Effects of calcium release from the sarcoplasmic reticulum on intramembrane charge movement in skeletal muscle

Alterations in calcium homeostasis reduce membrane excitability in amphibian skeletal muscle

The effects of alterations in intracellular calcium homeostasis on surface membrane excitability were investigated in resting Rana temporaria sartorius muscle. This was prompted by initial results from a fatiguing stimulation protocol study that demonstrated a fibre subpopulation in which action potential generation in response to a standard 1.5 V electrical stimulus failed despite mean membrane potentials [E (m), -69+/-2.3 mV (n=14)] compatible with spike firing in a control set of quiescent muscle fibres. Intracellular micro-electrode recordings showed a similar reversible loss of excitability, attributable to an increased threshold, despite only small (7.1+/-1.8 mV) positive changes in E (m) after approximately 60-min exposures to nominally 0 Ca(2+) Ringer solutions in which Ca(2+) was replaced by Mg(2+). This effect was not reproduced by addition of Mg(2+) to the Ringer solution and persisted under conditions of Cl(-) deprivation. The effects of three pharmacological agents, cyclopiazonic acid (CPA), caffeine and 4-chloro-m-cresol (4-CmC), each known to deplete store Ca(2+) and increase cytosolic Ca(2+) through contrasting mechanisms without influencing E (m), were then investigated. All three agents produced a more rapid, but nevertheless still reversible, loss of membrane excitability than in 0 Ca(2+) Ringer solution alone. This reduction in membrane excitability persisted in fibres studied in solutions containing a normal [Ca(2+)] following prior depletion of store Ca(2+) using CPA- and 4-CmC-containing solutions. These novel findings suggest that sarcoplasmic reticulum Ca(2+) content profoundly influences surface membrane excitability, thereby providing a potential mechanism by which spike firing fails in well-polarised fibres during fatigue.

Modulation of caffeine contractures in mammalian skeletal muscles by variation of extracellular potassium

Caffeine contractures were induced after K(+)-conditioning of skeletal muscles from pigs and mice. K(+)-conditioning is defined as the partial depolarization caused by increasing external potassium (K+0) with [K+]x[Cl-] constant. Conditioning depolarizations that rendered muscles refractory to brief electrical stimulation still enhanced the contracture tension elicited by subsequent direct caffeine stimulation of sarcoplasmic reticulum (SR) calcium release. The effects of K(+)-conditioning on caffeine-induced contractures of intact cell bundles reached a maximum at 15-30 mM K+0 and then progressively declined at higher [K+]0. Conditioning with 30 mM K+ for 5 min, which inactivates excitation-contraction (EC) coupling in response to action potentials, both increased the magnitude of caffeine contractures 2-10-fold and shifted the contracture threshold toward lower caffeine concentrations. Enhanced sensitivity to caffeine was inhibited by dantrolene (20 microM) and its watersoluble analogue azumolene (150 microM). These drugs decreased caffeine-induced contractures following depolarization with 4-15 mM K+ to 25-50% of control tension. The inorganic anion perchlorate (CIO-4), which like caffeine potentiates twitches, increased caffeine-induced contractures approximately twofold after K(+)-conditioning (> 4 mM). The results suggest that CIO-4 and dantrolene, in addition to caffeine, also influence SR calcium release either directly or by mechanism(s) subsequent to depolarization of the sarcolemma. Moreover, since CIO-4 is known to shift the voltage-dependence of intramembrane charge movement, CIO-4 may exert effects on the transverse-tubule voltage sensors as well as the SR.

Role of calcium feedback in excitation-contraction coupling in isolated triads

There is a considerable controversy in the literature concerning the effects of higher concentrations of calcium chelators (e.g. BAPTA (1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) or fura-2) on the intracellular Ca2+ transients in muscle. We induced calcium release from sarcoplasmic reticulum (SR) in the triad preparation by chemical depolarization of the T-tubule in the presence of various concentrations of BAPTA-calcium buffer ([Ca2+] = 0.1 microM) and investigated the effects of the BAPTA concentration on the time courses of conformational changes in the junctional foot protein (JFP) and calcium release from SR. Upon stimulation, the JFP underwent biphasic conformational changes, as determined by stopped-flow fluorometry of the JFP-bound conformational probe. The first phase of protein conformational change, which preceded calcium release from SR, was virtually unaffected by the BAPTA concentration. However, the magnitude of the second phase increased in an inversely proportional fashion to the BAPTA concentration. An abrupt increase in [Ca2+] from 0.1 microM up to 1.0 microM (delta Ca2+), concurrently with T-tubule depolarization, produced biphasic protein conformational changes: a delta Ca(2+)-independent first phase and a delta Ca(2+)-dependent second phase. Similar Ca2+ jump experiments under non-depolarizing conditions produced a slow monophasic conformational change equivalent to the second phase described above. These results suggest that the first phase of protein conformational change represents the activation of JFP by T-tubule depolarization to induce calcium release, and the second phase the secondary activation by the released Ca2+. Activation of the JFP by the released Ca2+ resulted in an acceleration of both (i) the rate of initial calcium release, and (ii) the subsequent attenuation of calcium release. The acceleration of both was suppressed by higher concentrations of BAPTA. These results provide a reasonable explanation for both of the apparently contradictory views in the literature; high concentrations of calcium buffer (a) suppress the initial activation and (b) prevent the subsequent attenuation of calcium release.