The time-dependent and dose-dependent effects of caffeine on the contraction of the ferret heart

1. Trabeculae isolated from ferret heart and from other mammalian hearts have been mounted in a way that enables the tension generated to be measured while the composition of the bathing fluid is rapidly altered.2. Application of caffeine to these trabeculae initiates a rapid transient contracture and depresses the strength of regularly evoked heart beats.3. The strength of the contractures, the rate of tension development and the rate of spontaneous relaxation are all increased by raising the concentration of the applied caffeine.4. The strength of the caffeine contracture is relatively unaffected by changes in the bathing Na(+), K(+) or Ca(2+) concentrations, but is reduced by exposure to the free-base form of local anaesthetics.5. Lowering of the temperature has complex effects on the amplitude of the caffeine contracture due to the differing temperature sensitivities of the contraction and spontaneous relaxation.6. Following a caffeine contracture, a period of perfusion by caffeinefree solution is required before a full-sized contracture can be evoked by the re-application of caffeine. This re-priming of the caffeine contracture has a sigmoidal time course that can be fitted by a two compartment model. The rate constants of the filling of each of the compartments can be obtained analytically, and are found to be increased by raising the extracellular calcium concentration, [Ca](o), by stimulating the preparation or by raising the temperature. Reducing the [Na](o) or raising the [K](o) has little effect on these processes.7. The presence of traces of caffeine in the perfusing fluid between the conditioning and test challenges with the caffeine contracture solution reduces the extent of the re-priming without much affecting its rate.8. The behaviour of several model systems have been compared with that of the heart with the aid of an analogue computer. A four compartment closed system has been found to simulate the results presented in this paper.9. It appears that caffeine has its effects by acting to increase the rate of release of activator calcium from one part of a non-homogeneous intracellular relaxing system present within the mammalian heart, which is likely to be the sarcoplasmic reticulum.

Nandrolone decanoate pre-treatment attenuates unweighting-induced functional changes in rat soleus muscle

The effect of nandrolone decanoate pre-treatment (15 mg kg(-1) week(-1), for 6 weeks) was tested on the changes in mass and contractile properties of soleus muscle associated with 3 weeks of hindlimb suspension. Male rats were assigned to four groups (eight animals/group): control, nandrolone decanoate hindlimb-loaded, hindlimb-unweighted and nandrolone decanoate hindlimb-unweighted. Compared with age-matched control values, suspension induced a reduction in relative muscle mass and a shift in tension characteristics from slow-towards fast-twitch type. Nandrolone decanoate pre-treatment of suspended animals (nandrolone decanoate hindlimb-unweighted vs. nandrolone decanoate hindlimb-loaded) partially spared the relative soleus mass. Furthermore, (1) the relative twitch tension (nandrolone decanoate hindlimb-loaded: 5.4 +/- 0.7%; nandrolone decanoate hindlimb-unweighted: 5.1 +/- 0.5%), (2) the time to peak tension (nandrolone decanoate hindlimb-loaded: 152 +/- 9 ms; nandrolone decanoate hindlimb-unweighted: 167 +/- 15 ms), (3) the time constant of relaxation (nandrolone decanoate hindlimb-loaded: 274 +/- 12 ms; nandrolone decanoate hindlimb-unweighted: 245 +/- 20 ms), (4) the relative K+ contracture tension (nandrolone decanoate hindlimb-loaded: 81.7 +/- 3.8%; nandrolone decanoate hindlimb-unweighted: 86.9 +/- 4.2%) and (5) the relative caffeine contracture tension (0.5 mM) (nandrolone decanoate hindlimb-loaded: 5.2 +/- 0.8%; nandrolone decanoate hindlimb-unweighted: 5.9 +/- 1.1%) were not significantly modified. The present results demonstrate that exogenously provided nandrolone decanoate pre-treatment attenuates functional changes occurring in soleus muscle subject to unweighting.

Nandrolone decanoate treatment induces changes in contractile responses of rat untrained fast-twitch skeletal muscle

This investigation was designed to examine whether short-term administration of anabolic-androgenic steroids (AAS) (nandrolone decanoate) could produce changes in contractile responses of untrained rat fast- (edl) and slow- (soleus) twitch skeletal muscle. Twenty male rats were divided into two groups, one group received weekly (for 6 weeks) an intramuscular injection of AAS, nandrolone decanoate (15 mg kg(-1)) and the second group received weekly the similar doses of vehicle (sterile peanut oil). In edl intact isolated small bundles (two to four cells), it was found that nandrolone decanoate treatment increases the K+ contracture tension (146 mM) relative to maximum tension by 56%, whereas no change was observed in the time to peak tension and in the time constant of relaxation. By contrast, in treated soleus muscle, compared with control, no significant modification was found in the K+ contracture characteristics. The change in edl contractile responses was associated with a shift to more negative potential of the voltage-dependence activation and the steady-state inactivation curves which also shifted leftward in treated soleus fibres. Furthermore, in edl skinned Triton X-100 fibres, the Ca2+ sensitivity of contractile proteins (pCa50) was increased, while electrophoresis analysis indicates no significant effect of nandrolone decanoate treatment on myosin heavy chain (MHC) isoforms. The present results show that nandrolone decanoate treatment produces more pronounced changes in untrained fast muscle function rather than soleus by acting at different levels of the excitation-contraction coupling mechanism without changes in the MHC isoforms and that contractile responses became similar to those found in soleus muscle.

A comparative analysis of the effects of exercise training on contractile responses in fast- and slow-twitch rat skeletal muscles

The effects of 5 weeks treadmill-exercise training on isometric tension and contractile proteins were studied in intact and skinned isolated small bundles of rat skeletal soleus and extensor digitorum longus (edl) fibers. In soleus and edl muscles, 5 weeks exercise training: (i) increased twitch amplitude by 25% and 8%, respectively, without modification in the time-to-peak tension and the time constant of relaxation, (ii) increased the amplitude of K(+) contracture by 93% and 88%, respectively, and accelerated its relaxation by 17% and 43%, respectively, and (iii) increased the amplitude of caffeine contractures (soleus: 0.5 mM: 86%, 10 mM: 77%; edl: 0.5 mM: 89%, 10 mM: 87%). In conclusion, changes in contractile responses were associated with shifts in the steady state inactivation curves and in the voltage-dependent activation curve to a more negative potential, with increases in soleus and edl caffeine sensitivity, without changes in the Ca(2+) sensitivity of contractile proteins and myosin heavy chain isoforms.

Sarcoplasmic reticulum Ca(2+) content affects 4-CmC and caffeine contractures of rat skinned skeletal muscle fibers

This study investigated whether the sarcoplasmic reticulum Ca(2+) content of rat skeletal muscle fibers affected contractile responses obtained by an application of 4-chloro-m-cresol (4-CmC) and caffeine. Contractures were elicited on saponin-skinned fibers under different Ca(2+) loading conditions. The amplitude of 4-CmC and caffeine contractures of fast-twitch muscle fibers (edl, extensor digitorum longus) differed between the different loading conditions, and this is associated with a greater change in sensitivity to 4-CmC. When the sarcoplasmic reticulum was loaded with a low Ca(2+) concentration for a short period, the 4-CmC concentration providing half-maximal response was tenfold higher than with a larger sarcoplasmic reticulum Ca(2+) loading for a longer period, whereas for caffeine this concentration was only twofold higher in the same conditions. These findings indicate that 4-CmC contractile responses of edl muscle fibers are more dependent on luminal Ca(2+) activity than those of caffeine are. Thus 4-CmC would appear to be of greater interest than caffeine for the study of muscle contractile responses where variations in intracellular Ca(2+) activity exist.

