Simultaneous monitoring of changes in magnesium and calcium concentrations in frog cut twitch fibers containing antipyrylazo III

Magnesium modulates actin binding and ADP release in myosin motors

We examined the magnesium dependence of five class II myosins, including fast skeletal muscle myosin, smooth muscle myosin, β-cardiac myosin (CMIIB), Dictyostelium myosin II (DdMII), and nonmuscle myosin IIA, as well as myosin V. We found that the myosins examined are inhibited in a Mg(2+)-dependent manner (0.3-9.0 mm free Mg(2+)) in both ATPase and motility assays, under conditions in which the ionic strength was held constant. We found that the ADP release rate constant is reduced by Mg(2+) in myosin V, smooth muscle myosin, nonmuscle myosin IIA, CMIIB, and DdMII, although the ADP affinity is fairly insensitive to Mg(2+) in fast skeletal muscle myosin, CMIIB, and DdMII. Single tryptophan probes in the switch I (Trp-239) and switch II (Trp-501) region of DdMII demonstrate these conserved regions of the active site are sensitive to Mg(2+) coordination. Cardiac muscle fiber mechanic studies demonstrate cross-bridge attachment time is increased at higher Mg(2+) concentrations, demonstrating that the ADP release rate constant is slowed by Mg(2+) in the context of an activated muscle fiber. Direct measurements of phosphate release in myosin V demonstrate that Mg(2+) reduces actin affinity in the M·ADP·Pi state, although it does not change the rate of phosphate release. Therefore, the Mg(2+) inhibition of the actin-activated ATPase activity observed in class II myosins is likely the result of Mg(2+)-dependent alterations in actin binding. Overall, our results suggest that Mg(2+) reduces the ADP release rate constant and rate of attachment to actin in both high and low duty ratio myosins.

Paying the piper: the cost of Ca2+ pumping during the mating call of toadfish

Superfast fibres of toadfish swimbladder muscle generate a series of superfast Ca(2+) transients, a necessity for high-frequency calling. How is this accomplished with a relatively low rate of Ca(2+) pumping by the sarcoplasmic reticulum (SR)? We hypothesized that there may not be complete Ca(2+) saturation and desaturation of the troponin Ca(2+) regulatory sites with each twitch during calling. To test this, we determined the number of regulatory sites by measuring the concentration of troponin C (TNC) molecules, 33.8 μmol per kg wet weight. We then estimated how much SR Ca(2+) is released per twitch by measuring the recovery oxygen consumption in the presence of a crossbridge blocker, N-benzyl-p-toluene sulphonamide (BTS). The results agreed closely with SR release estimates obtained with a kinetic model used to analyse Ca(2+) transient measurements. We found that 235 μmol of Ca(2+) per kg muscle is released with the first twitch of an 80 Hz stimulus (15(o)C). Release per twitch declines dramatically thereafter such that by the 10th twitch release is only 48 μmol kg(-1) (well below the concentration of TNC Ca(2+) regulatory sites, 67.6 μmol kg(-1)). The ATP usage per twitch by the myosin crossbridges remains essentially constant at ∼25 μmol kg(-1) throughout the stimulus period. Hence, for the first twitch, ∼80% of the energy goes into pumping Ca(2+) (which uses 1 ATP per 2 Ca(2+) ions pumped), but by the 10th and subsequent twitches the proportion is ∼50%. Even though by the 10th stimulus the Ca(2+) release per twitch has dropped 5-fold, the Ca(2+) remaining in the SR has declined by only ∼18%; hence dwindling SR Ca(2+) content is not responsible for the drop. Rather, inactivation of the Ca(2+) release channel by myoplasmic Ca(2+) likely explains this reduction. If inactivation did not occur, the SR would run out of Ca(2+) well before the end of even a 40-twitch call. Hence, inactivation of the Ca(2+) release channel plays a critical role in swimbladder muscle during normal in vivo function.

Calcium indicators and calcium signalling in skeletal muscle fibres during excitation-contraction coupling

During excitation-contraction coupling in skeletal muscle, calcium ions are released into the myoplasm by the sarcoplasmic reticulum (SR) in response to depolarization of the fibre's exterior membranes. Ca(2+) then diffuses to the thin filaments, where Ca(2+) binds to the Ca(2+) regulatory sites on troponin to activate muscle contraction. Quantitative studies of these events in intact muscle preparations have relied heavily on Ca(2+)-indicator dyes to measure the change in the spatially-averaged myoplasmic free Ca(2+) concentration (Δ[Ca(2+)]) that results from the release of SR Ca(2+). In normal fibres stimulated by an action potential, Δ[Ca(2+)] is large and brief, requiring that an accurate measurement of Δ[Ca(2+)] be made with a low-affinity rapidly-responding indicator. Some low-affinity Ca(2+) indicators monitor Δ[Ca(2+)] much more accurately than others, however, as reviewed here in measurements in frog twitch fibres with sixteen low-affinity indicators. This article also examines measurements and simulations of Δ[Ca(2+)] in mouse fast-twitch fibres. The simulations use a multi-compartment model of the sarcomere that takes into account Ca(2+)'s release from the SR, its diffusion and binding within the myoplasm, and its re-sequestration by the SR Ca(2+) pump. The simulations are quantitatively consistent with the measurements and appear to provide a satisfactory picture of the underlying Ca(2+) movements.

Role of calsequestrin evaluated from changes in free and total calcium concentrations in the sarcoplasmic reticulum of frog cut skeletal muscle fibres

Calsequestrin is a large-capacity Ca-binding protein located in the terminal cisternae of sarcoplasmic reticulum (SR) suggesting a role as a buffer of the concentration of free Ca in the SR ([Ca2+](SR)) serving to maintain the driving force for SR Ca2+ release. Essentially all of the functional studies on calsequestrin to date have been carried out on purified calsequestrin or on disrupted muscle preparations such as terminal cisternae vesicles. To obtain information about calsequestrin's properties during physiological SR Ca2+ release, experiments were carried out on frog cut skeletal muscle fibres using two optical methods. One - the EGTA-phenol red method - monitored the content of total Ca in the SR ([Ca(T)](SR)) and the other used the low affinity Ca indicator tetramethylmurexide (TMX) to monitor the concentration of free Ca in the SR. Both methods relied on a large concentration of the Ca buffer EGTA (20 mM), in the latter case to greatly reduce the increase in myoplasmic [Ca2+] caused by SR Ca2+ release thereby almost eliminating the myoplasmic component of the TMX signal. By releasing almost all of the SR Ca, these optical signals provided information about [Ca(T)](SR) versus [Ca2+](SR) as [Ca2+](SR) varied from its resting level ([Ca2+](SR,R)) to near zero. Since almost all of the Ca in the SR is bound to calsequestrin, this information closely resembles the binding curve of the Ca-calsequestrin reaction. Calcium binding to calsequestrin was found to be cooperative (estimated Hill coefficient = 2.95) and to have a very high capacity (at the start of Ca2+ release, 23 times more Ca was estimated to initiate from calsequestrin as opposed to the pool of free Ca in the SR). The latter result contrasts with an earlier report that only approximately 25% of released Ca2+ comes from calsequestrin and approximately 75% comes from the free pool. The value of [Ca2+](SR,R) was close to the K(D) for calsequestrin, which has a value near 1 mm in in vitro studies. Other evidence indicates that [Ca2+](SR,R) is near 1 mM in cut fibres. These results along with the known rapid kinetics of the Ca-calsequestrin binding reaction indicate that calsequestrin's properties are optimized to buffer [Ca2+](SR) during rapid, physiological SR Ca2+ release. Although the results do not entirely rule out a more active role in the excitation-contraction coupling process, they do indicate that passive buffering of [Ca2+](SR) is a very important function of calsequestrin.

