A carbamate-type cholinesterase inhibitor 2-sec-butylphenyl N-methylcarbamate insecticide blocks L-type Ca2+ channel in guinea pig ventricular myocytes.

2-sec-Butylphenyl N-methylcarbamate (BPMC) is a carbamate-type cholinesterase (ChE) inhibitor with unique toxicological properties such as noncholinergic cardiovascular collapse. Effects of BPMC on L-type Ca2+ channel currents (ICa(L)) were studied in isolated guinea pig ventricular myocytes using the whole-cell patch-clamp technique, since the examination of cardiovascular responses indicated its Ca2+ antagonistic action. BPMC induced bradycardic and hypotensive responses in vivo and inhibited contraction of isolated papillary muscles (IC50 = 1.3 x 10(-4) M) in guinea pigs. BPMC produced reversible block of ICa(L) in the concentration range of 10(-4) - 10(-3) M. At test potentials between -30 mV and +20 mV, BPMC at 3 x 10(-4) M caused marked acceleration of decay rate of ICa(L) with moderate reduction of peak ICa(L) amplitude. BPMC (3 x 10(-4) M) shifted the steady-state inactivation curve to the hyperpolarizing direction by 12.7 mV. Decay rate of Ba2+ currents (IBa(L)) was also accelerated by BPMC. Fitting analysis of inactivation kinetics of IBa(L) with a two-exponential equation revealed that BPMC accelerates the slow inactivation component. At concentrations for blocking peak IBa(L) by ca. 30%, the inactivation kinetics of IBa(L) were significantly accelerated by BPMC, but merely slightly accelerated by Ca2+ channel antagonists such as diltiazem, nifedipine, or verapamil. These results indicate that BPMC, in addition to the inhibition of ChE, blocks L-type Ca2+ channels by accelerating voltage-dependent inactivation.