Cyclopiazonic acid and thapsigargin reduce Ca2+ influx in frog skeletal muscle fibres as a result of Ca2+ store depletion

We have investigated the influence of the sarcoplasmic reticulum (SR) Ca2+ content on the retrograde control of skeletal muscle L-type Ca2+ channels activity by ryanodine receptors (RyR). The effects of cyclopiazonic acid (CPA) and thapsigargin (TG), two structurally unrelated inhibitors of SR Ca(2+)-adenosine triphosphatase (ATPase), were examined on the SR Ca2+ content, the calcium current and contraction in single frog semitendinosus fibres using the double mannitol-gap technique. At moderate concentrations that only partially inhibited Ca2+ sequestration by the SR, CPA (2-4 microM) induces a concentration dependent depression of contraction and Ca2+ current amplitudes. When Ba2+ is the charge carrier, the inward current is not changed by CPA suggesting that this Ca(2+)-pump inhibitor does not directly affect dihydropyridine Ca2+ channels. Similar effects were obtained with TG (1-5 microM). Changes in Ca2+ currents and contraction were accompanied by a reduced Ca2+ loading of the SR. We attribute the modulation of the Ca2+ current to the selective inhibition of the SR Ca2+ ATPase, resulting in a decreased Ca2+ release and thereby a reduced activation of calcium inward currents. This is therefore taken to represent a calcium release-dependent modulation of skeletal muscle L-type Ca2+ channels.

Inhibition of caffeine-induced Ca2+ release by adenosine in mammalian skinned slow- and fast-twitch fibres

The present study performed on chemically skinned skeletal fibres was designed to compare the effects of adenosine on the Ca2+ sensitivity of contractile proteins and on caffeine-induced Ca2+ release in rat slow- (soleus) and fast-twitch (edl) muscles. The tension-pCa relationships were obtained by exposing triton X-100 (1% v/v) skinned fibres sequentially to solutions of decreasing pCa in the presence or in absence of adenosine. Then, changes in caffeine contracture due to adenosine were recorded on saponin (50 microg/ml) skinned fibres. The results show that the sensitivity to Ca2+ of contractile proteins in the presence of different concentrations of caffeine was not significantly modified by adenosine. However, it was proposed that adenosine (0.1-2 mM) reduced the Ca2+ released by caffeine (0.1-10 mM) from the sarcoplasmic reticulum in slow- and fast-twitch fibres and that the soleus was more sensitive to adenosine than edl muscle. The effects of specific A2a and A1 agonists and antagonists were also tested on caffeine contractures. It was found that the A1 antagonist reduced adenosine effect on caffeine response. Then it is proposed that adenosine modulates the sarcoplasmic reticulum Ca2+ release by a direct effect on the RyR1 receptors and/or by an indirect effect mediated by A1 receptors located at the sarcoplasmic level.

Skeletal muscle disuse induces fibre type-dependent enhancement of Na(+) channel expression

Slow-twitch and fast-twitch muscle fibres have specific contractile properties to respond to specific needs. Since sodium current density is higher in fast-twitch than in slow-twitch fibres, sodium channels contribute to the phenotypic feature of myofibres. Phenotype determination is not irreversible: after periods of rat hindlimb unloading (HU), a model of hypogravity, a slow-to-fast transition occurs together with atrophy in the antigravity slow-twitch soleus muscle. Using cell-attached patch-clamp and northern blot analyses, we looked at sodium channel expression in soleus muscles after 1-3 weeks of HU in rats. We found that sodium channels in fast-twitch flexor digitorum brevis muscle fibres, soleus muscle fibres and 1- to 3-week HU soleus muscle fibres showed no difference in unitary conductance, open probability and voltage-dependencies of activation, fast inactivation and slow inactivation. However, muscle disuse increased sodium current density in soleus muscle fibres 2-fold, 2.5-fold and 3-fold after 1, 2 and 3 weeks of HU, respectively. The concentration of mRNA for the skeletal muscle sodium channel alpha subunit increased 2-fold after 1 week of HU but returned to the control level after 3 weeks of HU. In contrast, the concentration of mRNA for the ubiquitous sodium channel beta(1) subunit was unchanged after 1 week and had increased by 30% after 3 weeks of HU. The tetrodotoxin sensitivity of sodium currents in 3-week HU soleus muscles and the lack of mRNA signal for the juvenile skeletal muscle sodium channel alpha subunit excluded denervation in our experiments. The observed increase in sodium current density may reduce the resistance to fatigue of antigravity muscle fibres, an effect that may contribute to muscle impairment in humans after space flight or after long immobilization.

Caffeine stimulates the reverse mode of Na/Ca2+ exchanger in ferret ventricular muscle

This study investigated the effect of caffeine on the sarcolemmal mechanisms involved in intracellular calcium control. Ferret cardiac preparations were treated with ryanodine and thapsigargin in order to eliminate the sarcoplasmic reticulum (SR) function. This treatment abolished caffeine contracture irreversibly in normal solution. The perfusion with K-free medium that blocked the Na+--K+ pump resulted in a recovery of slow relaxing caffeine contractures similar to Na-free contractures. The amplitude of caffeine contractures was dependent on the bathing [caffeine]o and [Ca2+]o. Divalent cations Ni2+ and Cd2+, which have an inhibitory effect on the Na+/Ca2+ exchanger, produced dose-dependent inhibition of caffeine responses with apparent Ki of 780 +/- 19 and 132 +/- 5 microM, respectively. Caffeine also caused dose-dependent inhibition of Na-free contractures (Ki=4.62 +/- 1.5 mM), and the reduction or removal of [Na+]o exerted an inhibitory effect on caffeine contractures (Ki=73.5 +/- 17.12 mM). These experiments indicate that the increase in resting tension following exposure to caffeine was mediated by Na+/Ca2+ exchanger, which represents an additional element of complexity in caffeine action on cardiac muscle.

Differential effects of nandrolone decanoate in fast and slow rat skeletal muscles

PURPOSE

We studied the effects of high doses of an anabolic-androgenic steroid, exercise training, and a combination of steroid and training on mammalian fast- and slow-twitch skeletal muscles at the cellular level.

METHODS

Thirty-two male rats were divided into sedentary and treadmill-trained groups (increased speed and time: 18 m.min-1, 0.5 h.d-1, 5 d.wk-1). Eight animals of each group were treated with nandrolone decanoate (ND) (15 mg.kg-1.wk-1), and others received the same doses of solvent. The animals were killed after 5 wk, and the contractile parameters for isolated small bundles of soleus and extensor digitorum longus (edl) fibers were estimated.

RESULTS

Muscle mass, twitches, and K+ contractures were increased in soleus and edl muscles after the drug treatment and after the exercise training. Caffeine contractures were increased only after the exercise training. The combination of exercise with ND treatment produced greater effects, particularly a significant increase in sensitivity to caffeine and the amplitude of K+ contractures as well as a shortening of the time required to restore contracture. These modifications were more marked in slow than fast muscle.

CONCLUSION

These results show that 5 wk of exercise training produced changes in the contractile responses developed by isolated skeletal muscle cells. The combination of exercise training with ND treatment potentiated these effects, suggesting that there was some modification in the excitation-contraction coupling mechanism. ND treatment also produced a more potent effect in soleus than edl sedentary muscle.

Rapid cooling-induced contractures in rat skinned skeletal muscle fibres originate from sarcoplasmic reticulum Ca2+ release through ryanodine and inositol trisphosphate receptors

Previous reports have shown that cooling striated muscles induces contractile responses that are related to Ca2+ release from the sarcoplasmic reticulum. However, the effect of cooling has generally been studied in the presence of pharmacological agents that potentiate rapid cooling-induced contractures. The present study shows that in saponin-skinned rat skeletal muscle preparations, a drop in temperature from 22 degrees C to 2 degrees C per se induces a contracture which relaxes on return to 22 degrees C. In fast-twitch fibres, rapid cooling-induced contractures are fully blocked by ryanodine, an inhibitor of ryanodine receptors. By contrast, in slow-twitch fibres, ryanodine partially inhibits the rapid cooling-induced contractile response, leaving a residual tension that dissipates after application of inositol 1,4,5-trisphosphate (InsP3). At low concentrations, heparin, an inhibitor of InsP3 receptors, decreases rapid cooling-induced contractures in both types of muscle. The present results suggest that in skeletal muscle, rapid cooling-induced contractures are due to both ryanodine-sensitive and InsP3-sensitive Ca2+ release from the sarcoplasmic reticulum.