Slowing effects of Mg2+ on contractile kinetics of skinned preparations of rat hearts depending on myosin heavy chain isoform content

The effects of changes in Mg2+ concentration on the kinetics of stretch activation were investigated on skinned rat heart preparations under maximal Ca2+ activation. Muscle strips of hyper- and hypothyroid rat hearts were investigated at 0.5 and 1 mM free Mg2+; the total ATP concentration was 8 mM which resulted in saturating MgATP2- concentrations above 5 mM. Preparations containing exclusively the cardiac alpha-myosin heavy chain (hyper- and hypothyroid atria, hyperthyroid ventricles) showed an acceleration of the kinetics of stretch activation by a factor of about 1.5 (P<0.01, paired t-test) when free Mg2+ was decreased from 1 to 0.5 mM. Conversely, preparations containing exclusively the beta-myosin heavy chain isoform showed only a small acceleration by a factor of 1.05 (P<0.05, paired t-test) under the same conditions. The fact that the Mg2+ sensitivity was dependent on the myosin heavy chain isoform excludes the possibility that Mg2+ exhibits only unspecific effects on contractile proteins. Several hypotheses for explaining the observed Mg2+ effects are discussed. The conditions used in our experiments might be close to the physiological situation and, thus, changes of Mg2+ concentration must be considered as possible factors modulating the contractile kinetics especially of atrial muscle tissue.

Simulation of calcium sparks in cut skeletal muscle fibers of the frog

Spark mass, the volume integral of Delta F/F, was investigated theoretically and with simulations. These studies show that the amount of Ca2+ bound to fluo-3 is proportional to mass times the total concentration of fluo-3 ([fluo-3T]); the proportionality constant depends on resting Ca2+ concentration ([Ca2+]R). In the simulation of a Ca2+ spark in an intact frog fiber with [fluo-3T] = 100 microM, fluo-3 captures approximately one-fourth of the Ca2+ released from the sarcoplasmic reticulum (SR). Since mass in cut fibers is several times that in intact fibers, both with similar values of [fluo-3T] and [Ca2+]R, it seems likely that SR Ca2+ release is larger in cut fiber sparks or that fluo-3 is able to capture a larger fraction of the released Ca2+ in cut fibers, perhaps because of reduced intrinsic Ca2+ buffering. Computer simulations were used to identify these and other factors that may underlie the differences in mass and other properties of sparks in intact and cut fibers. Our spark model, which successfully simulates calcium sparks in intact fibers, was modified to reflect the conditions of cut fiber measurements. The results show that, if the protein Ca2+-buffering power of myoplasm is the same as that in intact fibers, the Ca2+ source flux underlying a spark in cut fibers is 5-10 times that in intact fibers. Smaller source fluxes are required for less buffer. In the extreme case in which Ca2+ binding to troponin is zero, the source flux needs to be 3-5 times that in intact fibers. An increased Ca2+ source flux could arise from an increase in Ca2+ flux through one ryanodine receptor (RYR) or an increase in the number of active RYRs per spark, or both. These results indicate that the gating of RYRs, or their apparent single channel Ca2+ flux, is different in frog cut fibers--and, perhaps, in other disrupted preparations--than in intact fibers.

Intracellular EDTA mimics parvalbumin in the promotion of skeletal muscle relaxation

Parvalbumin (PA) is an intracellular Ca2+-binding protein found in some muscle and nerves. Its ability to bind Ca2+ and facilitate skeletal muscle relaxation is limited by its Mg2+ off-rate. EDTA serves as an "artificial" PA in that it exhibited similar rate constants for Mg2+ (3 s-1) and Ca2+ (0.7 s-1) dissociation at 10 degrees C. When introduced into frog skeletal muscle, EDTA increased the relaxation rate by approximately 2.7-fold, and with increasing tetanus duration, EDTA lost its ability to contribute to relaxation (and Ca2+ sequestration) at its Mg2+ off-rate. Intracellular EDTA recovered its ability to contribute to muscle relaxation and Ca2+ sequestration at its Ca2+ off-rate. Like PA, EDTA's contribution to muscle relaxation and Ca2+ sequestration was more clearly observed when the SR Ca-ATPase was inhibited. Introduction of EDTA into rat soleus muscle, which has low [PA], increased the relaxation rate in a manner that was analogous to the way in which PA facilitates relaxation of frog skeletal muscle. Thus intracellular EDTA serves as an effective mimic of PA, and its use should aid in our understanding of PA's function in muscle and nerve.

Cytoplasmic free concentrations of Ca2+ and Mg2+ in skeletal muscle fibers at rest and during contraction

This review summarizes estimates for cytoplasmic-free concentrations of Ca2+ ([Ca2+]i) and Mg2+ ([Mg2+]i) at rest and during contraction of skeletal muscles, from which substantial quantitative information about them has been accumulated. Although the estimates of resting [Ca2+]i in the literature widely differ, which is because of the variety of difficulties related to different methodologies used, recent studies suggest that estimates of resting [Ca2+]i of approximately 0.05-0.1 microM are likely to be correct. Following action potential propagation, the Ca2+ release from the sarcoplasmic reticulum causes a transient rise of [Ca2+]i (Ca2+ transient). The large peak amplitude and brief time course of the Ca2+ transients have been established only recently by studies with low-affinity Ca2+ indicators developed in the past decade. These technical improvements in [Ca2+]i measurements have made it possible to study relationships between [Ca2+]i and force in intact muscle fibers. In the second part of this review, various estimates of [Mg2+]i in the resting muscle are discussed. Relatively recent estimates of the [Mg2+]i level appear to be about 1.0 mM. Using the current knowledge of concentrations and reaction properties of intracellular Ca2+-Mg2+ binding sites, we constructed a model for dynamic Mg2+ movement following Ca2+ transients. The model predicts that with a train of action potentials, the sustained rise of [Ca2+]i produces an elevation of [Mg2+]i of about 200 microM.

Effect of sarcoplasmic reticulum (SR) calcium content on SR calcium release elicited by small voltage-clamp depolarizations in frog cut skeletal muscle fibers equilibrated with 20 mM EGTA

Cut muscle fibers from Rana temporaria (sarcomere length, 3.5-3.9 micro(m); 14-16 degreesC) were mounted in a double Vaseline-gap chamber and equilibrated with an external solution that contained tetraethyl ammonium- gluconate and an internal solution that contained Cs as the principal cation, 20 mM EGTA, and 0 Ca. Fibers were stimulated with a voltage-clamp pulse protocol that consisted of pulses to -70, -65, -60, -45, and -20 mV, each separated by 400-ms periods at -90 mV. The change in total Ca that entered into the myoplasm (Delta[CaT]) and the Ca content of the SR ([CaSR]) were estimated with the EGTA/phenol red method (Pape, P.C., D.-S. Jong, and W.K. Chandler. 1995. J. Gen. Physiol. 106:259-336). Fibers were stimulated with the pulse protocol, usually every 5 min, so that the resting value of [CaSR] decreased from its initial value of 1,700-2, 300 microM to values near or below 100 microM after 18-30 stimulations. Three main findings for the voltage pulses to -70, -65, and -60 mV are: (a) the depletion-corrected rate of Ca release (release permeability) showed little change when [CaSR] decreased from its highest level (>1,700 microM) to approximately 1,000 microM; (b) as [CaSR] decreased below 1,000 microM, the release permeability increased to a maximum level when [CaSR] was near 300 microM that was on average about sevenfold larger than the values observed for [CaSR] > 1,000 microM; and (c) as [CaSR] decreased from approximately 300 microM to <100 microM, the release permeability decreased, reaching half its maximum value when [CaSR] was approximately 110 microM on average. It was concluded that finding b was likely due to a decrease in Ca inactivation, while finding c was likely due to a decrease in Ca-induced Ca release.

