Metabolic correlates of fatigue and of recovery from fatigue in single frog muscle fibers

Fatigue and recovery from fatigue were related to metabolism in single fibers of the frog semitendinosus muscle. The fibers were held at a sarcomere length of 2.3 microm in oxygenated Ringer solution at 15 degrees C and were stimulated for up to 150 s by a schedule of 10-s, 20-Hz tetanic trains that were interrupted by 1-s rest periods, after which they were rapidly frozen for biochemical analysis. Two kinds of fatigue were produced in relation to stimulus duration. A rapidly reversed fatigue occurred with stimulation for under 40 s and was evidenced by a decline in tetanic tension that could be overcome by 1 s of rest. A prolonged fatigue was caused by stimulation for 100-150 s. It was evidenced during stimulation by a fall in tetanic tension that could not be overcome by 1 s of rest, and after stimulation by a reduction, lasting for up to 82 min, in the peak tension of a 200-ms test tetanus. Fiber phosphocreatine (PCr) fell logarithmically in relation to stimulus duration, from a mean of 121 +/- 8 nmol/mg protein (SEM, n = 12) to 10% of this value after 150 s of stimulation. PCr returned to normal levels after 90-120 min of rest. Stimulation for 150 s did not significantly affect fiber glycogen and reduced fiber ATP by at most 15%. It is suggested that the prolonged fatigue caused by 100-150 s of tetanic stimulation was caused by long-lasting failure of excitation-contraction coupling, as it was not accompanied by depletion of energy stores in the form of ATP. One possibility is that H+ accumulated in fatigued fibers so as to interfere with the action of Ca2+ in the coupling process.     

Changes in force and stiffness during stretch of skeletal muscle fibers, effects of hypertonicity.

Slow stretch ramps (velocity: 0.17 fiber lengths s-1) were imposed during fused tetanic contractions of intact muscle fibers of the frog (1.4-3.0 degrees C; sarcomere length: 2.12-2.21 microns). Instantaneous force-extension relations were derived both under isometric conditions and during slow stretch by applying fast (0.2 ms) length steps to the fiber. An increase in tonicity (98 mM sucrose added to control Ringer solution) led to significant reduction of the maximum isometric tension but at the same time to marked increase in the force enhancement during slow stretch. The maximum force level reached during the stretch was affected very little. Experiments on relaxed fibers showed that recruitment of passive parallel elastic components were of no relevance for these effects. Hypertonicity slightly increased the instantaneous stiffness of the active fiber both in the presence and in the absence of stretch. The total extension of the undamped fiber elasticity was considerably reduced by increased tonicity under isometric conditions but was only slightly affected during slow stretch. The change in length of the undamped cross-bride elasticity upon stretch was thus greater in the hypertonic than in the normotonic solution suggesting a greater increase in force per cross-bridge in the hypertonic medium. The contractile effects are consistent with the assumptions that hypertonicity reduces the capability of the individual cross-bridge to produce active force and, furthermore, that hypertonicity has only minor effects on the number of attached cross-bridges and the maximum load-bearing capacity of the individual bridge.     

Platelet-derived-growth-factor-stimulated heterogeneous polyphosphoinositide metabolism and phosphate uptake in C3H fibroblasts.

1. Gated 31P-n.m.r. spectra were obtained from the ankle flexor muscles of the rat at various times after 3 s isometric tetanic contraction. This allowed the time course of changes in phosphocreatine (PCr), Pi and free ADP concentrations and intracellular pH to be monitored in skeletal muscle in vivo with 1 s time resolution. 2. ATP concentration did not change significantly, either during the recovery from a 3 s tetanus or during the overall protocol. 3. The calculated rate of recovery of ADP towards pre-stimulation levels was very rapid (t1/2 less than 5 s). The rate of Pi disappearance (t1/2 = 14 s) was more rapid than the rate of PCr synthesis (t1/2 = 24 s), resulting in a significant transient decrease in n.m.r.-visible PCr + Pi between 25 and 45 s after tetanic contraction. 4. The rates of PCr, Pi and ADP recovery are higher than those previously reported for recovery from steady-state exercise in humans or twitch isometric contraction in animals.     

Tension and instantaneous stiffness of single muscle fibers immersed in Ringer solution of decreased tonicity.