Differential effects of 4-chloro-m-cresol and caffeine on skinned fibers from rat fast and slow skeletal muscles

Contractile responses to 4-chloro-m-cresol (4-CmC) were tested in saponin- and Triton X-100-skinned fibers from soleus and edl (extensor digitorum longus) muscles of adult rats and compared with those to caffeine. The testing of different concentrations of 4-CmC on saponin-skinned fibers showed that 4-CmC induced a dose-dependent caffeine-like transient contractile response in edl and soleus due to an activation of the ryanodine receptor. Both types of skeletal muscles showed a 10 to 20 times lower 4-CmC threshold concentration and EC(50) value (concentration providing 50% of the maximal 4-CmC contracture) than for caffeine. The results indicate that edl is more sensitive than soleus to 4-CmC and that this difference in sensitivity is more marked than with caffeine. Furthermore, an increase in cytosolic Ca(2+) activity induced a more marked shift of dose-response curves toward lower concentrations for 4-CmC than caffeine. Experiments conducted on Triton X-100-skinned fibers showed that in both muscles, 4-CmC decreased in a dose-dependent manner the Ca(2+)-activated force of contractile apparatus, particularly in edl. Furthermore, the tension pCa curves indicated that 4-CmC induced a dose-dependent sensitizing (soleus) or desensitizing (edl) effect on the Ca(2+) sensitivity of myofibrils. These results indicate that edl and soleus contractile responses can be discriminated with 4-CmC instead of caffeine and that care must be taken in interpreting results because muscular pathology could be due in part to an increase in intracellular Ca(2+).

Ciliary neurotrophic factor prevents unweighting-induced functional changes in rat soleus muscle

The purpose of the present work was to see whether changes in rat soleus characteristics due to 3 wk of hindlimb suspension could be modified by ciliary neurotrophic factor (CNTF) treatment. Throughout the tail suspension period, the cytokine was delivered by means of an osmotic pump (flow rate 16 microg. kg(-1). h(-1)) implanted under the hindlimb skin. In contrast to extensor digitorum longus, CNTF treatment was able to reduce unweighting-induced atrophy in the soleus. Twitch and 146 mM potassium (K) tensions, measured in small bundles of unloaded soleus, decreased by 48 and 40%, respectively. Moreover, the time to peak tension and the time constant of relaxation of the twitch were 48 and 54% faster, respectively, in unloaded soleus than in normal muscle. On the contrary, twitch and 146 mM K contracture generated in CNTF-treated unloaded and normal soleus were not different. CNTF receptor-alpha mRNA expression increased in extensor digitorum longus and soleus unloaded nontreated muscles but was similar in CNTF-treated unloaded muscles. The present results demonstrate that exogenously provided CNTF could prevent functional changes occurring in soleus innervated muscle subject to unweighting.

Inositol 1,4,5-trisphosphate-sensitive Ca2+ release in rat fast- and slow-twitch skinned muscle fibres

Inositol 1,4,5-trisphosphate (InsP3), an intracellular messenger, induces Ca2+ release in various types of cells, particularly smooth muscle cells. Its role in skeletal muscle, however, is controversial. The present study shows that the application of InsP3 to rat slow- and fast-twitch saponin-skinned fibres induced contractile responses that were not related to an effect of InsP3 on the properties of the contractile proteins. The amplitude of the contractures was dependent upon the Ca(2+)-loading period, and was larger in slow- than in fast-twitch muscle. In both types of skeletal muscle, these responses, unlike caffeine contractures, were not inhibited by ryanodine (100 microM), but were abolished by heparin (20 micrograms.ml-1). In soleus muscle, the concentration of heparin required to inhibit the response by 50% (IC50) was 5.7 micrograms.ml-1, a similar value to that obtained previously in smooth muscle. Furthermore, the results show that in slow-twitch muscle, the InsP3 contractures have a "bell-shaped" dependency on the intracellular Ca2+ concentration. These results show that InsP3 receptors should be present in skeletal muscle. Thus, it is possible that InsP3 participates in the regulation of sarcoplasmic reticulum Ca2+ release in skeletal muscle, particularly in slow-twitch fibres.

Na(+)-Ca2+ exchange induces low Na+ contracture in frog skeletal muscle fibers after partial inhibition of sarcoplasmic reticulum Ca(2+)-ATPase

Contractile responses due to reduction in external sodium concentration ([Na+]o) were investigated in twitch skeletal muscle fibers of frog semitendinosus. Experiments were conducted after partial inhibition of sarcoplasmic reticulum Ca(2+)-ATPase by cyclopiazonic acid (CPA). In the absence of CPA, Na+ withdrawal failed to produce any change in resting tension. In the presence of CPA (2-10 microM), [Na+]o reduction induced a transient contracture without a significant change in the resting membrane potential. The amplitude of the contracture displayed a step dependence on [Na+]o, was increased by K(+)-free medium and was prevented in Ca(2+)-free medium. This contracture was inhibited by various blockers of the Na(+)-Ca2+ exchange but was little affected by inhibitors of sarcolemmal Ca(2+)-ATPase or mitochondria. When sarcoplasmic reticulum function was impaired, low-Na+ solutions caused no contracture. These results provide evidence that skeletal muscle fibers possess a functional Na(+)-Ca2+ exchange which can mediate sufficient Ca2+ entry to activate contraction by triggering Ca2+ release from sarcoplasmic reticulum when the sodium electrochemical gradient is reduced, and sarcoplasmic reticulum Ca(2+)-ATPase is partially inhibited. This indicates that when the sarcoplasmic reticulum Ca(2+)-ATPase is working (no CPA), Ca2+ fluxes produced by the exchanger are buffered by the sarcoplasmic reticulum. Thus the Na(+)-Ca2+ exchange may be one of the factors determining sarcoplasmic reticulum Ca2+ content and thence the magnitude of the release of Ca2+ from the sarcoplasmic reticulum.

Changes in voltage activation of contraction in frog skeletal muscle fibres as a result of sarcoplasmic reticulum Ca2+-ATPase activity

The effects of cyclopiazonic acid, a specific sarcoplasmic reticulum Ca2+-ATPase inhibitor, on isometric tension were studied in response to prolonged steady-state depolarization induced by a rapid change in extracellular potassium concentration (potassium contractures) in frog semitendinosus muscle fibres. Cyclopiazonic acid (1-10 microM) enhanced the amplitude and time-course of relaxation of 146 mM potassium contracture. In the presence of cyclopiazonic acid 0.5 microM, the relationship between the amplitude of potassium contractures and the membrane potential shifted to more negative potentials, whereas the steady-state inactivation curve was unchanged. These observations suggest that cyclopiazonic acid has no effect on voltage sensors. The difference between potassium contractures in the absence and presence of cyclopiazonic acid in skeletal muscle fibres implies that the amplitude and slow relaxation of tension during prolonged steady-state depolarization may be expected to depend not only on inactivation of the process regulating calcium release from the sarcoplasmic reticulum but also on the ability of the sarcoplasmic reticulum to pump calcium.

