Biomedical subjects
Publications and source records attributed to H Sugi.
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1. The electrical and mechanical responses of the anterior byssal retractor muscle (ABRM) of Mytilus edulis to acetylcholine (ACh), high [K]O or the removal of external Ca were examined under a variety of conditions. 2. ACh (10(-6)--10(-3)M) produced contracture tensions larger than those produced by high [K]O (30-300 mM) for a given amount of depolarization. In Ca-free solution, the rate of decline of ACh-contractures was much smaller than that of K contractures, though both ACh- and K-contractures eventually disappeared. 3. 5-HT (10(-4)M) of procaine (1 mM) markedly reduced the height of ACh-contractures, but had little or no effect on K-contractures. The height of K contractures was markedly decreased by Mn ions (20 mM) or low pH (4-5), while ACh-contractures remained unaffected. 4. Partial replacement of [Na]o by choline (30-100 mM) reduced both ACh-induced depolarization and contracture tension, whereas K-contractures remained unchanged even after total replacement of [Na]o by choline. 5. ACh could produce little or no tension when applied during the relaxation phase of K-contractures, while high [K]o produced the maximal contracture tension when applied during the relaxation phase of ACh-contractures. 6. Following the removal of external Ca from solutions containing less than 10 mM-Mg, the ABRM showed a marked tension development associated with repetitive electrical activity superimposed on a gradual decline of membrane potential. 7. These results suggest that ACh-contractures are mainly due to the release of intracellularly stored Ca, while K-contractures are mainly associated with the inward movement of external Ca.
1. The localization of Ca-accumulating structures in the anterior byssal retractor muscle (ABRM) of Mytilus edulis and their role in the contraction-relaxation cycle were studied by fixing the ABRM at rest or during various phases of mechanical activity with a 1% osmium tetroxide solution containing 2% potassium pyroantimonate. 2. In the resting ABRM, electron-opaque pyroantimonate precipitate was observed at the inner surface of the plasma membrane, the vesicles and the mitochondria. 3. Electron X-ray microanalysis showed the presence of Ca in the precipitate, indicating that the precipitate provides a valid measure for Ca localization. 4. In the ABRM fixed at the peak of mechanical response to the Ca-removal or to acetylcholine, the precipitate was found to be diffusely distributed in the myoplasm in the form of a number of particles. At the completion of spontaneous relaxation, the precipitate was again seen at the inner surface of the plasma membrane. 5. During the catch state, the precipitate was found to be re-accumulated in the peripheral structures with a corresponding decrease of the precipitate in the myoplasm. 6. These results not only provide evidence for the involvement of the Ca-accumulating structures in the contraction-relaxation cycle in the ABRM, but also suggest that the transition from active to catch contractions is related to a decrease in myoplasmic free Ca ion concentration.
1. The length changes of the anterior byssal retractor muscle (ABRM) of Mytilus edulis, following step changes in load, were studied at various phases of active and catch contractions produced by acetylcholine. 2. The load-extension curves of the series elastic component (SEC) were found to be scaled down in proportion to the isometric tension immediately before step changes in load, but remain unchanged irrespective of whether the ABRM was in active or in catch contraction. 3. In hypertonic solutions the compliance of the SEC was reduced in the same manner as that of the SEC in frog skeletal muscle. 4. These results seem to favour the linkage hypothesis for the catch mechanism, though the SEC in the ABRM is suggested to be composed not only of the cross-linkages, but also of the compliance of the myofilaments.
