The action of N-ethylmaleimide on the membrane currents and contractile tension in the bullfrog atrium.
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.
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D001001:Anura; D002118:Calcium / Q000502:physiology; D003545:Cysteine / Q000494:pharmacology; D005033:Ethylmaleimide / Q000008:administration & dosage / Q000494:pharmacology*; D006321:Heart / Q000187:drug effects*; D006325:Heart Atria / Q000187:drug effects; D066298:In Vitro Techniques; D009200:Myocardial Contraction / Q000187:drug effects*; D011892:Rana catesbeiana; D013268:Stimulation, Chemical
D003545:Cysteine; D005033:Ethylmaleimide; D002118:Calcium