Measurement of sarcomere length during fast contraction of muscle fibers by digital image analysis

A low-cost, high-resolution (spatial and temporal) image analysis system was developed to measure sarcomere length (Sl) during fast twitch of isolated striated muscle fibers at different temperatures. Fiber images were examined during twitch with an imaging rate of 220 Hz. To increase temporal resolution beyond 220 Hz, consecutive temporally shifted image sequences (N sequences) were acquired. Individual or average Sl was directly measured from a horizontal profile without spatial-frequency assessment. Measurement precision (E) was determined and expressed as: E(%) = 100xPs/(IsxSl), where Ps is the pixel size and Is the involved sarcomere number. At 18 degrees C during isometric twitch, Sls were measured with 220 Hz temporal and 0.2% spatial resolutions. Sl shortened in the central region (0.21+/-0.12 microm) as tension developed, reaching a maximal shortening of 8.09 + 2.05% (at rest, Sl = 2.59+/-0.05 microm, n = 4) in 32.5+/-1.96 ms. At 30 degrees C, Sl variations were examined with 880 Hz temporal resolution, in which case maximal S1 shortening was reached in 15.74+/-1.99 ms, and then decreased to 5.19+/-1.97% (at rest, S1 = 2.6+/-0.06 microm). The twitch tension developed by the whole fiber was recorded and compared with sarcomere length behavior. Sarcomere length variations in the central region were representative of overall developed tensions at 18 and 30 degrees C.

Changes in CNTF receptor alpha expression in rat skeletal muscle during the recovery period after hindlimb suspension

This study investigated whether modifications in the contractile function of rat muscle after 3 weeks of hindlimb suspension followed by different recovery periods were associated with changes in ciliary neurotrophic factor (CNTF) receptor alpha. CNTF receptor alpha was stronger in control extensor digitorum longus (EDL) than soleus. CNTF receptor alpha was increased in unweighted as compared to control soleus muscle, while no significant changes occurred in EDL muscle. Furthermore, CNTF level was not significantly modified in adult sciatic nerve.

Cyclopiazonic acid-induced changes in the contraction and Ca2+ transient of frog fast-twitch skeletal muscle

The effects of cyclopiazonic acid (CPA) were investigated on isolated skeletal muscle fibers of frog semitendinosus muscle. CPA (0.5-10 microM) enhanced isometric twitch but produced little change in resting tension. At higher concentrations (10-50 microM), CPA depressed twitch and induced sustained contracture without affecting resting and action potentials. In Triton-skinned fibers, CPA had no significant effect on myofibrillar Ca2+ sensitivity but decreased maximal activated force at concentrations > 5 microM. In intact cells loaded with the Ca2+ fluorescence indicator indo 1, CPA (2 microM) induced an increase in Ca(2+)-transient amplitude (10 +/- 2.5%), which was associated with an increase in time to peak and in the time constant of decay. Consequently, peak force was increased by 35 +/- 4%, and both time to peak and the time constant of relaxation were prolonged. It is concluded that CPA effects, at a concentration of up to 2 microM, were associated with specific inhibition of sarcoplasmic reticulum Ca(2+)-adenosinetriphosphatase in intact skeletal muscle and that inhibition of the pump directly affected the handling of intracellular Ca2+ and force production.

Negative inotropic effect of heparin on tension development in rat skinned skeletal muscle fibres

Heparin inhibits inositol trisphosphate receptors, particularly in smooth muscle, but its effect on skeletal muscle is controversial. Our study showed that heparin induced a decrease in the amplitude of 10 mM caffeine-induced contracture in slow and fast saponin-skinned fibres. Moreover, measurements on Triton X-100-skinned fibres in soleus muscle showed that heparin alone decreased maximal Ca2(+)-activated tension and Ca2+ sensitivity of contractile proteins, whereas no significant effect was observed in extensor digitorum longus muscle. However, in the presence of caffeine, heparin decreased maximal Ca2(+)-activated tension in both muscles. It would appear that the heparin-induced decrease in the amplitude of caffeine contracture in rat skeletal muscle was not related to a direct inhibition of Ca2+ release from sarcoplasmic reticulum but to a desensitising effect of heparin and caffeine on myofilaments.

An estimate of the participation of the sarcoplasmic reticulum in the intracellular Ca2+ regulation in adult and newborn ferret hearts

The aim of the present study was to estimate the participation of the sarcoplasmic reticulum in the Ca2+ regulation of the contraction of newborn ferret heart. Cyclopiazonic acid has been used to block the sarcoplasmic reticulum Ca2+ pump in adult and newborn (1 month and 5-6 day old) ferret ventricles of intact and saponin-treated preparations. Cyclopiazonic acid induced a decrease of the amplitude of the caffeine contractures generated in saponin skinned fibers. The sensitivity of the sarcoplasmic reticulum Ca2+ uptake to cyclopiazonic acid was similar in adult and newborn hearts. In intact preparations, cyclopiazonic acid (1-20 microM) induced a negative inotropic effect on the twitch with a prolongation in its kinetics. The maximal decrease in the amplitude of the twitch was larger in adult (92.4%) than in 1 month old (86.5%) and 5-6 day newborns (72.5%). Contrary to other species, where the sarcoplasmic reticulum Ca2+ pump is not functional in neonatal heart, it is proposed that ferret myocardium shows an early maturation of sarcoplasmic reticulum function.

Effects of caffeine and cyclopiazonic acid on contractile proteins of adult and newborn ferret cardiac fibres

Ventricular trabeculae from adult and newborn ferret heart were chemically skinned by Triton X-100 which disrupts all cellular membranes. In newborn preparations, maximal Ca(2+)-activated tension was two times smaller than in adult fibres while Ca2+ sensitivity was higher. Caffeine (10 mM) and cyclopiazonic acid (100 and 200 microM) increased the Ca2+ sensitivity in the two groups of preparations. The caffeine effects were more pronounced in adult than in newborn fibres while the cyclopiazonic acid effects were larger in newborn preparations. In the presence of both drugs. the increase of Ca2+ sensitivity was equal to the addition of the effect of each drug applied separately. These results suggest that the drugs act on two different binding sites. Consequently, care must be taken when caffeine and cyclopiazonic acid are used to study the function of the sarcoplasmic reticulum in intact or saponin-skinned fibres.

Hypogravity increases cyclopiazonic acid sensitivity of rat soleus muscle

The functional capacity of skeletal muscle sarcoplasmic reticulum was explored in slow rat soleus muscle after 21 days of hindlimb suspension. The sarcoplasmic reticulum function was assessed in intact and saponin-skinned fibers by using cyclopiazonic acid, a specific Ca(2+)-adenosinetriphosphatase inhibitor. After hindlimb unweighting, the sensitivity to cyclopiazonic acid of intact and skinned soleus fibers becomes similar to that found in fast-twitch muscles. This change could be related to the expression of fast Ca2(+)-adenosinetriphosphatase-pump protein in unloaded soleus muscles and agrees with a transformation of soleus muscle from slow- to fast-twitch type. These results also indicate that specific pharmacological tools, like cyclopiazonic acid, could be used to analyze subcellular functional changes due to hindlimb unweighting.

Effects of cyclopiazonic acid on membrane currents, contraction and intracellular calcium transients in frog heart

The effects of cyclopiazonic acid, a specific inhibitor of the sarcoplasmic reticulum Ca(2+)-ATPase, on membrane currents and contraction were investigated under voltage clamped conditions on frog atrial trabeculae using the double mannitol-gap technique. In ringer solution, cyclopiazonic acid (10 microM) decreased the phasic contraction at all levels of depolarization without significant change in the time to peak of the contraction and time constant of relaxation. In a low-sodium the amplitude and the current/voltage relationship of L-type and T-type Ca2+ currents were not modified while their inactivation phase was markedly slowed by cyclopiazonic acid. At all levels of depolarization the tonic contraction was increased by cyclopiazonic acid. In the presence of cyclopiazonic acid, no significant variation in diastolic level of intracellular calcium ([Ca2+]i) was found while the maximum amplitude of the Ca2+ transient associated with an action potential was reduced and its time course slowed. All the reversible changes observed could be interpreted by assuming that, in frog atria, sarcoplasmic reticulum was present and cyclopiazonic acid inhibits the sarcoplasmic reticulum Ca(2+)-ATPase. It is proposed that under normal conditions relaxation is due mainly to Na+/Ca2+ exchange. The increase in [Ca2+]i during the contraction phase is due to ICa that provokes a fast release of Ca2+ from the sarcoplasmic reticulum and to an influx of Ca2+ provided by reverse Na+/Ca2+ exchange.

