Decline In Twitch Tension Research Articles

Morphological correlates of physiological responses in partially denervated mouse muscle during aging

To evaluate the effect of partial denervation on age changes, morphological and physiological parameters were studied in young versus old extensor digitorum longus (EDL) muscles. The aims of this study were to elucidate synaptic maintenance in general and specifically to assess the adaptability of motor neurons from young and old animals to an enlarged field of innervation. Partial denervation was carried out in two groups of 30–40 g mice, aged within 6 or 24 months, by sectioning of the nerve supplying EDL muscle under methoxyfluorane anaesthesia. Sham operations were carried out on additional animals which served as controls. Five weeks' post-surgery, the safety factor of synaptic transmission was estimated by dividing the nerve-evoked twitch tension generated in 1 mM Ca2+ Krebs by that generated in 2.5 mM Ca2+ Krebs. Low Ca2+ Krebs caused a more pronounced decline in twitch tension in young control EDL muscles than old control. After partial denervation, young EDL twitch tensions were not significantly different while old were 20% of those in normal Krebs. Zinc Iodide Osmium (ZIO) stained nerve terminal parameters were significantly increased with aging. After partial denervation, nerve terminal areas were 46 and 39% larger in young and old mice, respectively, compared to their corresponding controls. Present data suggest that motor neurons of old mice are unable to develop and maintain an enlarged field of innervation.

The role of mitochondria and sarcoplasmic reticulum calcium handling upon reoxygenation of hypoxic myocardium.

The mechanism of twitch prolongation of hypoxic myocardium after reoxygenation was studied before and after interventions that affect cellular cyclic nucleotide levels, subcellular calcium handling, or oxygen-derived free radical production/survival. Right ventricular ferret papillary muscles were subjected to two 20-minute periods of hypoxia, each followed by 1 hour of reoxygenation. The first sequence of hypoxia/reoxygenation was done without intervention. Before the second sequence, the pharmacological agent under study was added to the superfusate or the superfusate calcium concentration was increased from 2.5 to 8 mM. Time from peak to 80% decline in twitch tension was measured in the presence and absence of each intervention immediately before each period of hypoxia and after reoxygenation at maximal twitch prolongation. Interventions that affect Ca2+ flux across the sarcolemma (verapamil and 8 mM [Ca2+]o) or agents that affect oxygen free radical production/survival (dimethyl sulfoxide and allopurinol) did not affect twitch prolongation. Pharmacological agents that increase cyclic AMP levels (forskolin and milrinone) or those that inhibit mitochondrial activity (oligomycin B and ruthenium red) attenuated twitch prolongation. Pharmacological agents that decrease cyclic AMP levels (carbachol) or inhibit function of the sarcoplasmic reticulum (ryanodine) augmented twitch prolongation. The effect of mitochondrial inhibitors on intracellular calcium handling during hypoxia and reoxygenation was examined using muscles loaded with the bioluminescent calcium indicator aequorin. Mitochondrial inhibitors abbreviated the calcium transient and maximal twitch prolongation after hypoxia. We conclude that alterations in sarcoplasmic reticulum and mitochondria calcium handling contribute to the prolonged relaxation seen upon reoxygenation of hypoxic myocardium.

SR Ca loading in cardiac muscle preparations based on rapid-cooling contractures

The influence of rest periods on twitches and rapid-cooling contractures (RCCs) was examined in trabeculae from rabbit, rat, guinea pig, and frog ventricle and rabbit atrium. RCCs were used as a relative index of sarcoplasmic reticulum (SR) Ca content. After increasing rest duration, rabbit and guinea pig ventricles exhibit a decline of both twitch force and RCC force (rest decay). When stimulation is resumed, both twitches and RCCs recover to steady-state levels. The SR (and cells) in these tissues may lose Ca during quiescence and become reloaded with progressive stimulation. Rat ventricle and rabbit atrium exhibited an increase in both twitch and RCC tension as a function of rest duration (rest potentiation). Resumption of stimulation resulted in parallel declines of both twitch and RCC tension approaching steady state. Thus stimulation in rat ventricle and rabbit atrium may lead to a net Ca loss from the SR (and the cell) and quiescence may lead to replenishment of cellular Ca. This major difference in Ca metabolism in mammalian cardiac muscles might be due to a fundamental difference in SR properties or, alternatively, different sarcolemmal transport properties (e.g., action potential configuration, Na-pump). After long rest intervals in rabbit and guinea pig ventricle, RCCs return toward their steady-state value in considerably fewer beats than does twitch tension. This implies that something other than SR refilling is responsible for the slow phase of twitch recovery after rest. In rabbit ventricle increasing frequency or extracellular Ca concentration ([Ca]o) generally increases both twitch and RCC tension. However, decreasing [Ca]o (to 0.2 mM) does not decrease RCCs much despite a dramatic decline in twitch tension (suggesting low twitch tension despite a loaded SR). Rapid rewarming during an RCC usually results in a transient rise in tension (or rewarming "spike"), which is due to a warming-induced increase in myofilament Ca sensitivity. Differences in rewarming spikes among the tissues studied suggest differences in temperature effects on myofilament Ca sensitivity.

