Excitation-contraction coupling in a barnacle muscle fiber as examined with voltage clamp technique

Voltage clamp methods for the study of membrane currents and SR Ca(2+) release in adult skeletal muscle fibres

Skeletal muscle excitation-contraction (E-C)(1) coupling is a process composed of multiple sequential stages, by which an action potential triggers sarcoplasmic reticulum (SR)(2) Ca(2+) release and subsequent contractile activation. The various steps in the E-C coupling process in skeletal muscle can be studied using different techniques. The simultaneous recordings of sarcolemmal electrical signals and the accompanying elevation in myoplasmic Ca(2+), due to depolarization-initiated SR Ca(2+) release in skeletal muscle fibres, have been useful to obtain a better understanding of muscle function. In studying the origin and mechanism of voltage dependency of E-C coupling a variety of different techniques have been used to control the voltage in adult skeletal fibres. Pioneering work in muscles isolated from amphibians or crustaceans used microelectrodes or 'high resistance gap' techniques to manipulate the voltage in the muscle fibres. The development of the patch clamp technique and its variant, the whole-cell clamp configuration that facilitates the manipulation of the intracellular environment, allowed the use of the voltage clamp techniques in different cell types, including skeletal muscle fibres. The aim of this article is to present an historical perspective of the voltage clamp methods used to study skeletal muscle E-C coupling as well as to describe the current status of using the whole-cell patch clamp technique in studies in which the electrical and Ca(2+) signalling properties of mouse skeletal muscle membranes are being investigated.

Temperature compensation in the escape response of a marine copepod, Calanus finmarchicus (Crustacea)

Calanus finmarchicus, the dominant mesozooplankter of the North Atlantic, is an important food source for many fishes and other planktivores. This species, which has limited diel vertical migration, depends on its fast-start escape response to evade predators. It has myelinated neuronal axons, which contribute to its rapid and powerful escape response. The thermal environment that C. finmarchicus inhabits ranges from below 0 degrees C to 16 degrees C. Previous studies have shown that respiration, growth, and reproductive rates are strongly dependent on temperature, with Q10 > 2.5. A comparable dependence of the escape response could place the animal at higher risk for cold-compensated predators. Our work focused on the temperature dependence of the behavioral response to stimuli that mimic predatory attacks. We found that in contrast to other biological processes, all aspects of the escape response showed a low dependence on temperature, with Q10 values below 2. This low temperature dependence was consistent for escape parameters that involved neural as well as muscle components of the behavioral response. These findings are discussed in the contexts of the predator-prey relations of copepods and the thermal dependence of behavior in other taxa.

Voltage-clamp analysis of membrane currents and excitation-contraction coupling in a crustacean muscle

A single fibre preparation from the extensor muscle of a marine isopod crustacean is described which allows the analysis of membrane currents and simultaneously recorded contractions under two-electrode voltage-clamp conditions. We show that there are three main depolarisation-gated currents, two are outward and carried by K+, the third is an inward Ca2+ current, I(Ca). Normally, the K+ currents which can be isolated by using K+ channel blockers, mask I(Ca). I(Ca) activates at potentials more positive than -40 mV, is maximal around 0 mV, and shows strong inactivation at higher depolarisation. Inactivation depends on current rather than voltage. Ba2+, Sr2+ and Mg2+ can substitute for Ca2+. Ba2+ currents are about 80% larger than Ca2+ currents and inactivate little. The properties of I(Ca) characterise it as a high threshold L-type current. The outward current consists primarily of a fast, transient A current, I(K(A)) and a maintained, delayed rectifier current, I(K(V)). In some fibres, a small Ca2+-dependent K+ current is also present. I(K(A)) activates fast at depolarisation above -45 mV, shows pronounced inactivation and is almost completely inactivated at holding potentials more positive than -40 mV. I(K(A)) is half-maximally blocked by 70 microM 4-aminopyridine (4-AP), and 70 mM tetraethylammonium (TEA). I(K(V)) activates more slowly, at about -30 mV, and shows no inactivation. It is half-maximally blocked by 2 mM TEA but rather insensitive to 4-AP. Physiologically, the two K+ currents prevent all-or-nothing action potentials and determine the graded amplitude of active electrical responses and associated contractions. Tension development depends on and is correlated with depolarisation-induced Ca2+ influx mediated by I(Ca). The voltage dependence of peak tension corresponds directly to the voltage dependence of the integrated I(Ca). The threshold potential for contraction is at about -38 mV. Peak tension increases with increasing voltage steps, reaches maximum at around 0 mV, and declines with further depolarisation.