Sarcoplasmic reticulum function in newborn ferret cremaster skeletal muscles

A study of the properties of the sarcoplasmic reticulum was performed with newborn ferret cremaster muscles at two different development stages: at 8 and 21 days. The effects of extracellular Ca2+, caffeine and cyclopiazonic acid, a specific sarcoplasmic reticulum Ca(2+)-ATPase inhibitor, were examined on intact cremaster skeletal muscles. The uptake and release of Ca2+ were explored on saponin-skinned fibres with or without cyclopiazonic acid and some results obtained were compared with those obtained with adult cremaster muscle. The results have shown that skeletal muscle sarcoplasmic reticulum of newborn animals possesses the ability to accumulate and release Ca2+. Furthermore, application of cyclopiazonic acid modified the twitch, the caffeine responses and decreased the amount of Ca2+ taken up by the sarcoplasmic reticulum in saponin-skinned fibres. In contrast to adult skeletal muscle, in newborn cremaster muscles, the Ca2+ dependence of the twitch suggests that the Ca2+ influx at the sarcolemma level was mainly involved in the activation of the contraction. Furthermore, the results obtained in the presence of cyclopiazonic acid were in favour, as in adult muscle, of a participation of the sarcoplasmic reticulum in the relaxation process.

Potentiation of the twitch responses by inhibitors of sarcoplasmic reticulum Ca(2+)-ATPase in frog atrial fibres

In frog atrial fibres, cyclopiazonic acid as well as thapsigargin, which are inhibitors of sarcoplasmic reticulum Ca(2+)-ATPase, induced a significant increase in the twitch amplitude without detectable changes in its kinetics. The measurements performed on chemically skinned fibres show that cyclopiazonic acid has no effect on the properties of contractile proteins. In the presence of a T-type Ca2+ channel blocker or L-type Ca2+ channel blocker, cyclopiazonic acid still induced a potentiation of the twitch while no effect was found in the presence of a Na(+)-Ca2+ exchange blocker. The effect of cyclopiazonic acid was not related to any modification in myofibrillar Ca2+ sensitivity or in Ca2+ influx through Ca2+ channels. It is proposed that the inhibition of the sarcoplasmic reticulum Ca(2+)-ATPase resulted in a potentiation of the effect of the Ca2+ influx and that the major role of the sarcoplasmic reticulum was to limit the intracellular Ca2+ concentration.

Calcium currents and contraction in frog atrial trabeculae

The properties of two types of calcium current (ICa(T) and ICa(L)) and their relationship with contraction were studied in isolated frog atrial trabeculae using the double mannitol gap technique. In Ringer solution containing TTX the calcium currents observed are similar to the ICa(T) and ICa(L) found in single frog atrial cells. The results obtained by using inorganic calcium channel blockers have shown that the phasic contraction appears only when ICa(T) and ICa(L) are simultaneously present. It is proposed that ICa(T) loads the sarcoplasmic reticulum (SR) while ICa(L) triggers Ca2+ release from the SR.

Effect of cyclopiazonic acid on contractile responses in slow and fast bundles of cremaster skeletal muscle from the ferret

The effects of cyclopiazonic acid (CPA) on twitch force, calcium (Ca2+) uptake and release by the sarcoplasmic reticulum (SR), and Ca2+ sensitivity of contractile apparatus were studied using intact and chemically skinned cremaster fibers and compared with those on the extensor digitorum longus and soleus. In cremaster muscles treated with CPA (0.5-5 microM) a potentiation of the twitch was observed, associated with an increase in time to peak and in time of relaxation. In Triton-skinned fibers, CPA, at concentrations less than 10 microM, exerted no significant effect on the contractile apparatus of either slow- or fast-twitch fibers. In slow-twitch fibers, a dose-dependent increase in Ca2+ sensitivity was associated with a decrease in maximal tension, at CPA concentrations > 10 microM. In saponin-skinned fibers, during the uptake phase, CPA at > 10 microM induced a dose-dependent decrease in caffeine contracture. The possibility of an action on the SR Ca2+ release channel was excluded by testing the effect of CPA during the releasing phase. The enhancing effect of CPA (0.5-5 microM) on mechanical activity could be explained by an inhibition of the SR Ca2+ ATPase in skeletal muscle cells without an effect on the contractile proteins. Our results strongly suggest that CPA (< 10 microM) has a highly specific effect on the SR Ca2+ pump in the fast- and slow-twitch fibers and therefore could be a good tool to study the mechanisms of Ca2+ regulation in skeletal muscles.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes of contractile responses due to simulated weightlessness in rat soleus muscle

Some contractile and electrophysiological properties of muscle fibers isolated from the slow-twitch soleus (SOL) and fast-twitch extensor digitorum longus (EDL) muscles of rats were compared with those measured in SOL muscles from suspended rats. In suspended SOL (21 days of tail-suspension) membrane potential (Em), intracellular sodium activity (aiNa) and the slope of the relationship between Em and log [K]o were typical of fast-twitch muscles. The relation between the maximal amplitude of K-contractures vs Em was steeper for control SOL than for EDL and suspended SOL muscles. After suspension, in SOL muscles the contractile threshold and the inactivation curves for K-contractures were shifted to more positive Em. Repriming of K-contractures was unaffected by suspension. The exposure of isolated fibers to perchlorate (ClO4-)-containing (6-40 mM) solutions resulted in a similar concentration-dependent shift to more negative Em of activation curves for EDL and suspended SOL muscles. On exposure to a Na-free TEA solution, SOL from control and suspended rats, in contrast to EDL muscles, generated slow contractile responses. Suspended SOL showed a reduced sensitivity to the contracture-producing effect of caffeine compared to control muscles. These results suggested that the modifications observed due to suspension could be encounted by changes in the characteristics of muscle fibers from slow to fast-twitch type.

Effects of cyclopiazonic acid on Ca(2+)-activated tension production in skinned skeletal muscle fibres of the ferret

The effect of cyclopiazonic acid, an inhibitor of sarcoplasmic reticulum Ca(2+)-ATPase, was investigated on force generation by the contractile apparatus in Triton-skinned fibres from extensor digitorum longus and soleus. Concentrations of cyclopiazonic acid lower than 10 microM were without effect on Ca(2+)-activated tension in both types of muscles. In contrast, in soleus, cyclopiazonic acid (20, 50, 100 microM) was found to shift reversibly the relation-tension pCa (-log[Ca2+]) towards lower free Ca2+ and to decrease maximal Ca(2+)-activated tension, in a dose-dependent manner. In extensor digitorum longus, the Ca2+ sensitivity was significantly increased only at a high cyclopiazonic acid concentration (100 microM) and for all the concentrations tested between 5 to 100 microM maximal Ca(2+)-activated tension was unchanged. These results suggest that cyclopiazonic acid has a direct effect on contractile proteins, in a dose-dependent manner. Ca2+ sensitivity and Ca(2+)-activated maximal tension of the contractile apparatus were differentially affected in fast- (extensor digitorum longus) and slow-twitch (soleus) fibres from ferret skeletal muscle.

