Effects of hypertonic solutions on calcium transients in frog twitch muscle fibres

The effect of extracellular tonicity on the anatomy of triad complexes in amphibian skeletal muscle

Ultrastructural features of tubular-sarcoplasmic (T-SR) triad junctions and measures of cell volume following graded increases of extracellular tonicity were compared under physiological conditions recently shown to produce spontaneous release of intracellularly stored Ca2+ in fully polarized amphibian skeletal muscle fibres. The fibres were fixed using solutions of equivalent tonicities prior to processing for electron microscopy. The resulting anatomical sections demonstrated a partially reversible cell shrinkage corresponding to substantial increases in intracellular solute or ionic strength graded with extracellular tonicity. Serial thin sections through triad structures confirmed the presence of geometrically close but anatomically isolated transverse (T-) tubular and sarcoplasmic reticular (SR) membranes contrary to earlier suggestions for the development of luminal continuities between these structures in hypertonic solutions. They also quantitatively demonstrated accompanying decreases in T-SR distances, increased numbers of sections that showed closely apposed T and SR membranes, tubular luminal swelling and reductions in luminal volume of the junctional SR, all correlated with the imposed increases in extracellular osmolarity. Fully polarized fibres correspondingly showed elementary Ca(2+)-release events ('sparks', in 100 mM-sucrose-Ringer solution), sustained Ca2+ elevations and propagated Ca2+ waves (> or = 350-500 mM sucrose) following exposure to physiological Ringer solutions of successively greater tonicities. These were absent in hypotonic, isotonic or less strongly hypertonic (approximately 50 mM sucrose-Ringer) solutions. Yet exposure to hypotonic solutions also disrupted T-SR junctional anatomy. It increased the tubular diameters and T-SR distances and reduced their area of potential contact. The spontaneous release of intracellularly stored Ca2+ thus appears more closely to correlate with the expected changes in intracellular solute strength or a reduction in absolute T-SR distance rather than disruption of an optimal anatomical relationship between T and SR membranes taking place with either increases or decreases in extracellular tonicity.

Non-cross-bridge calcium-dependent stiffness in frog muscle fibers

At the end of the force transient elicited by a fast stretch applied to an activated frog muscle fiber, the force settles to a steady level exceeding the isometric level preceding the stretch. We showed previously that this excess of tension, referred to as "static tension," is due to the elongation of some elastic sarcomere structure, outside the cross bridges. The stiffness of this structure, "static stiffness," increased upon stimulation following a time course well distinct from tension and roughly similar to intracellular Ca(2+) concentration. In the experiments reported here, we investigated the possible role of Ca(2+) in static stiffness by comparing static stiffness measurements in the presence of Ca(2+) release inhibitors (D600, Dantrolene, (2)H(2)O) and cross-bridge formation inhibitors [2,3-butanedione monoxime (BDM), hypertonicity]. Both series of agents inhibited tension; however, only D600, Dantrolene, and (2)H(2)O decreased at the same time static stiffness, whereas BDM and hypertonicity left static stiffness unaltered. These results indicate that Ca(2+), in addition to promoting cross-bridge formation, increases the stiffness of an (unidentified) elastic structure of the sarcomere. This stiffness increase may help in maintaining the sarcomere length uniformity under conditions of instability.

Differential effects of sarcoplasmic reticular Ca(2+)-ATPase inhibition on charge movements and calcium transients in intact amphibian skeletal muscle fibres

A hypothesis in which intramembrane charge reflects a voltage sensing process allosterically coupled to transitions in ryanodine receptor (RyR)-Ca(2+) release channels as opposed to one driven by release of intracellularly stored Ca(2+) would predict that such charging phenomena should persist in skeletal muscle fibres unable to release stored Ca(2+). Charge movement components were accordingly investigated in intact voltage-clamped amphibian fibres treated with known sarcoplasmic reticular (SR) Ca(2+)-ATPase inhibitors. Cyclopiazonic acid (CPA) pretreatment abolished Ca(2+) transients in fluo-3-loaded fibres following even prolonged applications of caffeine (10 mM) or K(+) (122 mM). Both CPA and thapsigargin (TG) transformed charge movements that included delayed (q(gamma)) "hump" components into simpler decays. However, steady-state charge-voltage characteristics were conserved to values (maximum charge, Q(max) approximately equal to 20-25 nC microF(-1); transition voltage, V* approximately equal to -40 to-50 mV; steepness factor, k approximately equal to 6-9 mV; holding voltage -90 mV) indicating persistent q(gamma) charge. The features of charge inactivation similarly suggested persistent q(beta) and q(gamma) charge contributions in CPA-treated fibres. Perchlorate (8.0 mM) restored the delayed kinetics shown by "on" q(gamma) charge movements, prolonged their "off" decays, conserved both Q(max) and k, yet failed to restore the capacity of such CPA-treated fibres for Ca(2+) release. Introduction of perchlorate (8.0 mM) or caffeine (0.2 mM) to tetracaine (2.0 mM)-treated fibres, also known to restore q(gamma) charge, similarly failed to restore Ca(2+) transients. Steady-state intramembrane q(gamma) charge thus persists with modified kinetics that can be restored to its normally complex waveform by perchlorate, even in intact muscle fibres unable to release Ca(2+). It is thus unlikely that q(gamma) charge movement is a consequence of SR Ca(2+) release rather than changes in tubular membrane potential.