A dihydropyridine-sensitive voltage-dependent calcium channel in the sarcolemmal membrane of crustacean muscle

Single-channel currents through calcium channels in muscle of a marine crustacean, the isopod Idotea baltica, were investigated in cell-attached patches. Inward barium currents were strongly voltage-dependent, and the channels were closed at the cell's resting membrane potential. The open probability (Po) increased e-fold for an 8.2 mV (+/- 2.4, n = 13) depolarization. Channel opening were mainly brief (< 0.3 ms) and evenly distributed throughout 100-ms pulses. Averaged, quasimacroscopic currents showed fast activation and deactivation and did not inactivate during 100-ms test pulses. Similarly, channel activity persisted at steadily depolarized holding potentials. With 200 mM Ba2+ as charge carrier, the average slope conductance from the unitary currents between +30 and +80 mV, was 20 pS (+/- 2.6, n = 12). The proportion of long openings, which were very infrequent under control conditions, was greatly increased by preincubation of the muscle fibers with the calcium channel agonist, the dihydropyridine Bay K8644 (10-100 microM). Properties of these currents resemble those through the L-type calcium channels of mammalian nerve, smooth muscle, and cardiac muscle cells.

Effect of pentachlorophenol on calcium accumulation in barnacle muscle cells

1. The effect of extracellularly applied pentachlorophenol (PCP) was studied on the membrane potential (Vm) and Ca2+ uptake in isolated single skeletal muscle cells of Balanus nubilus. 2. When compared with the controls, 0.1 mM PCP induced a significant (P < 0.05) increase in Ca2+ uptake accompanied by membrane depolarization (9 mV at 45 min incubation). This depolarization was reduced by 11% of extracellular Ca2+ (Cao2+) was replaced by Tris+ and by 50% if extracellular Na+ was also replaced by Tris+. 3. The Ca2+ channel blocker, verapamil (0.1 mM), completely inhibited the PCP-induced Ca2+ uptake as well as the membrane depolarization either in the absence or presence of Cao2+. Experiments on voltage-clamped cells show that the PCP-induced Ca2+ uptake was independent of the PCP-induced depolarization. 4. The results indicate that PCP induces activation of a verapamil-sensitive Ca2+ influx pathway (presumably L-type Ca2+ channels) independent of Vm. The permeation of Ca2+, Na+ and Tris+ through this pathway produces membrane depolarization in the following order of effectiveness: Ca2+ > Na+ > Tris+.

Barnacle muscle: Ca2+, activation and mechanics

In this review, aspects of the ways in which Ca2+ is transported and regulated within muscle cells have been considered, with particular reference to crustacean muscle fibres. The large size of these fibres permits easy access to the internal environment of the cell, allowing it to be altered by microinjection or microperfusion. At rest, Ca2+ is not in equilibrium across the cell membrane, it enters the cell down a steep electrochemical gradient. The free [Ca2+] at rest is maintained at a value close to 200 nM by a combination of internal buffering systems, mainly the SR, mitochondria, and the fixed and diffusible Ca(2+)-binding proteins, as well as by an energy-dependent extrusion system operating across the external cell membrane. This system relies upon the inward movement of Na+ down its own electrochemical gradient to provide the energy for the extrusion of Ca2+ ions. As a result of electrical excitation, voltage-sensitive channels for Ca2+ are activated and permit Ca2+ to enter the cell more rapidly than at rest. It has been possible to determine both the amount of Ca2+ entering by this step, and what part this externally derived Ca2+ plays in the development of force as well as in the free Ca2+ change. The latter can be determined directly by Ca(2+)-sensitive indicators introduced into the cell sarcoplasm. A combination of techniques, allowing both the total and free Ca2+ changes to be assessed during electrical excitation, has provided valuable information as to how muscle cells buffer their Ca2+ in order to regulate the extent of the change in the free Ca2+ concentration. The data indicate that the entering Ca2+ can only make a small direct contribution to the force developed by the cell. The implication here is that the major source of Ca2+ for contraction must be derived from the internal Ca2+ storage sites within the SR system, a view reinforced by caged Ca2+ methods. The ability to measure the free Ca2+ concentration changes within a single cell during activation has also provided the opportunity to analyse, in detail, the likely relations between free Ca2+ and the process of force development in muscle. The fact that the free Ca2+ change precedes the development of force implies that there are delays in the mechanism, either at the site of Ca2+ attachment on the myofibril, or at some later stage in the process of force development that were not previously anticipated.(ABSTRACT TRUNCATED AT 400 WORDS)

Ultrastructural investigations on the muscular systems in the barnacle, Tetraclita squamosa japonica

Four muscular systems of the Tetraclita squamosa barnacle were observed by means of an electron microscope and it was revealed that these systems each bore different types of muscle cells. The four systems were the adductor (A), the lateral scutal depressor (LSD), the ventral scutal depressor (VSD), and the tergal depressor (TD). The A-system included cross stiated muscle cells which showed long sarcomeres (about 10 mum) and rather disordered arrays of myofilaments. The LSD-system included cross striated muscles which had medium length sarcomeres (about 6.7 mum) and rather ordered myofilamental arrays. The VSD-system was constructed of cross striated muscle cells which bore shorter sarcomeres (4.6 mum) than the previous three systems and ordered myofilamental arrays. This last type of cell also bore well-developed sarcoplasmic reticular systems. The TD-system included smooth muscle cells which showed rather ordered arrays of myofilaments and dense-bodies. Each muscular system, as described above, included to its advantage one type of cross striated or smooth muscle cell for its characteristic contraction. The relations between ultrastructures and functions of each muscular system will now be discussed.