Calcium sensitivity of skinned ferret EDL, soleus, and cremaster fibers

The properties of the contractile system at different times of the year in the ferret extensor digitorum longus (EDL), soleus and cremaster muscles were examined by using chemically skinned (Triton X-100) preparations. The results show clear differences in calcium sensitivity between these skeletal muscles. The apparent calcium threshold for activation was lower in soleus than in EDL, while calcium concentrations ([Ca2+]) required to obtain the half-maximal tension, expressed as pCa50 (-log[Ca2+]), was lower in EDL than in soleus muscle. In fact, pCa50 obtained in fast and slow fibers by fitting the experimental data points by a modified Hill equation was 5.92 +/- 0.02 (n = 9) and 6.09 +/- 0.03 (n = 11) respectively. So EDL appears to be a typical fast-twitch muscle and soleus a typical slow-twitch muscle. Adult ferret cremaster muscle was composed of two types of fibers during the quiescent period similar to EDL and soleus, and only one type that was intermediate between EDL and soleus in the breeding period, as assessed by pCa50 values. These annual modifications in calcium activation of adult ferret cremaster muscle could be related to changes in the function of these muscles and may be correlated with seasonal variations of sexual activity.

Effect of haemodynamic pressure overload of the adult ferret right ventricle on inotropic responsiveness to external calcium and rest periods

The inotropic effects of external calcium concentration ([Ca2+]o] and rest periods have been compared in papillary muscles isolated from control (n = 4) and pressure-overloaded right (n = 5) ventricles of adult ferrets. Hypertrophy was induced by pulmonary artery clipping for 30-45 days. Under control conditions (3 mM [Ca2+]o, 0.1 Hz), the isometric twitch force of hypertrophied muscles was decreased by 75%, time to peak was increased by 30% and time to half-relaxation was increased by 50% compared with non-hypertrophied preparations. The sensitivity of contraction to [Ca2+]o was decreased in hypertrophied muscles compared with control ([Ca2+] required for half-maximal contraction: 4.1 mM vs 1.7 mM) and the maximal contraction reached at high [Ca2+]o was smaller in pressure-overloaded muscles compared with control (8.3 +/- 2.0 mN mm-2 vs 19.0 +/- 2.1 mN mm-2 respectively). In both groups, rest periods longer than the steady-state interval were initially accompanied by a potentiation of the first post-rest contraction compared with steady-state. Peak potentiation occurred after a rest of 120 s in hypertrophied muscles and after a rest of 60 s in control. The maximal relative potentiation, i.e. compared with the steady-state twitch, was higher in hypertrophied muscles (+75%) than in control (+20%). After peak potentiation, the amplitude of the first post-rest contraction progressively decreased with increasing periods of rest, although at a slower rate in hypertrophy compared with control. The time constants of post-rest decay were 1203 +/- 99 s and 528 +/- 24 s respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of two types of fibres in ferret, Mustela putorius furo, cremaster muscle

Some contractile, histochemical, morphological and electrophysiological properties of ferret, Mustela putorius furo, cremaster muscle have been estimated. Histochemical fibre typing revealed the presence of two types of fibres (type I 66.2%, type II 33.8%). Morphometry performed on ATPase-stained transverse sections showed that type I was composed of a large amount (40%) of small (less than 1400 microns2) cells. In mammalian Ringer two groups of fibres could be recognized on the basis of the values of resting potential (-69.7 mV and -59.1 mV) intracellular sodium activity (8.3 mmol.l-1 and 14.1 mmol.l-1, respectively). In experiments on fibre bundles, the elevation of extracellular potassium concentration to 15-200 mmol.l-1 produced contractures that consisted of a well-defined transient or phasic tension followed by a sustained or tonic tension. Properties of activation and inactivation of the tension analysed in small bundles of cut fibres (lengths 0.5-1.0 cm) were of fast- and slow-twitch type for phasic and tonic phase, respectively. In contrast to the phasic component of K contractures, the tonic phase was abolished by Ca2+ withdrawal and inhibited by Ni2+, Cd2+, Co2+, Gd3+ and gallopamil (D600). In Ca(2+)-free medium the sustained tension was restored by adding Sr2+. It is concluded that in ferret cremaster muscle the presence of slow-twitch fibres would give rise to the tonic component of the K contracture in which an extracellular source of activator Ca2+ is involved. The ability of these fibres to contract with a maintained tension for prolonged periods of time might participate in the temperature regulation of the testes.

Effects of cyclopiazonic acid, an inhibitor of the Ca++ ATPase of sarcoplasmic reticulum, on Ca++ transport, contraction and relaxation in cardiac muscle

Cyclopiazonic acid is a potent inhibitor of cardiac sarcoplasmic reticulum Ca++ ATPase. It scarely affects inotropism but significantly impairs lusitropism suggesting a greater role for cardiac sarcoplasmic reticulum in the control of cardiac relaxation than in the control of cardiac contraction.

Intracellular Na activity measurements in the control and hypertrophied heart of the ferret: an ion-sensitive micro-electrode study

Because of the role of intracellular Na on cardiac contractility and of the depressed isometric contractile response of the hypertrophied myocardium, the effects of pressure overload on the intracellular Na activity (aiNa) have been investigated in papillary muscles isolated from the ferret right ventricle. In animals subjected to pulmonary artery clipped for 1-2 months, right ventricle-to-body weight ratio was increased by about 39% in comparison with the control group. aiNa was measured in quiescent papillary muscles, by means of Na-sensitive micro-electrodes, at room temperature (19-22 degrees C). aiNa values were, in the control ventricular cells, 7.8 +/- 1.1 mM (mean +/- SD; n = 20) and in the hypertrophied ones, 8.0 +/- 1.2 mM (n = 49). During superfusion by medium with a reduced extracellular Na concentration ([Na]0), aiNa declined in control and pressure-overloaded muscles to similar steady-state levels at a given [Na]0. aiNa fall was mono-exponential and was characterized by a smaller time constant in the hypertrophied group upon total withdrawal of Na0 (control 209 +/- 19 s, n = 4; hypertrophied 128 +/- 42 s, n = 6). In the absence of external K, aiNa increased to levels that were not significantly different between both groups. It was concluded that, in quiescent preparations, steady-state aiNa was not modified by the hypertrophic process. However, pressure overload induced a modification of aiNa regulation by a possible alteration of the sarcolemmal Na/Ca exchange, although other mechanisms, such as mitochondrial Ca transport, could be involved in the differential response to Na0 removal.

Dual effects of charged amphiphiles on depolarization-contraction coupling in denervated rat soleus muscle

Ionic currents and contraction were recorded under voltage clamp conditions in single fibres isolated from rat soleus muscles denervated for more than 20 days. The effects of amphiphiles on depolarization-contraction (d.c.) coupling in Na-free TEA-containing solutions were analyzed. An anionic amphiphile, sodium dodecyl sulfate (1-10 microM), caused a dose-dependent reduction of the contractile response at all amplitudes of depolarization while a cationic amphiphile, dodecyltrimethylamine (1-10 microM), increased the maximum developed tension with a shift in the contractile threshold. A neutral amphiphile, lauryl acetate (20 microM), induced no significant variation. The effects of charged amphiphiles were found to be strongly dependent on the external calcium concentration and on membrane potential. The effect of sodium dodecyl sulfate to decrease tension was reduced or changed to positive inotropy following hyperpolarization of the membrane by, respectively, +10 and +20 mV. In hyperpolarized (+20 mV) cells, dodecyltrimethylamine reduced the amplitude of the contraction. The results demonstrated that changes in Ca-binding properties of surface membrane modified d.c. coupling in denervated slow twitch skeletal muscle.

Na+-Ca2+ exchange in limb muscles of dystrophic (C57 BL/6J dy2J/dy2J) mice

Isometric tension of Na+-withdrawal contractures, membrane potential and intracellular Na+ activity (aiNa) have been measured in vitro under conditions which modify the activity of the Na+-Ca2+ exchanger of dystrophic limb muscles of (dy2J/dy2J) C57 BL/6J mice. In dystrophic soleus (Sol) and extensor digitorum longus (EDL) muscles exposed to Na+-free medium, there was a decrease in aiNa associated with a hyperpolarization of the membrane and the generation of a transient contracture. By contrast, exposing normal or denervated EDL muscles to Na+-free solution induced no change in aiNa nor in the resting tension. This study shows that Na+-Ca2+ exchange is still present in dystrophic Sol muscle. However from thermodynamic considerations, it could be suggested that the higher resting aiNa compared with that in normal muscle, may decrease the efficacy of the exchange.

