Decline In Tension Research Articles

A comparison of excitation-contraction coupling in heart and skeletal muscle: An examination of “calcium-induced calcium release”

Vascularly perfused guinea pig semimembranosus accessorius (SMA) and frog gastrocnemius (FG) muscles and perfused interventricular septa of guinea pig heart were used to examine the hypothesis which proposes the physiological regenerative release of Ca. Membrane resting potentials and excitability were maintained by including 100 μ m La in Ca-free perfusion media (0-Ca, 100-La). Tension decline in 0-Ca, 100-La was rapid in the heart with halftime (t 1 2 ) = 11 s and correlated with the most rapid component of 45Ca efflux. In contrast, decline of the major component of tension in SMA and FG muscles was much slower with t 1 2 = 22 to 33 min . The rate of tension decline was virtually identical to the t 1 2 of the slow component of 45Ca efflux from these muscles. Neither perfusion with 0-Ca, 2 m m EGTA solutions after 30 min 0-Ca exposure (at which time interstitial [Ca 2+] was estimated < 3 × 10 −9 m) nor addition of 100 μ m La to 0-Ca, 100-La perfusates after 60 min (SMA) or 120 min (FG) significantly altered the rate of tension decline. The data emphasize the difference in source of contractile Ca in heart and skeletal muscle. It is concluded that in these physiologically perfused muscles with sarcolemma intact there is no evidence for a role of “trigger Ca” in the process of E-C coupling.

Heat effects on a marine snail.

Littorina littorea enters a reversible state of heat coma at temperatures below the lethal temperature of the species. The threshold of heat coma can be shifted as much as 8.5° C by acclimation; a linear relation exists between heat coma temperature and acclimation temperature over the experimental range.Deganglionated columellar muscle preparations of 5° C and 25° C acclimated Littorina littorea showed an approximately linear decline in tetanic tension when tested from 5° C to 40° C. There was no statistically significant difference in the amplitude of muscle contraction or of tension-length curves of 5° C- and 25° C-acclimated muscles at any experimental temperature; this excludes failure of muscle contraction as the cause of heat coma. Speed of contraction was similar and relaxation slower in muscles from 5° C-acclimated snails than in muscles from 250 C-acclimated snails when measured at 10° C but not at 25° C.Attempts to ascertain by the use of drugs and nerve degeneration experiments whether muscle ...

POSTMORTEM GLYCOLYSIS AND ISOMETRIC THAW TENSION DEVELOPMENT AND DECLINE IN BOVINE SKELETAL MUSCLE UNDERGOING THAW RIGOR

Journal of Food ScienceVolume 40, Issue 5 p. 955-959 POSTMORTEM GLYCOLYSIS AND ISOMETRIC THAW TENSION DEVELOPMENT AND DECLINE IN BOVINE SKELETAL MUSCLE UNDERGOING THAW RIGOR A. O. OKUBANJO, A. O. OKUBANJO Dept. of Animal Science, N. Y. State College of Agriculture & Life Sciences Dept. of Animal Science, University of Ibadan, Ibadan, Nigeria. AfricaSearch for more papers by this authorJ. R. STOUFFER, J. R. STOUFFER Cornell University, Ithaca. NY 14853Search for more papers by this author A. O. OKUBANJO, A. O. OKUBANJO Dept. of Animal Science, N. Y. State College of Agriculture & Life Sciences Dept. of Animal Science, University of Ibadan, Ibadan, Nigeria. AfricaSearch for more papers by this authorJ. R. STOUFFER, J. R. STOUFFER Cornell University, Ithaca. NY 14853Search for more papers by this author First published: September 1975 https://doi.org/10.1111/j.1365-2621.1975.tb02242.xCitations: 2 AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Citing Literature Volume40, Issue5September 1975Pages 955-959 RelatedInformation

Cross bridges as the major source of compliance in contracting skeletal muscle.

