Mitsuyoshi Ohba

The slow wave in the circular muscle of the guinea‐pig stomach.

The slow wave in the circular muscle of guinea‐pig stomach was investigated with the double sucrose‐gap method. 2. The amplitude of the slow wave was reduced by depolarization, and it was increased by a small hyperpolarization (5‐10 mV). With hyperpolarization greater than 15 mV the amplitude decreased, and the slow wave became reduced, and less dependent on polarization. This residual was not abolished by strong hyperpolarizing current pulses. 3. The frequency of the slow waves was not much affected by membrane polarization. The change was only 15‐20% by depolarization or hyperpolarization of 12mV. 4. Rythmic inward currents could be recorded under voltage‐clamp conditions. The frequency of the inward currents was the same as that of the slow wave. The intensity of inward current was little affected by membrane polarization. 5. Lowering the temperature reduced the frequency of the slow wave. The rates of rise and fall of the component which remained during strong hyperpolarization were similarly decreased by lowering the temperature. The Q10 of the frequency was about 2‐7. 6. It is suggested that the slow wave consists of two different components. One is generated by a potential independent process, and triggers the second component which is potential dependent. The first component may be controlled by some metabolic process.

Effects of sodium, potassium and calcium ions on the slow wave in the circular muscle of the guinea-pig stomach

1. The contribution of Na, K and Ca ions to the generation of slow waves in the circular muscle of the guinea‐pig stomach was studied.

Calcium antagonistic action involved in vasodilation by brovincamine.

Possible mode of vasodilative action of brovincamine was assessed in isolated cardiovascular preparations in comparison with verapamil and papaverine. Brovincamine (IC50: 1.2 x 10(-5) M), verapamil (IC50: 3.5 x 10(-7) M) or papaverine (IC50: 2.5 x 10(-5) M) caused a dose-dependent relaxation of potassium (30 mM)-contracture in the rabbit pulmonary arterial segment. This relaxation by verapamil or brovincamine, but not by papaverine, was antagonized by increasing external Ca2+ concentration to 12.4mM. Duration of slow action potentials of partially depolarized guinea-pig papillary muscle was reduced by 14 min exposure to brovincamine (5 x 10(-5) M) or verapamil (10(-5) M. These results suggest that brovincamine produces a vasodilation via a slow Ca2+-channel blockade.

Impedance components in longitudinal direction in the guinea-pig taenia coli.

1. When the tissue impedance of the guinea‐pig taenia coli was measured across a 2 mm sucrose gap in a longitudinal direction, the impedance locus could be fitted by two different circular arcs. Their characteristic frequencies were about 0‐6 and 240 Hz after 60 min superfusion with sucrose solution. From the effects of changing the width of sucrose gap and of transection of tissue, and also from taking the difference between impedances measured at two distances, it was concluded that the low‐frequency locus corresponds to the transverse impedance of the plasma membrane and the high‐frequency locus to the longitudinal tissue impedance. 2. A change in the longitudinal tissue impedance was measured during superfusion with sucrose solution, using a frequency range between 5 Hz and 10 kHz. The admittance decreased with time of superfusion and this time course could be expressed by the sum of three exponential terms. The fastest component, having a time constant of 1‐3 min at 10 kHz, was interpreted to correspond to a process of wash‐out of extracellular medium. 3. Admittances at zero and infinite frequencies were obtained from the impedance locus. The decrease in these admittances with the time was analysed and the values at the start of washing were obtained by extrapolating the admittance change to zero time. 4. From these values it was estimated that the myoplasmic resistance was 214 omega cm, the junctional resistance 372 omega cm, and the junctional capacity 3‐1 muF/cm at 25 degrees C. In these calculations the equivalent circuit of tissue was assumed to be expressed by two components in series: one for the myoplasmic resistance and the other for the junction which has the junctional resistance and capacity in parallel. 5. After 90 min superfusion with sucrose solution, the total tissue impedance measured at zero frequency was increased from 586 to 3034 omega cm. In the total impedance the myoplasmic resistance was increased from 214 to 914 omega cm and the junctional resistance from 372 to 2120 omega cm. Thus, the change in junctional resistance was greater than that in myoplasmic resistance during superfusion of sucrose solution.