Calcium waves induced by hypertonic solutions in intact frog skeletal muscle fibres

1. Regenerative Ca2+ waves and oscillations indicative of calcium-induced calcium release (CICR) activity were induced in fully polarized, fluo-3-loaded, intact frog skeletal muscle fibres by exposure to hypertonic Ringer solutions. 2. The calcium waves persisted in fibres exposed to EGTA-containing solutions, during sustained depolarization of the membrane potential or following treatment with the dihydropyridine receptor (DHPR)-blocker nifedipine. 3. The waves were blocked by the ryanodine receptor (RyR)-specific agents ryanodine and tetracaine, and potentiated by caffeine. 4. In addition to these pharmacological properties, the amplitudes, frequency and velocity of such hypertonicity-induced waves closely resembled those of Ca2+ waves previously described in dyspedic skeletal myocytes expressing the cardiac RyR-2. 5. Quantitative transmission and freeze-fracture electronmicroscopy demonstrated a reversible cell shrinkage, transverse (T)-tubular luminal swelling and decreased T-sarcoplasmic reticular (SR) junctional gaps in fibres maintained in and then fixed using hypertonic solutions. 6. The findings are consistent with a hypothesis in which RyR-Ca2+ release channels can be partially liberated from their normal control by T-tubular DHPR-voltage sensors in hypertonic solutions, thereby permitting CICR to operate even in such fully polarized skeletal muscle fibres.

Peptide-induced Ca(2+) movements in a tonic insect muscle: effects of proctolin and periviscerokinin-2

Although most of the characterized insect neuropeptides have been detected by their actions on muscle contractions, not much is known about the mechanisms underlying excitation-contraction coupling. Thus we initiated a pharmacological study on the myotropic action of the peptides periviscerokinin-2 (PVK-2) and proctolin on the hyperneural muscle of the cockroach Periplaneta americana. Both peptides required extracellular Ca(2+) to induce muscle contraction, and a blockage of sarcolemmal Ca(2+) channels by Mn(2+) or La(3+) inhibited myotropic effects. The peptides were able to induce contractions in dependence on the extracellular Ca(2+) concentration in muscles depolarized with high K(+) saline. A reduction of extracellular Na(+), K(+), or Cl(-) did not effect peptide action. Nifedipine, an L-type Ca(2+)-channel blocker, partially blocked the response to both peptides but to a much lesser extent than contractions evoked by elevated K(+). Using calcium imaging with fluo-3, we show that proctolin induces an increase of the intracellular Ca(2+) concentration. In calcium-free saline, no increase of the intracellular Ca(2+) concentration could be detected. The inhibiting effect of ryanodine, thapsigargin, and TMB-8 on peptide-induced contractions suggests that Ca(2+) release from the sarcoplasmic reticulum plays a major role during peptide-induced contractions. Preliminary experiments suggest that the peptides do not employ cyclic nucleotides as second messengers, but may activate protein kinase C. Our results indicate that the peptides induce Ca(2+) influx by an activation or modulation of dihydropyridine-sensitive and voltage-independent sarcolemmal Ca(2+) channels. Ca(2+)-induced Ca(2+) release from intracellular stores, but not inositol trisphosphate-induced Ca(2+) release, seems to account for most of the observed increase in intracellular Ca(2+). Additionally, both peptides were able to potentiate glutamate-induced contractions at threshold concentrations.

Stimulation by polyols of the two ryanodine receptor isoforms of frog skeletal muscle