IT has been established that the elastic properties of contracting skeletal muscle are dominated by structures within the sarcomeres. We have demonstrated1 a decrease in the instantaneous stiffness of a contracting toad sartorius at lengths greater than body length (l0) coinciding directly with the decline in maximum isometric tetanic tension. This finding suggested that the cross bridges between actin and myosin filaments may be the major site of compliance in an active muscle, a conclusion in accordance with that reached by Huxley and Simmons2 on the basis of a study of tension transients following rapid releases in single fibres of frog semitendinosus. Also in agreement with this conclusion is the work of Cleworth and Edmond3 who used a light diffraction technique to demonstrate that any change in the length of the sarcomere during isometric contraction was less than 50 A, or less than 0.1 %. These studies suggest that the cross links could be the major site of elasticity within the sarcomere, but it is possible that significant compliance resides in the unbonded thin filaments, although X-ray diffraction studies of contracting muscle4 indicate that the I-filament length does not increase by more than 0.02%. The experiments reported here were carried out to extend previous observations of muscle stiffness into the range of lengths below the resting length.

Tension decline during isometric contractions without visual cues

Subjects tried to maintain 15%, 30% and 50% maximum voluntary contraction (MVC) of their handgrip for 60 sec and 70% MVC for 30 sec. When the subjects watched the tension record, they could maintain the contraction for the required time. However, without visual cues (uncued), the tests resulted in a tension decline. Ae-alpha t described the tension decline in the 15% tests, Be-beta t the 70% test, while the form in the 30% and 50% tests was Ae-alpha t + Be-beta t. In all cases alpha and/or beta were found to be dependent on the starting tension. The electrical activity obtained from surface electromyograms did not increase during the uncued tests which would be expected if muscle fatigue were involved in the tension decline. Instead the electrical activity was directly proportional to the decline in tension suggesting that the fall in tension was not due entirely to muscle fatigue. 15% MVC uncued tests in which fatigue is known not to be a factor also showed a tension decline. Precooling the hand in an attempt to alter sensory input produced no systematic change from the original uncued tests, indicating that sensory information from the hand was probably not a major factor in the tension decline. It is suggested that during the uncued tests the fast component of the tension decline may be related to a sensory adaptation involving the Golgi tendon organs. The slow component is due to forearm muscle fatigue which is related to the ischemia produced during the isometric contractions.

Effects of tonicity on the resting tension in bullfrog ventricle.

A study of the effects of hyper -and hypotonic media on resting tension were examined in ventricular muscle of the bullfrog was undertaken because only few studies on resting tension from the viewpoint of E-C coupling in the heart muscle are available. Thin strips obtained from the bullfrog ventricle were perfused in the sucrose-gapped chamber, which allowed the effective stimulation and the simultaneous measurement of the contractile tension and the intracellular membrane potential. Hypertonic solutions up to 3.0 T, prepared by adding NaCl or sucrose, induced a triphasic response, namely an initial rapid decline, a secondary slow development of contracture and a final transient off-contraction. Both the second and the final responses were strongly sensitive to the calcium concentration of the bathing medium and were completely abolished by depleting external calcium ions. The initial decreasing phase was not sensitive to external calcium but was progressively augmented by increasing the initial muscle length, whereas the dehydration process, estimated by measuring the muscle weight, was unaffected by muscle stretch. The changes in the membrane potential were not essential for these responses. It was concluded that the initial tension decline results from the passive process due to volume change while the sustained contracture as well as the transient off-contraction are produced by some active process involved in E-C coupling. Hypotonic solutions induced a monophasic contracture which is insensitive both to the external calcium concentration and muscle stretch. The possible mechanism for the genesis of the response was discussed.

42K efflux, EKG and tension in isolated perfused hearts of white-footed mice

1. 1. The EKG and systolic tension of isolated, perfused hearts from Peromyscus leucopus is a function of the perfusion temperature. 2. 2. The marked loss of 42K during perfusion at lower temperatures indicates a loss of intracellular potassium and suggests a breakdown in the ability to maintain electrolyte balance. 3. 3. The decline in EKG potential and tension as well as the increased 42K efflux which occur at perfusion temperatures similar to the lower range of body temperature during torpidity of the intact animal indicate that cardiac performance may limit the depth and length of torpor in P. leucopus.

