The 4-aminopyridine (4AP) sensitive outward current of enzymatically dispersed single smooth muscle cells of the rabbit main pulmonary artery were investigated using the voltage clamp method. When the cell was exposed to physiological salt solution (PSS) in the bath and high K+ in the pipette no inward current was generated by depolarization of the membrane, but when 4AP was present in the bath or when Cs+ with tetraethylammonium+ (Cs+-TEA+) in the pipette, an inward current was generated. This current was enhanced by Ba2+ or high Ca2+ and was blocked by inorganic or organic Ca2+ channel blockers. The outward current was partly inhibited by the Ca2+ channel blockers, Ca2+-free or Mn2+ containing solution. The residual outward current was blocked by external application of 10 mM 4AP, whereas it was inhibited by half with 100 mM TEA+. To investigate further natures of 4AP sensitive outward current, the following experiments were done in the bath solution containing 2.5 mM Mn2+. The reversal potential of this outward current, estimated from the tail current, remained the same in Na+-deficient solution, but shifted to near the K+-equilibrium potential in Cl- deficient solution. Thus, the main current carrier for the outward current seems to be K+, but Cl- may participate to some extent. The amplitude of the outward current decreased slowly. However, the reversal potential was not changed, suggesting the reduction in amplitude of the outward current was not due to the accumulation of K+ on the outer surface of the membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

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 of Ifast. Both nicardipine (30 nM) and diltiazem (1-10 microM) inhibited Islow but had no effect on Ifast. Application of tetrodotoxin (greater than 1 nM) in Ca free solution inhibited the amplitude of Ifast 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 of Ifast and completely inhibited Islow. 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.

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1 The effects of cromakalim (BRL 34915) on the smooth muscle cells of guinea-pig mesenteric artery and vein were investigated with microelectrode and tension recording methods. 2 Cromakalim (>10 IM) produced membrane hyperpolarization with an increase in ionic conductance. The hyperpolarization occurred to a greater extent and lasted longer in the vein than in the artery.3 The hyperpolarization induced by cromakalim in mesenteric vein comprised two components, one of which was Mn sensitive. In mesenteric artery, the hyperpolarization was relatively insensitive to Mn. 4 From the current-voltage relationship measured from arterial smooth muscle membranes, the reversal potential of cromakalim was estimated to be -80mV. The cromakalim-induced hyperpolarization was not modified in Na-or Cl-deficient solution. 5 In both mesenteric artery and vein, cromakalim relaxed tissues precontracted with high K with (below 40 mM) or without (above 40 mM) hyperpolarization of the membrane. 6 In the mesenteric artery, action potentials evoked by electrical stimulation ceased before the generation of hyperpolarization. 7 Cromakalim produced a cross-desensitization with nicorandil on the evoked membrane hyperpolarization in mesenteric artery. 8 It is concluded that the relaxing actions of cromakalim result from the hyperpolarization which follows the opening of Ca-dependent K channels. The inhibition of a voltage-dependent Ca current may also be involved in this inhibitory effect.

The effects of intracellular perfusion of inositol 1,4,5-trisphosphate (InsP3) or inositol 1,3,4,5-tetrakisphosphate (InsP4) on electrical responses of smooth muscle cell membranes of the rabbit portal vein were studied using the whole cell voltage clamp technique. Depolarisation to 0 mV from a holding potential of -60 mV, evoked inward Ca (Ica), transient outward (ISO), oscillatory outward (IOO) and sustained outward (ISO) currents. Generation of IOO was dependent on the [Ca]o, but it was also generated in 0 mM Ca solution for over 10 min. From amplitude histograms, IOO was divided into two components. Reduction in [Ca]o inhibited the appearance of but not the amplitudes of both IOO components. However, the larger component of IOO was more resistant to a reduction in [Ca]o than the smaller one. InsP3 (10 microM) increased the frequency of both IOO components to a greater extent than their amplitude, but the larger component was more sensitive to InsP3 than the smaller one. The increase in the occurrence of IOO induced by InsP3 did not occur following pretreatment with 3 mM caffeine or 1 nM A23187. In normal PSS, InsP3 was evoked by a depolarising pulse positive to -40 mV, whereas following perfusion with InsP3 (10 microM), IOO was evoked at -60 mV. In normal PSS, intracellular perfusion with 10 microM InsP4 changed neither the frequency nor the amplitude of IOO, and the amplitudes of ICa, ITO and ISO were also unchanged. However, in 10 mM Ca solution, 10 microM InsP4 generated IOO at a membrane potential of -60 mV.(ABSTRACT TRUNCATED AT 250 WORDS)