CAFFEINE-INDUCED BLOCK OF NA+ CURRENT IN GUINEA-PIG SINGLE VENTRICULAR CELLS

Effects of caffeine on Na+ current (I(Na)) were investigated in single ventricular cells from guinea pigs using the whole cell clamp method. With a Ca2+ -containing internal solution (pCa 8.2), 10 mM caffeine blocked I(Na) by 17.5 +/- 4.6% at a -120-mV holding potential. It was accompanied by 3- to 5-mV shifts of the steady-state inactivation curve and time constant-voltage relationship toward hyperpolarization. The inactivation kinetics spontaneously shifted toward hyperpolarization at 0.30 +/- 0.17 mV/min. The spontaneous shift was accompanied by a similar negative shift of the threshold potential, whereas caffeine did not affect it. Caffeine retarded the recovery of I(Na) from inactivation, and a 4-mV positive shift in the recovery potential produced a similar retardation in I(Na) recovery without caffeine. The I(Na) block by caffeine was not influenced by reinforcing the internal buffering capacities using internal solutions containing 40 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid or 50 mM N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid or by pretreating the cell with 1-mu-M ryanodine. Neither pretreatment with isoproterenol or H 7 nor prestimulation of G(s) protein by nonhydrolyzable GTP (GTP-gamma-S) altered the effects of caffeine on I(Na). It is concluded that caffeine inhibits I(Na) and shifts the inactivation kinetics without being mediated by changes in intracellular ionic composition or intracellular signaling systems. Direct action on the channel proteins may be involved.