Gradient in excitation-contraction coupling in canine gastric antral circular muscle

Slow waves decay in amplitude as they propagate through the thickness of circular muscle of the canine antrum. Slow waves are the excitable events that initiate contractions in the antrum. Excitation-contraction coupling occurs if slow wave depolarizations surpass a 'mechanical threshold'. The amplitude of slow waves recorded from circular muscle cells near the submucosa was insufficient to reach the mechanical threshold previously determined for muscle near the myenteric plexus, suggesting that either submucosal cells are normally mechanically quiescent, or that contractions of submucosal cells are initiated at more polarized levels. Experiments were performed to determine the voltage-tension relationships in adjacent 'myenteric' and 'submucosal' circular muscles. Membrane potentials of the muscles were depolarized by elevated concentrations of potassium. Submucosal muscles were stimulated to contract at lower potassium concentrations than were myenteric muscles. Contractions of submucosal muscles at each potassium concentration studied were more forceful than contractions of myenteric muscles. Plots of membrane potential vs. potassium concentration on a logarithmic scale showed that the membrane potential of myenteric cells was more dependent upon the potassium gradient than the membrane potential of submucosal cells. The potassium permeability of both groups of cells increased when depolarized, and the slopes of these plots approached Nernstian levels when depolarized below -55 mV. Force developed in submucosal strips at more polarized levels than in myenteric muscles. The 'mechanical threshold' of submucosal muscles was 5-10 mV above resting potential, whereas myenteric muscles had to be depolarized by 25-30 mV before contraction was initiated. The mechanisms responsible for the difference in mechanical thresholds are not known, but differences in the voltage dependence of calcium channels, in calcium release mechanisms, or in the sensitivity of the contractile proteins to calcium could be involved.

Muscle calcium transient. Effect of post-stimulus length changes in single fibers

We examined the effects of post-stimulus length changes on voltage-clamped, aequorin-injected single muscle fibers from the barnacle Balanus nubilus. Extra light (extra calcium) is seen when the fiber is allowed to shorten (a small percentage) during the declining phase of the calcium transient. The opposite is observed when the fiber is stretched. Increasing the extent of shortening increases the amount of extra calcium, as does decreasing the temperature. The extra calcium probably comes from the myofilaments and not from the sarcoplasmic reticulum because (a) there is a strong correlation between the extra calcium and the level of activation; (b) there is a strong correlation between the extra calcium and the amount of force redeveloped after a length change; and (c) the time course of the appearance of the extra calcium is intermediate between that of the free calcium concentration and that of force. We suggest (a) that the calcium binding to the activating myofibrillar proteins is sensitive to muscle length or muscle force, and (b) that there is a pool of bound calcium (activating calcium) that waxes and wanes with a time course intermediate between the free calcium concentration and force.

Excitation-contraction coupling: role of K-activation within the transverse tubular system

Long-duration Ca action potentials induced in crustacean muscle fibers after prolonged exposure to quaternary ammonium ions are accompanied by attenuated tensions with unique time courses. The tensions have three phases. The initial phase, correlated with the upstroke of the spike, is a rapid increase in tension followed by relaxation to or near to resting level (on-tension). In the second phase, tension rises slowly as the spike plateau declines. The final phase is another rapid increase and decay in tension that is correlated with termination of the action potential (off-tension). To observe these tensions, fibers must be exposed to 50-100 mM tetrabutylammonium ion for about 1 hr or to lower concentrations for longer periods (e.g., 5 mM for 20-30 hr). To obtain a similar response in fibers treated with tetraethylammonium ion, higher concentrations or longer soaking periods, or both, are required. Because neither caffeine-induced tensions in intact fibers nor contractile protein and sarcoplasmic reticulum function in skinned fibers were modified by quaternary ammonium ions, their site of action appears to be limited to surface or transverse tubular system membranes, or both. The unique tensions can be explained by considering the mode by which quaternary ammonium ions block K channels in conjunction with a scheme in which activation of K channels within the transverse tubular system controls the driving force for influx of Ca ions.

Activation of the contractile system in crustacean muscle: ultrastructural evidence for the role of the T system

Freeze-fracture and thin sections of lobster abdominal fast flexor muscle were used to study the morphology of the sarcoplasmic reticulum (SR) and T system of crustacean muscle. Tannic acid mordanting, which can result in a dense black deposit in the T system lumen, was used to distinguish T system from SR membranes. Ferritin was also used as an extracellular tracer to confirm the tannic acid method. The T system consists of an extensive network of flattened sacs which fills most of the space between the myofibrils and is in close contact with them. The SR also appears as flattened sacs, sometimes with fenestrations. There is extensive junctional contact between the SR and T system. Quantitative estimates of the volume and surface area of the membranes show that the T system has about 50% more surface area than the SR. The intramembrane particle (IMP) density of the PF face of the T system is about 1100/micron 2 membrane, while the IMP density of the PF face of SR is about 4800/micron 2 membrane. In morphology, extent, and IMP density, the T system of lobster abdominal fast flexor muscle appears (AFF) adapted to provide at least part of the Ca2+ for muscle activation and the transport system for relaxation.