Isometric tension and instantaneous stiffness were measured in frog semitendinosus single muscle fibers in both isotonic and hypotonic Ringer solution. In 0.7 and 0.5 x normal Ringer tension increased 17 and 20%, respectively. There was no corresponding increase in the measured stiffness. The increase in tension in hypotonic Ringer could be reversed by the addition of an osmotic equivalent of sucrose to the bathing solution. These findings suggest that the potentiated tension observed in hypotonic Ringer is due to an increased tension per cross-bridge and not to an increase in the number of attached cross-bridges.     

甲状腺功能亢进大鼠比目鱼肌肌浆网Ca2+-ATP酶活性增高可加速强直收缩疲劳

甲状腺功能亢进(甲亢)时甲状腺素分泌增加,不仅使具有神经支配的慢缩型肌纤维向快缩型转化,而且改变骨骼肌的强直收缩功能.因此,甲亢性肌病的肌肉乏力可能与骨骼肌强直收缩易发生疲劳有关.本实验在离体条件下,观测甲亢4周引起的大鼠慢缩肌--比目鱼肌(soleus, SOL)单收缩与间断强直收缩功能的变化.结果显示,甲亢4周大鼠体重明显低于同步对照组[(292±13)g vs (354±10)g],但SOL湿重没有明显改变[(107.3±8.6)mg vs (115.1±6.9)mg].甲亢大鼠SOL单收缩张力达到峰值的时间(time to peak tension, TPT)、从峰值降至75%舒张时间(time from peak tension to 75% relaxation, TR75)均明显缩短;强直收缩的TR75也明显缩短[(102.8±4.1)ms vs (178.8±15.8)ms];强直收缩的最适频率从对照组的100Hz增加到140Hz;间断强直收缩期间容易发生疲劳.甲亢大鼠SOL肌浆网Ca2 -ATP酶(sarcoplasmic-reticulum Ca2 -ATPase, SERCA)活性增高.采用SERCA特异性抑制剂CPA (1.0μmol/L)处理后,对照组与甲亢大鼠SOL间断强直收缩的TR75均延长,同时不易出现疲劳.5.0μmol/L CPA灌流虽可进一步抵抗甲亢大鼠SOL间断强直收缩引起的疲劳,但强直收缩期间的静息张力却明显升高.将CPA浓度增至10.0μmol/L,甲亢大鼠SOL间断强直收缩又趋向易发生疲劳.这些结果提示,与心肌相同,骨骼肌肌纤维SERCA活性亦可影响单收缩与强直收缩的舒张时间,SERCA活性升高可加速间断强直收缩发生疲劳.     

ULTRASTRUCTURE OF THE RESTING AND CONTRACTED STRIATED MUSCLE FIBER AT DIFFERENT DEGREES OF STRETCH

Passive stretch, isometric contraction, and shortening were studied in electron micrographs of striated, non-glycerinated frog muscle fibers. The artifacts due to the different steps of preparation were evaluated by comparing sarcomere length and fiber diameter before, during, and after fixation and after sectioning. Tension and length were recorded in the resting and contracted fiber before and during fixation. The I filaments could be traced to enter the A band between the A filaments on both sides of the I band, creating a zone of overlap which decreased linearly with stretch and increased with shortening. This is consistent with a sliding filament model. The decrease in the length of the A and I filaments during isometric contraction and the finding that fibers stretched to a sarcomere length of 3.7 µ still developed 30 per cent of the maximum tetanic tension could not be explained in terms of the sliding filament model. Shortening of the sarcomeres near the myotendinous junctions which still have overlap could account for only one-sixth of this tension, indicating that even those sarcomeres stretched to such a degree that there is a gap between A and I filaments are activated during isometric contraction (increase in stiffness). Shortening, too, was associated with changes in filament length. The diameter of A filaments remained unaltered with stretch and with isometric contraction. Shortening of 50 per cent was associated with a 13 per cent increase in A filament diameter. The area occupied by the fibrils and by the interfibrillar space increased with shortening, indicating a 20 per cent reduction in the volume of the fibrils when shortening amounted to 40 per cent.     