Membrane Ca2+ interactions and contraction in denervated rat soleus muscle

Under voltage clamp conditions contractile responses and ionic currents of single fibres isolated from rat soleus, denervated for more than 20 days, were recorded in Na-free TEA containing solutions. The relationship between membrane potential and contraction has been analysed under various conditions. The addition of trivalent cations (La3+, Gd3+) resulted in a dose dependent reduction of the contractile response and similar effects were produced by polymyxin B (0.05-0.5 mM). By contrast in the presence of phospholipase D (1-5 U/ml) contractions were significantly increased for all values of depolarization. The time course of the change of tension amplitude after the application of Ca-free medium, was dependent on the amplitude, the duration and the frequency of the depolarization. Upon depolarization glycerol-treated fibres generated contractile responses which were similar to those recorded in normal muscle and were also dependent on [Ca]o. It is proposed that in denervated soleus muscle the negatively charged phospholipids at the outside of the membrane were involved in the depolarization-contraction coupling by means of their Ca binding properties. The quantity of Ca binding sites would be dependent on [Ca]o and membrane potential and their binding properties modified during and/or following variation in membrane potential.

Effects of external cations and calcium-channel blockers on depolarization-contraction coupling in denervated rat twitch skeletal muscles

In the double mannitol gap arrangement the contraction was estimated in single fibres isolated from rat extensor digitorum longus (e.d.l.) muscles that had been denervated for 2-48 days. Denervation induced large changes in the characteristics of the action potential and of the twitch. Up to 15-20 days after denervation the contraction-depolarization curve was sigmoidal and the maximum amplitude of the contraction was not modified by variation of [Ca]o or [Na]o. After 15-20 days of denervation a bell-shaped curve described the relation between contraction and potential. The maximum amplitude was dependent upon the [Ca]o. In Ca-free solution no contractile response was obtained. In Na-free, Ca-containing solution the relationship between contraction and potential was not modified by the addition of divalent cations Co2+, Cd2+, Mn2+, Mg2+, Ni2+, or Ba2+. The contraction, which appeared in Ca-free solution, was restored by adding Sr2+. D600, verapamil and bepridil failed to change the amplitude of the contraction while a marked reduction was found with dihydropyridines. The reduction was overcome by increasing [Ca]o. The present results suggest that the slow calcium current is not involved in the generation of the contractile responses developed by denervated muscles in Na-free (TEA) solution.

Acetylcholine-sensitivity in fast and slow twitch muscle of normal and dystrophic (C57 BL/6J dy2J/dy2J) mice

Small bundles of muscle fibres were isolated from diaphragm, extensor digitorum longus (EDL) and soleus (SOL) muscles of normal and dystrophic (C57 BL/6J dy2J/dy2J) mice, and their isometric tension developed in response to acetylcholine (ACh) was recorded. For each type of muscle the relationship between the maximum amplitude of the ACh-contracture and log [ACh] was similar in normal and dystrophic animals. However, this relationship was steeper for normal and dystrophic SOL than for EDL and diaphragm muscles. Dystrophy did not induce changes in the time course of the ACh-contractures, except a significant 'speeding' of dystrophic SOL that appeared in the time to peak of the contractile response. The amplitude of ACh-contractures of both normal and dystrophic diaphragm preparations increased by about 50% after perfusion for 80-90 min in physiological solution containing phospholipase C 5 mU/ml. ACh-sensitivity was measured in normal and dystrophic diaphragm preparations by iontophoretic application of ACh from high resistance pipettes. ACh-potentials were similar in time course in the two types of muscle fibres, and there was also no significant difference in the length of sensitive fibre segments and maximum sensitivity values. Extrajunctional ACh-sensitivity was absent in normal as well as in dystrophic fibres. It is concluded that the absence in dystrophic muscles of stronger ACh-contractures and of extrajunctional sensitivity can be considered as evidence against a primary neuronal involvement in murine dystrophy of the dy2J strain.

Changes in the ionic currents sensitivity to inhibitors in twitch rat skeletal muscles following denervation

Under voltage clamp conditions, using the double mannitol gap technique, ionic currents developed by fast (e.d.l.) and slow (soleus) twitch muscle fibers of the rat were analysed at different times following denervation and the results compared with those obtained in normal cells. In slow fibers, denervation caused the appearance of a new population of TTX-resistant Na+ channels (dissociation constant K2 = 2,800 nM) compared with the normal TTX-sensitive Na+ channels (K1 = 9 nM). This new population of Na channels appeared in 5 days and contributed about 32% of the total Na conductance. Denervated fast fibres developed a slow component in the delayed outward current which was found to be typical of slow innervated muscles. This component appeared 5 to 20 days after nerve section. These changes are associated with modifications of potassium channels' sensitivity for specific inhibitors (TEA and 4-AP). After denervation, the delayed outward current in the two types of muscles becomes resistant to 4-AP whereas TEA, which blocks the total delayed outward current in innervated fibers (dissociation constant of 21.4 mM) becomes more effective in blocking the fast component (dissociation constant of 0.61 mM) and less effective in blocking the slow component in denervated cells. The analysis of the characteristics of the TEA sensitive and TEA insensitive components of the outward current leads to the proposal that these components were related to the fast and to the slow components previously described in fast and slow twitch mammalian skeletal muscles.

Potassium and caffeine contractures in limb muscles of normal and dystrophic (C57 BL/6J dy2J/dy2J) mice

The strength of contractures, produced by 15 to 146 mM [K]0 (as L-glutamate), was measured in isolated small bundles of muscle fibers from the fast-twitch extensor digitorum longus and from the slow-twitch soleus of normal and dystrophic (C57 BL/6J dy2J/dy2J) mice. The analysis of the relation between the maximal amplitude of the contracture vs the membrane potential and the time constant of relaxation of the K-contractures has shown that dystrophy induced an attenuation of the differences between fast- and slow-twitch muscles. The repriming of K-contractures was more affected by changes in [Ca]0 in normal soleus than in normal extensor digitorum longus and this difference was unaffected by dystrophy. For both types of muscles, the ability of caffeine to produce contractures was reduced in dystrophic muscle and this modification was not related to a change in the fiber typing.

Sodium withdrawal contractures in rat slow twitch skeletal muscle

Contractile responses due to alterations in [Na+]o have been investigated in fast (iliacus) and slow (soleus) twitch muscles of the rat. On exposure to a Na-free solution, the soleus in contrast to the iliacus cells, generated contractile responses without depolarizing the surface membrane. Following glycerol treatment, the twitch and a part of the Na-withdrawal contracture were abolished. The amplitude of the remaining contracture was between 5 and 50% of the original response and the time to peak was 0.4 to 2 times longer. In intact and detubulated preparations, the amplitude of the zero-Na contracture was modified by changes in [Ca2+]o and a linear relationship was found if the reciprocal of tension was plotted against 1/square root[Ca2+]o. In intact and detubulated fibres, a steep dependence of the Na-withdrawal contracture on [Na+]o was found and [Na+]o which induced the half maximal response at each [Ca2+]o was the same, the responses were inhibited by Mg-ions in a competitive way. It is assumed that the activator Ca is triggered at the tubular and sarcolemmal membrane level by lowering [Na+]o and that a calcium-induced calcium release mechanism at the S.R. level may also be involved.

Which active forms of physostigmine generate contractions in diaphragm muscles of mice?

