Ken Hotta

Silver ion-induced tension development and membrane depolarization in frog skeletal muscle fibres

Silver ions elicit dose-dependently a transient contracture in single fibres of bull-frog toe muscle placed in 0-Ca2+, Cl−-free MOPS solution containing 3 mM Mg2+ and NO3−. To elucidate the mechanisms involved, changes in membrane potential and in tension development were continuously measured following exposure to Ag+. The effect of Ag+ on contraction in fibres in which the membrane had been depolarized by elevating the external K+ concentration was also examined. The major findings of this investigation are as follows. (1) The mechanical threshold was shifted towards more negative potentials by 5 mV (−51 to −56 mV), when Ca2+ and Cl− in the Ringers solution were replaced with Mg2+ and NO3−, respectively. (2) On the exposure of the fibres to 5 μM Ag+, the membrane potential decreased by 1.6 mV from −87.8 mV and tension was developed. (3) In fibres soaked in a solution containing 10 mM K+ (corresponding to a membrane potential of −69.5 mV), 5 μM Ag+ produced a large contracture similar to that seen in the control solution. (4) The Ag+-induced contracture was inactivated when more than 20 mM K+ was used. (5) The membrane depolarization evoked by either 20 or 50 μM Hg2+ did not produce contraction. (6) Muscle fibres which had been exposed to 20 μM Hg2+ for 5 min responded to 5 μM Ag+ by a transient tension development. These findings strongly suggest that Ag+-induced tension development is not associated with depolarization of the surface membrane but rather is caused by specific actions of Ag+ on membrane proteins in the T-tubules.

Effect of methionine-enkephalin on the spontaneous electrical and mechanical activity of the smooth muscle of the rat portal vein

In the longitudinal smooth muscle of the isolated rat portal vein, methionine-enkephalin (Met-enkephalin) increased the spontaneous contraction with a concentration as low as 10(-8)M. When the membrane activity was recorded using a microelectrode, Met-enkephalin enhanced the spike burst activity but without any effect on the resting membrane potential. Naloxone, phentolamine, atropine and reserpine pre-treatment did not inhibit the excitatory effect of Met-enkephalin on the spontaneous contraction. These results suggest that the excitatory effect of Met-enkephalin on the mechanisms involved in the automaticity may be a direct action on smooth muscle or relate to presynaptic action on a non-adrenergic non-cholinergic system.

Electromechanical and morphological observations on single muscle fibers in developing dystrophic mouse

Abstract Electrophysiologic characteristics and ultrastructural features of single muscle fibers of the dystrophic mouse (C57BL/6J-dy2J) at various stages of disease were investigated and comparison with those of normal muscle. Resting membrane potentials of the extensor digitorum longus muscle fibers from both normal and dystrophic young mice (to 3 weeks) were between −90 and −40 mV. With advancing ages to 7 to 9 weeks, the membrane potentials of normal fibers increased and tended to be within a narrow range around the mean value (−74 mV), whereas those of dystrophic fibers remained at the same value as at 3 weeks (−63 mV), with large deviations. The twitch tension of single fibers induced by direct stimulation was drastically reduced in dystrophic muscle, approximately from one-half to one-third of what it had been in normal fibers. Three-dimensional structures of the T-system in normal muscle observed under a conventional electron microscope revealed the formation of a dense network of tubules along the A-I boundaries of the myofibrils. In dystrophic muscle, the network was poorly developed and the tubules looked disrupted, although the arrangement of myofilaments was preserved relatively well even in the advanced stages. It seems likely that the drastic reduction of contractile force in dystrophic muscle is due to the defect of the internal membrane system, rather than the degeneration of contractile elements.

Autonomic nervous control of the heart in exercising man

Effects of pharmacological denervation with propranolol (0.2 mg· kg−1) and/or atropine (0.04 mg· kg−1) on the control system of heart rate during exercise were studied using a frequency domain analysis, to evaluate the cardiac control function of autonomic nervous systems in man. Propranolol decreased the gain of the system in low frequency range and increased the gain slightly in middle and high frequency ranges. The phase angle of the system was advanced over the whole frequency range observed. Atropine decreased and delayed the gain and phase angle, respectively, especially in high frequency range. The combined administration of blockades significantly decreased the gain and delayed the phase angle in the whole and high frequency ranges, respectively. These findings suggest that sympathetic and parasympathetic nervous systems act nearly independently in controlling heart rate during exercise; that they act in different manners; and that the characteristics of the systems can feasibly be used to estimate clinically the cardiac control function of autonomic nerves in man.

Change of membrane potential in rat urinary bladder epithelium treated with sodium l-ascorbate

We previously showed that 5% sodium L-ascorbate (Na-AsA) in the diet promotes rat urinary bladder carcinogenesis, whereas 5% ascorbic acid (AsA) and 1% sodium chloride (NaCl) in the diet do not. In order to cast light on basic properties of these compounds regarding the rat urinary bladder, we examined membrane potential levels in the bladder epithelium of F344 rats treated with 5% Na-AsA, 5% AsA or 1% NaCl in the diet for 2, 4 or 8 weeks. Microelectrode measurement showed Na-AsA to induce hyperpolarization of the membrane potential, which was, however, lacking with AsA and NaCl from week 2 until the end of experiment. Thus a good correlation between the effects on membrane potential and tumor promoting activity could be established.

