EFFECTS OF LOW-MOLECULAR GTP BINDING PROTEINS AND TYROSINE KINASE ACTIVATION ON ION CHANNELS OF MYOCARDIAL CELLS.

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The aim of this research project is to determine whether low-molecular GTP binding proteins (small G protein) and activation of intracellular tyrosine kinase play crucial roles in signal transduction mechanism of cell-surface receptors.Using a patch clamp method, we observed influences of modified small G protein by botulinum C_3 toxin and of insulin on membrane current system (I_<Na>, I_<Ca>, I_K, I_<K1> and I_<to>) of single myocardial cells obtained from rabbit hearts. In some ex13EA\ : periments, action potentials of rabbit and guinea-pig papillary muscles hearts were recorded by means of glass microelectrodes, and intracellular _pH was monitored with hydrogen-ion selective microelectrodes. The following results were obtained.1) ADP-ribosylation of small G protein (rho protein) by botulinum C_3 toxin did not affect I_<Na>, I_<Ca>, I_K, I_<K1> and I_<to>, suggesting that rho ptotein is not an active member of signal transduction pathway controlling receptor-mediated alterat13EA\ : ions of ion-channels.2) Insulin did not alter in any rate major membrane currents of single ventricular cells of rabbits and also did not affect Na^+/H^+ exchangers and anion transporters in guinea-pig papillary muscles. This finding implies that tyrosine kinase prese13EA\ : nt in intracellular domain of insulin receptor does not modulate ion channel functions. However, insulin did inhibit Na^+ pump current. This unexpected results warrant further study.3) Functional inactivation of high-molecular G protein (probably G_i) induced by pertussis toxin potentiated alpha_1-adrenoceptor mediated cellular responses such as prolonged action potential duration, positive inotropic effect and hydrosis of ph13EA\ : osphatidyl inositides. This finding denies G_i as a candidate of high-molecular G protein coupling to alpha_1-adrenoceptor and suggests dynamic inhibition of G_i on intracellular signal transduction system.