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PubMed1959

Potassium ions and the inhibitory process in the crayfish stretch receptor.

The effect of the absence of potassium in the bathing solution on the synaptic inhibitory potentials of the crayfish stretch receptor has been studied. The inhibitory potentials were increased in size, i.e. became more hyperpolarizing, in the absence of potassium. Since the resting potential of the cell is increased in the absence of potassium, the alteration of the inhibitory potentials implies that the potassium conductance of the membrane is increased. While other ions, e.g. Cl(-), may also be involved, it seems that the membrane potential during inhibition is mainly dominated by K(+).

Animals↗
PubMed1959

The mechanism of dual responsiveness in muscle fibers of the grasshopper Romalea microptera.

Dually innervated Romalea muscle fibers which respond differently to stimulation of their fast and slow axons are excited by intracellularly applied depolarizing stimuli. The responses, though spike-like in appearance, are graded in amplitude depending upon the strength of the stimuli and do not exceed about 30 mv. in height. In other respects, however, these graded responses possess properties that are characteristic of electrically excitable activity: vanishingly brief latency; refractoriness; a post-spike undershoot. They are blocked by hyperpolarizing the fiber membrane; respond repetitively to prolonged depolarization, and are subject to depolarizing inactivation. As graded activity, these responses propagate decrementally. The fast and slow axons of the dually responsive muscle fibers initiate respectively large and small postsynaptic potentials (p.s.p.'s) in the muscle fiber. These responses possess properties that characterize electrically inexcitable depolarizing activity. They are augmented by hyperpolarization and diminished by depolarization. Their latency is independent of the membrane potential. They have no refractory period, thus being capable of summation. The fast p.s.p. evokes a considerable or maximal electrically excitable response. The combination, which resembles a spike, leads to a twitch-like contraction of the muscle fiber. The individual slow p.s.p.'s elicit no or only little electrically excitable responses, and they evoke slower smaller contractile responses. The functional aspects of dual responsiveness and the several aspects of the theoretical importance of the gradedly responsive, electrically excitable component are discussed.

Animals↗
PubMed1960

Radiocalcium release by stimulated and potassium-treated sartorius muscles of the frog.

Stimulation of frog (Rana pipiens) sartorius muscle accelerates release of Ca(45), but only during the period of stimulation. No appreciable difference is obtained in the calcium released per impulse whether stimulation is at a rate of 20/sec. or 0.5/sec. However, prior stimulation may appreciably increase the loss per impulse. In unfatigued muscles, the minimum amount of calcium liberated during an isotonic twitch is estimated to be about that previously calculated to enter, viz. 0.2 micromicromole/cm(2). The time course of radiocalcium release during potassium depolarization depends on the nature of the contracture. When contracture is isometric, the rate of escape is doubled and declines only slowly; if isotonic, the rate is quadrupled but declines in a few minutes to a level maintained at about double that before potassium. The minimal calcium release during the first 10 minutes of potassium treatment is estimated to be about the same in both cases and about one-half to one-third the uptake. This, and especially the close equality of calcium entry and exit during electrical stimulation, are pointed out as not necessarily inconsistent with a transitory net entry of calcium, comparable to the influx, into restricted regions of the individual fibers.

Animals↗
PubMed1960

The basis for prolonged contractions in molluscan muscles.

Two basically different hypotheses have been advanced to explain the behavior of molluscan muscles in cases in which relaxation of the muscle is extraordinarily prolonged. In one hypothesis, tetanic activation due to prolonged activity in an intrinsic ganglion network is postulated; in the other, changes in the mechanical properties of the muscle capable of maintaining tension generated by the contractile system are proposed. Experiments reported here were designed to test these hypotheses. Recordings were made of electrical activity in a number of circumstances in which the muscle relaxes slowly, and this activity was absent in some cases and in others was not found to correlate well with rate of relaxation. Quick release of the muscle during and after a stimulus which induced slow relaxation showed disappearance of the active state long before decay of tension. Contractile tension decreases with length below rest length whereas passive tension due to stretch following D. C. stimuli remains approximately independent of length. The latter has the same mechanical basis as prolonged relaxation following D. C. stimuli. Thus initial contractile tension and the tension remainder during prolonged relaxation appear to originate through different mechanisms. These results lead us to favor the second hypothesis above. A means by which this could be achieved in vivo is discussed.

Muscle Contraction↗
PubMed1960

The mechanochemistry of muscular contraction. I. The isometric twitch.

