Ca2+ entry blocking and contractility promoting actions of norbormide in single rat caudal artery myocytes

1 Aim of the present study was to investigate the effects of norbormide, a selective vasoconstrictor agent of the rat peripheral vessels, on the whole-cell voltage-dependent L-type Ca(2+) current (I(Ca(L))) of freshly isolated smooth muscle cells from the rat caudal artery, using either the conventional or the amphotericin B-perforated whole-cell patch-clamp method. 2 Norbormide decreased L-type Ca(2+) current in a concentration- and voltage-dependent manner, without modifying the threshold and the maximum of the current-voltage relationship. Norbormide-induced I(Ca(L)) inhibition was reversible upon wash-out. 3 Norbormide both shifted the voltage dependence of the steady-state inactivation curve to more negative potentials by about 16 mV, without affecting the activation curve, and decreased the slope of inactivation. Norbormide, however, did not modify both the activation and the inactivation kinetics of the I(Ca(L)). 4 Norbormide decreased I(Ca(L)) progressively during repetitive step depolarizations, with inhibition depending on the stimulation frequency (use-dependent block) as well as on the holding potential. 5 Addition of 50 micro M norbormide caused the contraction of all freshly isolated cells and also of those impaled with the perforated method, but not of those impaled with the conventional method (i.e. dialysed). 6 In conclusion, these results prove norbormide to be a vascular L-type Ca(2+) channel inhibitor, which preferentially acts on the inactivated and/or open state of the channel. In rat caudal artery smooth muscle, however, this mechanism does not result in a vasodilating effect since it is overwhelmed by the mechanism underlying norbormide-induced vasoconstriction.