Measurements of intracellular Mg2+ concentration in mouse skeletal muscle fibers with the fluorescent indicator mag-indo-1

Measurements of intracellular free magnesium concentration ([Mg2+]i) were performed on enzymatically isolated skeletal muscle fibers from mice, using the fluorescent ratiometric indicator mag-indo-1. An original procedure was developed to calibrate the dye response within the fibers: fibers were first permeabilized with saponin in the presence of a given extracellular magnesium concentration and were then embedded in silicone grease. The dye was then pressure microinjected into the saponin-permeabilized silicone-embedded fibers, and fluorescence was measured. The results show that for all tested [Mg2+], the value of the measured fluorescence ratio was higher than that found in aqueous solutions. Furthermore, the apparent binding curve that could be fit to the in vivo ratio data was shifted toward higher [Mg2+] by a factor of approximately 2. Using the in vivo calibration parameters, the mean resting [Mg2+]i was found to be 1.53 +/- 0.16 mM (n = 7). In an attempt to gain insight into the myoplasmic magnesium buffering capacity, we measured, together with mag-indo-1 fluorescence, the current elicited by the application of carbamylcholine (CCh) to the endplate of isolated fibers, in the presence of a high extracellular magnesium concentration. The results show that, under these conditions, a change in [Mg2+]i displaying a time course and amplitude qualitatively consistent with the CCh-induced inward current can be measured.

Steady state relation between cytoplasmic free Ca2+ concentration and force in intact frog skeletal muscle fibers

The steady state relation between cytoplasmic Ca2+ concentration ([Ca2+]i) and force was studied in intact skeletal muscle fibers of frogs. Intact twitch fibers were injected with the dextran-conjugated Ca2+ indicator, fura dextran, and the fluorescence signals of fura dextran were converted to [Ca2+]i using calibration parameters previously estimated in permeabilized muscle fibers (Konishi and Watanabe. 1995. J. Gen. Physiol. 106:1123-1150). In the first series of experiments, [Ca2+]i and isometric force were simultaneously measured during high K+ depolarization. Slow changes in [Ca2+]i and force induced by 15-30 mM K+ appeared to be in equilibrium, as instantaneous [Ca2+]i versus force plot tracked the common path in the rising and relaxation phases of K+ contractures. In the second series of experiments, 2,5-di-tert-butylhydroquinone (TBQ), an inhibitor of the sarcoplasmic reticulum Ca2+ pump, was used to decrease the rate of decline of [Ca2+]i after tetanic stimulation. The decay time courses of both [Ca2+]i and force were dose-dependently slowed by TBQ up to 5 micro M; the instantaneous [Ca2+]i- force relations were nearly identical at >/=1 micro M TBQ, suggesting that the change in [Ca2+]i was slow enough to reach equilibrium with force. The [Ca2+]i-force data obtained from the two types of experiments were consistent with the Hill curve using a Hill coefficient of 3.2-3.9 and [Ca2+]i for half activation (Ca50) of 1.5-1.7 micro M. However, if fura dextran reacts with Ca2+ with a 2.5-fold greater Kd as previously estimated from the kinetic fitting (Konishi and Watanabe. 1995. J. Gen. Physiol. 106:1123-1150), Ca50 would be 3.7-4.2 micro M. We also studied the [Ca2+]-force relation in skinned fibers under similar experimental conditions. The average Hill coefficient and Ca50 were estimated to be 3.3 and 1.8 microM, respectively. Although uncertainties remain about the precise levels of [Ca2+]i, we conclude that the steady state force is a 3rd to 4th power function of [Ca2+]i, and Ca50 is in the low micromolar range in intact frog muscle fibers, which is in reasonable agreement with results obtained from skinned fibers.

Regulation of mouse skeletal muscle L-type Ca2+ channel by activation of the insulin-like growth factor-1 receptor

We investigated the modulation of the skeletal muscle L-type Ca2+ channel/dihydropyridine receptor in response to insulin-like growth factor-1 receptor (IGF-1R) activation in single extensor digitorum longus muscle fibers from adult C57BL/6 mice. The L-type Ca2+ channel activity in its dual role as a voltage sensor and a selective Ca2+-conducting pore was recorded in voltage-clamp conditions. Peak Ca2+ current amplitude consistently increased after exposure to 20 ng/ml IGF-1 (EC50 = 5.6 +/- 1.8 nM). Peak IGF-1 effect on current amplitude at -20 mV was 210 +/- 18% of the control. Ca2+ current potentiation resulted from a shift in 13 mV of the Ca2+ current-voltage relationship toward more negative potentials. The IGF-1-induced facilitation of the Ca2+ current was not associated with an effect on charge movement amplitude and/or voltage distribution. These phenomena suggest that the L-type Ca2+ channel structures involved in voltage sensing are not involved in the response to the growth factor. The modulatory effect of IGF-1 on L-type Ca2+ channel was blocked by tyrosine kinase and PKC inhibitors, but not by a cAMP-dependent protein kinase inhibitor. IGF-1-dependent phosphorylation of the L-type Ca2+ channel alpha1 subunit was demonstrated by incorporation of [gamma-32P]ATP to monolayers of adult fast-twitch skeletal muscles. IGF-1 induced phosphorylation of a protein at the 165 kDa band, corresponding to the L-type Ca2+ channel alpha1 subunit. These results show that the activation of the IGF-1R facilitates skeletal muscle L-type Ca2+ channel activity via a PKC-dependent phosphorylation mechanism.

Regulation of mouse skeletal muscle L-type Ca2+ channel by activation of the insulin-like growth factor-1 receptor

We investigated the modulation of the skeletal muscle L-type Ca2+ channel/dihydropyridine receptor in response to insulin-like growth factor-1 receptor (IGF-1R) activation in single extensor digitorum longus muscle fibers from adult C57BL/6 mice. The L-type Ca2+ channel activity in its dual role as a voltage sensor and a selective Ca2+-conducting pore was recorded in voltage-clamp conditions. Peak Ca2+ current amplitude consistently increased after exposure to 20 ng/ml IGF-1 (EC50 = 5.6 +/- 1.8 nM). Peak IGF-1 effect on current amplitude at -20 mV was 210 +/- 18% of the control. Ca2+ current potentiation resulted from a shift in 13 mV of the Ca2+ current-voltage relationship toward more negative potentials. The IGF-1-induced facilitation of the Ca2+ current was not associated with an effect on charge movement amplitude and/or voltage distribution. These phenomena suggest that the L-type Ca2+ channel structures involved in voltage sensing are not involved in the response to the growth factor. The modulatory effect of IGF-1 on L-type Ca2+ channel was blocked by tyrosine kinase and PKC inhibitors, but not by a cAMP-dependent protein kinase inhibitor. IGF-1-dependent phosphorylation of the L-type Ca2+ channel alpha1 subunit was demonstrated by incorporation of [gamma-32P]ATP to monolayers of adult fast-twitch skeletal muscles. IGF-1 induced phosphorylation of a protein at the 165 kDa band, corresponding to the L-type Ca2+ channel alpha1 subunit. These results show that the activation of the IGF-1R facilitates skeletal muscle L-type Ca2+ channel activity via a PKC-dependent phosphorylation mechanism.