On the role of extracellular calcium in triggering contraction in muscle fibres from barnacle under membrane potential control

1. Single giant barnacle muscle fibres from Megabalanus psittacus (Darwin) were used to establish the possible role of extracellular Ca2+ in triggering the contraction. 2. Peak tension p0 during twitches elicited by depolarizing voltage clamp pulses decreases after the removal of Ca2+ from the external saline. p0 becomes too small to measure after a few minutes, when the model-estimated level of total intracellular Ca has been reduced to 99.5% of its original value and the extracellular Ca in the clefts and/or tubules to about 10%. 3. In fibres where the outward currents were blocked by internal perfusion with Cs+, the curves of the integral of the inward current versus membrane potential and of peak tension versus membrane potential follow a similar pattern. 4. When Ca2+ in the external saline was replaced by Ba2+ or Sr2+, no twitches were generated although depolarizing voltage clamp pulses induced large inward currents comparable to those measured when Ca2+ was present. 5. It is concluded that an increase in Ca2+ in the fibre during the Ca2+ inward current is a necessary condition for the triggering of the development of tension. The data on Ba2+ inhibition is consistent with the hypothesis that there are two different intracellular sites of action for Ca2+.

Calcium fluxes in single muscle fibres measured with a glass scintillator probe

1. An intracellular glass scintillator (Caldwell & Lea, 1973) has been used to obtain a continuous record of the influx of 45Ca into single muscle fibres of the barnacle, Balanus nubilus. 2. In the presence of intracellular EGTA (final concentration greater than 3 mM/kg), the scintillator detected an initial fast phase to the influx (half-time = 18.3 min, compartment size = 4.1% fibre volume) followed by a slow, linear phase which gave a value for the Ca influx of 1.2 p-mole . cm-2 . sec-1. The efflux of 45Ca was also measured with the scintillator by transferring a 45Ca-loaded fibre into 45Ca-free saline. Two exponential phases of efflux were detected with half-times of 16.2 and 500 min. 3. The characterisitics of the fast phase of the influx and efflux are similar to those of the influx of the impermeant sucrose and inulin, suggesting that the fast phase represents exchange with the extracellular 'cleft space'. This phase was insensitive to external La3+ (2 mM). 4. The slow phase is considered to represent the flux of Ca across the surface membrane. It was inhibited by external La3+ (2 mM) and stimulated by replacing external Na+ with Li+. 5. When EGTA-injected fibres were depolarized using an axial, intracellular electrode the Ca influx, measured from the slow phase, was increased. At higher concentrations of intracellular EGTA (6--22 mM/kg), the extra Ca influx due to a rectangular, depolarizing current pulse was proportional to the number of Ca spikes it produced. A single Ca spike gave an extra Ca influx of 19--48 p-mole . cm-2. External D600 (5 x 10(-4)M) inhibited both Ca spike and the extra Ca influx. 6. At lower intracellular EGTA concentrations (3.6--11 mM/kg), a 50 mV depolarization of 250 msec duration gave a mean extra Ca influx of 80 p-mole . cm-2. The upper value was 145 p-mole . cm-2 and this would increase the total internal Ca by 4.1 micrometer/kg. It is calculated that if all this extra Ca was bound to the myofibrillar sites for tension, it would only produce 6.2% of the force expected for a similar depolarization in a fibre with no intracellular EGTA.

Contractile activation phenomena in voltage-clamped barnacle muscle fiber

Tension development in voltage-clamped barnacle muscle fibers occurs with depolarizing pulses so small as not to activate the potassium and calcium conductance systems. Peak tension and the tension time integral appear to be graded by both amplitude and duration of the depolarizing pulses. Subthreshold depolarizing conditioning pulses shorter than 500 ms potentiate the response to a given test pulse. This effect diminishes and reverts when the duration of the conditioning pulse is increasingly prolonged. The relationship between fiber membrane potential and tension developed in response to depolarizing pulses is described by an S-shaped curve. The tension saturates at a membrane potential of about +10 mV (inside positive). For a given pulse duration the saturation value remains constant even when the fiber interior reaches a value of +230 mV, which is well above what may be estimated to be the equilibrium potential of calcium ions (Eca = +120). In the presence of 5 mM external procaine, the shape of the tension-potential curve changes; the maximum value tension besides being diminished is not sustained by falls when the potential approaches the estimated value for Eca. These results suggest that under physiological conditions the contractile activator is probably released from an internal store, and that the calcium entering the fiber as inward current does not play a direct major role in contractile activation.

Muscle activation: effects of small length changes on calcium release in single fibers

In single muscle fibers, small (1 percent) changes of length have a marked effect of both the calcium activation and the tension elicited by a constant current stimulus. The decrease in tension with shortening is accounted for almost entirely by a decrease in calcium release, rather than by changes in mechanical factors, such as filament geometry.