Effects of nitroglycerin on left ventricular cavitary size and cardiac performance determined by ultrasound in man

Controversy remains whether the antianginal action of nitroglycerin is the result of change in coronary blood flow or decrease in myocardial oxygen demand (MVO 2) by decline in left ventricular systolic tension. Although left ventricular radius is an important determinant of left ventricular tension, clinical data have not been available on the effects of nitroglycerin on left ventricular cavitary size. Thus we evaluated the action of the organic nitrate on endocardial left ventricular dimensions determined by echograms. Left ventricular echoes were obtained just below the mitral valve in 6 normal subjects and in 14 patients with coronary disease before and at 2 minute intervals for 10 minutes after the sublingual administration of 0.6 mg nitroglycerin. Mean heart rate rose from 71 to 83 beats/min (p < 0.001) and mean arterial pressure fell from 92 to 87 mm Hg (p < 0.001) whereas cardiac output and peripheral vascular resistance were not significantly altered. Nitroglycerin reduced left ventricular end-diastolic dimension (4.94 to 4.69 cm, p < 0.001) and end-systolic dimension (3.63 to 3.37 cm, p < 0.001) accounting for declines in calculated left ventricular end-diastolic volume (129 to 112 cc, p < 0.001) and end-systolic volume (57 to 47 cc, p < 0.001) with fall in stroke volume from 72 to 65 cc, p < 0.01. Mean fiber shortening velocity (V CF) decreased from 1.08 to 1.02 circumferences/sec (p < 0.01), indicating reduced preload as we found with V CF on head-up tilting. Since stroke volume fell in patients with minimal changes in heart rate whereas peripheral vascular resistance remained unaltered, the principal extracardiac dilator effect of nitroglycerin is on the systemic venous bed. Thus, sublingual administration of nitroglycerin in man reduces end-diastolic dimension and end-systolic dimension and thereby is capable of lowering MVO 2 by diminishing left ventricular tension. These findings support the concept that a decrease in MVO 2 plays an important role in the relief of angina pectoris by nitroglycerin.

Influence of hyperosmolar agents on hypoxic cat papillary muscle function.

The influence of hypertonic agents on 64 isolated hypoxic cat papillary muscles was examined under isometric conditions in Krebs-Ringer bicarbonate solution with 18 mM glucose at 29°C. Nine papillary muscles pretreated before hypoxia and eight muscles exposed to hypertonic mannitol 6 min following hypoxia demonstrated a significantly reduced decline from prehypoxia control values for developed tension and its first derivative compared to the same muscles in the control solution. There was no significant change in resting tension. This improvement in muscle performance during hypoxia was present at osmolalities of 60 mOsm above control values. The performance of papillary muscles during hypoxia was similar for hypertonic mannitol, sucrose, and urea when the muscles were exposed to the agents for 30 min before hypoxia. However, longer periods (2 hours) of exposure to hypertonic mannitol or urea before hypoxia resulted in a reduced decline in developed tension and its first derivative during hypoxia only in the case of mannitol. These studies suggest that the beneficial effect of hyperosmolar agents on muscle performance during hypoxia may not be totally dependent on blood flow changes and may be associated with changes in cell size.

Tension in skinned frog muscle fibers in solutions of varying ionic strength and neutral salt composition.

The maximal calcium-activated isometric tension produced by a skinned frog single muscle fiber falls off as the ionic strength of the solution bathing this fiber is elevated declining to zero near 0.5 M as the ionic strength is varied using KCl. When other neutral salts are used, the tension always declines at high ionic strength, but there is some difference between the various neutral salts used. The anions and cations can be ordered in terms of their ability to inhibit the maximal calcium-activated tension. The order of increasing inhibition of tension (decreasing tension) at high ionic strength for anions is propionate(-) approximately SO(4) (--) < Cl(-) < Br(-). The order of increasing inhibition of calcium-activated tension for cations is K(+) approximately Na(+) approximately TMA(+) < TEA(+) < TPrA(+) < TBuA(+). The decline of maximal calcium-activated isometric tension with elevated salt concentration (ionic strength) can quantitatively explain the decline of isometric tetanic tension of a frog muscle fiber bathed in a hypertonic solution if one assumes that the internal ionic strength of a muscle fiber in normal Ringer's solution is 0.14-0.17 M. There is an increase in the base-line tension of a skinned muscle fiber bathed in a relaxing solution (no added calcium and 3 mM EGTA) of low ionic strength. This tension, which has no correlate in the intact fiber in hypotonic solutions, appears to be a noncalcium-activated tension and correlates more with a declining ionic strength than with small changes in [MgATP], [Mg], pH buffer, or [EGTA]. It is dependent upon the specific neutral salts used with cations being ordered in increasing inhibition of this noncalcium-activated tension (decreasing tension) as TPrA(+) < TMA(+) < K(+) approximately Na(+). Measurements of potentials inside these skinned muscle fibers bathed in relaxing solutions produced occasional small positive values (<6 mV) which were not significantly different from zero.