Dual actions of tetracaine on intramembrane charge in amphibian striated muscle

1. The effects of graded concentrations of tetracaine on the steady-state and kinetic properties of intramembrane charge were examined in intact voltage-clamped amphibian muscle fibres. 2. The micromolar tetracaine concentrations that were hitherto reported to abolish Ca2+ transients in skeletal muscle failed to affect significantly the steady-state charge. Maximal reductions of such intramembrane charge required relatively high, 1-2 mM, concentrations of tetracaine. 3. The plots of maximum charge against tetracaine concentration suggested a saturable 1:1 drug binding that spared a fixed amount of tetracaine-resistant (q beta) charge but inhibited a discrete fraction of susceptible (q gamma) charge with a KD between 0.1 and 0.2 mM. 4. The q beta charge thus isolated by 2 mM tetracaine was conserved through a wide range of applied test voltages and pulse durations and regardless of whether the imposed transition from the holding potential (-90 mV) to the test potential took place in one or more steps. 5. Similarly, 'on' and 'off' q beta currents that were elicited by voltage steps from fixed conditioning to varying test levels mapped onto non-linear phase-plane trajectories that nevertheless depended uniquely upon voltage. In contrast, the currents that followed voltage steps made from varying prepulse levels to fixed -90 or -20 mV test potentials mapped onto identical q beta phase-plane trajectories that were independent of the prepulse history. 6. The charge movements that followed strong depolarizing voltage clamp steps to test potentials in the range -50 to 0 mV approximated simple monotonic decays that could empirically be described by a single time constant. Nevertheless, a complete inhibition of a tetracaine-sensitive (q gamma) charge movement by 2 mM tetracaine that left only q beta charge, sharply altered both the magnitude and the voltage dependence of these time constants. This establishes a distinct contribution of the q gamma species to overall charge kinetics even at such test voltages. 7. Under such a criterion for the voltage dependence of charging kinetics, even the micromolar (0.05-0.2 mM) tetracaine concentrations that failed to markedly alter the steady-state charge consistently increased the charging time constants yet did not influence their voltage sensitivity. 8. These findings demonstrate the existence of separate kinetic and steady-state effects of tetracaine on intramembrane charge movements, at micromolar and millimolar anaesthetic concentrations, respectively. These parallel earlier effects of tetracaine that have been reported upon the transient and sustained components of sarcoplasmic reticular Ca2+ release. They also establish that maximally effective concentrations of tetracaine isolate a single distinct species of conserved (q beta) intramembrane charge.

Properties of tri- and tetracarboxylate Ca2+ indicators in frog skeletal muscle fibers

Recently a number of lower-affinity fluorescent Ca2+ indicators have become available with principal absorbance bands at visible wavelengths. This article evaluates these indicators, as well as two shorter wavelength indicators, mag-fura-5 and mag-indo-1, for their suitability as rapid Ca2+ indicators in frog skeletal muscle fibers. With three lower-affinity tricarboxylate indicators (mag-fura-5, mag-indo-1, and magnesium orange), the change in fluorescence in response to an action potential (delta F) appeared to track the myoplasmic Ca2+ transient (delta[Ca2+]) without delay. With three lower-affinity tetracarboxylate indicators (BTC, calcium-orange-5N, and calcium-green-5N) and one tricarboxylate indicator (magnesium green), delta F responded to delta[Ca2+] with a small delay. Unfortunately, with the tetracarboxylate indicators, other problems were detected that appear to limit their usefulness as reliable Ca2+ indicators. Surprisingly, delta F from mag-fura-red, another tricarboxylate indicator, was biphasic (with 480 nm excitation), a feature that also greatly limits its usefulness. With several of the indicators, estimates were obtained for the myoplasmic value of KD, Ca (the indicator's dissociation constant for Ca2+) and found to be elevated severalfold in comparison with the value measured in a simple salt solution. These and other problems related to the quantitative use of Ca2+ indicators in the intracellular environment are evaluated and discussed.

Kinetic separation of charge movement components in intact frog skeletal muscle

1. Procedures for a complete charge movement separation employed a combination of its steady-state inactivation and activation properties in intact frog skeletal muscle fibres in gluconate-containing solutions. 2. Holding potential shifts from -70 to -50 mV reduced the total charge available between -90 and -20 mV from 16.76 +/- 1.70 nC microF-1 (mean +/- S.E.M.; n = 4 fibres) to 9.25 +/- 1.43 nC microF-1 without significant loss of tetracaine-resistant charge (q beta). 3. The steady-state and kinetic properties of tetracaine-sensitive charge (q gamma) persisted through holding potential changes from -90 to -70 mV in the presence of gluconate and generally resembled activation properties established hitherto in sulphate-containing solutions. 4. Further holding potential displacement to -50 mV abolished q gamma charge movements and depressed the charge-voltage curve. 5. Test voltage steps applied from a -70 mV prepulse level gave rapid monotonic q beta decays and similarly depressed activation functions in 2 mM tetracaine unchanged by holding potential shifts between -70 and -50 mV. 6. The isolated 'on' q gamma charge movements, I(t), always included early transients that preceded any prolonged charging phases and which increased with depolarization. They decayed to stable baselines in the absence of prolonged time-dependent or inward-current phases and yielded integrals, Q(t), that monotonically increased with test voltage. 7. 'Off' steps always elicited rapid monotonic q gamma decays that fully returned the 'on' charge. 8. 'On' and 'off' q gamma currents, I(t), following voltage steps from fixed conditioning to varying test levels mapped onto topologically distinct higher-order phase-plane trajectories, I(Q), that steeply varied with test voltage. 9. In contrast, voltage steps to fixed test potentials of either -70 or -20 mV elicited identical q gamma phase-plane trajectories independent of prepulse history. 10. The q gamma current thus reflects an independent, capacitative process driven uniquely by higher-order dependences upon charge distribution, Q(t), and test voltage, V(t), autonomous of prepulse history or time, t.

Relaxation, [Ca2+]i and [Mg2+]i during prolonged tetanic stimulation of intact, single fibres from mouse skeletal muscle