A non-cross-bridge stiffness in activated frog muscle fibers

Force responses to fast ramp stretches of various amplitude and velocity, applied during tetanic contractions, were measured in single intact fibers from frog tibialis anterior muscle. Experiments were performed at 14 degrees C at approximately 2.1 microm sarcomere length on fibers bathed in Ringer's solution containing various concentrations of 2,3-butanedione monoxime (BDM) to greatly reduce the isometric tension. The fast tension transient produced by the stretch was followed by a period, lasting until relaxation, during which the tension remained constant to a value that greatly exceeded the isometric tension. The excess of tension was termed "static tension," and the ratio between the force and the accompanying sarcomere length change was termed "static stiffness." The static stiffness was independent of the active tension developed by the fiber, and independent of stretch amplitude and stretching velocity in the whole range tested; it increased with sarcomere length in the range 2.1-2.8 microm, to decrease again at longer lengths. Static stiffness increased well ahead of tension during the tetanus rise, and fell ahead of tension during relaxation. These results suggest that activation increased the stiffness of some sarcomeric structure(s) outside the cross-bridges.     

Isometric tension and instantaneous stiffness in amphibian skeletal muscle exposed to solutions of increased tonicity.

The instantaneous elasticity and maximum isometric tetanic tension of isolated frog and toad sartorii have been measured in hypertonic Ringer solution. Although the mechanical response of contraction muscle continued to decrease as the tonicity of the bathing solution was increased to 1.26 X R, 1.52 X R, and 2.04 X R, a similar change in the instantaneous stiffness could not be shown. This finding was not expected on the basis of our current model of the cross-bridge mechanism which predicts that the instantaneous stiffness is a measure of the total number of tension-generating cross-bridges formed in a stimulated muscle. The compatability of our findings with an electrostatic theory of the cross-bridge mechanism proposed by Iwazumi (1970) is discussed.     

Continuum rheology of muscle contraction and its application to cardiac contractility.

A set of constitutive equations is proposed to describe the mechanics of contraction of skeletal and heart muscle. Fiber tension is assumed to depend on the degree of chemical activation, the stretch ratio, and the rate of stretching of the fibers. The time rate of change of activation is governed by a differential equation. The proposed constitutive equations are used to model the time courses of isotonic and isometric twitches during contraction and relaxation phases of the muscle response to stimulation. Various contractility indices of the left ventricle are considered next by using the proposed constitutive equations. The present analysis introduces a new interpretation of the index of contractility (dP/dt)/P used in cardiac literature. It is shown that this index may not be related at all to the maximum speed of shortening and that it may be dependent on both preload and afterload. The development of pressure during isovolumetric contraction of the left ventricle is shown to be governed by a differential equation describing the time rate of change of tension during isometric contraction of myocardium fibers.     

Metabolic underpinnings of the paradoxical net phosphocreatine resynthesis in contracting rat gastrocnemius muscle

Net phosphocreatine (PCr) resynthesis during muscle contraction is a paradoxical phenomenon because it occurs under conditions of high energy demand. The metabolic underpinnings of this phenomenon were analyzed non-invasively using 31P-magnetic resonance spectroscopy in rat gastrocnemius muscle (n=11) electrically stimulated (7.6 Hz, 6 min duration) in situ under ischemic and normoxic conditions. During ischemic stimulation, [PCr] initially fell to a steady state (9+/-5% of resting concentration) which was maintained for the last 5 min of stimulation, whereas isometric force production decreased to a non-measurable level beyond 3 min. Throughout normoxic stimulation, [PCr] and force production declined to a steady state after respectively 1 min (5+/-3% of resting concentration) and 3.25 min (21+/-8% of initial value) of stimulation. Contrary to the observations under ischemia, a paradoxical net PCr resynthesis was recorded during the last 2 min of normoxic stimulation and was not accompanied by any improvement in force production. These results demonstrate that the paradoxical net PCr resynthesis recorded in contracting muscle relies exclusively on oxidative energy production and could occur in inactivated fibers, similarly to PCr resynthesis during post-exercise recovery.     

The effect of diazepam on potassium contractures, contraction threshold, and resting tension in rat skeletal muscles

The effects of diazepam on potassium contractures, contraction threshold, and resting tension have been examined in rat soleus muscle fibres. Two actions of the drug were defined that could not be attributed to changes in the resting membrane potential or depolarization in high potassium solutions. The major effect was an increase in the amplitude of submaximal tension during either twitches or potassium contractures and an increase in resting tension. At 400 microM diazepam, there was (a) a fourfold increase in 40 mM potassium contracture tension, (b) a negative shift of 8 mV in the membrane potential for half maximum tension estimated from the best fit of a Boltzmann-type equation to average potassium contracture data, (c) a negative shift of 8 mV in the threshold for contraction measured under voltage clamp conditions, and (d) a contracture of variable amplitude to a level that was occasionally equivalent to maximum tetanic tension. These potentiating actions of diazepam depended on drug concentration within the range of 100-800 microM. In contrast, the second effect of diazepam, depression of maximum tension by 10-15%, was independent of drug concentration between 100 and 400 microM. The results support the idea that diazepam produces an increase in resting myoplasmic calcium concentrations.     

