Kazuhisa Nakao

Some electrical properties of human pregnant myometrium

Membrane properties of the human pregnant myometrium were investigated with the conventional microelectrode and patch clamp methods. The majority of preparations produced spontaneous action potentials at a very low frequency, and action potentials were inhibited in sodium-deficient or calcium-free solutions. With the patch clamp technique with 120 mmol/L cesium-20 mmol/L tetraethylammonium in the pipette, the inward current was evoked by a depolarizing pulse above -40 mV from a holding potential of -60 mV and maximum amplitude was obtained at 0 mV. At a holding potential of -100 mV, the inward current could be evoked with less positive depolarizing pulses, and the membrane potential that evoked the maximum amplitude of inward current was shifted to a more negative potential. Single-channel current recording revealed that two types of calcium channels existed in human pregnant myometrium, with single-channel conductances of 12 and 29 pS. One of the calcium channels (12 pS) was inactivated at a holding potential of -60 mV.

Characteristics of cromakalim-induced relaxations in the smooth muscle cells of guinea-pig mesenteric artery and vein.

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 μm) 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 −80 mV. 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.

Factors inducing endothelium‐dependent relaxation in the guinea‐pig basilar artery as estimated from the actions of haemoglobin

1 Factors inducing dilatation of guinea‐pig basilar artery were investigated in intact and endothelium‐denuded tissues by measurement of isometric tension and by electrophysiological methods. 2 The amplitudes of contractions induced by 9,11,epithio‐11,12‐methanothromboxane A2 (STA2) and by high K+ were enhanced by haemoglobin (oxyhaemoglobin, Hb) in a concentration‐dependent fashion (above 1 μm). For the high K+‐induced contraction, the initial tonic component was enhanced to a greater extent than the secondary phasic component. Mechanical responses evoked by STA2 and by high K+ were greater in endothelium‐denuded tissues, but Hb (below 10 μm) had no effect on them. 3 Hb (10 μm) had no effect on the contractile proteins as estimated from the actions of Hb on Ca2+‐induced contractions in skinned muscle tissues. Further, Hb had no effect on the release of Ca2+ from intracellular stores but it accelerated the Ca2+ accumulation into the sarcoplasmic reticulum as judged from the caffeine‐ or STA2‐induced contraction generated in intact tissues. 4 Acetylcholine (ACh) relaxed tissues that were precontracted by STA2 but Hb prevented this relaxation, in a concentration‐dependent fashion. The ACh‐induced relaxation was sustained for over 10 min in the absence of Hb, but following application of Hb, ACh caused only a transient relaxation. 5 STA2 (up to 100 nm) did not modify the resting membrane potential of smooth muscle cells of the basilar artery. ACh (10 μm) caused transient hyperpolarization which was only slightly inhibited by Hb (10 μm) whether or not STA2 was present. The hyperpolarization induced by ACh required the presence of endothelial cells. 6 A23187 (0.01–1 μm) relaxed tissues which were precontracted by STA2, in a concentration‐dependent fashion but had no effect on the membrane potential. 7 These results suggest that in guinea‐pig basilar artery, ACh induces relaxation of tissues that were precontracted by STA2 by causing release of both endothelium‐derived relaxing (EDRF) and endothelial dependent hyperpolarizing factor (EDHF) (sustained and initial transient relaxation, respectively), but via different mechanisms. Hb inhibits the former and to a lesser extent, the latter. Since A23187 produced relaxation of pre‐contracted tissue but caused no detectable change in the membrane potential, this agent may release EDRF but not EDHF.

Oxytocin enhances action potentials in pregnant human myometrium—A study with microelectrodes ☆ ☆☆ ★ ★★

OBJECTIVE Our purpose was to quantitatively assess the effects of oxytocin on membrane properties in the pregnant human myometrium. STUDY DESIGN Specimens were obtained from the lower uterine segment during cesarean section at term. Electrical activity was recorded from individual cells by a conventional microelectrode method and the membrane functions were analyzed. RESULTS Two types of spontaneous action potentials were seen: a long plateau potential and a spike-like action potential. With no change in the resting membrane potential, low concentrations of oxytocin either evoked an action potential with a plateau phase, increased the amplitude and duration of the plateau potential, or increased the frequency of generation of action potentials. Oxytocin also lowered the threshold for evoking an action potential. Higher concentrations depolarized the membrane with an associated reduction in membrane resistance. CONCLUSION Oxytocin augments the excitability of pregnant human myometrial cells by multiple actions on the membrane, affecting both frequency and amplitude of action potentials.