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1. Single fast muscle fibres of the frog were locally activated by applying current pulses to a pipette whose tip was in contact with the fibre surface, and the resulting local contractions were recorded with a high-speed ciné-camera at 1000-3000 frames/sec.2. In some but not all of the fibres examined, a moderate membrane depolarization of 20-30 mV initiated a phasic type of local contraction, which showed a definite threshold and relaxed spontaneously while the depolarization still continued.3. The phasic contraction was sometimes followed by a smaller steady contraction, which lasted as long as the depolarization went on and was regarded to be indentical with the graded type of local contraction.4. With pipettes of 20-40 mum diameter, the phasic contraction was first initiated at the depolarized fibre surface, and spread to some extent inwards with a velocity of 0.7-2 cm/sec at 18-26 degrees C. The velocity of inward spread of contraction had a Q(10) of about 2.3.5. With larger pipettes of more than 50-60 mum diameter, the phasic response was first initiated at the inner part definitely distant from the fibre surface, and the extent of contraction was greater at this part than at the superficial part during the course of the response. Furthermore, with depolarizations of nearly the threshold value, the steady contraction following the phasic one was seen only at the inner part, suggesting the reversal of the gradient of depolarization along the T tubules.6. The phasic contraction spread inwards or transversely with a considerable decrement. The response spreading across the whole diameter of the fibre could be observed only in some cases.7. The phasic response was not always sensitive to tetrodotoxin or to the removal of external sodium ions.8. These results not only give information about the nature of the regenerative mechanism within the T system, but also suggest that the organization of the T tubule network is such that the electrotonic depolarization becomes maximum at the T tubules located distant from the fibre surface when a sufficiently large area of surface membrane is depolarized.
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1. Small fibre bundles from the semitendinosus muscles of the frog were stretched during tetanic stimulation, and the resulting tension changes were studied over a wide range of stretch velocities from 0.1 to 150 cm/sec (0.1-100 length/sec). The experiments were performed within the range of fibre lengths where the resting tension was negligible.2. With stretch velocities of more than 30 cm/sec (20 length/sec), the tension rose abruptly at first, and then started to fall while the stretch still continued, indicating the ;slip' of the contractile component. When the fibres were stretched at 80-150 cm/sec (70-100 length/sec), the tension fell quickly below the initial isometric level at the end of the stretch, and then began to rise again to the initial isometric value.3. Following stretches of 30-60 cm/sec (20-50 length/sec), the tension showed a delayed transient rise. The delayed rise of tension became more marked as the amount of stretch was increased.4. In some preparations, oscillatory tension changes were observed following stretches of 50-100 cm/sec (40-70 length/sec).5. The tension developed above the isometric level during moderate-velocity stretches of less than 15 cm/sec (10 length/sec) increased by lowering temperature, and showed a tendency to decay when the stretch velocity was suddenly reduced during a stretch.6. These results are discussed in relation to the sliding filament hypothesis, which provides an explanation for the findings of the present work.
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1. Single fast muscle fibres isolated from the semitendinosus muscles of the frog were locally activated by applying linearly rising current pulses to a pipette whose tip (diameter, 10-40 mu) was in contact with the fibre surface.2. In normal or choline-Ringer solution with 1.8 mM-Ca, the threshold depolarization for producing a just perceptible local contraction was nearly constant over a wide range of the duration of linearly rising currents (0.5-20 sec or more).3. If [Ca](o) was reduced to 0.25-0.1 mM, the threshold depolarization increased steeply with the increase in the duration of linearly rising currents, indicating a marked increase in the rate of an accommodation process in excitation-contraction coupling mechanism.4. In the low-Ca media, the threshold depolarization was observed to rise exponentially during the application of a linearly rising current of 3-10 sec duration, and to return exponentially to its initial value within 3-10 sec after the removal of the current.5. The marked accommodation in the low-Ca media was partially inhibited by the addition of Mg (5-10 mM), and completely eliminated in the presence of caffeine (1 mM), suggesting that the accommodation process may occur in the link between membrane depolarization and release of Ca from sarcoplasmic reticulum.6. Microscopic observation of local contractions in the low-Ca media indicated that the accommodative rise of threshold depolarization was most marked at the superficial layer of the fibre.7. From these results, it is suggested that the removal of bound Ca or some other kind of charged particle from the transverse tubular system or from its vicinity might be the cause of accommodation in excitation-contraction coupling.
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