Calcium fluxes in internally dialyzed giant barnacle muscle fibers

Calcium-45 fluxes have been examined in isolated giant barnacle muscle fibers subjected to internal solute control by means of "internal dialysis". The 45Ca efflux was dependent upon the concentrations of both total and ionized internal Ca (Ca2+ buffered with EGTA). With a total Ca concentration of 2.0 mM and a 1:2 Ca/EGTA ratio (nominal [Ca2+]i =0.13 muM), the Ca efflux averaged 1.2 pmoles/cm2 sec. Under identical conditions, the mean Ca influx was only 0.36 pmoles/cm2 sec. The Ca efflux may not be attributed to leak of the CaEGTA complex, since a 2.5-fold increase in the EGTA concentration (nominal [Ca2+]i=0.032 muM) reduced the 45Ca efflux by one-third. Furthermore, when EDTA was used to buffer the internal Ca concentration (in the absence of internal Mg), the steady efflux of 14C-EDTA was only about 10% of the 45Ca efflux (in parallel experiments). The timecourse of the 45Ca fluxes also appeared anomalous in the 45Ca influx reached a steady level much more rapidly than 45Ca efflux in fibers of comparable diameters. If the muscle fibers are treated as right circular cylinders, these data imply that the apparent diffusion coefficient for inwardly-moving Ca is much larger than for outwardly-moving Ca. In contrast to Ca efflux, the outward diffusion of 22Na, 14C-EDTA and 3H2O appears to be limited primarily by the permeability of the dialysis tube wall. Some, but not all, of the anomalous behavior of the Ca fluxes can be reconciled if the deep, branched infoldings of the barnacle muscle surface membrane are taken into account.

Changes in intracellular free calcium concentration during illumination of invertebrate photoreceptors. Detection with aequorin

Aequorin, which luminesces in the presence of calcium, was injected into photoreceptor cells of Limulus ventral eye. A bright light stimulus elicited a large increase in aequorin luminescence, the aequorin response, indicating a rise of intracellular calcium ion concentration, Ca(i). The aequorin response reached a maximum after the peak of the electrical response of the photoreceptor, decayed during a prolonged stimulus, and returned to an undetectable level in the dark. Reduction of Ca(o) reduced the amplitude of the aequorin response by a factor no greater than 3. Raising Ca(o) increased the amplitude of the aequorin response. The aequorin response became smaller when membrane voltage was clamped to successively more positive values. These results indicate that the stimulus-induced rise of Ca(i) may be due in part to a light-induced influx of Ca and in part to release of Ca from an intracellular store. Our findings are consistent with the hypothesis that a rise in Ca(i) is a step in the sequence of events underlying light-adaptation in Limulus ventral photoreceptors. Aequorin was also injected into photoreceptors of Balanus. The aequorin responses were similar to those recorded from Limulus cells in all but two ways: (a) A large sustained aequorin luminescence was measured during a prolonged stimulus, and (b) removal of extracellular calcium reduced the aequorin response to an undetectable level.

Calcium influxes and tension development in perfused single barnacle muscle fibres under membrane potential control

1. Single giant barnacle muscle fibres from Megabalanus psittacus (Darwin) were used to measure the Ca entry and the development of tension in the fibres under membrane potential control.2. Fibres bathing in 60 mm-MgCl(2) sea water, free of Ca, did not develop tension with sudden displacements of the membrane potential towards more positive values. This failure to develop tension with depolarizations was observed with and without the internal application of Ca buffers.3. Fibres bathing in artificial sea water with either 10, 20, 60 or 100 mm-CaCl(2) developed tension with depolarization even after 60 min of internal perfusion of the fibres with solution containing no Ca buffers. In this case the maximum tension recorded during a voltage clamp run decreased with time from nearly 2.5 to 0.2 kg/cm(2). However, addition of 10 mm-Tris-EGTA (ethyleneglycol-bis (beta-aminoethyl ether) N, N' - tetraacetic acid) to the perfusing solution rapidly eliminated the development of tension; after 10 min of internal perfusion with Ca buffers no tension could be elicited by electrical stimulation.4. Ca-influx determinations were carried out only in the fibres in which the outward K(+) currents were blocked by internal application of TEA (tetra-ethylammonium). The ratio of ;measured extra Ca influx/computed ionic flux of divalent cations during the inward current' was 1.06 +/- 0.41.5. For fibres bathed in either natural sea water or in artificial sea water with various concentrations of Ca, the temporal course of development of isometric tension was similar to the temporal course of the integral of the inward current due to Ca(2+).6. In a fibre from M. psittacus bathing in natural sea water the calculated extra entry of Ca required to increase its internal concentration to about 50 mum was 500 p-mole per depolarization (60 mV); while the corresponding average influx calculated from the inward current record in natural sea water is 474 p-mole.7. Evidence was obtained for the accumulation of Ca in an internal compartment.

