The Action of N-Ethylmaleimide on the Membrane Currents and Contractile Tension in the Bullfrog Atrium

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  • Action of N Ethylmaleimide on the Membr

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The effects of N-ethylmaleimide (NEM), a sulfhydryl blocking reagent, on the membrane currents, action potentials and contractile tension in bullfrog atrial muscle were studied by using the double sucrose gap technique. In concentrations of 10-3 to 10-4 M, NEM led to a transient enhancement of twitch contraction followed by a late inhibition, while 10-2 M NEM merely produced an inhibitory effect. The positive inotropism was accompanied by a prolongation of action potential and the negative inotropism, by a depression of action potential. Under voltage clamp conditions, 10-3 M NEM reduced the fast inward current and increased the steady state outward and background currents. The slow inward current and the inward Ca current in Na-free conditions were transiently increased by 10-3 M NEM with an enhancement of contractile tension. In voltage-tension relationships, NEM-induced augmentation of tension appeared in depolarizing pulses of 60-70 mV. The results indicate that NEM increases transiently the Ca conductance of the cell membrane. Preincubation with L-cysteine blocked the production of positive inotropic effect by NEM, suggesting that the effects of NEM are due to the blockade of protein sulfhydryl group in the atrial muscle.

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