Characteristics of Na(+)-Ca2+ exchange in frog skeletal muscle

Ryanodine receptor modulation by caffeine challenge modifies Na+ current properties in intact murine skeletal muscle fibres

We investigated effects of the ryanodine receptor (RyR) modulator caffeine on Na⁺ current (I_Na) activation and inactivation in intact loose-patch clamped murine skeletal muscle fibres subject to a double pulse procedure. I_Na activation was examined using 10-ms depolarising, V₁, steps to varying voltages 0–80 mV positive to resting membrane potential. The dependence of the subsequent, I_Na inactivation on V₁ was examined by superimposed, V₂, steps to a fixed depolarising voltage. Current-voltage activation and inactivation curves indicated that adding 0.5 and 2 mM caffeine prior to establishing the patch seal respectively produced decreased (within 1 min) and increased (after ~2 min) peak I_Na followed by its recovery to pretreatment levels (after ~40 and ~30 min respectively). These changes accompanied negative shifts in the voltage dependence of I_Na inactivation (within 10 min) and subsequent superimposed positive activation and inactivation shifts, following 0.5 mM caffeine challenge. In contrast, 2 mM caffeine elicited delayed negative shifts in both activation and inactivation. These effects were abrogated if caffeine was added after establishing the patch seal or with RyR block by 10 μM dantrolene. These effects precisely paralleled previous reports of persistently (~10 min) increased cytosolic [Ca²⁺] with 0.5 mM, and an early peak rapidly succeeded by persistently reduced [Ca²⁺] likely reflecting gradual RyR inactivation with ≥1.0 mM caffeine. The latter findings suggested inhibitory effects of even resting cytosolic [Ca²⁺] on I_Na. They suggest potentially physiologically significant negative feedback regulation of RyR activity on Na_v1.4 properties through increased or decreased local cytosolic [Ca²⁺], Ca²⁺-calmodulin and FKBP12.

Role of sarcoplasmic reticulum Ca2+ content in Ca2+ entry of bovine airway smooth muscle cells

Depletion of intracellular Ca(2+) stores induces the opening of an unknown Ca(2+ )entry pathway to the cell. We measured the intracellular free-Ca(2+) concentration ([Ca(2+)]i) at different sarcoplasmic reticulum (SR) Ca(2+) content in fura-2-loaded smooth muscle cells isolated from bovine tracheas. The absence of Ca(2+) in the extracellular medium generated a time-dependent decrement in [Ca(2+)]i which was proportional to the reduction in the SR-Ca(2+) content. This SR-Ca(2+) level was indirectly determined by measuring the amount of Ca(2+) released by caffeine. Ca(2+) restoration at different times after Ca(2+)-free incubation (2, 4, 6 and 10 min) induced an increment of [Ca(2+)]i. This increase in [Ca(2+)]i was considered as Ca(2+) entry to the cell. The rate of this entry was slow (~0.3 nM/s) when SR-Ca(2+) content was higher than 50% (2 and 4 min in Ca(2+)-free medium), and significantly ( p<0.01) accelerated (>1.0 nM/s) when SR-Ca(2+) content was lower than 50% (6 and 10 min in Ca(2+)-free medium). Thapsigargin significantly induced a higher rate of this Ca(2+) entry ( p<0.01). Variations in Ca(2+) influx after SR-Ca(2+) depletion were estimated more directly by a Mn(2+) quench approach. Ca(2+) restoration to the medium 4 min after Ca(2+) removal did not modify the Mn(2+) influx. However, when Ca(2+) was added after 10 min in Ca(2+)-free medium, an increment of Mn(2+) influx was observed, corroborating an increase in Ca(2+) entry. The fast Ca(2+) influx was Ni(2+) sensitive but was not affected by other known capacitative Ca(2+) entry blockers such as La(3+), Mg(2+), SKF 96365 and 2-APB. It was also not affected by the blockage of L-type Ca2(+) channels with methoxyverapamil or by the sustained K(+)-induced depolarisation. The slow Ca(2+) influx was only sensitive to SKF 96365. In conclusion, our results indicate that in bovine airway smooth muscle cells Ca(2+) influx after SR-Ca(2+) depletion has two rates: A) The slow Ca(2+) influx, which occurred in cells with more than 50% of their SR-Ca(2+) content, is sensitive to SKF 96365 and appears to be a non-capacitative Ca(2+) entry; and B) The fast Ca(2+) influx, observed in cells with less than 50% of their SR-Ca(2+) content, is probably a capacitative Ca(2+) entry and was only Ni(2+)-sensitive.