The dependence of PC(1) and ATP(1) dephosphorylation on the number of isometric twitches in the iodoacetate-nitrogen-poisoned muscle has been examined. There is no net dephosphorylation of adenosinetriphosphate. PC dephosphorylation varies linearly with the number of twitches and produces equivalent amounts of C(1) and P(1i).(1) Iodoacetate concentrations which block the enzyme, creatine phosphokinase, render the muscle non-contractile. A value of 0.286 micromole/gm. for the amount of PC split per twitch is obtained which gives a value of -9.62 kcal./mole for the "physiological" heat of hydrolysis of PC in agreement with expectations based on thermochemical data. In a single maximal isometric twitch it is estimated that 2 to 3 PC molecules are dephosphorylated per myosin molecule, or 1 per actin molecule. The results support the view that under the conditions of these experiments PC dephosphorylation is the net energy yielding reaction. The in vivo stoichiometry of the mechano-chemistry of contraction revealed by these studies on the one hand, and the known stoichiometry of actin polymerization and its coupling to the creatine phosphokinase system on the other are strikingly similar and strongly suggest that the reversible polymerization of actin is involved in a major way in the contraction-relaxation-recovery cycle of muscle.

Adenosine Triphosphate↗
PubMed1961

Effect of inhibitors on D-xylose permeability in rat diaphragm muscle.

The influence of metabolic inhibitors and low temperatures upon D-xylose transfer has been studied in rat diaphragm muscle preparations in vitro. Using intact fiber preparations, it has been confirmed that at body temperature metabolic inhibitors like DNP have an insulin-like action in that they permit D-xylose to distribute into previously unavailable intracellular aqueous regions; inhibitors, unlike insulin, disturb cation distribution in association with increased sugar penetration. Although the studies with inhibitors suggest an energy requirement for maintenance of D-xylose exclusion, the D-xylose exclusion mechanism is effectively maintained at 0 degrees for many hours, and under these conditions, inhibitors have little or no effect on D-xylose distribution, though they do produce potassium loss. In cut muscle fiber preparations, in which insulin significantly increases the rate at which D-xylose equilibrates between cell water and external medium, DNP does not increase the rate of D-xylose entry, but does abolish the effect of insulin in this preparation. The results suggest that insulin action upon sugar permeability in muscle involves two barrier systems; some of the characteristics of these systems have been defined.

Animals↗
PubMed1961

The photoactivated relaxation of smooth muscle of rabbit aorta.

Smooth muscle of strips of rabbit aorta, placed in a state of active tonic contraction by addition of a stimulating drug, relaxes during exposure to light. The relaxation is reversible. The extent of relaxation produced by a standard exposure depends on the preexposure level of active contraction but not on the nature of the stimulating drug used to produce contraction. With strips brought to an intermediate level of contraction, the degree of relaxation (steady state levels) is a rectangular hyperbolic function of radiation intensity. The kinetics of the relaxation process during irradiation and the recovery process following irradiation are consistent with the hypothesis that the primary photoactivated material initiates a reaction or reactions leading to a product which inhibits some process involved in the production of active contraction. The photorelaxation does not require the presence of oxygen. It is potentiated by reducing the temperature of the aortic strip. The action spectrum of the photorelaxation shows relatively low effectiveness at wavelengths above 450 mmicro. The effectiveness increases markedly and progressively as the wavelength is lowered below 450 mmicro, reaching a peak at 310 mmicro. A deep trough occurs at 280 mmicro. However, both peak and trough probably result from internal filtering due to absorption by proteins in the aortic strip. It is surmised that if a correction could be made for this internal filtering, the action spectrum would rise continuously down to wavelengths at least as low as 250 mmicro.

Animals↗
PubMed1961

Permeability of frog skeletal muscle cells to choline.

Using choline-methyl-C(14) as a tracer, it has been shown that choline(+) penetrates into the cells of resting frog skeletal muscle at a rate similar to that of Na(+), and that it escapes from these cells much more slowly than does Na(+). Some implications of these findings are discussed.

Animals↗
PubMed1961

Studies on the flight mechanism of insects. I. The electrophysiology of fibrillar flight muscle.