Effect of cold storage on calcium-related responses and electrolyte content of rabbit aortic strips

Rabbit aortic strips stored 4 days at 6°C in Ringer solution had potentiated responses to 10 −9−10 −8M norepinephrine. After 24 hr, or 4 days at 6°C in a high calcium Ringer (4.8 mM) responses to all concentrations of norepinephrine were potentiated. Responses of fresh and cold stored aortae were depressed in calcium-free Ringer and by 10 −3 M LaCl 3. Maximum tension in a 60 mM potassium, calcium-free Ringer was maintained longer by cold stored aortie than by fresh aortae. Rate of tension decline in this solution was the same for both groups of aortae. The ED 50 for calcium-induced responses was increased in the cold-stored aortic strips. Calcium required for norepinephrine responses was greater in the cold-stored aortae. In normal Ringer at 37°C fresh aortae did not respond to increases in calcium. After cold storage, 7 out of 12 aortae developed tension in response to calcium in normal Ringer. After 24 hr or 4 days at 6°C sodium and calcium contents of aortae were increased, potassium decreased. After 2 hr incubation at 37°C of cold-stored aortae, sodium contents were same as that of fresh aortae; calcium was still increased and potassium decreased. These data suggest potentiation after cold storage is due to a depolarization of the muscle membrane and not to an increase in effectiveness of calcium.

Histochemical composition, contraction speed and fatiguability of rat soleus motor units

The relation between the histochemical myosin ATPase and SDH activities and the speed and endurance of contraction at 34.5–35.5°C was studied in 28 rat soleus motor units. 1. (1) Myosin ATPase activity was associated with speed and from the intensity of the reaction it was possible to predict the contraction time of a fast unit with unexpected accuracy. Units with a strong myosin ATPase reaction after formaldehyde fixation and an inhibited reaction after acid preincubation at pH 4.35 had a contraction time of 15–20 msec. Units with moderate staining reactions had a contraction time of 21–23 msec and those with an inhibited reaction after fixation and strong reaction after preincubation had contraction times of 26–44 msec. The fibres of fast and slow units appeared histochemically uniform; fibres of intermediate units revealed subtle variations in the intensity of the ATPase staining. Three units were histochemically transitional between two adjacent groups. 2. (2) The direct correlation between SDH activity and endurance was confirmed. Resistance to fatigue was in addition inversely related to speed. Despite the higher SDH activity in fast and intermediate motor units compared with slow units, the decline in tetanus tension in the three groups was proportionate to their speed of contraction. The greater stability of slow units in a fused tetanus was the result of better matching between the capacity of neuromuscular transmission and tetanic fusion frequency and in a moderate, incomplete tetanus was associated with a larger increase in plateau tetanus-twitch ratio. 3. (3) The intense SDH activity in many fibres with high myosin ATPase activity typical for limb muscles of small animals with great rapidity of limb movements serves to compensate for the higher energy expenditure demanded by their operation at high speed; they therefore combine endurance with speed, particularly apparent in strong repetitive phasic contractions. The independent variability of myosin ATPase and mitochondrial enzymes enables a wide range of specifications of motor units with regard to speed and endurance, required by muscles with different functions.

Radiotelemetry of cardiovascular responses to exercise and diving in penguins

1. 1. Cardiovascular adjustments to voluntary exercise and diving have been studied in two species of penguins ( Pygoscelis papua and P. adeliae) using techniques for telemetry of blood flow, blood pressure and heart rate. 2. 2. Free exercise resulted in a rise in heart rate (from 122 to 278 beats/min), mean blood pressure (from 92 to 163 mm Hg) and femoral blood flow (from 24 to 59 ml/min). This response was little modified when birds were exercised on a treadmill. 3. 3. Arterial oxygen tension during exercise rose from 70 to 80 mm Hg while venous oxygen tension fell from 38 to 28 mm Hg, resulting in increased oxygen utilization. Severe exercise reduced arterial pH from 7·55 to 7·25. 4. 4. In free swimming, heart rate was 227 beats/min, femoral flow, 40 ml/min and carotid flow, 55 ml/min. During voluntary dives heart rate fell to approximately one-third as did femoral flow to one-fourth and carotid flow by 30 per cent. Forced dives differed by causing considerable struggling accompanied by transient changes in heart rate and blood flows. 5. 5. Only minor changes in blood oxygen tensions and pH were observed after free diving. Forced dives of 90–120 sec caused reductions of arterial oxygen tension from 82·4 to 33·5 mm Hg, venous oxygen tension from 43·5 to 27·5 mm Hg while pH fell from 7·48 to 7·37. Recovery showed a continued but transient decline in pH and venous oxygen tension. At 30 sec pH was 7·27 and venous pO 2, 25 mm Hg, while arterial oxygen tension had risen near control, 71·5 mm Hg.

