1) The effects of reducing external Na, excess Ca and Mnon pregnant mouse myometrium were investigated by studying theintracellular membrane activity and mechanical response.
2) Excess Ca (22, 66 mM) in Locke solution caused hyperpolarizationand reduction of spontaneous spike activity. The amplitude and rate ofrise of the spike were increased, however the overshoot was practicallyunaltered.
3) Reduction of external Na to 16.2 mM by replacement with Tris orsucrose caused initial depolarization for about 15 min, then a repolarization. When external Na was replaced with sucrose the overshootpotential at the steady state was increased. In Na-free solution, themembrane was depolarized and contracture developed.
4) Mn (0.1, 0.3 mM) reduced the spontaneous spike activity. Higherconcentration (0.6-1.8 mM) blocked the action potential, and causedhyperpolarization accompanying a decrease in membrane conductance.The spike potential was restored by increasing external Ca concentration.
5) Mn suppressed the depolarization and contracture normally producedin low Na media. Membrane which had been depolarized in Ca-freesolution containing the normal Na concentration was repolarized bytreatment with 1.2 mM Mn.
6) It is argued that contracture and depolarization, which are producedtransiently in low Na and maintained in Na-free solutions, may be due inpart to an increase in Ca inflow across the membrane.

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The tension development of rapid cooling contracture (RCC) of toe muscle of Rana nigromaculata treated with 1.0 mM caffeine invarious hypertonic or hypotonic solutions was measured by mechanoelectrictransducer. The tension output of RCC decreased in hypertonic NaCl solution in which the concentration of NaCl was increasedand in hypertonic solution prepared by addition of KCl to the normalRinger solution. The RCC tension output was not altered in isotonicsolution in which NaCl was isotonically substituted for KCl in normalRinger solution, but it decreased in test solution in which KCl washypertonically increased. However, the RCC tension output was notaltered in a solution in which urea was hypertonically added to thenormal Ringer solution.
The RCC tension output increased in hypotonic solution prepared byreducing the concentration of NaCl in normal Ringer solution; its valuewas maintained at an osmotic strength of 200-210m O sm/kg H₂O andwas reduced at 160m O sm/kg H₂O.
In conclusion, the tension development of rapid cooling contracturein hypertonic and hypotonic solutions was similar to the data on crayfishmuscle (APRIL et al., 1968), in which the ionic strength of the intracellularspace is a major determinant of the mechanical response to agiven activation of Ca mechanical coupling.

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In isolated preparations from the axolotl and turtle hearts thepropagation of impulses through different cardiac tissues was studied. Intracellular records were taken from the various types of fibers. Notchedaction potentials were recorded in two junctional regions: the atrioventricularring (A-V. R.) and the ventricle-bulbus cordis (V-B. C.).
The action potential recorded in either the A-V. R. or the V-B. C.regions has a notch in its depolarization phase. The temporal occurrenceof the notch was determined by the activation of the correspondingneighboring tissues. The two components given by the notch can beseparated out by using different experimental variables.
The A-V. R. and the V-B. C. junctional regions are constitutedby groups of horizontal fibers. When these are severed from the otheradjacent cardiac tissues, it can be seen that the junctional fibers presentspontaneous activity and that the action potential recorded from themis characterized by a slow rate of rise, absence of a notch, and a slowdiastolic depolarization.
In these junctional regions it was found that the conduction velocitywas lower than that of any of the other cardiac tissues (Table 1). Thesejunctional regions are preferential sites for delays and blockages inpropagation.
The morphological studies revealed that the cells of the A-V intermediateregion are characterized by:(a) small cross sectional diameter ofcytoplasmic extensions;(b) isolated and scanty myofibrils;(c) abundantglycogen deposits; and (d) sparsity of junctional contacts.
The electrophysiological and ultrastructural characteristics reportedhere, point towards the conclusion that in the axolotl and turtle hearts themechanisms that underlie the delays in propagation in the junctional regions are similar to those of the mammalian in spite of the absenceof specialized conductive tissues in the lower species.

