The effects of the alkylating agent N-ethylmalemide (NEM) on the L-type Ca current (ICa) of guinea-pig ventricular myocytes were studied with the whole-cell voltage-clamp technique at 21-23 degrees C. NEM (50-100 microM) produced an increase in ICa comparable to that produced by cAMP-increasing agents (isoprenaline, forskolin, phosphodiesterase inhibitors). The effects of NEM on ICa were not additive with those of the phosphodiesterase blockers papaverine and isobutylmethylxanthine. They were neither suppressed nor prevented by carbachol (100 microM) and could also not be prevented by pertussis toxin treatment or by cell dialysis with GDP beta S (300 microM) or GTP gamma S (300 microM). NEM, at low concentration (50 microM), increased twitch contraction measured in right ventricular strips. At a higher concentration (> or = 100 microM), this effect was followed by a decrease of the twitch and an increase in basal tension. It is proposed that NEM acts by transiently increasing cAMP levels to increase ICa and the twitch. Our experiments are not consistent with a mechanism involving Gi inhibition for NEM effect.