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

Ca2+ overload and sarcoplasmic reticulum instability in tric-a null skeletal muscle

The sarcoplasmic reticulum (SR) of skeletal muscle contains K(+), Cl(-), and H(+) channels may facilitate charge neutralization during Ca(2+) release. Our recent studies have identified trimeric intracellular cation (TRIC) channels on SR as an essential counter-ion permeability pathway associated with rapid Ca(2+) release from intracellular stores. Skeletal muscle contains TRIC-A and TRIC-B isoforms as predominant and minor components, respectively. Here we test the physiological function of TRIC-A in skeletal muscle. Biochemical assay revealed abundant expression of TRIC-A relative to the skeletal muscle ryanodine receptor with a molar ratio of TRIC-A/ryanodine receptor ∼5:1. Electron microscopy with the tric-a(-/-) skeletal muscle showed Ca(2+) overload inside the SR with frequent formation of Ca(2+) deposits compared with the wild type muscle. This elevated SR Ca(2+) pool in the tric-a(-/-) muscle could be released by caffeine, whereas the elemental Ca(2+) release events, e.g. osmotic stress-induced Ca(2+) spark activities, were significantly reduced likely reflecting compromised counter-ion movement across the SR. Ex vivo physiological test identified the appearance of "alternan" behavior with isolated tric-a(-/-) skeletal muscle, i.e. transient and drastic increase in contractile force appeared within the decreasing force profile during repetitive fatigue stimulation. Inhibition of SR/endoplasmic reticulum Ca(2+ ATPase) function could lead to aggravation of the stress-induced alternans in the tric-a(-/-) muscle. Our data suggests that absence of TRIC-A may lead to Ca(2+) overload in SR, which in combination with the reduced counter-ion movement may lead to instability of Ca(2+) movement across the SR membrane. The observed alternan behavior with the tric-a(-/-) muscle may reflect a skeletal muscle version of store overload-induced Ca(2+) release that has been reported in the cardiac muscle under stress conditions.

ATP consumption by sarcoplasmic reticulum Ca2+ pumps accounts for 50% of resting metabolic rate in mouse fast and slow twitch skeletal muscle

In this study, we aimed to directly quantify the relative contribution of Ca(2+) cycling to resting metabolic rate in mouse fast-twitch (extensor digitorum longus, EDL) and slow-twitch (soleus) skeletal muscle. Resting oxygen consumption of isolated muscles (Vo(2), microl.g wet wt(-1).s(-1)) measured polarographically at 30 degrees C was approximately 25% higher in soleus (0.61 +/- .03) than in EDL (0.46 +/- .03). To quantify the specific contribution of Ca(2+) cycling to resting metabolic rate, cyclopiazonic acid (CPA), a highly specific inhibitor of sarco(endo)plasmic reticulum Ca(2+) ATPases (SERCAs), was added to the bath at different concentrations (1, 5, 10, and 15 microM). There was a concentration-dependent effect of CPA on Vo(2), with increasing CPA concentrations up to 10 microM resulting in progressively greater reductions in muscle Vo(2). There were no differences between 10 and 15 microM CPA, indicating that 10 microM CPA induces maximal inhibition of SERCAs in isolated muscle preparations. Relative reduction in muscle Vo(2) in response to CPA was nearly identical in EDL (1 microM, 10.6 +/- 3.0%; 5 microM, 33.2 +/- 3.4%; 10 microM, 49.2 +/- 2.9%; 15 microM, 50.9 +/- 2.1%) and soleus (1 microM, 11.2 +/- 1.5%; 5 microM, 37.7 +/- 2.4%; 10 microM, 50.0 +/- 1.3%; 15 microM, 49.9 +/- 1.6%). The results indicate that ATP consumption by SERCAs is responsible for approximately 50% of resting metabolic rate in both mouse fast- and slow-twitch muscles at 30 degrees C. Thus SERCA pumps in skeletal muscle could represent an important control point for energy balance regulation and a potential target for metabolic alterations to oppose obesity.

Effect of cyclopiazonic acid, an inhibitor of the sarcoplasmic reticulum Ca-ATPase, on skeletal muscles from normal and mdx mice

AIM

In this study, we investigated Ca2+ loading by the sarcoplasmic reticulum in skeletal muscle from mdx mice, an animal model of human Duchenne's muscular dystrophy, at two stages of development: 4 and 11 weeks.

