Tyrosine kinase inhibition reduces i(f) in rabbit sinoatrial node myocytes

We studied pacemaker current (i(f)), the inward current activated by hyperpolarization in rabbit sinoatrial (SA) node myocytes, with the permeabilized-patch-clamp technique. The tyrosine kinase inhibitors genistein (50 microM) or herbimycin A (35 microM) reduced the amplitude of i(f) in response to step hyperpolarizations in the diastolic range of potentials. A two-step voltage-clamp protocol revealed that the reduction in i(f) is due to a decrease in maximal i(f) conductance. The observed effects are due to tyrosine kinase inhibition since an inactive analog of genistein did not reduce i(f). To further examine the mechanism of action, we added 2 mM chlorophenylthio cAMP (CPTcAMP, a membrane-permeant cAMP analog) to the bathing Tyrode, which increased i(f). Genistein still reduced i(f) in the presence of CPTcAMP. This suggests that the pathway mediating the actions of tyrosine kinase inhibition on i(f) is independent of cAMP- or protein-kinase-A-mediated phosphorylation.