The effect of changing free Ca2+ on light diffraction intensity and correlation with tension development in skinned fibers of frog skeletal muscle

The relationship between the diffraction intensity change of the first order line and tension development was examined in mechanically skinned single fibers from the dorsal head of the semitendinosus of frogs. Passive stretch of the fibers resulted in an increase in intensity over the range of sarcomere lengths from 2.5 to 3.6 μm, indicating that the intensity is a function of sarcomere length. Activation of skinned fibers caused a decrease in the intensity, at all sarcomere lengths, where the thick and thin filaments overlapped. The magnitude of the intensity decrease and that of the tension development depended on the Ca2+ concentration in the medium. The drop of intensity-pCa and the tension-pCa curves showed a similarly steep S-shape within a range of 0.5 pCa unit, although the intensity-pCa curve shifted to the left; the pCa for 50% decrease in light signal was 6.48 and that for 50% tension development was 6.40. Caffeine (25 mM) added to the medium produced a decrease in the intensity of skinned fibers with the simultaneous development of tension, thereby indicating that caffeine induces a release of Ca2+ from the sarcoplasmic reticulum and disorder in the filaments ensues. Changes in diffraction intensity with electrical stimulation to the intact single fiber were similar, although a more striking summation was observed in the optical response, as compared to the tension development. These results suggest that tension development upon stimulation can be monitored by assessing the magnitude of diffraction intensity decrease in the first order line, except for some shift in the short fiber.

Similar inhibitory effects of dantrolene sodium on twitch tension and on silver ion-induced contracture in skeletal muscle

To determine the mechanism by which Ag+ induces a transient contracture in skeletal muscle, the effect of dantrolene sodium on the Ag+ contracture was examined and the findings compared with those for the twitch, tetanus and caffeine contracture. The inhibition of twitch by dantrolene was equivalent to that of the Ag+ contracture at concentrations of 1, 2 or 5 microM of dantrolene. The tetanus tension was slightly inhibited by dantrolene, but not the caffeine contracture. These observations suggest that the Ag+ contracture may be governed by the same mechanism as that involved in the development of twitch tension.

Transient effect of intracellular dantrolene on E--C coupling in skeletal muscle.

The effect of dantrolene-Na (DAN) on electrical and mechanical responses was investigated to single fibers of frog semitendinosus. Twitch tension was potentiated over 2--4 min by intracellular application of DAN and was rapidly decreased thereafter. Extracellular DAN depressed the tension with all dosages used. Although DAN had no effect on membrane potential, it exerted a biphasic action on membrane excitability. From these results, a mechanism is postulated for the release of trigger Ca2+ from T-tubules.

Relationship between ATP-induced contraction of muscle model and conformation change of muscle proteins

Abstract Responses of myosin B and myofibril suspensions to additions of ATP were examined by means of turbidity and volume changes. The turbidity of myofibrillar suspensions increased transiently upon addition of ATP, followed by subsequent decrease. Destruction of myofibrillar structure by brief application of sonic vibration changed this characteristic turbidity response to that of myosin B, which is a monophasic increase. Increase of turbidity may represent the molecular reorganization of myosin and actin; however, the decrease of turbidity cannot be attributed to the reactions occurring at the molecular level. ATP induces considerable volume change of myofibrillar fragments. Sonication of myofibrils destroyed their structure and depressed their ATP-induced volume change. The myosin B-ATP system did not show such a response during the superprecipitation process. These results suggest myofibrillar structure is essential for the volume change and, presumably, contraction of muscle. It was concluded that there are two or more consecutive reactions between muscle proteins and ATP during muscle contraction, a small conformational change in the muscle proteins (around ATPase active center), and a larger intermolecular change induced by the preceding reaction, which occur in the subcellular structure.

Inhibition of Zn2+-induced potentiation of twitch tension by Ni2+ in frog muscle.

Effect of Ni2+ on Zn2+-induced potentiation of twitch tension was studied electrophysiologically in the toe muscle fibers of Rana catesbeiana. The major findings of this investigation are as follows. When 2 mM Ni2+ was applied to fibers in a normal Ringers solution containing 50 microM Zn2+ (Zn2+ solution), the Zn2+-potentiated twitch tension decreased remarkably to about one-third of that before Ni2+ treatment. This concentration of Ni2+ caused a 23% decrease in the duration of action potential which had been prolonged by Zn2+ (6.61-5.09 ms). Ni2+ (2 mM) added to normal Ringers solution led to increases of about 30 and 42% in twitch tension and in the duration of action potential, respectively. A slight increase in the mechanical threshold was induced by 2 mM Ni2+. The inhibitory action of Ni2+ on the twitch tension in Zn2+ solution was larger than that in the case of tetanus tension. Diltiazem (40 microM), a Ca2+ channel blocker, did not inhibit the twitch tension potentiated in Zn2+ solution. These results suggest that the decrease in Zn2+-potentiated twitch tension by Ni2+ may possibly derive from impairment of the propagation of action potential along the T tubules.