While the stimulating effect of concentrated salts on ryanodine receptor (RyR) is widely accepted in Ca(2+)-induced Ca2+ release (CICR) and [3H]ryanodine binding, the effect of non-ionic solutes on RyR is controversial. We investigated the effects of polyols on [3H]ryanodine binding to alpha- and beta-RyR purified from bullfrog skeletal muscle, and on CICR from sarcoplasmic reticulum (SR) in a skinned frog skeletal muscle fibre. Addition of polyols (glucose, sucrose, sorbitol, glycerol and ethylene glycol) in submolar to molar concentrations to an isotonic salt medium increased dose-dependently Ca(2+)-activated [3H]ryanodine binding to alpha- and beta-RyR of a similar magnitude. The increase is due to the rise in both apparent affinity (1/KD) and maximal numbers of binding sites (Bmax) for ryanodine. In addition to this stimulating effect, glucose sensitized both isoforms to Ca2+ in the Ca(2+)-activated reaction, which is distinct in mechanism(s) from caffeine. These stimulating effects of polyols were not observed unless some NaCl was present, which might explain the discrepancy among reported results. Consistent with these findings, polyols reversibly enhanced the rate of CICR from SR in skinned fibres with an increase in the Ca2+ sensitivity. The enhanced CICR was still sensitive to well-known modulators for CICR (Ca2+, Mg2+, adenine nucleotides and procaine), as with [3H]ryanodine binding. The results of this study reveal that polyols stimulate alpha- and beta-RyR in frog skeletal muscle, bringing about increased CICR activity. The finding that the specific activity of polyols in stimulation of [3H]ryanodine binding was approximately proportional to their molecular weights leads us to discuss the possible modification of protein surface-water molecule interaction as an underlying mechanism.

Hyperosmotic modulation of the cytosolic calcium concentration in a rat osteoblast-like cell line

1. The effects of hyperosmotic stress on cytosolic calcium concentration ([Ca2+]i) were studied by ratio image analysis in single cells of an osteoblast-like bone cell line (RCJ 1.20) loaded with fura-2 AM. 2. The ratio (340 nm/380 nm) of steady-state [Ca2+]i in resting osteoblasts kept in Hepes-buffered medium was 0.82 +/- 0.04. A hyperosmotic stimulus (200 mosmol l-1 sucrose) produced a [Ca2+]i transient with a peak ratio of 1.28 +/- 0.09, which decayed with an apparent half-life (t1/2) of 42.7 +/- 2.6 s. 3. The hyperosmotically induced [Ca2+]i transients were insensitive to verapamil, diltiazem or nifedipine, which excludes the involvement of dihydropyridine-sensitive Ca2+ channels in the process. Non-specific Ca2+ channel blockers (Mn2+, Ni2+, La3+ or Gd3+) partially abolished the hyperosmotically induced [Ca2+]i elevation, indicating the contribution of extracellular Ca2+ influx. 4. A hyperosmotic stimulus applied in Ca(2+)-free medium (0.5 mM EGTA) lowered the [Ca2+]i peak to a ratio of 0.96 +/- 0.08 (P < 0.001) compared with a Ca(2+)-containing medium. This suggests that the [Ca2+]i increase is due to extracellular influx, as well as release from an intracellular Ca2+ pool. 5. Application of thapsigargin (0.5 microM), a specific inhibitor of endoplasmic reticulum Ca(2+)-ATPase, in Ca(2+)-free medium caused transient [Ca2+]i elevation to peak ratios of 1.33 +/- 0.09, and completely abolished the [Ca2+]i response to a hyperosmotic stimulus. This implies the existence of a thapsigargin-sensitive intracellular pool of Ca2+ that is mobilized by hyperosmotic stimulus.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of ryanodine receptors

Recent findings on the ryanodine receptor of vertebrates, a Ca-release channel protein for the caffeine- and ryanodine-sensitive Ca pools, are reviewed in this article. Three distinct genes, i.e., ryr1, ryr2, and ryr3, express different isoforms in specific locations: Ryr1 in skeletal muscle and Purkinje cells of cerebellum; Ryr2 in cardiac muscle and brain, especially cerebellum; Ryr3 in skeletal muscle of nonmammalian vertebrates, the corpus striatum, and limbic cortex of brain, smooth muscles, and the other cells in vertebrates. While only one isoform (Ryr1) is expressed in mammalian skeletal muscles, two isoforms (alpha- and beta-isoforms expressed by ryr1 and ryr3, respectively) are found in nonmammalian vertebrate skeletal muscles. Although the coexistence of two isoforms may merely be related to differentiation and specialization, the biological significance remains to be clarified. Ryanodine receptors in vertebrate skeletal muscles are believed to mediate two different modes of Ca release: Ca(2+)-induced Ca release and action potential-induced Ca release. All results obtained so far with any isoform of ryanodine receptor are related to Ca(2+)-induced Ca release and show very similar characteristics. Ca(2+)-induced Ca release, however, cannot be the underlying mechanism of Ca release on skeletal muscle activation. Susceptibility of the ryanodine receptor's ryanodine-binding activity to modification by physical factors, such as osmolality of the medium, might be related to action potential-induced Ca release. A hypothesis of molecular interaction in view of the plunger model of action potential-induced Ca release is discussed, suggesting that the model could be compatible with Ryr1 and Ryr3, but incompatible with Ryr2. The functional relevance of ryanodine receptor isoforms, especially Ryr3, in brain also remains to be clarified. Among ryr1 gene-related diseases, malignant hyperthermia was the first to be identified; however, there is still the possibility of involvement of the other genes. Central core disease has been added to the list recently. A molecular approach for the diagnosis and treatment of diseases is now in progress.