1. In skeletal muscle there is generally a slowing of relaxation with increasing tetanus duration and it has been suggested that this is due to Ca2+ loading of parvalbumin (PA). To study this we have produced prolonged tetani in intact, single fibres from a mouse foot muscle which contain a high concentration of PA. We measured the rate of tension relaxation and also various aspects of Ca2+ handling. 2. During 'interrupted' tetani (15 repeated cycles of 100 ms with stimulation and 50 ms without) we observed a marked slowing of the relaxation both under control conditions and in acidosis (obtained by increasing the bath CO2 content). This slowing was not accompanied by any reduction of the initial rate of decline of the free myoplasmic Ca2+ concentration ([Ca2+]i), which was measured with indo-1. 3. The functioning of the sarcoplasmic reticulum (SR) pump after tetani of various durations was analysed by plotting d[Ca2+]i/dt vs. [Ca2+]i during the final slow decline of [Ca2+]i after tetani. This analysis showed that the rate of SR Ca2+ pumping after a 1 s tetanus is less than half of that after a 100 ms tetanus. 4. The amplitude of the tail of [Ca2+]i 250 ms into relaxation was measured after tetani of various durations. This amplitude increased with tetanus duration and could be fitted to the sum of one exponential and one linear function. The exponential component increased with a time constant of 0.17 s and probably reflects Ca2+ loading of PA. 5. Ca2+ binding to PA will displace Mg2+ and hence the free myoplasmic concentration of Mg2+ ([Mg2+]i) will increase. To study this we used the fluorescent Mg2+ indicator furaptra. The results showed an increase of [Mg2+]i during prolonged tetani which, after removing the Ca2+ component of the fluorescent signal, amounted to about 0.5 mM. 6. A model of Ca2+ movements and tension production in skeletal muscle was used. The model showed that the increase of the amplitude of [Ca2+]i tails after tetani of various durations can be explained by the combined effect of Ca2+ loading of PA and slowed SR Ca2+ pumping. In contrast to the experimental data, the model predicted a slight reduction of the initial rate of [Ca2+]i decline with increased tetanus duration. 7. In conclusion, we observed a marked slowing of relaxation during prolonged tetanic stimulation. Altered Ca2+ handling, including Ca2+ loading of PA, seems not to be important for this slowing. Thus, the slowing appears to be due to altered cross-bridge kinetics.

Charge conservation in intact frog skeletal muscle fibres in gluconate-containing solutions

1. The conservation of intramembrane charge was investigated in intact voltage-clamped frog skeletal muscle fibres under conditions that minimized time-dependent ionic currents and so facilitated precise determination of capacitative charge. 2. Prolonged (q gamma) transients were demonstrated in 3,4-diaminopyridine and tetraethyl-ammonium gluconate-containing low [Ca2+] solutions in response to 125 ms pulses that explored the voltage range -90 to -20 mV. The tetracaine-sensitive, q gamma, component then accounted for a significant proportion (over 50%) of available charge. 3. Both delayed 'on' q gamma currents and 'off' current tails decayed to steady direct current (DC) baselines without significant residual ionic current slopes in the chosen extracellular solutions. This suggested that the current transients represented capacitative decays. It was also compatible with the precise determination of effective charge by integration. 4. The advent of 'on' q gamma current was accompanied by increased 'off' charge. Thus, charge was conserved through all 'on' and 'off' steps and through test voltages that extended from the threshold appearance of q gamma as a slow transient to its full merger with the earlier q beta decay at stronger depolarizations. 5. Charge conservation persisted through a wide range of 'on' pulse durations between 60 and 370 ms and was therefore independent of the interval following the q gamma decay. 6. The quantity of q gamma charge remained a monotonic single-valued function of test voltage, whether this potential was reached directly from the -90 mV holding potential or following a prepulse to -10 mV. 7. These findings suggest that the q gamma charge movement represents the electrical signature of an intramembrane entity whose transitions are primarily driven by, and therefore conserved with, the steady-state potential.

Ca transients in cardiac myocytes measured with a low affinity fluorescent indicator, furaptra

Intracellular calcium ion ([Ca2+]i) transients were measured in single rat ventricular myocytes with the fluorescent indicator furaptra. Cells were voltage clamped with a single patch electrode containing the K+ salt of furaptra and fluorescence at 500 nm was measured during illumination with 350 and 370 nm light. Depolarizing voltage-clamp pulses elicited [Ca2+]-dependent fluorescent transients in 30 of 33 cells tested. The peak change in [Ca2+]i elicited by 50-ms depolarizations from -70 to +10 mV was 1.52 +/- 0.25 microM (mean +/- SEM, n = 7). The size of the [Ca2+]i transient increased in response to 10 microM isoproterenol, prolongation of the depolarization, and increasing pipette [Na+]. Because furaptra is sensitive to Ca2+ and Mg2+, changes in [Mg2+]i during the [Ca2+]i transient could not be measured. Instead, a single-compartment model was developed to simulate changes in [Mg2+] during [Ca2+] transients. The simulations predicted that a 2 microM [Ca2+] transient was accompanied by a slow increase in [Mg2+] (14-29 microM), which became larger as basal [Mg2+] increased (0.5-2.0 mM). The [Mg2+] transient reached a peak approximately 1 s after the peak of the [Ca2+] transient with the slow changes in [Mg2+] dominated by competition at the Ca2+/Mg2+ sites of Troponin. These changes in [Mg2+], however, were so small and slow that they were unlikely to affect the furaptra fluorescence signal at the peak of the [Ca2+]i transient. The [Ca2+]i transient reported by furaptra appears to be larger than that reported by other Ca2+ indicators; however, we conclude this larger transient is at least as accurate as [Ca2+]i transients reported by the other indicators.

Calcium transients in single mammalian skeletal muscle fibres

1. We studied the transient changes in myoplasmic Ca2+ concentration under current- and voltage-clamp (double Vaseline-gap technique) in cut fibres of rat extensor digitorum longus muscle using mag-fura-2 (furaptra) as Ca2+ indicator, at 3.6-3.8 microns sarcomere length and 17 degrees C. Mag-fura-5 and fura-2 were also used in order to characterize some aspects of the Ca2+ transients. 2. The peak [Ca2+] in response to a single action potential was 4.6 +/- 0.4 microM (n = 5). The time to peak of the Ca2+ transient was 4.6 +/- 0.42 ms, with half-width of 8.2 +/- 1.5 ms, time constant of the rising phase 1.15 +/- 0.25 ms, time constant of the decaying phase 3.26 +/- 0.65 ms, and delay between action potential and Ca2+ transient 2.0 +/- 0.2 ms. 3. Ca2+ transients were studied under voltage-clamp conditions at different voltages and pulse durations. The rising phase showed a complex temporal course with a fast initial increase and a second component. Both components were separated by a plateau or a brief decrease of the Ca2+ concentration. The peak Ca2+ transient was 10.5 +/- 1.3 microM (n = 22). 4. After interrupting the pulse, Ca2+ concentration decayed exponentially. The time constant of decay of the Ca2+ transient increased with the pulse voltage and duration, reaching a maximum value at potentials more positive than +10 mV and pulses longer than 200 ms. An analysis of the decaying phases of the Ca2+ transients suggests that only the removal process operates after fibre repolarization. 5. The rate of Ca2+ release from the sarcoplasmic reticulum was calculated using the Melzer, Ríos & Schneider model. The value of 17.2 +/- 3.1 micronM ms-1 (n = 10) estimated in these calculations was intermediate between those obtained by other authors from cut frog muscles (10 microM ms-1) and intact frog fibres (100 microM ms-1) using antipyrylazo III (AP III) as the Ca2+ indicator.

