抄録
In contrast to cardiac myocytes, sympathetic stimulation does not largely enhance L-type Ca2+ channel current in smooth muscle cells. In the present study, we assessed possible mechanisms underlying this discrepancy, using a whole-cell clamp technique. In guinea-pig detrusor cells, only L-type Ca2+ channels occur. During depolarizations of large positivity, the conformation of the majority of Ca2+ channels is converted from the normal (O1) to a second open state (O2), in which Ca2+ channels do not, or only slowly inactivate during depolarization. This feature of the O2 state produces U-shaped inactivation. In order to estimate the population of Ca2+ channels that can be converted to the O2 state, we applied a paired pulse protocol: Test steps with and without preconditioning step (+80 mV, 4s) were alternately applied. Extracellular application of genistein decreased the amplitudes of both conditioned and unconditioned test inward currents (Acond and Auncond), accompanied by significant reduction of Acond/Auncond, while genistin, an inactive analogue, did not. Intracellular application of genistein caused similar or more pronounced effects, when ATP was removed from the patch pipette. This result is consistent with the fact that ATP antagonizes the inhibitory effect of genistein on tyrosine kinase activity. It is concluded that even under normal conditions smooth muscle L-type Ca2+ channels are already in a "stimulated mode° due to a tyrosine-kinase-related mechanism(s). [J Physiol Sci. 2006;56 Suppl:S79]