Effect of acetylstrophanthidin on twitches, microscopic tension fluctuations and cooling contractures in rabbit ventricle.

1. We have measured the effect of the aglycone acetylstrophanthidin (ACS) on twitches, cooling contractures and microscopic tension fluctuations in rabbit ventricular muscle. 2. Both developed twitches and cooling contractures are strengthened by applications of ACS in the range 1-4 microM. This positive inotropy averages 150-160% of control (zero ACS) in both twitches and cooling contractures. Cooling contracture magnitude is assumed to reflect the availability of sarcoplasmic reticulum (SR) Ca2+ for contraction (Bridge, 1986). We infer that ACS increases the availability of SR Ca2+ by enlarging SR Ca2+ stores and this may contribute to the positive inotropy. 3. However, twitches appear to increase at lower concentrations of ACS than those required to increase cooling contractures. This observation suggests that the initial ACS inotropy may be achieved without an increase in SR Ca2+. Furthermore, low doses of ACS produce positive inotropy in the presence of 10.0 mM-caffeine where cooling contractures are abolished. This also suggests that positive inotropy occurs in the absence of SR Ca2+ accumulation. 4. Rest decay of both cooling contractures and twitches is significantly slowed in 4 and 8 microM-ACS. We infer that ACS slows the rate of decline of SR Ca2+ available for contraction by slowing the rate at which Ca2+ is lost from the cell during rest. This suggests that ACS produces a net slowing of Ca2+ efflux during activity which in the absence of altered Ca2+ influx will result in net Ca2+ gain and presumably enlarged SR Ca2+ stores. 5. Increasing the concentration of ACS (6-10 microM) results in a decline in developed twitch tension, total tension and an increase in rest tension. Measurement of microscopic tension fluctuations indicates that as developed twitches decline, the root mean square (r.m.s.) of the tension fluctuations increases in a reciprocal manner. This supports the suggestion of others that the decline in developed twitch tension and the appearance of tension fluctuations are causally related. 6. Although ACS (6-10 microM) causes a decline in twitch tension, rapid cooling contractures remain elevated. We suggest that in the presence of Ca2+ oscillations the magnitude of cooling contractures reflects the sum of cytosolic Ca2+ and Ca2+ that is available for release. If microscopic tension fluctuations do represent Ca2+ moving between the SR and cytosol the sum of SR and cytosolic Ca2+ and hence cooling contracture might not decline.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of sodium on the calcium paradox in rat hearts.

Reduction of the Na concentration in the Ca-free perfusion solution reduces the amount of myoglobin released by the cells when Ca is readmitted if sucrose is used to replace NaCl under mild hypothermia. When salts like cholinechloride or LiCl are used instead of sucrose, no protection is seen at any temperature. The temperature threshold above which myoglobin loss sharply increases is lowered by prolonged Ca depletion or by the addition of EGTA to the Ca-free solution. Protection by sucrose does not occur in the presence of EGTA. An increase of cell Na induced by strophanthidin during the Ca depletion phase has no effect on myoglobin release. The exponential decline in twitch tension in the early phase of Ca deprivation has the same half-live (T1/2) for Ca-free solutions containing 145 mM Na or 35 mM Na (110 mM Li or choline), but its T1/2 is prolonged if sucrose is used to replace NaCl. When 5 mM EGTA is added to the Ca-free solutions, the T1/2 is shortened and is not changed by the replacement of NaCl with sucrose. The rate of washout of Ca within the first 20 s of Ca depletion has a similar time course in a normal Na or in a Li and low Na solution. In a sucrose and low Na solution the rate of the Ca efflux is reduced. The addition of EGTA increases this rate and abolishes the slowing effect of a sucrose and low Na solution. Therefore myoglobin release during the Ca paradox does not depend on the Na gradient across the sarcolemma.(ABSTRACT TRUNCATED AT 250 WORDS)

Denervated frog skeletal muscle. Some electrical and mechanical properties.

