TTX-sensitive Na+ and nifedipine-sensitive Ca2+ channels in rat vas deferens smooth muscle cells

The inward currents in single smooth muscle cells (SMC) isolated from epidi dymal part of rat vas deferens have been studied using whole-cell patch-cla mp method. Depolarising steps from holding potential -90 mV evoked inward c urrent with fast and slow components. The component with slow activation po ssessed voltage-dependent and pharmacological properties characteristic for Ca2+ current carried through L-type calcium channels (I-Ca). The fast comp onent of inward current was activated at around -40 mV, reached its peak at 0 mV, and disappeared upon removal of Na ions from bath solution. This cur rent was blocked in dose-dependent manner by tetrodotoxin (TTX) with an app arent dissociation constant of 6.7 nM. On the basis of voltage-dependent ch aracteristics, TTX sensitivity of fast component of inward current and its disappearance in Na-free solution it is suggested that this current is TTX- sensitive depolarisation activated sodium current (I-Na) Cell dialysis with a pipette solution containing no macroergic compounds resulted in signific ant inhibition of I-Ca (depression of peak I-Ca by about 81% was observed b y 13 min of dialysis), while I-Na remained unaffected during 50 min of dial ysis. These data draw first evidence for the existence of TTX-sensitive Na current in single SMC isolated from rat vas deferens. These Na+ channels d o not appear to be regulated by a phosphorylation process under resting con ditions. (C) 1999 Elsevier Science B.V. All rights reserved.