Fibrillar type flight muscle powers the flight machinery of the more phylogenetically advanced groups of flying insects. A comparison of responses from single fibers in insects from various orders having fibrillar muscle reveals fundamental differences. In single fibers of flies and wasps the response to a single threshold stimulus is an all-or-none, uniformly rising, in most cases overshooting action potential. Beetles give variable responses, some of which appear similar to the type mentioned above, and others which summate and facilitate. Some of the latter responses vary with time in a cyclic manner, and some are altered by the intensity of the stimulus. Further differences appear when the two types of muscle are exposed to ether and carbon dioxide. In the wasp and fly ether produces a neuromuscular block, while CO(2) effects a rapid depolarization of the resting fiber membrane. Both reactions are completely reversible. The electrical responses of beetle muscle are somewhat affected but only by massive doses. The implications of these data are discussed relative to the existence of fibrillar muscle "types."

Action Potentials↗
PubMed1961

The ionic mechanisms of hyperpolarizing responses in lobster muscle fibers.

Lobster muscle fibers develop hyperpolarizing responses when subjected to sufficiently strong hyperpolarizing currents. In contrast to axons of frog, toad, and squid, the muscle fibers produce their responses without the need for prior depolarization in high external K(+). Responses begin at a threshold polarization (50 to 70 mv), the potential reaching 150 to 200 mv hyperpolarization while the current remains constant. The increased polarization develops at first slowly, then becomes rapid. It usually subsides from its peak spontaneously, falling temporarily to a potential less hyperpolarized than at threshold for the response. As long as current is applied there can be oscillatory behavior with sequential rise and subsidence of the polarization, repeating a number of times. Withdrawal of current leads to rapid return of the potential to the resting level and a small, brief depolarization. Associated with the latter, but of longer duration, is an increased conductance whose magnitude and duration increase with the antecedent current. Hyperpolarizing responses of lobster muscle fibers are due to increased membrane resistance caused by hyperpolarizing K inactivation. The oscillatory characteristic of the response is due to a delayed superimposed and prolonged increase in membrane permeability, probably for Na(+) and for either K(+) or Cl(-). The hyperpolarizing responses of other tissues also appear to result from hyperpolarizing K inactivation, on which is superimposed an increased conductance for some other ion or ions.

Animals↗
PubMed1961

Effects of insulin on the permeability of D- and L-xylose and D- and L-arabinose in rat diaphragm muscle.

The permeability characteristics of D- and L-xylose and D- and L-arabinose have been compared in isolated intact rat diaphragm muscle preparations, in the absence and presence of exogenous insulin. In the absence of added insulin, these pentoses distribute in less than a third of the total cell water. In the presence of added insulin, intracellular distribution of all these pentoses is increased. L-Xylose and D-arabinose distribute in 50 per cent of the intracellular water, whereas D-xylose and L-arabinose distribute in 80 per cent of the cell water. A significant lag period was observed before the insulin effect upon the penetration of L-xylose and D-arabinose was evident whereas the effect upon D-xylose and L-arabinose was rapid. The lag period with L-xylose could be abolished by pretreating the tissues with insulin for 1 hour, but such pretreatment had little effect on D-xylose. These results indicate that insulin has a biphasic effect upon the monosaccharide exclusion system in diaphragm muscle. In dinitrophenol-treated tissues, in which all permeability processes are irreversibly damaged and in which sucrose and pentoses penetrate into most of the cell water, the entry rate of pentoses and sucrose is initially similar but subsequently D-xylose and L-arabinose penetrate more rapidly than their corresponding optical isomers.

Animals↗
PubMed1962

The coefficient of thermal conductivity of blood and of various tissues.

A method for measuring K, the coefficient of thermal conductivity, for a variety of dog tissues is described. The values of K for muscle and liver are larger than that for water, the value of K for lung is smaller than that for water and the values of K for brain, plasma, and blood are about the same as that of water. The values found for K are given in a table.

Animals↗
PubMed1962

The pattern of activation in the sartorius muscle of the frog.

The development of isometric twitch tension has been compared with the redevelopment of isometric tension in the fully active frog sartorius muscle following release. At 0 degrees C the rate of rise of isometric twitch tension is the same as that for the muscle in the fully active state at the same tension but not until about 40 msec. after the stimulus and then only for a few milliseconds. The rates of rise of tension in the twitch and in the redevelopment of tension in the fully active muscle appear to be nearly the same at low tensions. Substitution of nitrate for chloride in the Ringer's solution bathing the muscle retards the development of tension during the early part of the contraction phase of the twitch and the effect reaches a maximum within 3 minutes after changing the solutions. These observations have been discussed in connection with some possible patterns of activation and the hypothesis has been advanced that the rate of activation of a sarcomere is determined mainly by the rate at which the transverse component of the link between excitation and contraction is propagated inwards from the periphery to the center of the fiber. This hypothesis has been discussed in relation to others concerning the nature of excitation-contraction coupling.