Oxygen flow through a soft contact lens on a living eye.

The polarographic oxygen sensor when applied to a soft contact lens worn in the eye shows that oxygen moves through the lens material to the cornea. The rate of oxygen tension decline recorded by the sensor is significantly different if the cornea under the soft contact lens is denuded of its epithelium. A method for monitoring epithelial regrowth under a soft contact lens when worn as a bandage is suggested.

A myotoxin secreted by some piscivorous Conus species.

1. Toxins isolated from the venom apparatus of Conus magus and Conus achatinus have the same pharmacological properties, but differ from the toxins of several other piscivorous species of cone shells.2. C. magus and C. achatinus toxins are heat labile at pH 8.5. A single lethal component with an approximate molecular weight of 10,000 was isolated from C. achatinus toxin by exclusion chromatography.3. Animals died from a characteristic spastic paralysis after intravenous injection of the toxin.4. Nerve transmission was unaffected by the toxin; skeletal muscle appeared to be the primary site of action. The toxin caused a persistent contracture of rat diaphragm muscle, and a dose-dependent decline in twitch tension. The contracture was potentiated by caffeine.5. The decline in twitch tension was associated with depolarization of the cell membrane. Action potential height and the maximum rate of rise declined, and spike propagation failed when the resting potential had declined to approximately 60 mV. The muscle recovered very slowly on washout of the toxin. The depolarization could be reversed by exposure of the preparation to 5 mM Na(+) solution or tetrodotoxin or saxitoxin.6. Miniature end-plate potential frequency in the rat diaphragm decreased, as did the quantal content of the end-plate potential. The acetylcholine-induced contraction and depolarization of the chronically denervated rat diaphragm were increased by low doses of toxin and reduced by higher, depolarizing toxin doses. The K(+)-induced contraction and depolarization of innervated diaphragm were similarly affected by the toxin.7. Cardiac and smooth muscle were relatively resistant to the toxin. The isotonic contraction of the isolated perfused guinea-pig heart was increased by the toxins from both Conidae. The heart rate decreased. Guinea-pig atrial cells showed a small decrease in action potential height and maximum rate of rise. Rabbit sino-atrial cells showed increases in action potential height, maximum diastolic potential and maximum rate of rise of the spike at low toxin levels, and no change in any of these parameters at high levels. There was a decrease in the rate of the spontaneously beating atrium. Atrioventricular nodal potentials showed no change other than a slight increase in the maximum rate of rise of the action potential.8. It is postulated that the action of the toxin may be related to a change in the Ca(++) permeability of the excitable membrane, which makes it unstable, leading to a secondary, depolarizing entry of Na(+). The effects of the toxin are compared with those of batrachotoxin, which it somewhat resembles.

Excitation, contraction, and excitation-contraction coupling of frog muscles in hypotonic solutions

Twitch, tetanus, K contracture, and caffeine contracture tensions, and extracellularly recorded action potentials were measured in frog toe muscles bathed in hypotonic Ringer solutions. Tetanic tension is potentiated in these solutions down to 0.4 normal tonicity (0.4T), but falls off sharply to about 15% of the 1T value at 0.25T. The extracellularly recorded action potential also falls off at low tonicities but still is at 99% of the 1T value at 0.25T where the tetanus is only 15% of the 1T value. By contrast, the caffeine contracture tension is maintained at all tonicities indicating that hypotonic solutions have little direct effect on contractile proteins. It is shown that the decline of tension in very hypotonic solutions is due both to a loss in excitability and a true uncoupling of excitation and contraction. Raising [Ca] 0 to 3 times normal (5.4mM) allows recording of action potentials with no contraction. The loss of excitability and the decline of tension are not purely due to the decreased [Na] 0 because when NaCl is replaced with an osmotic equivalent of sucrose, tension and compound action potentials did not decline to zero. The loss of excitability may be due to the membrane depolarization observed in microelectrode studies.