Caffeine-induced depolarization in amphibian skeletal muscle fibres: role of Na+/Ca2+ exchange and K+ release

Caffeine (4 mM) produces a depolarization of about 10 mV in frog muscle fibres (Leptodactylus ocellatus). The aim of this work was to study the mechanisms of this effect. An approximately threefold rise in membrane resistance [Cl--free (SO(4)2-) medium] substantially increased, and both Na+-free medium and Ni2+ (5 mM) reduced, the caffeine-induced depolarization. In voltage-clamped (-60 mV) short fibres from lumbricalis muscle of the toad (Buffo arenarum), caffeine generated an inward current of 4.13 +/- 0.48 microA cm(-2). This caffeine-induced current was reduced by 60% in Na+-free medium, 44% in the presence of 5 mM amiloride and 48% by 5 mM Ni2+, suggesting that the activation of the Na+-Ca2+ exchanger in its forward mode may play a role in the observed electrical effects of the drug. Caffeine also produced a marked release of K+. Net K+ efflux increased from 3.5 +/- 0.2 (control) to 22.1 +/- 2.3 pmol s(-1) cm(-2) (caffeine). It is shown that in the presence of the drug, [K+] in the lumen of the T tubules may well increase to levels which could produce, in part, both the observed depolarization and the caffeine-induced current under voltage clamp conditions. The caffeine-induced K+ efflux was not reduced by 5 mM Ni2+. At a holding potential of 30 mV the caffeine-induced current was reversed (outward) and roughly halved by 5 mM Ni2+. The Ni2+-sensitive fraction of the caffeine-induced current, assumed to represent the Na+-Ca2+ exchanger current, had an estimated reversal potential close to 12 mV ([Na+]o = 115 mM; [Ca2+]o = 1 mM). In conclusion, the depolarizing effect of caffeine described here would be produced by two mechanisms: (a) an inward current generated by the activation of the Na+-Ca2+ exchanger in its forward mode, and (b) the rise of the external [K+] in restricted spaces like the T tubules.

Dynamics and consequences of potassium shifts in skeletal muscle and heart during exercise

Since it became clear that K(+) shifts with exercise are extensive and can cause more than a doubling of the extracellular [K(+)] ([K(+)](s)) as reviewed here, it has been suggested that these shifts may cause fatigue through the effect on muscle excitability and action potentials (AP). The cause of the K(+) shifts is a transient or long-lasting mismatch between outward repolarizing K(+) currents and K(+) influx carried by the Na(+)-K(+) pump. Several factors modify the effect of raised [K(+)](s) during exercise on membrane potential (E(m)) and force production. 1) Membrane conductance to K(+) is variable and controlled by various K(+) channels. Low relative K(+) conductance will reduce the contribution of [K(+)](s) to the E(m). In addition, high Cl(-) conductance may stabilize the E(m) during brief periods of large K(+) shifts. 2) The Na(+)-K(+) pump contributes with a hyperpolarizing current. 3) Cell swelling accompanies muscle contractions especially in fast-twitch muscle, although little in the heart. This will contribute considerably to the lowering of intracellular [K(+)] ([K(+)](c)) and will attenuate the exercise-induced rise of intracellular [Na(+)] ([Na(+)](c)). 4) The rise of [Na(+)](c) is sufficient to activate the Na(+)-K(+) pump to completely compensate increased K(+) release in the heart, yet not in skeletal muscle. In skeletal muscle there is strong evidence for control of pump activity not only through hormones, but through a hitherto unidentified mechanism. 5) Ionic shifts within the skeletal muscle t tubules and in the heart in extracellular clefts may markedly affect excitation-contraction coupling. 6) Age and state of training together with nutritional state modify muscle K(+) content and the abundance of Na(+)-K(+) pumps. We conclude that despite modifying factors coming into play during muscle activity, the K(+) shifts with high-intensity exercise may contribute substantially to fatigue in skeletal muscle, whereas in the heart, except during ischemia, the K(+) balance is controlled much more effectively.

Sodium/calcium exchange in amphibian skeletal muscle fibers and isolated transverse tubules

The Na(+)/Ca(2+) exchanger participates in Ca(2+) homeostasis in a variety of cells and has a key role in cardiac muscle physiology. We studied in this work the exchanger of amphibian skeletal muscle, using both isolated inside-out transverse tubule vesicles and single muscle fibers. In vesicles, increasing extravesicular (intracellular) Na(+) concentration cooperatively stimulated Ca(2+) efflux (reverse mode), with the Hill number equal to 2.8. In contrast to the stimulation of the cardiac exchanger, increasing extravesicular (cytoplasmic) Ca(2+) concentration ([Ca(2+)]) inhibited this reverse activity with an IC(50) of 91 nM. Exchanger-mediated currents were measured at 15 degrees C in single fibers voltage clamped at -90 mV. Photolysis of a cytoplasmic caged Ca(2+) compound activated an inward current (forward mode) of 23 +/- 10 nA (n = 3), with an average current density of 0.6 muA/muF. External Na(+) withdrawal generated an outward current (reverse mode) with an average current density of 0.36 +/- 0.17 muA/muF (n = 6) but produced a minimal increase in cytosolic [Ca(2+)]. These results suggest that, in skeletal muscle, the main function of the exchanger is to remove Ca(2+) from the cells after stimulation.