Physostigmine-induced contractions of isolated small bundles (100 micron diameter) of muscle fibres isolated from the diaphragm of C57 BL mice were studied under various conditions. At different external pH the changes in the maximal amplitude of the contraction were related to the external activity of the permeant neutral form of the drug. However changes in the internal pH showed that the amplitude of the contractile response depended directly on the internal activity of the protonated form of the drug. The amplitude of the contraction was enhanced by caffeine treatment and depended on the external calcium concentrations. In mammalian skeletal muscle, physostigmine appeared to produce contractions due to its anticholinesterase property. The release of calcium seemed to occur from an intracellular store different from that involved in the action of caffeine.

A comparative study of contractile responses in diaphragm muscles of normal and dystrophic (C57BL/6J dy2J/dy2J) mice

Potassium and caffeine contractures of isolated small bundles (100 to 200 micron diameter) of muscle fibers isolated from the diaphragm of normal and dystrophic (C57BL/6J dy2J/dy2J) mice were compared. In diaphragms of pathologic mice (3 to 5 months old) the resting potential, the characteristics of the twitch, and some histological examinations were typical of dystrophic muscles. The slopes of the relationships between the steady membrane potential and log [K]0 were similar for the two types of cells. In 110 mM and 146 mM K there were no significant differences in the time course of the contractures and reduction in [Ca]0 decreased the time to peak and the time constant of relaxation to the same extent; the relative efficiency of [Mg]0 compared with [Ca]0 was equivalent. Repriming of K contractures at different external calcium concentrations indicated that the normal diaphragm did not have any special advantage. The exposure of isolated strips to a solution containing caffeine resulted in a similar increase of the strength of the regularly evoked twitch responses. However, the contractures elicited by 1.25 to 20 mM caffeine showed a subsensitivity of the dystrophic diaphragm (KmDys = 9.3 KmN) and the rate of relaxation was significantly slower than in normal muscle (in 20 mM caffeine, 50% decay time for normal muscle was 25.2 +/- 7.6 s and for dystrophic muscle 54.8 +/- 11.2 s. These results suggest an absence of major alterations in the mechanism of excitation-contraction coupling associated with dystrophy, except for a change in the specific element of the sarcoplasmic reticulum where caffeine acts.

Membrane potential and contractures in segments cut from rat fast and slow twitch muscles

Contractile responses associated with depolarization caused by an increase in [K]0 or by voltage-clamp steps were compared for fast and slow mammalian twitch muscles. The contractions and contractures of isolated mammalian muscle fibres cut into 0.5-1.0 cm lengths are similar to those obtained from intact cells. The slope of the relationship between the membrane potential and the log [K]0 is similar in slow (46 mV +/- 0.8) and in fast fibres (48 mV +/- 1.1). This relation is not significantly modified in sodium-free or Cl-free solution. The K-contractures of cut sections of slow and fast fibres are transient and a full repriming of the response is only observed when the [K] x [Cl] product is kept constant. The contractile threshold in fast fibres is at 20-30 mM [K]0 (-52 to -43 mV) and in slow muscle at 10-15 mM [K]0 (-62 to -55 mV). Under voltage-clamp conditions, the contractile responses of both types of muscle show two components. In Na-free solution or in presence of TTX (5 x 10(-7) g/ml) the first component is abolished and the second remaining component is similar to that developing during K-contractures. In iliacus fibres, the contracture threshold is between -50.5 mV and -40.5 mV and in soleus fibres between -66 mV and -56 mV; these values are close to those obtained with K-rich depolarizing fluids. The S-shaped curve of the contracture vs membrane potential relation is similar to that found in frog muscles except that the contractile responses are graded over a larger range of membrane potentials (-50 to + 30 mV in fast and -55 to + 10 mV in slow muscle).

Comparison between the delayed outward current in slow and fast twitch skeletal muscle in the rat

1. A comparison of the delayed outward current of isolated fibres from rat soleus and iliacus muscle has been made using a double sucrose-gap voltage-clamp method. 2. The fast and slow components of the outward current were separated using time constants of the tail currents. The results indicate that in both iliacus and soleus fibres there is a shift in reversal potential which depends on the quantity of current that flows during depolarization. 3. The shift is larger in iliacus than in soleus; it is absent in glycerol-treated muscles. 4. The results obtained in normal and in detubulated fibres show that the shift is due to an accumulation process of potassium ions in the lumen of the T-tubules. 5. In detubulated soleus fibres the outward current is composed of a fast and a slow component, each with the same reversal potential; in detubulated iliacus the slow component is absent. 6. In both types of muscles TEA produces a dose-dependent block of the total outward current. 4-aminopyridine has different effects; it inhibits the total outward current in iliacus fibres and only the fast component in soleus fibres. 7. These results show that in soleus fibres a fast and a slow component participate in the potassium outward current, while only a fast component is present in iliacus muscle.

Ionic currents in slow twitch skeletal muscle in the rat

1. The ionic currents in slow fibres isolated from rat soleus muscle have been studied under voltage-clamp conditions with a double sucrose-gap method and the results are compared to those obtained from fast fibres isolated from the iliacus muscle. 2. The mean value of the resting potential in slow fibres is -70 mV. a value 8 mV more positive that the mean resting potential of fast fibres (-78 mV). 3. In slow muscle, a fast inward current which is blocked by tetrodotoxin and which depends on external sodium concentration is presumed to be carried by sodium ions. The characteristics of this current, which are time- and voltage-dependent, are similar to those of the iliacus fibres. From a holding potential at -86 mV, this inward current is maximal (2.6 mA/cm2 +/- 0.3) at +49.1 mV +/- 1.5 (mean +/- S.E. of mean), reverses at +127.3 mV +/- 2.2 (mean +/- S.E. of mean), and its half inactivation occurs at +23.2 mV +/- 0.8 (mean +/- S.E. of mean). 4. The delayed outward current in slow fibres is unchanged by exposure to chloride free solution and has a time course very different from that found in fast fibres. This current reaches an initial peak in 5-10 msec and a second peak or steady level after 40-150 msec. The decay of the outward current is also very different, being ten times slower than that in fast fibres (1500-3000 msec). 5. Analysis of the tail currents reveals the existence of two components of delayed current in slow fibres. The faster component reverses at a potential of 11.3 mV +/- 0.9 (mean +/- S.E. of mean) positive to the holding potential (equivalent to a membrane potential of about -75 mV), in contrast to a reversal potential of 35.4 mV +/- 2.5 (mean +/- S.E. of mean) positive to the holding potential for the slower component (equivalent to a membrane potential of about -51 mV. 6. In L-glutamate solution the characteristics of the inward-going rectification are the same in the two types of muscle.

Ionic currents in mammalian fast skeletal muscle

1. The double sucrose-gap technique has been applied to rat skeletal muscle fibres to study the ionic currents under voltage-clamp conditions. 2. The iliacus muscle was found to be of 'fast' type according to the characteristics of the twitch generated by an action potential. 3. Micro-electrode measurements have shown that the intracellular potential is under good control even when an inward current develops. 4. The components of an equivalent circuit with two time constants have been estimated from the records of the capacitive current. 5. In rat muscle, between 15 and 21 degrees C, inward and outward currents are similar to sodium and potassium currents found in frog muscle at lower temperature (1--3 degrees C). 6. The inward current which depends on [Na]o and is abolished by tetrodotoxin is carried by sodium ions. Related to the mean value for the holding potential (-90.5 mV) this current reaches its maximum amplitude a +40 and +50 mV, reverses between +130 and +150 mV and its half inactivation occurs between +14 and +22 mV. The effect of low doses of tetrodotoxin suggests that two components participate in the sodium current. 7. The delayed outward current which shows inactivation is divided in two components: (i) the fast has a linear instantaneous current-voltage relation and differs from the fast component of frog muscle in that its equilibrium potential is more negative than the resting potential; (ii) the slow has a linear instantaneous current-voltage relation and the mean value for its equilibrium potential is 26 mV less negative than the resting potential. 8. Inward-going rectification is present in rat muscle.