Calcium efflux from barnacle muscle fibers. Dependence on external cations

Calcium-45 was injected into single giant barnacle muscle fibers, and the rate of efflux was measured under a variety of conditions. The rate constant (k) for (45)Ca efflux into standard seawater averaged 17 x 10(-4) min(-1) which corresponds to an efflux of about 1-2 pmol/cm(2).s. Removal of external Ca (Ca(o)) reduced the efflux by 50%. In most fibers about 40% of the (45)Ca efflux into Ca-free seawater was dependent on external Na (Na(o)); treatment with 3.5 mM caffeine increased the magnitude of the Na(o)-dependent efflux. In a few fibers removal of Na(o), in the absence of Ca(o), either had no effect or increased k; caffeine (2-3.5 mM) unmasked an Na(o)-dependent efflux in these fibers. The Na(o)-dependent Ca efflux had a Q(10) of about 3.7. The data are consistent with the idea that a large fraction of the Ca efflux may be carrier-mediated, and may involve both Ca-Ca and Na-Ca counterflow. The relation between the Na(o)-dependent Ca efflux and the external Na concentration is sigmoid, and suggests that two, or more likely three, external Na(+) ions may activate the efflux of one Ca(+2). With a three-for-one Na-Ca exchange, the Na electrochemical gradient may be able to supply sufficient energy to maintain the Ca gradient in these fibers. Other, more complex models are not excluded, however, and may be required to explain some puzzling features of the Ca efflux such as the variable Na(o)-dependence.

Calcium and potassium systems of a giant barnacle muscle fibre under membrane potential control

1. Single barnacle muscle fibres from Megabalanus psittacus (Darwin) were internally perfused and the effects of various internal and external solutions on voltage clamp currents were examined.2. The usual internal solution was 180 mM-K(+) aspartate (osmotic pressure adjusted to 1000 m-osmole by adding sucrose). Fibres perfused with this solution gave an average resting potential of -55 +/- 5 mV (all potentials are referred to the external solutions as ground). Further increase in internal K concentration depolarized the fibres.3. With membrane current control the total capacitance, referred to apparent membrane surface area, was 21.2 +/- 2.4 muF/cm(2).4. Under normal conditions, with demonstrably good longitudinal space clamp control, voltage clamp currents associated with certain depolarizing pulses showed oscillations. These oscillations were reduced in frequency and magnitude by lowering the temperature from 20 to 10 degrees C, by eliminating the inward currents with external Ca-free saline or by reducing the outward currents with internal tetraethylammonium (TEA) or replacement of internal K by Cs.5. With a Na- and Ca-free, 60 mM-MgCl(2) solution outside depolarizing voltage clamp pulses produced only outward currents. On repolarization the current tail reversed direction at about -70 mV for pulses of less than 50 msec duration. For longer pulses this reversal potential was less negative, suggesting an accumulation of external, or depletion of internal K.6. Both the size of the outward currents and the rate at which they reached their maximum value increased with temperature. The activation energy for the rate constant was about 63 kJ/mole.7. Fibres bathed in Na- and Mg-free, 60 mM-CaCl(2) saline were excitable. After replacement of the internal K(+) with Cs(+) or adding 60 mM-TEA to the internal solution only sustained inward currents were recorded with depolarization.8. Sustained inward currents could be reduced by external application of 5 mM-LaCl(3). Tetrodotoxin was not effective even at a concentration of 1000 nM.9. The rate at which these inward currents reached a maximum value increased with increase in temperature of the bathing solution with an activation energy of the order of 42 kJ/mole.10. The reversal potential of the inward currents changed with the level of internal Ca ions. For a fibre perfused without ethyleneglycol-bis (beta-aminoethyl ether) N,N'-tetraacetic acid (EGTA) this reversal potential was 175 mV (internal free Ca 5 x 10(-7)M), and was 196 mV for a fibre perfused with 20 mM-Tris EGTA (internal free Ca 0.26 x 10(-8)M).11. We propose an electrical equivalent circuit to account for most of the observed electrical properties of barnacle muscle fibres. In this model the Ca and the K system are located at different anatomical places and they interact through a series resistance.

Ultrastructure of barnacle giant muscle fibers

Increasing use of barnacle giant muscle fibers for physiological research has prompted this investigation of their fine structure. The fibers are invaginated by a multibranched system of clefts connecting to the exterior and filled with material similar to that of the basement material of the sarcolemmal complex. Tubules originate from the surface plasma membrane at irregular sites, and also from the clefts They run transversely, spirally, and longitudinally, making many diadic and some triadic contacts with cisternal sacs of the longitudinal sarcoplasmic reticulum. The contacts are not confined to any particular region of the sarcomere. The tubules are wider and their walls are thicker at points of contact with Z material. Some linking of the Z regions occurs across spaces within the fiber which contain large numbers of glycogen particles. A-band lengths are extremely variable, in the range 2.2 microm-20.3 microm (average 5.2 microm) Individual thick filaments have thin (110 A) hollow regions alternating with thick (340 A) solid ones. Bridges between thick filaments occur at random points and are not concentrated into an M band The thin:thick filament ratio is variable in different parts of a fiber, from 3:1 to 6:1. Z bands are basically perforated, but the number of perforations may increase during contraction.