The effects of denervation on several mechanical and electrical parameters of frog sartorius muscle have been investigated. In denervated muscles, there is no change in the resting potential and a relatively small change in the action potential. The first alteration in the action potential is a reduction of about 30% in the maximum rate of repolarization in muscles that have been denervated for 40 days or longer. Later, the overshoot and maximum rate of depolarization also decline. No tetrodotoxin resistant action potentials could be detected. Fibrillatory potentials were observed infrequently and in most cases in depolarized fibers. Twitch tension is significantly reduced by denervation while the tetanus tension is practically unaffected by denervation. The experiments suggest that the decline in twitch tension produced by denervation reflect a defect in some step of the excitation contraction coupling sequence. On the other hand, post-tetanic potentiation of the twitch is much larger in denervated than in control muscles. This potentiation in denervated muscles is paralleled by an increased action potential duration which returns to its pretetanic duration with a time course indistinguishable from that of the twitch potentiation.

Catecholamine effects on intracellular sodium activity and tension in dog heart.

Because catecholamines have been reported to stimulate the sodium pump and Na+-K+-ATPase in skeletal and cardiac muscle, we examined the effects of isoproterenol (0.2-1.0 X 10(-6) M) and norepinephrine (2-4 X 10(-7) M) on intracellular sodium activity (aiNa) and twitch tension in dog Purkinje strands and ventricular muscles, aiNa was measured with Na+-sensitive microelectrodes. During the initial rapid increase in tension induced by catecholamines in Purkinje strands, no changes in aiNa were found. After 5-10 min aiNa decreased by about 2 mM, coincident with a small decline in twitch tension. When the catecholamine was removed, tension declined rapidly to a level less than control. Recovery of tension to its control level less than control. Recovery of tension to its control level occurred simultaneously with recovery of aiNa. Comparable changes in aiNa occurred in ventricular muscle, but the biphasic effect of catecholamines on tension was not seen. These results are consistent with sodium pump stimulation in cardiac muscle. In Purkinje strands the resulting changes in aiNa may alter the direct positive inotropic effect of catecholamines, probably by influencing Na+-Ca2+ exchange.

Effect of venoms from Conidae on skeletal, cardiac and smooth muscles

J. Kobayashi, H. Nakamura, Y. Hirata and Y. Ohizumi. Effect of venoms from Conidae on skeletal, cardiac and smooth muscles. Toxicon 20, 823–830, 1982.—The effects of the venoms of 29 species of Conidae were studied on the mouse isolated diaphragm, the guinea-pig isolated left atria and ileum, and the rabbit isolated aorta. When the directly stimulated mouse diaphragm was exposed to the venom (10 −4 g/ml) of C. geographus, there occurred a marked decline in twitch tension. The venoms of C. magus (3 × 10 −6 g/ml) and C. striatus (10 −3 g/ml) also caused complete loss of contractile response to electrical stimulation. This was followed by a gradual rise in the baseline. The venoms (10 −6−3 × 10 −5 g/ml) of C. magus and C. striatus caused dose-dependent increases in the contractile force of the electrically driven guinea-pig left atria. The venoms (10 −5−10 −4 g/ml) of C. eburneus and C. tessulatus also elicited long-lasting positive inotropic actions on the atria. The inotropic actions induced by the venoms of C. magus and C. striatus were abolished by tetrodotoxin (TTX, 10 −6 M), but were not affected by verapamil (3 × 10 −7 M), whereas those induced by the venoms of C. eburneus and C. tessulatus were completely inhibited by verapamil (3 × 10 −7 M), but were not modified by TTX (10 −6 M). These results suggest that the inotropic actions of the venoms of C. magus and C. striatus may be due to an increase in Na + permeability of the cardiac cell membrane, while those of venoms of C. eburneus and C. tessulatus may be due to an increase in Ca 2+ permeability. The venoms (10 −6 g/ml) of C. magus and C. striatus elicited rhythmic, transient contractions of the guinea-pig ileum followed by relaxations, which were blocked by TTX (10 −6 M). This suggests that these biphasic responses were caused by increasing Na + permeability of the nerve cell membrane. On the other hand, the venoms (10 −4 g/ml) of C. eburneus and C. tessulatus caused transient contractions of the ileum followed by relaxations. The venoms (10 −4 g/ml) of C. tessulatus and C. eburneus caused marked, long-lasting contractions of the rabbit aorta which were abolished by verapamil (10 −6 M), but were not affected by TTX (5 × 10 −7 M) or phentolamine (10 −6 M). It is suggested that these venoms increase the verapamil-sensitive Ca 2+ influx across the smooth muscle membrane of the aorta, resulting in contractions. The active principles in the venoms of C. geographus, C. textile and C. imperialis were stable at 100°C for 15 min and passed through PM 10 filters, suggesting molecular weights lower than 10,000. On the other hand, the active principles in the venoms of C. magus, C. striatus, C. tessulatus and C. eburneus were partially destroyed with heating and were kept in the retentate of the PM 10 filters.

Isometric contractile properties and instantaneous stiffness of amphibian skeletal muscle in the temperature range from 0 to 20 degrees C.

