The existence of a highly tetrodotoxin sensitive Na channel in freshly dispersed smooth muscle cells of the rabbit main pulmonary artery

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Abstract
To characterize the inward current recorded from single smooth muscle cells of the rabbit main pulmonary artery, a voltage clamp procedure using patch pipettes filled with high Cs solution to inhibit K currents was employed. Under superfusion with normal physiological salt solution, application of a command potential to −10 mV from the holding potential of −80 mV elicited an inward current comprising fast and slow components. In Ca-free solution containing 2.5 mM Mn and 134 mM Na, the major part of the slow inwart current (Islow) ceased, but a transient fast inward current (Ifast) remained. A reduction in the Na concentration in the bath solution inhibited the amplitude ofIfast. Both nicardipine (30 nM) and diltiazem (1–10 μM) inhibitedIslow. but had no effect onIfast. Application of tetrodotoxin (>1 nM) in Ca free solution inhibited the amplitude ofIfast in a dose-dependent manner with a dissociation constant of 8.7 nM. Chloramine-T (0.3 mM) increased the peak amplitude and reduced the rate of decay ofIfast and completely inhibitedIslow. These results suggest that the inward curent generated in the smooth muscle cells of the rabbit main pulmonary artery is associated with activation of a voltage-dependent Ca channel and a tetrodotoxin-sensitive Na channel.