Actions of nicorandil on vascular smooth muscles.

The action of nicorandil on vascular smooth muscles has been studied in vitro using the microelectrode, Ca-transient, isometric tension recording methods, and the bioassay methods of cyclic nucleotides and inositol phospholipids. Nicorandil increased Ca-insensitive K conductance and hyperpolarized the membrane and thus prevented the activation of the voltage-dependent Ca channel. The hyperpolarization occurred to a greater extent in venous tissue than in arterial tissue. Nicorandil stimulated the synthesis of cyclic guanosine monophosphate (cGMP) in the polarized and depolarized muscle tissues, as did nitroglycerine. Consequently, nicorandil reduces the concentrations of free Ca in the myoplasm, due to acceleration of the Ca pump at the sarcolemma, and may prevent the phosphorylation of myosin through phosphorylation of myosin light chain kinase. These actions of nicorandil may not contribute to the synthesis of inositol-1,4,5-trisphosphate hydrolyzed from phosphatidylinositol-4,5-bisphosphate. The above actions of nicorandil—hyperpolarization and increase in the cyclic GMP—may cause relaxation of the tissues precontracted by various stimulants.

Actions of quinidine and apamin on after-hyperpolarization of the spike in circular smooth muscle cells of the guinea-pig ileum

SummaryThe effects of quinine and quinidine on membrane potential and action potential were investigated in circular smooth muscle of the guinea-pig ileum and the findings compared with the actions of apamin. In addition to results obtained from microelectrode experiments, the actions of quinidine and apamin on membrane currents were assessed using the single cell voltage clamp method. Quinine (above 0.2 mmol/l) and quinidine (above 0.08 mmol/l) depolarized the membrane, increased the membrane resistance and blocked generation of the after-hyperpolarization of the spike. Higher concentrations of both agents reduced the amplitude of the action potential and further depolarized the membrane. Quinidine and quinine possessed much the same action, with the former being more potent than the latter. Apamin, an inhibitor of the Ca-dependent K current, did not inhibit the after-hyperpolarization of the spike and had no effect on the membrane potential. In voltage clamp experiments, a depolarizing pulse (above −30 mV from −60 mV; 200 ms duration) elicited an inward current, followed by an outward current. With application of 2.5 mmol/l Mn instead of Ca, the outward current was subclassified into the Mn sensitive (Ca-dependent) and Mn resistant (volage-dependent) K currents. Apamin (0.1 μmol/l) did not modify membrane currents evoked in the circular muscle cell, while, 0.1 mmol/l quinidine inhibited both the Ca- and voltage-dependent K outward currents, and Ca inward current.Our observations suggest that apamin-insensitive Ca-dependent K channels are present in the smooth muscle membrane and that they probably participate in the falling phase and after-hyperpolarization of the action potential.

Mechanisms of endothelin-induced augmentation of the electrical and mechanical activity in rat portal vein.

Actions of porcine endothelin (ET) on the electrical and mechanical activity of the rat portal vein were investigated by means of the intracellular microelectrode and isometric tension recording techniques, ET (> 0.1 nM) enhanced the amplitude and frequency of the spontaneous contractions which ceased in the presence of 100 nM dihydropyridine derivatives (nifedipine or nicardipine). ET (0.15 nM) increased the frequency of the spontaneous action potentials, with no change in the basal membrane potential. Higher concentrations of ET (≧ 0.3 nM) further depolarized the membrane potential and increased the spike frequency. After blocking the spontaneous action potentials with nifedipine (100 nM), ET still depolarized the membrane. The depolarization was associated with a reduction in the electrotonic potential and was blocked in a Na-deficient solution (15.5 mM) but not in Ca-free, K-deficient or Cl-deficient solutions. In a Na-deficient solution, ET still evoked action potentials without depolarization. In Ca-free solution, ET depolarized the membrane potential with small oscillations, which were blocked by nifedipine (100 nM). The results indicate that in the rat portal vein, ET enhances electrical and mechanical responses through activation of the dihydropyridine-sensitive and voltage-dependent Ca channels. Acceleration of the Ca entry induced by ET can occur with or without depolarization of the membrane and can enhance the pacemaking mechanism.