Fast calcium removal during single twitches in amphibian skeletal muscle fibres

Fluorescence signals from the calcium sensitive dyes Fluo-3 or Rhod-2 were obtained simultaneously with isometric tension in single fibres isolated from the anterior tibialis muscle of Leptodactylus insularis (20-22 degrees C). Fluo-3 fluorescence signals were transformed into [Ca2+]i transients as previously described. Most of the decay phase of single twitch transient is well fitted by a single exponential (tau of about 10 ms), followed by a slower declining component lasting tens of milliseconds. During short periods, 10 to 20 s, of low frequency stimulation, between 0.2 and 5 Hz, the basal [Ca2+]i increased slowly from 0.1 to about 0.4 microM, with only minor changes in the exponentially decaying phase. In fibres poisoned with thapsigargin or cyclopiazonic acid (1-2 microM) the tau of decay of fluorescence or Ca2+ transients of single twitches was very similar to that observed in non-poisoned fibres. Nevertheless, in poisoned fibres challenged with repetitive stimulation. the tau of Ca2+ transients decay increased from about 10 ms to >40 ms, while the basal [Ca2+]i increased from 0.1 to 2 microM. Short rest periods (about 5 min) could reverse these effects, indicating that they were not a direct consequence of SR Ca 2+ -ATPase inhibition. The correlation coefficient between tau of decay and basal [Ca2+]i was >0.8 (P<0.0001). Qualitatively similar results were obtained measuring Rhod-2 fluorescence signals. A lumped, two-compartment model could account for these results. Loading the fibres with EGTA-AM, diminished the effects of prolonged stimulation observed in poisoned fibres. Moreover, we show that the Na+ - Ca2+ exchange mechanism does not participate appreciably in fast Ca2+ removal.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

Ni2+ transport by the human Na+/Ca2+ exchanger expressed in Sf9 cells

The mechanism of Ni2+ block of the Na+/Ca2+ exchanger was examined in Sf 9 cells expressing the human heart Na+/Ca2+ exchanger (NCX1-NACA1). As predicted from the reported actions of Ni2+, its application reduced extracellular Na+-dependent changes in intracellular Ca2+ concentration (measured by fluo 3 fluorescence changes). However, contrary to expectation, the reduced fluorescence was accompanied by measured 63Ni2+ entry. The 63Ni2+ entry was observed in Sf 9 cells expressing the Na+/Ca2+ exchanger but not in control cells. The established sequential transport mechanism of the Na+/Ca2+ exchanger could be compatible with these results if one of the two ion translocation steps is blocked by Ni2+ and the other permits Ni2+ translocation. We conclude that, because Ni2+ entry was inhibited by extracellular Ca2+ and enhanced by extracellular Na+, the Ca2+ translocation step moved Ni2+, whereas the Na+ translocation step was inhibited by Ni2+. A model is presented to discuss these findings.

Excitation-induced Ca2+ uptake in rat skeletal muscle

In isolated rat extensor digitorum longus (EDL) muscle mounted for isometric contractions, chronic low-frequency electrical stimulation was found to lead to an increased uptake of 45Ca (154% above control after 240 min) and a progressive accumulation of Ca2+ (85% above control after 240 min). In soleus, however, this treatment led to a small, but significant, increase in 45Ca uptake (30% above control after 180 min) but no significant accumulation of Ca2+. In muscles mounted for isotonic contractions without any external load, electrical stimulation gave rise to a larger 45Ca uptake and accumulation of Ca2+ in both EDL and soleus. These uptakes of Ca2+ coincided with an accumulation of Na+. During isometric or isotonic contractions, stimulation at 40 Hz increased the initial (60 s) rate of 45Ca uptake in soleus muscle 15- and 30-fold, respectively. The stimulation-induced increase in 45Ca uptake was only reduced by 17% by the Ca2+-channel blockers nifedipine and verapamil but was blocked by tetrodotoxin. The initial rate of stimulation-induced 22Na and 45Ca uptake was correlated (r = 0.80; P < 0.003). Stimulation of Na+ channels with veratridine increased 45Ca uptake by 93 and 139% in soleus and EDL, respectively (P < 0.001), effects that were abolished by tetrodotoxin. The results indicate that in skeletal muscle, excitation induces a considerable influx of Ca2+, mediated by Na+ channels.