Tension relaxation after stretch in resting mammalian muscle fibers: stretch activation at physiological temperatures.

Tension responses to ramp stretches of 1-3% Lo (fiber length) in amplitude were examined in resting muscle fibers of the rat at temperatures ranging from 10 degrees C to 36 degrees C. Experiments were done using bundles of approximately 10 intact fibers isolated from the extensor digitorum longus (a fast muscle) and the soleus (a slow muscle). At low temperatures (below approximately 20 degrees C), the tension response consisted of an initial rise to a peak during the ramp followed by a complex tension decay to a plateau level; the tension decay occurred at approximately constant sarcomere length. The tension decay after a standard stretch at approximately 3-4.Lo/s contained a fast, an intermediate, and a (small amplitude) slow component, which at 10 degrees C (sarcomere length approximately 2.5 microns) were approximately 2000.s-1, approximately 150.s-1, and approximately 25.s-1 for fast fibers and approximately 2000.s-1, approximately 70.s-1 and approximately 8.s-1 for slow fibers, respectively. The fast component may represent the decay of interfilamentary viscous resistance, and the intermediate component may be due to viscoelasticity in the gap (titin, connectin) filament. The two- to threefold fast-slow muscle difference in the rate of passive tension relaxation (in the intermediate and the slow components) compares with previously reported differences in the speed of their active contractions; this suggests that "passive viscoelasticity" is appropriately matched to contraction speed in different muscle fiber types. At approximately 35 degrees C, the fast and intermediate components of tension relaxation were followed by a delayed tension rise at approximately 10.s-1 (fast fibers) and 2.5.s-1 (slow fibers); the delayed tension rise was accompanied by sarcomere shortening. BDM (5-10 mM) reduced the active twitch and tetanic tension responses and the delayed tension rise at 35 degrees C; the results indicate stretch sensitive activation in mammalian sarcomeres at physiological temperatures.     

Metabolic underpinnings of the paradoxical net phosphocreatine resynthesis in contracting rat gastrocnemius muscle

Net phosphocreatine (PCr) resynthesis during muscle contraction is a paradoxical phenomenon because it occurs under conditions of high energy demand. The metabolic underpinnings of this phenomenon were analyzed non-invasively using ³¹P-magnetic resonance spectroscopy in rat gastrocnemius muscle (n=11) electrically stimulated (7.6 Hz, 6 min duration) in situ under ischemic and normoxic conditions. During ischemic stimulation, [PCr] initially fell to a steady state (9±5% of resting concentration) which was maintained for the last 5 min of stimulation, whereas isometric force production decreased to a non-measurable level beyond 3 min. Throughout normoxic stimulation, [PCr] and force production declined to a steady state after respectively 1 min (5±3% of resting concentration) and 3.25 min (21±8% of initial value) of stimulation. Contrary to the observations under ischemia, a paradoxical net PCr resynthesis was recorded during the last 2 min of normoxic stimulation and was not accompanied by any improvement in force production. These results demonstrate that the paradoxical net PCr resynthesis recorded in contracting muscle relies exclusively on oxidative energy production and could occur in inactivated fibers, similarly to PCr resynthesis during post-exercise recovery.     

Observation of fatigue unrelated to gross energy reserve of skeletal muscle during tetanic contraction--an application of 31P-MRS

The mechanism of muscle fatigue was studied by 31P-MRS. During tetanic contraction for 2 minutes(min), the tension measured with a strain gauge and Phosphocreatine(PCr)/Inorganic phosphate(Pi)+ Phosphomonoester(PME) ratio decreased to 31.5 +/- 4.4% of the control value and 0.6 +/- 0.1, respectively. The intracellular pH(pH) also decreased to 6.62 +/- 0.04. Toward the end of the stimulation, the tension decreased to 25.3 +/- 1.9% of the control value. However, during 20min stimulation, the PCr/(Pi+PME) ratio increased to 2.5 +/- 0.5 and the pH to 6.91 +/- 0.04. These results show that muscular fatigue is ascribable not to a decreased level of high energy metabolites required for actomyosin ATPase, but to an increase in the threshold intensity of excitation in excitation-contraction coupling.     