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1) Changes in the membrane potential of sodium-loaded frog'sskeletal muscle caused by a sudden rise of temperature (thermohyperpolarization) were observed with or without ouabain at different membranepotential levels by applying different anodal or cathodal currents.
2) The membrane was hyperpolarized when the applied current wasabsent. The amplitude of the thermohyperpolarization was decreasedwhen the original membrane potential level at low temperature (2°C) was hyperpolarized. The thermohyperpolarization finally disappeared ata certain level (the thermoreversal potential) and reversed its polaritywhen the membrane was hyperpolarized beyond this level.
3) The changes in the membrane conductances at different potentiallevels were also measured with and without 10^-5 M ouabain. The membraneconductance and potential were unchanged at low temperatureirrespective of the presence of ouabain. The membrane conductanceincreased with the rise of temperature and the presence of ouabaincaused the decrease in the membrane conductance at the high temperature (26°C). The current-voltage relationships were constructed fromconductance data.
4) The current required for compensating the thermohyperpolarization (the compensating current) was also measured. The compensatingcurrent as well as the thermohyperpolarization was consistently explainedin terms of the current-voltage relationships.
5) Assuming that ouabain supresses only the pump, the pump potentialdrop and the pump current were calculated as the difference betweenthe thermohyperpolarizations with and without ouabain and that betweencompensating currents with and without ouabain, respectively.Analysis of these data suggested the existence of the pump equilibriumpotential.
6) The thermohyperpolarization, the thermoreversal potential and thepump equilibrium potential were dependent on the external potassiumconcentration having a maximum near 3.5mm and decreased by eitherincreasing or decreasing the potassium concentration.
7) The equivalent electrical circuit for the sodium-loaded frog's skeletalmuscle was proposed.

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Exposing one end of a frog's toe muscle (ex. dig. long. IV) in a sucrose gap apparatus to a Ca⁺⁺ -free Ringer solution resulted in a depolarization of about 15 mV in about 20-25 min (or 40-45 min in a choline Ringer solution) with very little further depolarization with continued exposure (1 hr or more). As little as 0.05mm Ca⁺⁺ was sufficient to prevent or greatly reduce the depolarization. Means and frequency histograms of membrane potentials recorded with intracellular microelectrodes were plotted for fixed time periods of exposure to 0-Ca⁺⁺ The mean depolarizations were between 12 and 35mV when tested 15 min after exposure to 0-Ca⁺⁺ but were only about 10-15mV when first tested after a 1-hr exposure; the maximum fall observed was about 55 mV after 90 min and 4 periods of sampling. The most consistent change produced by 0-Ca⁺⁺ was a flattening and broadening of the frequency histogram. These results suggest that the large falls (>20 mV) in frog' skeletal muscle membrane potentials in 0-Ca⁺⁺ solutions seen here and often reported by other workers are an artifact produced by the intracellular microelectrodes on the cell membranes made fragile by the reduction in membrane Ca⁺⁺.

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The effects of caffeine on mouse myometrium and guineapig ileal muscle were investigated by recording the contractile responseand the membrane activity with an intracellular microelectrode.
1) Caffeine in concentrations of 2 to 20mM suppressed the spontaneouscontraction of mouse myometrium. In the ileal muscle, low concentration (2 to 6 mM) caused an initial potentiation of contraction, then a resulting decay. With high concentration (20 mM), the dominanteffect was a relaxation. K contracture of both tissues was suppressedby all caffeine concentrations.
2) In mouse myometrium, caffeine caused a hyperpolarization, anincrease in membrane conductance and the suppression of spontaneousspike activity. With high concentration, the response evoked by externalcurrent stimulation became abortive.
3) The spike potential in the guinea pig ileal muscle had an amplitudeof about 30 to 50mV and was followed by positive afterpotential. Lowconcentration of caffeine accelerated the normal pattern spike discharge. With high concentration, the membrane was only slightly depolarized, but the frequency of spike discharge increased. The amplitude of spikepotential became larger, and the repolarization slower.
4) The different membrane responses to caffeine in mouse myometriumand guinea pig ileal muscle due to their dependence on the interactionbetween the membrane and Ca are discussed.

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1) The time course of the active state was calculated bymathematical analysis based on the three-component model and comparedwith the experimental curve determined by quick release or stretchin the small bundle preparation dissected from frog semitendinosusmuscle at 10°C.
2) The three-component model contains a contractile component, which consists of a force generator and a viscous-like component, and aseries elastic component. The active state as the force of the force generatoris determined by substituting the tension and velocity curves of thecontractile component for the force-load-velocity relation.
3) By this analytical method, the entire active state curve can be determinednot only in the isometric but also in the isotonic contraction.
4) The active state of the isometric twitch does not reach its full extentof P_o but only 0.7-0.8 P_o.
5) In the isotonic contraction the duration of the active state decreaseswith decreasing load, and the rate of rise of the active state increasesat the transition from isometric to isotonic contraction.
6) The active state curve determined by quick release or quick stretchagrees approximately with the curve determined by the analytical methodwhen the amount of release or stretch is less than 2% of the muscle length, although the experimental curve varies with the amount of release orstretch.

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