METHOD

Experiments were conducted on fast- (extensor digitorum longus, EDL) and slow- (soleus) twitch muscles expressing different isoforms of Ca2+-ATPase, which is responsible for the uptake of Ca2+ by the sarcoplasmic reticulum.

RESULTS

In sarcoplasmic reticulum vesicles, the ATP-dependent activity and sensitivity to cyclopiazonic acid (CPA), an inhibitor of the sarcoplasmic reticulum Ca2+-ATPase, were similar in mdx and normal EDL muscle. Furthermore, in chemically-skinned fibres from both normal and mdx muscles, the presence of CPA induced a decrease in Ca2+ uptake by the sarcoplasmic reticulum. However, the sensitivity to CPA was lower in mdx EDL muscle than in normal muscle. In addition, in EDL muscle from 4-week-old mdx mice, the expression of the slow Ca2+-pump isoform (SERCA2a) was significantly increased, without any accompanying change in slow myosin expression. In contrast, the expression and function of the Ca2+-ATPase in mdx soleus muscles at 4- and 11-weeks of development did not differ from those in age-matched controls.

CONCLUSION

These findings show that in dystrophic muscle, where the Ca2+ homeostasis was perturbed, the Ca2+ handling by the sarcoplasmic reticulum was altered in fast-twitch muscle, and this was associated with the expression of the slow isoform of SERCA. In these muscles, reduced Ca2+ uptake could then contribute to an elevated concentration of Ca2+ in the cytosol, and also to Ca2+ depletion of the sarcoplasmic reticulum.

Comparison of catecholamine release in the isolated adrenal glands of SHR and WKY rats

1 The present study was designed to investigate the secretion of catecholamines (CA) evoked by stimulation of cholinergic receptors and membrane depolarization from the isolated perfused adrenal gland of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKYR) at adult age. 2 The wet weight of adrenal gland in SHR was greater than that in WKYR. The CA releasing responses evoked by acetylcholine (5.32 x 10-3 m), and high potassium (5.6 x 10-2 m), a membrane depolarizer, were significantly lower in WKYR than in SHR. 3 The secretory responses of CA evoked by DMPP (10-4 m for 2 min), a selective agonist of neuronal nicotinic receptors, and McN-A-343 (10-4 m for 2 min), a selective agonist of neuronal muscarinic receptors, were also significantly lower in WKYR than in SHR. 4 The CA release evoked by Bay-K-8644 (10-5 m), a dihydropyridine-sensitive Ca2+ channel activator, and cyclopiazonic acid (10-5 m), a selective inhibitor of Ca2+-ATPase in the endoplasmic reticulum, were also significantly greater in SHR than WKYR. 5 Taken together, these experimental results demonstrate that the CA secretion evoked by stimulation of cholinergic (nicotinic and muscarinic) receptors as well as membrane depolarization is enhanced more greatly in the perfused adrenal glands of SHR than in those of WKYR. It is suggested that the augmented CA release in SHR compared with WKYR was involved in essential hypertensive pathogenesis.

Effects of cyclopiazonic acid on contraction and intracellular Ca2+ in oesophageal striated muscle of normotensive and spontaneously hypertensive rats