Implication of ATP released from atrial, but not papillary, muscle segments of guinea pig by isoproterenol and forskolin

Effects of isoproterenol and forskolin, which increases intracellular cyclic AMP, on contraction and ATP release in atrial and papillary muscles of guinea pig were evaluated. In the electrically driven-left atrial muscle segments, isoproterenol and forskolin at 0.1 microM produced an ATP release coupled with a positive inotropic effect, the values of net ATP release at 5 min after these drugs being 5.20 +/- 0.59 and 5.37 +/- 0.55 nmoles/g wet weight, respectively. The forskolin evoked-ATP release was unaffected by prazosin plus propranolol or by guanethidine, implying that ATP is released from non-neuronal origin. In contrast, in papillary muscle segments, the test cardiotonics did not elicit any ATP release despite producing contractile response similar to that in atrial preparations. However, there is no difference in ectoATPase activities of both tissues. Adenosine added exogenously inhibited electrically evoked-contraction of the atrium, but not that of the papillary, although inhibitions by verapamil of the contractions were approximately equal in these preparations. These findings suggest that cardiotonics such as isoproterenol produce a liberation of ATP from auricle muscles, but not from ventricle muscles, and that the liberated ATP may mainly be catabolized to adenosine by ectoenzymes and the resultant nucleoside may serve as a functional modulator through stimulation of pre or postsynaptic A1-receptors.

Effects of dantrolene sodium on the excitation-contraction coupling of the mammalian and amphibian cardiac muscle.

Abstract Dantrolene sodium at concentrations which inhibit the twitch tension of frog toe muscle and is without effect on resting and action potentials exhibited a positive inotropic action in mammalian and amphibian myocardium. In guinea-pig atrial muscle, twitch tension was initially decreased and thereafter gradually increased above the control. In frog atrial muscle, twitch tension was potentiated by dantrolene sodium without a prior phase of decreased tension. In both guinea-pig and frog atrial muscles, the action potential duration was prolonged by the drug. The amplitude of the slow response was augmented by dantrolene sodium in guinea-pig and frog atria that were partially depolarized by KCl. These results suggest that dantrolene sodium increases the slow inward Ca current which consequently potentiates the twitch tension. The transient decrease of twitch tension in the guinea-pig atrial muscle could be due to inhibition of calcium release from the sarcoplasmic reticulum as postulated for skeletal muscle. The possibilities that the drug acts either on the Ca permeability of the membrane of the sarcoplasmic reticulum or on the junctional portion between the sarcolemma and the subsarcolemmal cisternae are discussed.

Tip Potential and Resistance of Micro-Electrodes Filled with KCI Solution by Boiling and Nonboiling Methods

Micro-electrodes were filled with 3-M KCI by two different methods. One group of electrodes was filled by boiling in KCI solution (boiled electrodes), and the other group was filled by capillarity, using glass fibers inside the electrode (glass fiber electrodes). The tip potential and the resistance of these electrodes were measured under various conditions. There was no significant correlation between the tip potential and the resistance in both groups of electrode. The boiled electrodes had larger tip potential (-12.7 mV) than the glass fiber electrodes (-0.2 mV) in sea water. The tip potential of boiled electrodes was decreased by increasing ionic concentrations and lowering the pH of external medium, by addition of divalent cation (Ca and Mg), and by lowering the temperature. The tip potential of glass fiber electrode was less affected by these changes. When electrodes containing glass fiber were filled with KCI solution by boiling, their tip potential was not much different from that in the boiled electrode without glass fiber. Furthermore, electrodes, filled with 3-M KCI without glass fiber and without boiling by using alcohol method, had a small tip potential (-2A mV). The tip potential of electrodes seems to be increased by boiling. Similar values of resting potential were obtained by both boiled and glass fiber electrodes. This is probably due to the fact that the tip potential in KCI solution was nearly the same as that in sea water which contained 465-mM NaCl, 10.8-mM CaC12, and 48.3-mM MgC12.

Negative inotropic effect of β-blockers in the guinea-pig atrium after preincubation with isoprenaline

In the isolated guinea-pig atrium, pretreated with a high concentration of isoprenaline (8 x 10(-7) M), beta-blockers produced a marked negative inotropic effect in a concentration which by itself was too low to have a direct depressing action. No negative inotropic effect was caused by isoprenaline when beta-blockers were applied first. The depressing action of beta-blockers was unaffected by phentolamine or atropine. During the depression, the electrical parameters of the action potential were not significantly different from those of the control, though the contractions were much smaller than control ones. The negative inotropic effect of beta-blockers was weaker in excess Ca (7.5--10 mM). The observations indicate that isoprenaline possesses a dual action but that the negative inotropic effect is usually masked by the positive inotropic effect and that beta-blockers, by quickly abolishing the positive inotropic effect, reveal the underlying negative inotropic effect.