Microinjection of strong calcium buffers suppresses the peak of calcium release during depolarization in frog skeletal muscle fibers

The effects of high intracellular concentrations of various calcium buffers on the myoplasmic calcium transient and on the rate of release of calcium (Rrel) from the sarcoplasmic reticulum (SR) were studied in voltage-clamped frog skeletal muscle fibers. The changes in intracellular calcium concentration (delta[Ca2+]) for 200-ms pulses to 0-20 mV were recorded before and after the injection of the calcium buffer and the underlying Rrel was calculated. If the buffer concentration after the injection was high, the initial rate of rise of the calcium transient was slower after injection than before and was followed by a slow increase of [Ca2+] that resembled a ramp. The increase in myoplasmic [Mg2+] that accompanies the calcium transient in control was suppressed after the injection and a slight decrease was observed instead. After the injection the buffer concentration in the voltage-clamped segment of the fiber decreased as the buffer diffused away toward the open ends. The calculated apparent diffusion coefficient for fura-2 (Dapp = 0.40 +/- 0.03 x 10(-6) cm2/s, mean +/- SEM, n = 6) suggests that approximately 65-70% of the indicator was bound to relatively immobile intracellular constituents. As the concentration of the injected buffer decreased, the above effects were reversed. The changes in delta[Ca2+] were underlined by characteristic modification of Rrel. The early peak component was suppressed or completely eliminated; thus, Rrel rose monotonically to a maintained steady level if corrected for depletion. If Rrel was expressed as percentage of SR calcium content, the steady level after injection did not differ significantly from that before. Control injections of anisidine, to the concentration that eliminated the peak of Rrel when high affinity buffers were used, had only a minor effect on Rrel, the peak was suppressed by 26 +/- 5% (mean +/- SE, n = 6), and the steady level remained unchanged. Thus, the peak component of Rrel is dependent on a rise in myoplasmic [Ca2+], consistent with calcium-induced calcium release, whereas the steady component of Rrel is independent of myoplasmic [Ca2+].

Role of parvalbumin in relaxation of frog skeletal muscle

Experiments were done to test the hypothesis that parvalbumin (PA) promotes relaxation in frog skeletal muscle. Single fibers and purified PA from R. temporaria skeletal muscle were used to determine the relationship between Ca2+ and Mg2+ dissociation rates from PA and changes in relaxation rate as a function of isometric tetanus duration at 0 degrees C. Relaxation rate slows as a function of tetanus duration with a rate constant of 1.18 s-1. Recovery of relaxation rate after a prolonged tetanus exhibits a rate constant of 0.12 s-1. Dissociation rate constants for Mg2+ and Ca2+ from purified PA are 0.93 s-1 and 0.19 s-1, respectively. Thus rates of slowing and recovery of relaxation rate may be controlled by Mg2+ and Ca2+ dissociation from PA, respectively. The influence of temperature on relaxation rate and on Ca2+ and Mg2+ dissociation rates from purified PA also was examined. The magnitude of slowing of relaxation rate with increasing tetanus duration, relative to the final, steady value of relaxation rate, is greater at 0 than at 10 degrees C. In the 0 to 10 degrees C range, the Q10 for relaxation rate increases with increasing tetanus duration. Both of these observations can be explained if the Q10 for Ca2+ uptake by the sarcoplasmic reticulum is greater than the Q10 for Ca2+ sequestration by PA during relaxation. When Ca2+ and Mg2+ dissociation rates from PA at various temperatures are compared to other proposed indicators of PA function, it is concluded that PA facilitates relaxation of frog skeletal muscle in the 0 to 20 degrees C range.

Effect of temperature on relaxation rate and Ca2+, Mg2+ dissociation rates from parvalbumin of frog muscle fibres

1. Influence of temperature on relaxation rate as a function of isometric tetanus duration and on Ca2+ and Mg2+ dissociation rates from purified parvalbumin (PA) was examined to test the hypothesis that PA promotes relaxation in frog skeletal muscle. Single fibres and PA IVB from Rana temporaria skeletal muscle were utilized. 2. The magnitude of slowing of relaxation rate with increasing tetanus duration, relative to the final, steady value of relaxation rate, is 3-fold greater at O than at 10 degrees C. 3. In the 0-10 degrees C range, the Q10 for relaxation rate increases from 2.3 to 3.7 with increasing tetanus duration. 4. Dissociation of Ca2+ and Mg2+ from PA exhibited: (i) rate constants of 1.03 +/- 0.03 s-1 (mean +/- S.D., n = 5) and 3.42 +/- 0.14 s-1 (n = 5) at 20 degrees C and (ii) Q10 values of 2.3 and 1.9 in the 0-20 degrees C range, respectively. 5. Time courses of slowing of relaxation rate with increasing tetanus duration and recovery of relaxation rate with rest after a prolonged tetanus at 10 degrees C are similar to rates of dissociation of Mg2+ and Ca2+ from PA, respectively, as previously reported at 0 degree C. 6. Both the temperature dependence of the relative magnitude of slowing of relaxation rate and the increased Q10 of relaxation rate with increased tetanus duration can be explained if the Q10 for Ca2+ uptake by the sarcoplasmic reticulum is greater than the Q10 for Ca2+ sequestration by PA during relaxation. When Ca2+ and Mg2+ dissociation rates from PA at various temperatures are compared to other proposed indicators of PA function, it is concluded that PA facilitates relaxation of frog skeletal muscle in the 0-20 degrees C range.

Ca(2+)-dependence of structural changes in troponin-C in demembranated fibers of rabbit psoas muscle

The Ca(2+)-dependence of structural changes in troponin-C (TnC) has been detected by monitoring the fluorescence from TnC labeled at Methionine-25, in the NH2-terminal domain, with danzylaziridine (TnC-DANZ) and then exchanged for endogenous TnC in glycerinated single fibers. The fluorescence-pCa relation obtained from fibers stretched to a sarcomere length greater than 4.0 microns evidenced two transitions: a small one, attributable to the binding of Ca2+ to the high affinity, Ca(2+)-Mg(2+)-binding sites of TnC; and a large one, attributable to the binding of Ca2+ to the low affinity, Ca(2+)-specific binding sites of TnC. In the fluorescence-pCa relation determined with fibers set to a sarcomere length of 2.4 microns, hence obtained in the presence of cycling cross-bridges, the large transition had the same Ca(2+)-dependence as did the development of tension. These results indicate that the NH2-terminal globular domain of TnC is modified by the binding of Ca2+ to sites located in both globular domains and that the structural changes in TnC resulting from the binding of Ca2+ to the low-affinity sites, but not to the high-affinity sites, are directly associated with the triggering of contraction.

Increased calcium influx in dystrophic muscle

We examined pathways which might result in the elevated resting free calcium [( Ca2+]i) levels observed in dystrophic mouse (mdx) skeletal muscle fibers and myotubes and human Duchenne muscular dystrophy myotubes. We found that mdx fibers, loaded with the calcium indicator fura-2, were less able to regulate [Ca2+]i levels in the region near the sarcolemma. Increased calcium influx or decreased efflux could lead to elevated [Ca2+]i levels. Calcium transient decay times were identical in normal and mdx fibers if resting [Ca2+]i levels were similar, suggesting that calcium-sequestering mechanisms are not altered in dystrophic muscle, but are slowed by the higher resting [Ca2+]i. The defect appears to be specific for calcium since resting free sodium levels and sodium influx rates in the absence of Na+/K(+)-ATPase activity were identical in normal and dystrophic cells when measured with sodium-binding benzofuran isophthalate. Calcium leak channels, whose opening probabilities (Po) were voltage independent, could be the major calcium influx pathway at rest. We have shown previously that calcium leak channel Po is significantly higher in dystrophic myotubes. These leak channels were selective for calcium over sodium under physiological conditions. Agents that increased leak channel activity also increased [Ca2+]i in fibers and myotubes. These results suggest that increased calcium influx, as a result of increased leak channel activity, could result in the elevated [Ca2+]i in dystrophic muscle.