1. The effects of cyclopiazonic acid (CPA), a selective inhibitor of sarcoplasmic reticulum (SR) Ca2+-ATPase, on twitch contraction and on the resting state of tension and intracellular Ca2+ level ([Ca2+]i) of the oesophageal striated muscle of stroke-prone spontaneously hypertensive rats (SHRSP) and normotensive Wistar Kyoto rats (WKY) were compared. 2. CPA (10 micronM) augmented the twitch contraction of oesophageal striated muscle preparations from both SHRSP and WKY, reducing the rate of relaxation (-dT/dt), and thus resulting in the prolongation of the time to 80% relaxation. The effect was significantly smaller in the SHRSP preparations. 3. In the resting state, CPA caused a sustained elevation of [Ca2+]i. The elevation was greater in the WKY preparations. Tension development accompanied by the elevation was observed in WKY preparations, but not in SHRSP preparations. 4. The sustained elevation of [Ca2+]i induced by CPA was eliminated by the removal of extracellular Ca2+. Both the elevated [Ca2+]i and tension in the preparations from WKY were reduced by flufenamic acid (100 micronM), mefenamic acid (100 micronM), lanthanum (La3+, 100 micronM), gadolinium (Gd3+, 100 micronM) and SK&F 96365 (100 micronM) but not by verapamil (10 micronM). 5. Thapsigargin (3 micronM), another SR Ca2+-ATPase inhibitor, produced similar effects on basal tension to those of CPA, although it reduced the amplitude of twitch contraction. 6. These results suggest that in the rat oesophageal striated muscle, (1) CPA extends the sequestrating time of Ca2+ into the SR, (2) CPA induces a Ca2+ influx mediated through verapamil-insensitive pathways, possibly nonselective cation channels, and (3) the mechanism of [Ca2+](i) modulation due to CPA-sensitive SR Ca2+-ATPase is deteriorated in the oesophageal striated muscle from SHRSP as compared with WKY preparations.

Malignant hyperthermia: effects of sarcoplasmic reticulum Ca2+ pump inhibition

In porcine malignant hyperthermia-susceptible (MHS) skeletal muscles, calcium release is abnormal and resting calcium may be elevated. Thus MHS muscles may have prolonged twitch relaxation and lower fusion frequencies, which would be augmented by inhibition of sarcoplasmic reticulum (SR) Ca2+ adenosine triphosphatase (ATPase) activity; bundles of intact muscle cells from MHS and normal pigs were used to investigate this possibility. Cooling and low-frequency stimulation, in combination, enhanced twitch fusion and prolonged twitch relaxation significantly more in MHS than in normal muscles (e.g., 34 +/- 4% versus 16 +/- 4% fusion, and 82.4 +/- 9.4 ms versus 43.2 +/- 7.8 ms half-relaxation time, for MHS and normal muscles, respectively). Similarly, inhibition of the SR Ca2+ ATPase by cyclopiazonic acid resulted in significantly greater twitch fusion in MHS muscles. These results were consistent with predicted effects of enhanced SR Ca2+ release and/or elevated resting calcium in MHS muscles and indicate that cooling during a malignant hyperthermia crisis could actually increase the force of muscle contractures.

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.

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.

The role of sarcoplasmic reticulum and sarcoplasmic reticulum Ca2+-ATPase in the smooth muscle tone of the cat gastric fundus

Circular smooth muscle strips isolated from cat gastric fundus were studied in order to understand whether the sarcoplasmic reticulum (SR) and SR Ca2+-ATPase could play a role in the regulation of the muscle tone. Cyclopiazonic acid (CPA), a specific inhibitor of SR Ca2+-ATPase, caused a significant and sustained increase in muscle tone, depending on the presence of extracellular Ca2+. Nifedipine and cinnarizin only partially suppressed the CPA-induced tonic contraction. Bay K 8644 antagonized the relaxant effect of nifedipine in CPA-contracted fundus. Nitric-oxide-releasing agents sodium nitroprusside and 3-morpholino-sydnonimine completely suppressed the CPA-induced tonic contraction. The blockers of Ca2+-activated K+ channels, tetraethylammonium, charybdotoxin and/or apamin, decreased the contractile effect of CPA. Vanadate increased the tone but did not change significantly the effect of CPA. CPA exerted its contractile effect even when Ca2+ influx was triggered through the Na+/Ca2+ exchanger and the other Ca2+ entry pathways were blocked. Thapsigargin, another specific SR Ca2+-ATPase inhibitor, also increased the muscle tone. The effect of thapsigargin was completely suppressed by sodium nitroprusside and 3-morpholino-sydnonimine and partially by nifedipine. In conclusion, under conditions when the SR Ca2+-ATPase is inhibited, the tissue develops a strong tonic contraction and a large part of this is mediated by Ca2+ influx presumably via nifedipine-sensitive Ca2+ channels. This study suggests the important role of SR Ca2+-ATPase in the modulation of the muscle tone and the function of SR as a "buffer barrier" to Ca2+ entry in the cat gastric fundus smooth muscle.

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.