Potassium ion accumulation near a pace-making cell of Aplysia

1. A delayed current decrease associated with prolonged depolarization was studied in R(15) (the parabolic burster) of Aplysia by using intracellular recording and voltage clamp techniques.2. For long duration command pulses (3 sec), the outward current shows a delayed decrease. The current goes from a maximum near 100 msec and falls until a steady-state outward current is reached between 1.5 and 2.5 sec after the beginning of the command step. This final steady-state current is usually only about 20-30% of the peak outward current.3. Double step voltage clamps show that this current decrease is associated with a large shift of e.m.f. Measurements of conductance, on the other hand, fail to show any significant difference in conductance associated with peak and steady-state currents.4. From the results of application of high K(+) ringer, the conclusion is reached that this shift in e.m.f. is due to an accumulation of K(+) near the exterior cell membrane. Several other experiments exclude the possibility of either metabolic events or compensating conductance changes producing the phenomenon.5. The location of the accumulation is considered on the basis of anatomical studies. It is concluded that the accumulation takes place in the extensive infoldings found in cells like R(15). An explanation of the difference in delayed current decrease between pace-makers and non-pace-makers is suggested, since the pace-makers apparently have more extensive invaginations than the non-pace-makers. This suggestion is lent support by measurements of capitance and current density.

Membrane calcium activation in excitation-contraction coupling

Depolarization thresholds for eliciting tension and Ca electrogenesis have been compared in isolated crayfish muscle fibers. Just-detectable tensions and Ca spikes induced after treatment with procaine were elicited with intracellularly applied depolarizing currents of fixed duration. Both thresholds were found to increase in a similar manner in fibers exposed to increased concentrations of Ca in the bathing solution or addition of other divalent cations (Mg, Mn, Ni). However, antagonistic effects between divalent cations were also demonstrated. Substitution of increasing amounts of NaSCN for NaCl in the standard saline produced a progressive decrease in both thresholds. The correlation in the change in thresholds for the two processes supports the hypothesis that a change in membrane Ca conductance is an integral step in excitation-contraction coupling.

Excitation-contraction coupling in amphioxus muscle cells

1. Excitation-contraction coupling was studied in myotomal muscles of amphioxus, Branchiostoma californiense.2. The action potential of a muscle cell produces a twitch with a rise time of 30-40 msec at 11 degrees C and its Q(10) is about 2.2.3. The twitch increases in amplitude with increasing external Ca concentration and is abolished in Ca-free saline (1 mM-EGTA and 55.7 mM-MgCl(2)); the twitch amplitude is suppressed by Co or La ions.4. Caffeine at concentrations above 1 mM in the external saline causes a prolongation of the action potential and a contracture which lasts several minutes.5. After exposure to caffeine the responsiveness of the muscle to subsequent applications of caffeine recovers in normal saline in 20-30 minutes but not in Ca-free saline.6. The amplitude of the caffeine contracture is independent of the external Ca concentration and is unaltered after the twitch is eliminated in Ca-free saline.7. After exposure to caffeine a full-sized twitch can be obtained before the responsiveness to caffeine shows any significant recovery.8. It is concluded that the twitch is produced by the Ca influx resulting from the increased permeability of the muscle cell membrane to Ca during the action potential and that the Ca mobilized by caffeine is not necessary to the initiation of the twitch.9. Electronmicroscopy shows the existence of sarcoplasmic reticulum.

The effect of quinine on tension development, membrane potentials and excitation-contraction coupling of crab skeletal muscle fibres

1. The effect of quinine on tension development and membrane potentials of crab skeletal muscle was examined using strain gauges and intracellular electrodes.2. In low concentrations (0.1-0.5 mM), quinine caused transient potentiation of twitch tension which then rapidly declined along with progressive depression of the tetanus. These actions are correlated with the decline of both action and resting potentials during quinine treatment.3. In moderate concentrations (1-5 mM), quinine induced phasic contractures, but the attendant depolarization made the muscles refractory to stimulation and potassium activation, but not to caffeine activation.4. Quinine did not induce contractures in depolarized muscle, which suggests that the action of quinine in inducing calcium release from the sarcoplasmic reticulum may be blocked by potassium depolarization, unlike the calcium-releasing action of caffeine. Quinine appeared to have no effect on the mechanical threshold of crab skeletal muscle fibres.5. To explain its depression of contractility in crab muscle, it is suggested that quinine may deplete the calcium store of the sarcoplasmic reticulum, leading to extinction of the terminal stages of the excitation-contraction coupling process and loss of contractility.

Magnesium in single skeletal muscle cells of Balanus

Single skeletal muscle cells of Balanus contain 48 +/- 1 mmoles magnesium/kg dry weight. Although (28)Mg can be shown either to enter the cells or to be bound to the cell surface within less than 10 min, only 2.1 +/- 0.3% of cellular or cell surface Mg exchanges with this isotope even after several hours. Glycerinated cells washed out in Tris buffer at low ionic strength retain approximately 70% of the Mg present in intact cells. About 85% of this Mg is removed by extraction with KCl or NaCl at concentrations of K and Na which prevail in intact cells, as well as by pyrophosphate, Tris-ATP, or reduction of the ionized Mg concentration to 1 microM. Lowering the ionized Mg concentration to 0.1 microM does not further reduce the Mg content of glycerinated cells. The pH dependence of KCl-inextractable Mg suggests that more than one class of binding sites is involved. A significant fraction of the KCl-inextractable Mg bound to glycerinated cells fails to exchange with (28)Mg even after long equilibration. It is suggested that this fraction may be actin-bound Mg incorporated into the thin filaments during the polymerization of actin.