Parvalbumin content and Ca2+ and Mg2+ dissociation rates correlated with changes in relaxation rate of frog muscle fibres

1. Experiments were done to test the hypothesis that parvalbumin (PA) promotes relaxation in frog skeletal muscle. Single fibres and purified PA from Rana temporaria skeletal muscle were used to determine the relationship between PA concentration ( [PA] ), Ca2+ and Mg2+ dissociation rates from PA and changes in rate of relaxation as a function of tetanus duration at 0 degrees C. 2. Total [PA] in fibres from tibialis anterior muscles is 0.76 +/- 0.20 mmol PA l-1 myoplasmic water (mean +/- S.D., n = 25) with 65% PA IVa and 35% PA IVb, where PA IVa and PA IVb are PA isoforms. 3. Relaxation rate from an isometric tetanus shows progressively as a function of tetanus duration with an exponential time course and a rate constant of 1.18 +/- 0.35 s-1 (n = 17). Time course of recovery of relaxation rate after a prolonged tetanus is exponential with a rate constant of 0.12 +/- 0.02 s-1 (n = 14). 4. The extent of recovery of relaxation rate after a prolonged tetanus was correlated with total [PA] in fibres (correlation coefficient (r) = 0.80, n = 7; P less than 0.05). 5. Dissociation rate constants for Mg2+ and Ca2+ from purified PA are 0.93 +/- 0.02 s-1 (n = 5) and 0.19 +/- 0.01 s-1 (n = 5), respectively. Dissociation rate constants were not significantly different for PA isoforms IVa and IVb. These rate constants are similar to the rate constants determined for the time courses of slowing and recovery of relaxation rate, respectively. 6. Results suggest that the time courses of slowing and recovery of relaxation rate may be controlled, to a large extent, by Mg2+ and Ca2+ dissociation from PA, respectively. This evidence supports a role for PA in facilitating relaxation during a tetanus in frog skeletal muscle at 0 degrees C.

Decline of myoplasmic Ca2+, recovery of calcium release and sarcoplasmic Ca2+ pump properties in frog skeletal muscle

1. The two calcium indicators Antipyrylazo III (AP III) and Fura-2 were used simultaneously to monitor free myoplasmic [Ca2+] in voltage-clamped cut segments of frog skeletal muscle fibres (8-10 degrees C). Antipyrylazo III was used for the relatively large [Ca2+] transients during 100-200 ms depolarizing pulses to -20 to 0 mV and for the rapid decline of [Ca2+] during the 200 ms after the pulses. Fura-2 was used to follow the slow decline of the small remaining elevation of [Ca2+] during the following 16 s (slow recovery period) and to monitor resting [Ca2+]. 2. From 1 to 16 s of the slow recovery period [Ca2+] declined with two exponential components, having time constants of 1.9 +/- 0.3 and 13.5 +/- 1.5 s (these and all other values are means +/- S.E.M. of eleven runs from seven fibres). At 1.2 s after the end of the pulses the amplitudes of the fast and slow exponential components of decline of [Ca2+] were 34 +/- 7 and 31 +/- 4 nM, respectively. The resting [Ca2+] in these runs was 40 +/- 4 nM. 3. The time course of calcium bound to parvalbumin [( Ca-Parv]) was calculated from the [Ca2+] records using literature values for the parvalbumin kinetic constants. From 1 to 16 s of the slow recovery period the total calcium [Ca]T outside the sarcoplasmic reticulum (SR) was assumed to equal [Ca-Parv] + [Ca-Fura]. During this period [Ca]T declined with two exponential components having time constants of 1.7 +/- 0.2 and 14.2 +/- 1.4 s, the same as those for [Ca2+]. Assuming the total concentration of parvalbumin cation binding sites to be 1000 microM, the fast and slow components of [Ca]T had amplitudes of 117 +/- 21 and 147 +/- 16 microM, respectively, at 1.2 s after the pulses. 4. The rate of decline of [Ca]T, -d[Ca]T/dt, was used as a measure of the net rate of removal of calcium from the myoplasm by the SR. From 3 to 16 s of the slow recovery period and in the resting fibre -d[Ca]T/dt varied with [Ca2+] according to A[Ca2+]n-L. The term A[Ca2+]n represents the pump rate and L represents a constant rate of calcium leak from the SR. 5. For 40 nM less than or equal to [Ca2+] less than or equal to 80 nM, the power n for the [Ca2+] dependence of pump rate was 3.9 +/- 0.6.(ABSTRACT TRUNCATED AT 400 WORDS)

Myoplasmic calcium transients in intact frog skeletal muscle fibers monitored with the fluorescent indicator furaptra

Furaptra (Raju, B., E. Murphy, L. A. Levy, R. D. Hall, and R. E. London. 1989. Am. J. Physiol. 256:C540-C548) is a "tri-carboxylate" fluorescent indicator with a chromophore group similar to that of fura-2 (Grynkiewicz, G., M. Poenie, and R. Y. Tsien. 1985. J. Biol. Chem. 260:3440-3450). In vitro calibrations indicate that furaptra reacts with Ca2+ and Mg2+ with 1:1 stoichiometry, with dissociation constants of 44 microM and 5.3 mM, respectively (16-17 degrees C; ionic strength, 0.15 M; pH, 7.0). Thus, in a frog skeletal muscle fiber stimulated electrically, the indicator is expected to respond to the change in myoplasmic free [Ca2+] (delta[Ca2+]) with little interference from changes in myoplasmic free [Mg2+]. The apparent longitudinal diffusion constant of furaptra in myoplasm was found to be 0.68 (+/- 0.02, SEM) x 10(-6) cm2 s-1 (16-16.5 degrees C), a value which suggests that about half of the indicator was bound to myoplasmic constituents of large molecular weight. Muscle membranes (surface and/or transverse-tubular) appear to have some permeability to furaptra, as the total quantity of indicator contained within a fiber decreased after injection; the average time constant of the loss was 302 (+/- 145, SEM) min. In fibers containing less than 0.5 mM furaptra and stimulated by a single action potential, the calibrated peak value of delta[Ca2+] averaged 5.1 (+/- 0.3, SEM) microM. This value is about half that reported in the preceding paper (9.4 microM; Konishi, M., and S. M. Baylor. 1991. J. Gen. Physiol. 97:245-270) for fibers injected with purpurate-diacetic acid (PDAA). The latter difference may be explained, at least in part, by the likelihood that the effective dissociation constant of furaptra for Ca2+ is larger in vivo than in vitro, owing to the binding of the indicator to myoplasmic constituents. The time course of furaptra's delta[Ca2+], with average values (+/- SEM) for time to peak and half-width of 6.3 (+/- 0.1) and 9.5 (+/- 0.4) ms, respectively, is very similar to that of delta[Ca2+] recorded with PDAA. Since furaptra's delta[Ca2+] can be recorded at a single excitation wavelength (e.g., 420 nm) with little interference from fiber intrinsic changes, movement artifacts, or delta[Mg2+], furaptra represents a useful myoplasmic Ca2+ indicator, with properties complementary to those of other available indicators.

Ca2+ and activation mechanisms in skeletal muscle

It has been known for a number of years that calcium ions play a crucial role in excitation-contraction (e-c) coupling (Sandow, 1952). The majority of the calcium required for this process is derived, at least in vertebrate striated muscle fibres, from discrete intracellular stores located at sites within the cell: the terminal cysternae (tc)/junctional SR of the sarcoplasmic reticulum (SR) (Fig. 1 a). These storage sites not only form a compartment that is distinct from the sarcoplasm of the fibre, but they are also closely associated with the contractile elements, the myofibrils. The SR release sites are activated following the spread of electrical activity (Huxley and Taylor, 1958) along the transverse (T) tubular system (Eisenberg and Gage, 1967; Adrian et al. 1969a, b; Peachey, 1973) from the surface membrane (Bm).