The isometric contractile properties of frog (Rana pipiens) and toad (Bufo bufo) sartorii have been studied over the temperature range from 0 to 20 degrees C. The isometric twitch tension was found to vary considerably between these two species and between muscles in the same species. Between 0 and 4 degrees C there was very little change in maximum isometric twitch tension. Between 4 and 12 degrees C several muscles from frog or toad showed a potentiation of twitch tension whereas others showed a decline. Over this temperature range the toad sartorii consistently demonstrated a greater potentiation. By 12 degrees C a steady decline in twitch tension in both muscles was seen as the temperature range the toad sartorii consistently demonstrated a greater potentiation. By 12 degrees C a steady decline in twitch tension in both muscles was seen as the temperature approached 20 degrees C. The maximum isometric tetanic tension recorded between 18 and 20 degrees C increased fractionally to an average of 1.504 +/- 0.029 (n = 4) for frog sartorii and to 1.377 +/- 0.008 (n = 5) for toad sartorii. The time to peak twitch tension and the half-relaxation time decreased markedly with an increase in temperature. Moreover, the half-relaxation time was reduced by a greater proportion than the time to peak twitch tension. Measurements of instantaneous stiffness by controlled velocity releases from the plateau of isometric tetani revealed that the large increase in isometric tetanus tension as the muscle was warmed was not accompanied by a corresponding increase in the total number of active cross-bridges. The possibility that a decreased availability of intracellular Ca2+ ions at the contractile sites contributing to the fall of isometric twitch tension at elevated temperatures is discussed. The possibility exists that at elevated temperatures a change inthe intrinsic contractile ability of the muscle occurs which produces an increased tension per cross-bridge.

Postsynaptic blockade of neuromuscular transmission by toxin II from the venom of the South African ringhals cobra ( Hemachatus haemachatus)

R. G. Bengis and D. F. Noble. Postsynaptic blockade of neuromuscular transmission by toxin II from the venom of the South African ringhals cobra ( Hemachatus haemachatus). Toxicon 14, 167–173, 1976.—Purified toxin II (2·5–25 μg per ml) from the venom of the South African ringhals cobra ( Hemachatus haemachatus) produces a slow decline in twitch tension of toad sciatic nerve-gastrocnemius preparations. These studies established that toxin II is a neurotoxin that eliminates the sensitivity of the post-junctional membrane to acetylcholine, indicating a primary postsynaptic site of action. Pretreatment with gallamine or curare afforded no protection against this toxin. Any effect toxin II may have on acetylcholinesterase is probably minor, since the effects of toxin II are not enhanced or reversed by acetylcholine, nor does a cholinesterase inhibitor reverse the effects of the toxin. The postsynaptic blockade is non-depolarizing and is irreversible by conventional methods.

Leucocidin from Pseudomonas aeruginosa and membrane functions.

Leucocidin from Pseudomonas aeruginosa (strain 158) induced loss of potassium from isolated hepatocytes. The (Na+-K+) stimulated ATPase activity of isolated rat liver plasma membranes showed dose-dependent activation up to 56%. Electron-spin-resonance (ESR) measurements gave no indication of toxin-induced changes in membrane fluidity. On isolated guinea pig heart auricles the toxin produced an increase in frequency from 180/min to about 300/min, with arrhythmia and transitory flutter. On isolated nerve-diaphragm preparations the toxin caused a contracture and a decline in twitch tension, with a loss of potassium into the bathing solution. The action potential of the electrically stimulated N. ischiadicus of rat or frog was not affected when leucocidin was added to the bathing solution up to a concentration of 10 micrograms/ml.

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.

Effects of calcium on the contraction of the hypodynamic frog heart.

1. The time course of build-up or decline of twitch tension in response to enhancement or reduction of the external calcium concentration was determined in frog heart ventricles, in the hypodynamic state as well as under non-hypodynamic conditions.2. The tension build-up induced by high calcium was greatly slowed when the heart passed into the hypodynamic state, whereas the decline in low calcium was little altered by this condition.3. The tension decline consisted of two approximately exponential phases, an initial rapid phase (t((1/2)) between 3 and 10 sec) and a later slow phase (t((1/2)) between 50 and 180 sec).4. A similar composite time course of tension build-up only occurred under certain conditions: (a) in ventricles in which the hypodynamic state had not developed; (b) after a conditioning period of exposure to enhanced calcium or reduced sodium concentrations.5. The results are explained on the assumption that contraction is brought about by the co-operative action inside heart cells of two calcium compounds whose concentrations change at different rates after variation of the external calcium concentration.6. Formal relationships describing the dependence of twitch tension on the concentration of these two hypothetical compounds are obtained.A tentative explanation of the development of the hypodynamic state is also proposed.