Double Sucrose-Gap Method Applied to Single Muscle Fiber of Xenopus laevis

Passive electrical properties (internal conductance, membrane conductance, low frequency capacity, and high frequency capacity obtained from the foot of the action potential) of normal and glycerol-treated muscle of Xenopus were determined with the intracellular microelectrode technique. The results show that the electrical properties of Xenopus muscle are essentially the same as those of frog muscle. Characteristics of the action potential of Xenopus muscle were also similar to those of frog muscle. Twitch tension of glycerol-treated muscle fibers of Xenopus recovered partially when left in normal Ringer for a long time (more than 6 h). Along with the twitch recovery, the membrane capacity increased. Single isolated muscle fibers of Xenopus were subjected to the double sucrose-gap technique. Action potentials under the sucrose gap were not very different from those obtained with the intracellular electrode, except for the sucrose-gap hyperpolarization and a slight tendency toward prolongation of the shape of action potential. Twitch contraction of the artificial node was recorded as a change of force from one end of the fiber under the sucrose gap. From the time-course of the recorded force and the sinusoidal stress-strain relationship at varying frequencies of the resting muscle fiber, the time-course of isotonic shortening of the node was recovered by using Fourier analysis. It was revealed that the recorded twitch force can approximately be regarded as isotonic shortening of the node.     

Aminophylline enhances contractility of frog skeletal muscle: an effect dependent on extracellular calcium

The effects of aminophylline (10-500 microM) on isometric twitch and tetanic forces were studied in vitro on frog semitendinosus muscle. Two hypotheses were tested: 1) that micromolar concentrations of aminophylline enhanced contractility of isolated skeletal muscle and 2) that the potentiating effect of aminophylline was dependent on the presence of extracellular calcium ions. Muscles were removed, placed in aerated Ringer solution at 20 degrees C, attached to a force transducer, and stimulated directly. Muscles in normal Ringer and aminophylline Ringer were compared throughout the frequency-force relationship from twitches to maximum tetanic force. Aminophylline increased twitch force significantly at concentrations as low as 25 microM. Over a range of stimulation frequencies, but especially at 10 and 20 Hz, aminophylline increased tetanic force. The potentiating effect of aminophylline (100 microM) was reduced or eliminated in calcium-free Ringer containing 10 mM magnesium. We conclude that aminophylline, at therapeutic concentrations, enhances muscle contractility, and the enhancement is dependent on the presence of extracellular calcium. These findings support the concept that aminophylline is effective in improving respiration in humans with airway obstruction by enhancing diaphragmatic contractility.     

Changes in intracellular pH caused by high K in normal and acidified frog muscle. Relation to metabolic changes

We examined the effect of depolarization on intracellular pH (pHi) of normal (pHi approximately 7.37) and acidified (pHi 5.90-6.70) frog semitendinosus muscle using microelectrodes. A small bundle was superfused with a Na(+)-free buffered solution (10 mM HEPES, 100% O2, pH 7.35) containing either 2.5 or 25 mM K+. An NH4Cl prepulse was used to lower pHi. At normal pHi, depolarization usually produced a slight (0.04) alkalinization, followed by a fall in pHi of approximately 0.2. In contrast, in all 25 acidified bundles pHi rose by 0.1-0.7. The rise was greater the lower the initial pHi. It could be imitated by caffeine and blocked by tetracaine and thus was, most likely, initiated by release of calcium. We ascribed the alkalinization to hydrolysis of phosphocreatine (PCr); 2,4-dinitrofluorobenzene abolished it. Biochemical analysis on fibers at the peak of alkalinization showed PCr to be reduced by one-half, while PCr in normal fibers that had been depolarized for the same period (4-6 min) showed no change. We postulated that low pHi slows glycolysis with its associated ATP formation by reducing glycogenolysis and particularly by reducing conversion of fructose-6-phosphate to fructose-1,6-diphosphate through inhibition of phosphofructokinase (PFK), an enzyme which is known to be highly pH sensitive. Thus PCr hydrolysis would be required to replace much of the hydrolyzed ATP. This postulated effect on PFK is in agreement with the finding that glucose-6-phosphate (in near-equilibrium with fructose-6-phosphate) was increased nearly fivefold in the depolarized acid fibers, but not in the depolarized normal fibers. However, fructose-1,6-diphosphate also increased significantly; 3-phosphoglycerate was not affected. This suggests an additional acid-induced bottleneck between the latter two substrates. We measured the intrinsic buffering power, beta, of frog semitendinosus muscle with small pulses of NH4Cl. It was found to vary with pHi according to beta = 144.6 - 17.2 (pHi).     