Animals↗
PubMed1962

Water loss during contracture of muscle.

The relationship of contracture and exudation of water in frozenthawed frog muscle was studied. With maximum shortening, there was a water loss of 35 per cent of the weight of muscle. By restricting the contraction, it was demonstrated that the amount of water loss was proportional to the degree of shortening, there being no significant loss with isometric contraction. Muscle already shortened by tetanic stimulation also exuded water on subsequent freezing and thawing. The force of contraction could be reduced by depleting the muscle of calcium and it was shown that the amount of water exuded was also proportional to the tensile ability of the muscle. In a smooth muscle (anterior byssus retractor of Mytilus) which did not contract vigorously only a little water exuded. Contracture produced by caffeine was similarly associated with a loss of water. Microscopic studies revealed a disruption of the sarcomeres of the frozen-thawed muscle which contracted; glycerol-extracted and calcium-depleted muscles, which did not contract on freeze-thawing, did not show such disruption. Freezing and thawing of actomyosin caused a reversible syneresis of the protein. It is concluded that the exudation of the water is not merely due to the freezing and thawing but is also dependent on the contractile events.

Actin Cytoskeleton↗
PubMed1962

Potassium and the recovery of arterial smooth muscle after cold storage.

The influence of K on the performance of vascular smooth muscle was studied by observing the mechanical performance of the muscle under conditions in which the magnitudes of [K(i)] and of the [K(i)]:[K(o)] ratio varied in opposite directions. During prolonged storage at 4 degrees C the artery strips lost K and their ability to respond to stimuli. Subsequently they were transferred to recovery solutions of various [K(o)] at 38 degrees C. The initial rate of K(i) reaccumulation and steady state [K(i)] were greater in solutions of higher [K(o)]. Conversely for any time during recovery, the greater [K(o)], the smaller the [K(i)]:[K(o)] ratio. When the strip was placed in the warm recovery solution it first contracted and then relaxed. The initial contraction was not relatable to [K(o)] of the recovery solution but the subsequent relaxation was greater in rate and magnitude as [K(o)] was greater. As the muscles recovered further they went into tonic contracture. As the [K(o)] in the recovery solutions was greater these contractures occurred after shorter recovery times, and attained greater amplitude at a faster rate. Solution-switching experiments indicated a dependence of responses to electrical shocks on both the [K(i)]:[K(o)] ratio and [K(i)]. Conclusions drawn were: (a) increased [K(i)] increases contractility, (b) increased [K(i)] increases the rate of relaxation, (c) excitability is decreased by too high or low a [K(i)]: [K(o)] ratio, and (d) the extent of tonic shortening depends on the [K(i)]:[K(o)] ratio.

Arteries↗
PubMed1962

Delayed rectification and anomalous rectification in frog's skeletal muscle membrane.

Delayed rectification was elicited in frog's skeletal muscles bathed in choline-Ringer's solution, in normal Ringer's solution with tetrodotoxin, in 40 mM Na(2)SO(4) solution with tetrodotoxin, and even in 40 mM K(2)SO(4) solution when the membrane had been previously hyperpolarized. However, after a sustained depolarization current-voltage relations in 40 mM K(2)SO(4) and in 40 mM Na(2)SO(4) solutions revealed a rectifier property in the anomalous direction. This indicates that the increase in potassium conductance which is brought about upon depolarization is a transient phenomenon and is inactivated by a maintained depolarization, and that this potassium inactivation process converts the delayed rectification into the anomalous rectification. In normal Ringer's solution with tetrodotoxin and in the 40 mM Na(2)SO(4) solution with tetrodotoxin the apparent resistance was increased when the membrane was hyperpolarized beyond about -150 mv. This is thought to be due to a decrease of K conductance caused by a strong hyperpolarizing current. In the 40 mM Na(2)SO(4) solution with tetrodotoxin a de- or hyperpolarizing current pulse induced a prolonged depolarizing response. During the early phase of this response the effective resistance was lower, and during the following phase greater than that in the resting fiber. An interpretation in terms of the ionic hypothesis was made of the nature of this response.

Animals↗