On the relationships between membrane potential, calcium transient and tension in single barnacle muscle fibres

1. The calcium-sensitive photoprotein aequorin has been used to follow the rapid changes in intracellular calcium concentration that occur during the contraction of single muscle fibres from the barnacle Balanus nubilus, Darwin.2. The transient change in calcium-mediated light emission (calcium transient) and the changes in membrane potential and tension were recorded simultaneously, thus permitting an examination of the relationships between the chemical, electrical, and mechanical events of excitation-contraction coupling.3. With short-duration stimuli (< 200 msec), the calcium transient shows an S-shaped rising phase reaching a maximum soon after the cessation of the stimulus pulse. During membrane repolarization the calcium transient begins an exponential falling phase which has a time constant of 50-80 msec at 11-12 degrees C.4. The shape of the calcium transient resembles the first derivative of the rising phase of the isometric tension response, thus suggesting that calcium controls the rate of tension development.5. There is no detectable increase of the light emission above resting values, during the falling phase of isometric tension.6. A plot of the calcium transient area (lumen x sec) versus peak isometric force (g. cm(-2)) is linear over, at least, a range of forces from ca. 50-400 g. cm(-2).7. When the fibre is capable of producing an active membrane response following the intracellular injection of potassium citrate, the onset and cessation of the calcium transient follow closely the onset and cessation of the active membrane response. Tension responses under these conditions are much suppressed, suggesting that excitation-contraction coupling may be partially blocked between calcium release and the development of tension.8. Hypertonic salines (1 M sucrose or 1 M glycerol) cause little change in the membrane response, but greatly suppress the calcium transient and completely abolish the tension responses. These effects are readily reversible when normal saline is reintroduced, suggesting that excitation-contraction coupling may be temporarily blocked between the membrane response and calcium release.9. If the stimulus is prolonged (> 250-300 msec), the calcium transient falls slowly from its maximum value despite continued membrane depolarization, suggesting a time-dependent change in the ratio of the rate of release of calcium to the rate of calcium binding. The results from brief tetanic stimulation also support this suggestion.

Membrane properties of a barnacle photoreceptor examined by the voltage clamp technique

1. Electrical properties of the membrane of photoreceptor cells in the lateral ocelli of barnacles, Balanus amphitrite and B. eburneus were investigated by intracellular recording, polarization and voltage-clamp techniques.2. The resting potential of a dark adapted cell was 36.3 +/- 6.6 mV (S.D.) and depended mainly on the external K(+) concentration.3. Current-voltage relations obtained from voltage-clamp experiments in the absence of light were non-linear and varied with time after the onset of a step change in membrane potential; the steady state was reached after about 0.5 sec.4. Illumination resulted in a membrane potential change under current clamp and in a change of membrane current (light-initiated membrane current (L.I.C.): total membrane current with illumination minus current without illumination) under voltage-clamp conditions. Amplitudes and time course of L.I.C. depended on the light intensity as well as membrane potential.5. The L.I.C.-voltage relation was non-linear and corresponded with a slope conductance increase with increasing positive membrane potential.6. The reversal potential of L.I.C. was independent of the light intensity and the time after onset of illumination; the average value obtained in normal saline was +26.9 +/- 5.0 mV.7. The membrane conductance estimated from instantaneous L.I.C.-voltage relations agreed with the chord conductance of the non-linear L.I.C.-voltage relation.8. Decreasing external Na(+) concentration decreased the inward component of L.I.C. but not the outward component.9. Decreasing external Ca(2+) concentration increased the inward as well as the outward component of L.I.C.10. The reversal potential shifted in the negative direction with decreasing external Na(+) concentration (the rate was 10-15 mV for a tenfold change in concentration) and the rate was augmented in the absence of Ca(2+) but did not exceed 21 mV.11. The change of reversal potential with changes of external Ca(2+) concentration was negligible in normal Na(+) media but was significant in the absence of Na(+) (rate as high as 20 mV).12. Alteration of the external K(+) or Cl(-) concentrations did not affect the amplitude or reversal potential of L.I.C.13. The results indicate that illumination increases the membrane permeability mainly to Na(+) ions and that the primary effect of Ca(2+) ions is suppression of the permeability increase; Ca(2+) permeability may increase slightly during illumination.

Calcium and potassium currents of the membrane of a barnacle muscle fibre in relation to the calcium spike

1. The behaviour of membrane currents in a giant muscle fibre of a certain barnacle Balanus nubilus, Darwin, was studied by using voltage clamp technique after treating fibres with a Ca-chelating internal solution.2. Membrane currents can be classified into an early transient current and a late outward current.3. The early transient current can be considered a sum of an inward and an outward component of different properties.4. A conditioning depolarization suppresses the inward component but not the outward component.5. Procaine suppresses the outward component and Co ions suppress the inward component.6. Changes in the external Ca ion concentration alter the inward component but not the outward component.7. The inward component is considered to be carried by Ca ions and the outward component mainly by K ions.8. The outward component of the early current is similar to the current expected from the resting membrane conductance.9. It is concluded that the early conductance increase of the barnacle muscle fibre membrane occurs only to Ca ions but not to K ions, although the membrane potential at the peak of spike is determined by both ions.