Tension in Isolated Frog Muscle Fibers Induced by Hypertonic Solutions

The effect of hypertonic solutions on the tension of isolated twitch muscle fibers of the frog has been investigated. Increased tonicity up to about 1.7 times normal (1.7 T) caused a very small, graded, maintained tension increase. Above about 1.7 T a large, transient contracture response was superimposed on the small tension. The contracture response was graded with tonicity and reached a maximum at 2.5 T of 108 ± 25 mN·mm² a third of the maximum tetanic tension in isotonic solution. Contracture tension developed with a delay which decreased with increased tonicity. The contracture threshold was lower and the delay shorter in small fibers than in large. Contractures were obtained equally well in depolarized as in polarized fibers. They were completely suppressed by 0.1–0.5 mM tetracaine. The possible mechanism responsible for the tension-inducing effect of hypertonic solutions is discussed in terms of the close similarity between the properties of these contractures and those caused by caffeine, and it is suggested that the effect is due to a release of calcium from internal stores.     

Changes of myoplasmic calcium concentration during fatigue in single mouse muscle fibers

Measurements of the intracellular free concentration of Ca2+ ([Ca2+]i) were performed during fatiguing stimulation of intact, single muscle fibers, which were dissected from a mouse foot muscle and loaded with fura-2. Fatigue, which was produced by repeated 100-Hz tetani, generally occurred in three phases. Initially, tension declined rapidly to approximately 90% of the original tension (0.9 Po) and during this period the tetanic [Ca2+]i increased significantly (phase 1). Then followed a lengthy period of almost stable tension production and tetanic [Ca2+]i (phase 2). Finally, both the tetanic [Ca2+]i and tension fell relatively fast (phase 3). The resting [Ca2+]i rose continuously throughout the stimulation period. A 10-s rest period during phase 3 resulted in a significant increase of both tetanic [Ca2+]i and tension, whereas a 10-s pause during phase 2 did not have any marked effect. Application of caffeine under control conditions and early during phase 2 resulted in a substantial increase of the tetanic [Ca2+]i but no marked tension increase, whereas caffeine applied at the end of fatiguing stimulation (tension depressed to approximately 0.3 Po) gave a marked increase of both tetanic [Ca2+]i and tension. The tetanic [Ca2+]i for a given tension was generally higher during fatiguing stimulation than under control conditions. Fatigue developed more rapidly in fibers exposed to cyanide. In these fibers there was no increase of tetanic [Ca2+]i during phase 1 and the increase of the resting [Ca2+]i during fatiguing stimulation was markedly larger. The present results indicate that fatigue produced by repeated tetani is caused by a combination of reduced maximum tension-generating capacity, reduced myofibrillar Ca2+ sensitivity, and reduced Ca2+ release from the sarcoplasmic reticulum. The depression of maximum tension-generating capacity develops early during fatiguing stimulation and it is of greatest importance for the force decline at early stages of fatigue. As fatigue gets more severe, reduced Ca2+ sensitivity and reduced Ca2+ release become quantitatively more important for the tension decline.     

Loss of the plateau of the cardiac action potential in hypertonic solutions

The effect of hypertonicity on the electrical properties of vertebrate myocardial cells was studied in ventricular muscle fibers of guinea pig, cat, frog, and chicken. The latter two species do not have a T-tubule system, whereas the former two do. In hypertonic solutions (2 x isotonic) produced by addition of sucrose or excess of NaCl, cell diameter decreased and there was a slight hyperpolarization and decrease in action potential overshoot. In guinea pig and cat, the hypertonic solution caused a decrease in input resistance and the plateau of the action potential to disappear in some of the cells; contractions of the entire ventricle also became depressed. These effects were reversed by returning the muscle fibers to isotonic solution. Addition of 5 mM SrCl₂ to the hypertonic solution also caused the plateau component and contraction to reappear. In frog and chick cells, loss of the plateau component and contraction never occurred in hypertonic solution, and input resistance increased. Urea and glycerol hyperosmolarity (2 x) caused no loss of the plateau component or contraction. If the frog and chicken ventricular, and guinea pig atrial myocardial cells (all of which lack T tubules) were to serve as an adequate control for possible effects of hypertonicity on the surface membrane and on contractile proteins, then the results suggest that swelling of the T tubules of mammalian myocardial cells leads to loss of the plateau component.