Effects of osmolality and ionic strength on the mechanism of Ca2+ release in skinned skeletal muscle fibres of the toad

1. The effects of increased osmolality and ionic strength on the mechanism of Ca2+ release were examined in mechanically skinned skeletal muscle fibres of the toad at 23 degrees C. Ca2+ release was induced by depolarizing the transverse tubular (T-) system by ionic substitution. 2. Increasing the osmolality of the 'myoplasmic' solution about four times (to 955 mosmol/kg), by addition of 700 mM sucrose to the standard potassium (K-)HDTA solution (HDTA: hexamethylenediamine-tetraacetate), only depressed the depolarization-induced response by about 46%. Much of this decrease could be attributed to a reduction in the Ca(2+)-sensitivity of the contractile proteins at this high osmolality. 3. Addition of > 400 mM sucrose itself often induced substantial Ca2+ release and a transient tension response. This 'spontaneous' release was (a) greatly enhanced when the sarcoplasmic reticulum (SR) had been heavily loaded with Ca2+, (b) little affected by inactivation of the voltage sensors by prolonged or permanent depolarization of the T-system and (c) blocked by Ruthenium Red (10 microM). 4. When both the osmolality and ionic strength were increased, by increasing the K-HDTA concentration, the depolarization-induced force was greatly reduced (to 35% at 818 mosmol/kg and 5% at 1095 mosmol/kg). Most of this reduction could be directly attributed to the substantially reduced maximum force and Ca2+ sensitivity of the contractile apparatus. 5. The small amount of releasable Ca2+ remaining in the SR after a single depolarization in a high-HDTA solution with 1 mM EGTA (to chelate the released Ca2+), indicated that depolarization could still elicit massive Ca2+ release at high ionic strength and osmolality (at 1 mM free Mg2+). 6. In contrast, when the total Mg2+ and ATP concentrations were raised about threefold (free [Mg2+] increased 2.7-fold) along with the osmolality and ionic strength, the ability of depolarization to elicit Ca2+ release was greatly hindered. 7. Osmotic compression of the skinned fibres to their in situ diameter by addition of 4% polyvinylpyrrolidone (PVP-40), substantially potentiated the depolarization-induced force responses, due partly to an increase in the Ca(2+)-sensitivity of the contractile apparatus. 8. These results indicate how increased intracellular osmolality, ionic strength and [Mg2+] produce the transient contraction and subsequent inhibition of tetanic tension in intact muscle fibres exposed to hypertonic solutions.

Intramembrane charge movements in frog skeletal muscle in strongly hypertonic solutions

Intramembrane charge movements were studied in intact, voltage-clamped frog (Rana temporaria) skeletal muscle fibers in external solutions made increasingly hypertonic by addition of sucrose. The marked dependence of membrane capacitance on test potential persisted with increases in extracellular sucrose concentration between 350 and 500 mM. Charge movements continued to show distinguishable early monotonic (q beta) decays and the strongly voltage-dependent delayed (q gamma) charging phases reported on earlier occasions. In contrast, a further increase to 600 mM sucrose abolished the most steeply voltage-sensitive part of the membrane capacitance. It left a more gradual variation with potential that closely resembled the function that resulted when q gamma charge was abolished by tetracaine in the presence of 500 mM sucrose. Charging transients were now simple monotonic (q beta) decays and lacked delayed (q gamma) transients. Furthermore, tetracaine (2 mM) altered neither the kinetic nor the steady-state features of the charge left in 600 mM sucrose. However, Ca2+ current activation in the same fibers persisted through such tonicity increases under identical conditions of temperature, external solution, and holding voltage. Tonicity changes thus accomplish an independent separation of q gamma and q beta charge as defined hitherto through their tetracaine sensitivity. Their effects on q gamma charge correlate with earlier observations of osmotic conditions on delta[Ca2+] signals (1987. J. Physiol. (Lond.) 383:615-627.) and the parallel effects of other agents on excitation-contraction coupling and q gamma charge. In contrast, they suggest that Ca2+ current activation does not require q gamma charge transfer whether by itself or as part of the excitation-contraction coupling process.

Minimal latency of calcium release in frog twitch muscle fibres

Intracellular release of calcium in frog skeletal muscle fibres was monitored by the use of arsenazo III, in response to voltage clamped depolarizing pulses. A latency of a few milliseconds was evident between the onset of depolarization and the first detectable rise in the arsenazo-calcium signal, and this decreased logarithmically as the depolarization was increased. The minimal latency with strong depolarization (to +20 to +100 mV) was about 2 ms at 5 degrees C. This delay appears to be sufficiently long to be compatible with a chemically mediated coupling mechanism between depolarization and calcium release from the sarcoplasmic reticulum.