are implicated in acquired and inherited long QT syndromes. The objective of the present study on I _Kr in guinea-pig ventricular myocytes was to evaluate whether the current is acutely regulated by tyrosine phosphorylation. 2 Myocytes configured for ruptured-patch or perforated-patch voltage-clamp were depolarised with 200-ms steps to 0 mV for measurement of I _Kr tail amplitude on repolarisations to -40 mV. 3 I _Kr in both ruptured-patch and perforated-patch myocytes was only moderately (14-20%) decreased by 100 μM concentrations of protein tyrosine kinase (PTK) inhibitors tyrphostin A23, tyrphostin A25, and genistein. However, similar-sized decreases were induced by PTK-inactive analogues tyrphostin A1 and daidzein, suggesting that they were unrelated to inhibition of PTK. 4 Ruptured-patch and perforated-patch myocytes were also treated with promoters of tyrosine phosphorylation, including phosphotyrosyl phosphatase (PTP) inhibitor orthovanadate, exogenous c-Src PTK, and four receptor PTK activators (insulin, insulin-like growth factor-1, epidermal growth factor, and basic fibroblast growth factor). None of these treatments had a significant effect on the amplitude of I _Kr. 5 We conclude that Kr channels in guinea-pig ventricular myocytes are unlikely to be regulated by PTK and PTP.