Evidence for Na+/Ca2+ exchange in intact single skeletal muscle fibers from the mouse

The myoplasmic free Ca2+ concentration ([Ca2+]i) was measured in intact single fibers from mouse skeletal muscle with the fluorescent Ca2+ indicator indo 1. Some fibers were perfused in a solution in which the concentration of Na+ was reduced from 145.4 to 0.4 mM (low-Na+ solution) in an attempt to activate reverse-mode Na+/Ca2+ exchange (Ca2+ entry in exchange for Na+ leaving the cell). Under normal resting conditions, application of low-Na+ solution only increased [Ca2+]i by 5.8 +/- 1.8 nM from a mean resting [Ca2+]i of 42 nM. In other fibers, [Ca2+]i was elevated by stimulating sarcoplasmic reticulum (SR) Ca2+ release with caffeine (10 mM) and by inhibiting SR Ca2+ uptake with 2,5-di(tert-butyl)-1,4-benzohydroquinone (TBQ; 0.5 microM) in an attempt to activate forward-mode Na+/Ca2+ exchange (Ca2+ removal from the cell in exchange for Na+ influx). These two agents caused a large increase in [Ca2+]i, which then declined to a plateau level approximately twice the baseline [Ca2+]i over 20 min. If the cell was allowed to recover between exposures to caffeine and TBQ in a solution in which Ca2+ had been removed, the increase in [Ca2+]i during the second exposure was very low, suggesting that Ca2+ had left the cell during the initial exposure. Application of caffeine and TBQ to a preparation in low-Na+ solution produced a large, sustained increase in [Ca2+]i of approximately 1 microM. However, when cells were exposed to caffeine and TBQ in a low-Na+ solution in which Ca2+ had been removed, a sustained increase in [Ca2+]i was not observed, although [Ca2+]i remained higher and declined slower than in normal Na+ solution. This suggests that forward-mode Na+/Ca2+ exchange contributed to the fall of [Ca2+]i in normal Na+ solution, but when extracellular Na+ was low, a prolonged elevation of [Ca2+]i could activate reverse-mode Na+/Ca2+ exchange. The results provide evidence that skeletal muscle fibers possess a Na+/Ca2+ exchange mechanism that becomes active in its forward mode when [Ca2+]i is increased to levels similar to that obtained during contraction.

3-Nitropropionic acid increases the intracellular Ca2+ in cultured astrocytes by reverse operation of the Na+-Ca2+ exchanger

The effect of 3-nitropropionic acid (3-NPA) on intracellular calcium ([Ca2+]i) in cultured cortical and striatal astrocytes was examined using a calcium imaging technique with fura-2. 3-NPA (1.7 mM) increased the [Ca2+]i in cortical and striatal astrocytes. The latency for the onset of the rise in [Ca2+]i in cortical astrocytes was 22.7 +/- 0.8 min and was significantly longer in striatal astrocytes (39.2 +/- 2.4 min; P < 0.001). The maximal increase in [Ca2+]i for both astrocytes was seen after 50 min and remained at that level even after extensive washing. The maximal responses were about 125 and 140% of the initial values for striatal and cortical cells, respectively. Pretreatment (2-3 h) with creatine (25 mM) significantly delayed the onset of increase in [Ca2+]i by 3-NPA in cortical (39.8 +/- 3.7 min) and in striatal (57.8 +/- 2.5 min) astrocytes from the respective untreated cells (P < 0.05). However, the [Ca2+]i increase was similar to that of the untreated cells at 60 or 90 min. The increase in [Ca2+]i by 3-NPA was not observed in Ca2+-free or low-Na+ medium, but there was rather a 10-15% decrease under the Ca2+-free condition (P < 0.05). Superfusion of normal Ca2+ (2 mM) medium after exposure of the cells to 3-NPA in Ca2+-free medium increased the [Ca2+]i dramatically and reversibly. This increase was significantly attenuated in the presence of 2',4'-dichlorobenzamil (100 microM), nifedipine (10 microM), or Ni2+ (2 mM) or after exposure to amiloride (1 mM). The blockade was total in the case of 2',4'-dichlorobenzamil. The results indicate that the 3-NPA-induced increase in [Ca2+]i in astrocytes was due to an influx of Ca2+ by the reverse operation of the Na+-Ca2+ exchanger system, which may be responsible for the gliotoxic action of 3-NPA.