Effects of calcium and lanthanum on glycerol removed rabbit atria

Continuously driven rabbit atria were equilibrated in Ringer-Locke solution, incubated in glycerol and then returned to normal Ringer's solution. Calcium was added (final concentration of 7.2 or 12.0 mM) at the beginning of glycerol removal or at the time of maximum decline in tension during the removal (10 min). There is an immediate increase in tension to a maximum followed by a slow decline to control levels after the addition of calcium. The glycerol removal effect is intensified by the addition of lanthanum (0.1–1.0 mM). Evidence would suggest that the glycerol removal effect results from a change of tissue calcium.

Effects of various glycerol or urea concentrations and incubation times on atrial contractions and ultrastructure

Continuously stimulated rabbit atria show a decline in contractile tension during and after exposure to special hypertonic solutions. Muscles were equilibrated in Ringer-Locke solution, incubated in hypertonic glycerol or urea media, and returned to normal Ringer's solution. Certain parameters of the contractile response were measured and compared quantitatively during the incubation and removal periods. The change of the contractile force was noted following different incubation times and different incubation concentrations. Effects of different tonicities of glycerol and urea were compared. The influence of the incubation concentration and time governing the effects during incubation and removal could be shown. The differences in action between low glycerol and urea concentrations during incubation and removal probably reflect a result caused by the difference in penetration speed of the molecules. The contractile changes following the removal of glycerol are accompanied by ultrastructural changes. There is a progressive swelling and disruption of the sarcotubular system and mitochondria with increasing incubation times and concentrations.

Activation heat, activation metabolism and tension-related heat in frog semitendinosus muscles.

1. Frog semitendinosus muscles were stretched to various lengths beyond the rest length (l(0)) and their initial heat and isometric tension production were measured.2. As the overlap between the thick and thin filaments is reduced, the initial twitch heat and tension decline in a linear manner. At a point at which the twitch tension approaches zero, the initial heat is 30% of that seen at l(0). It is concluded that this heat is the activation heat and reflects the energetics of calcium release and reaccumulation. The initial heat at shorter sarcomere lengths appears to be the sum of the activation heat plus a heat production associated with the interaction of the thick and thin filaments.3. A similar relationship between heat and tension production is seen in tetanic contractions.4. The time course of activation heat production in a twitch can be resolved into two phases: a temperature insensitive (Q(10) < 1.3) ;fast' phase (with a time constant of 45 msec) and a temperature sensitive (Q(10) = 2.8) ;slow' phase (with a time constant of 330 msec at 0 degrees C).5. Measurements of the creatine phosphate (PC) hydrolysis by muscles contracting isometrically at various muscle lengths at and beyond l(0), indicate an enthalpy change of -11.2 kcal/mole PC hydrolysed. The enthalpy change for the ATP hydrolysis by muscles stretched so that little or no tension was produced with stimulation was -9.9 kcal/mole ATP hydrolysed. It is concluded that the net activation heat is produced by the hydrolysis of PC or ATP.

Effects of glycerol treatment on contractility and transmembrane potentials in cardiac tissue

Recordings were made of transmembrane potentials and contractile activity before, during, and after treatment of guinea-pig left atria with Tyrode's solution made hypertonic with 400 m m-glycerol. During treatment with glycerol solution developed tension declined to a steady level while rest tension and transmembrane potentials were unchanged. After return to non-glycerol Tyrode's rest tension increased and developed tension declined further, sometimes to zero resulting in a period of quiescence. With time rest tension declined and developed tension increased, eventually returning to control values. Depolarization of transmembrane rest potentials was shown to accompany the decline of tension after return to non-glycerol Tyrode's. Field stimulation did not prevent these changes. However, if the tissue was placed in 400 m m-sucrose Tyrode's after treatment with 400 m m-glycerol solution, developed tension did not decline at all. In addition, it was shown that increased calcium or magnesium concentration prevents the decline in developed tension as well as depolarization of resting potentials after return to non-glycerol Tyrode's. It was also shown that similar changes in contractile activity occur after glycerol treatment of dog atrial tissue. However, glycerol treatment of frog cardiac tissue has little effect.