Changes in tetanic and resting [Ca2+]i during fatigue and recovery of single muscle fibres from Xenopus laevis

1. Single muscle fibres were dissected from the toe muscles of Xenopus laevis and microinjected with Fura-2 to measure myoplasmic calcium concentration ([Ca2+]i). Injected fibres were illuminated at 340 and 380 nm and the ratio of the resulting fluorescence at 505 nm (the Fura-2 ratio) was taken as a measure of [Ca2+]i. Fibres were fatigued at 21 degrees C by repeated tetani until developed tension had fallen to 50% of control. 2. Tetanic tension declined monotonically during fatiguing stimulation, whereas the tetanic Fura-2 ratio first increased and then declined. At the 10th tetanus, tension was 87% of control whereas the Fura-2 ratio was 106% of control. At the end of fatiguing stimulation, where tension was around 50% of control, the tetanic Fura-2 ratio was reduced to 71%. The rate of decline of both tension and the Fura-2 ratio after a tetanus slowed during fatigue. During recovery, the tension and the tetanic Fura-2 ratio recovered in parallel. 3. The resting Fura-2 ratio increased throughout fatigue reaching 237% of control when tension had declined to 50%. There was a rapid phase of recovery, complete within 1 min, by which time the resting Fura-2 ratio was 198% of control. Subsequent recovery was slower and took 20-30 min to reach a stable level which was 121% of control. 4. The resting Fura-2 ratio towards the end of fatiguing stimulation was greater than the tetanic Fura-2 ratio in the early part of recovery although there was no detectable increase of resting tension during fatiguing stimulation. This observation suggests that the Ca2+ sensitivity of the contractile proteins was reduced at the end of fatiguing stimulation. 5. Plots of the tetanic tension against tetanic Fura-2 ratios throughout fatiguing stimulation and recovery also suggested that Ca2+ sensitivity was reduced during fatiguing stimulation when compared to recovery. 6. The increases in resting [Ca2+]i caused by raised [K+]o (from 2.5 to 10 mM) and/or by application of 15% CO2 were much less than those produced by fatiguing stimulation. Much of the elevated [Ca2+]i in fatigue could be reversed by application of dantrolene (25 microM). 7. The results suggest that both reduced tetanic [Ca2+]i and reduced Ca2+ sensitivity contribute to the decline of tension during fatigue.(ABSTRACT TRUNCATED AT 400 WORDS)

Myoplasmic calcium transients monitored with purpurate indicator dyes injected into intact frog skeletal muscle fibers

Intact single twitch fibers from frog muscle were studied on an optical bench apparatus after microinjection with tetramethylmurexide (TMX) or purpurate-3,3' diacetic acid (PDAA), two compounds from the purpurate family of absorbance Ca2+ indicators previously used in cut muscle fibers (Maylie, J., M. Irving, N. L. Sizto, G. Boyarsky, and W. K. Chandler. 1987. J. Gen. Physiol. 89:145-176; Hirota, A., W. K. Chandler, P. L. Southwick, and A. S. Waggoner. 1989. J. Gen. Physiol. 94:597-631.) The apparent longitudinal diffusion constant of PDAA (mol wt 380) in myoplasm was 0.99 (+/- 0.04, SEM) x 10(-6) cm2 s-1 (16-17 degrees C), a value which suggests that 24-43% of the PDAA molecules were bound to myoplasmic constituents of large molecular weight. The corresponding values for TMX (mol wt 322) were 0.98 (+/- 0.05) x 10(-6) cm2 s-1 and 44-50%, respectively. Muscle membranes (surface and/or transverse-tubular) appear to be permeable to TMX and, to a lesser extent, to PDAA, since the total amount of indicator contained within a fiber decreased with time after injection. The average time constants for disappearance of indicator were 46 (+/- 7, SEM) min for TMX and 338 (+/- 82) min for PDAA. The fraction of indicator in the Ca2(+)-bound state in resting fibers was significantly different from zero for TMX (0.070 +/- 0.008) but not for PDAA (0.026 +/- 0.009). In in vitro calibrations PDAA but not TMX appeared to react with Ca2+ with 1:1 stoichiometry. In agreement with Hirota et al. (Hirota, A., W. K. Chandler, P. L. Southwick, and A. S. Waggoner. 1989. J. Gen. Physiol. 94:597-631), we conclude that PDAA is probably a more reliable myoplasmic Ca2+ indicator than TMX. In fibers that contained PDAA and were stimulated by a single action potential, the calibrated peak value of the myoplasmic free [Ca2+] transient (delta[Ca2+]) averaged 9.4 (+/- 0.6) microM, a value about fivefold larger than that calibrated with antipyrylazo III under otherwise identical conditions (Baylor, S. M., and S. Hollingworth. 1988. J. Physiol. 403:151-192). The fivefold difference is similar to that previously reported in cut fibers with antipyrylazo III and PDAA. Since in both intact and cut fibers the percentage of PDAA bound to myoplasmic constituents is considerably smaller than that found for antipyrylazo III, the PDAA calibration of delta[Ca2+] is likely to be more accurate. Interestingly, in intact fibers the peak value of delta[Ca2+] calibrated with either PDAA or antipyrylazo III is about half that calibrated in cut fibers.(ABSTRACT TRUNCATED AT 400 WORDS)

Intracellular free magnesium in frog skeletal muscle studied with a new type of magnesium-selective microelectrode: interactions between magnesium and sodium in the regulation of [Mg]i

The application of a new type of intracellular magnesium-selective microelectrode based on the neutral carrier ETH5214 to measure intracellular free magnesium ([Mg]i) in frog skeletal muscle fibers is reported. At room temperature (18-20 degrees C) the average values for [Mg]i was 0.93 mmol/l (pMgi = 3.03 +/- 0.42, SD; n = 38 experiments). The regulation of [Mg]i was studied by measuring [Mg]i and [Na]i with ion-selective microelectrodes during alterations of the membrane potential and the transmembrane sodium and magnesium gradients. Depolarization by increasing external [K] from 2.5 mmol/l to 12.5 mmol/l did not significantly influence [Mg]i. Increasing extracellular [Mg] from 1 mmol/l to 10 and 20 mmol/l caused a concentration-dependent rise in [Mg]i and a decrease in [Na]i, whereas removal of external magnesium did not affect [Mg]i. Removal of external [Na] caused an increase in [Mg]i and a decrease of [Na]i. The results show that [Mg]i in frog skeletal muscle is not in thermodynamic equilibrium and suggest that a Na/Mg exchange mechanism may be involved in maintaining low levels of [Mg]i.

Intracellular diffusion in the presence of mobile buffers. Application to proton movement in muscle

Junge and McLaughlin (1987) derived an expression for the apparent diffusion constant of protons in the presence of both mobile and immobile buffers. Their derivation applies only to cases in which the values of pH are considerably greater than the largest pK of the individual buffers, a condition that is not expected to hold in skeletal muscle or many other cell types. Here we show that, if the pH gradients are small, the same expression for the apparent diffusion constant of protons can be derived without such constraints on the values of the pK's. The derivation is general and can be used to estimate the apparent diffusion constant of any substance that diffuses in the presence of both mobile and immobile buffers. The apparent diffusion constant of protons is estimated to be 1-2 x 10(-6) cm2/s at 18 degrees C inside intact frog twitch muscle fibers. It may be smaller inside cut fibers, owing to a reduction in the concentration of mobile myoplasmic buffers, so that in this preparation a pH gradient, if established within a sarcomere following action potential stimulation, could last 10 ms or longer after stimulation ceased.