Effects of anions and cations on the resting membrane potential of internally perfused barnacle muscle fibres

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)

Internal citrate ions reduce the membrane potential for contraction threshold in mammalian skeletal muscle fibers

An effect of internal citrate ions on excitation-contraction coupling in skeletal muscle is described. The threshold for contraction was measured in rat extensor digitorum longus, (EDL), and soleus muscle fibers using a two microelectrode voltage clamp technique with either KCl-filled or K3 citrate-filled current electrodes. Contraction thresholds were stable for many minutes with KCl current electrodes. In contrast, thresholds fell progressively towards the resting membrane potential, by as much as -15 mV over a period of 10 to 20 min of voltage-clamp with citrate current electrodes. In addition, prepulse inhibition was suppressed, subthreshold activation enhanced and steady-state inactivation shifted to more negative potentials. Fibers recovered slowly from these effects when the citrate electrode was withdrawn and replaced with a KCl electrode. The changes in contraction threshold suggest that citrate ions act on the muscle activation system at an intracellular site, since the citrate permeability of the surface membrane is probably very low. An internal citrate concentration of 5 mM was calculated to result from citrate diffusion out of the microelectrode into the recording area for 20 min. 5 mM citrate added to an artificial cell lowered the free calcium concentration from 240 to 31 microM. It is suggested that citrate modifies excitation-contraction coupling either by acting upon an anion-dependent step in activation or by reducing the free calcium and/or free magnesium concentration in the myoplasm.

Kinetics of 13N-ammonia uptake in myocardial single cells indicating potential limitations in its applicability as a marker of myocardial blood flow

To study kinetics and principles of cellular uptake of 13N-ammonia, a marker of coronary perfusion in myocardial scintigraphy, heart muscle cells of adult rats were isolated by perfusion with collagenase and hyaluronidase. Net uptake of 13N, measured by flow dialysis, reached equilibrium within 20 sec in the presence of sodium bicarbonate and carbon dioxide (pH 7.4, 37 degrees C). Total extraction, 80 sec after the reaction start, was 786 +/- 159 mumol/ml cell volume. Cells destroyed by calcium overload were unable to extract 13N-ammonia. Omission of bicarbonate and carbon dioxide reduced total extraction to 36% of control. 13N-Ammonia uptake could also be reduced by 50 muM 4,4' diisothiocyanostilbene 2,2' disulfonic acid, by 100 micrograms/ml 1-methionine sulfoximine, and by preincubation with 5 muM free oleic acid. These results indicate that in addition to metabolic trapping by glutamine synthetase, the extraction of 13N-ammonia by myocardial cells is influenced by cell membrane integrity, intracellular-extracellular pH gradient, and possibly an anion exchange system for bicarbonate. For this reason, the uptake of 13N-ammonia may not always provide a valid measurement of myocardial perfusion.

Calcium carrying system in the giant muscle fibre of the barnacle species, Balanus nubilus

1. Single barnacle muscle fibres from Balanus nubilus were internally perfused with an isotonic solution containing 180 mM-tetraethylammonium acetate and the effects of Ca concentration in the external solution on the voltage-clamp currents, especially the initial inward current, were examined.2. Muscle fibre in external solution containing no added Ca (concentration of Ca < 10(-5)M) gave a current-voltage curve that showed a small inward current followed by a small outward current. An identical curve was obtained when the chloride in the external solution was replaced by acetate.3. The peak inward current increased with increase in Ca concentration in the external solution; but the increase in peak current for equal increments of Ca concentration was reduced and attained saturation conforming to an adsorption regime which can be described by the Langmuir equation.4. A double reciprocal plot of peak inward current due to Ca as a function of Ca concentration gave values of 20.7 mM and 200% (the current due to 20 mM-Ca normalized to 100%) for the dissociation constant and the maximum current respectively. These values were found to depend on the concentration of Mg used in the external solutions.5. The peak inward Ca currents measured at two Ca concentrations as a function of pH were unaffected in the pH range 6.5-8.8; however when the pH was reduced below 6.5, the currents were depressed.

Extracellular space and diffusion barriers in muscle fibres from Megabalanus psittacus (Darwin)

1. Muscle fibres from Megabalanus psittacus (Darwin) were used to measure the exchange of Na+ and Ca2+ between intracellular and extracellular compartments. 2. The size of the extracellular space was evaluated directly from electron micrographs at 6.1 +/- 0.5% (percentage of the fibre volume). The more conventional estimates using Na+ and 134Cs+ as space markers gave higher values, namely, 8.9 +/- 0.7 and 9.5 +/- 1.5%. 3. An average value of 9.2% was used to correct the total Na+ and K+ in the muscle fibres and to estimate the intracellular concentrations of Na+ and K+ as 39 +/- 4 and 202 +/- 11 mM respectively. 4. Na+ (and similar Ca2+) washout curves could be described using a three compartments diffusion model. The data analysed in terms of this model enabled us to estimate membrane permeabilities as PNa+ = 2.7 x 10(-7) and PCa2+ = 2.7 x 10(-7) cm/sec.

Some electrical properties of the membrane of the barnacle muscle fibers under internal perfusion

Intracellular perfusion technique has been applied to the muscle fibers of the barnacle species, Balanus nubilus. In these fibers, generation and the form of the calcium spike was governed by the frequency of stimulation and intra- and extracellular calcium concentrations. Voltage-clamp experiments showed that the magnitude of the potassium outward current was controlled by the intracellular calcium concentration whose increase, nearly 10(3)-fold, raised the resting membrane conductance and the outward potassium current. On the other hand, application of 10 mM zinc ions inside the muscle fiber had no effect on either the resting potential or the outward potassium current but suppressed the early inward calcium current. Similarly, the inward calcium current was decreased by low concentration of sodium ions in the extracellular fluid only when its ionic strength was made low by substituting sucrose for the sodium salt. Measurement of outward current with the muscle fiber in calcium-free ASW solution and intracellularly perfused with several cationic solutions established the selectivity sequence TEA less than Cs less than Li less than Tris less than Rb less than Na less than K for the potassium channel.

A mutant of Paramecium with increased relative resting potassium permeability

Fast-2, a membrane mutant of Paramecium aurelia, is due to a single-gene mutation and has behavioral abnormalities. Intracellular recordings through changes of external solutions were made. The mutant membrane hyperpolarized when it encountered solutions with low K+ concentration. This hyperpolarization and other associated activities were best observed in Ca- or Na-solutions devoid of K+. Membrane potential was plotted against the concentration of K+ (0.5 to 16 mM) in solutions of fixed Na+ or Ca++ concentration. The slopes of the curves for the mutant membrane were steeper than those for the wild type at the lower concentrations of K+. Inclusion of 2 mM tetraethylammonium chloride (TEA-Cl) counteracted the mutational effects. Spontaneous action potentials in Ba-solution and the electrically evoked action potentials in various solutions are normal in this mutant. We conclude that the resting permeability to K+ relative to the permeabilities to Na+ and Ca++ has been increased by the mutation.

Estimation of surface charges in some biological membranes

The resting membrane potential data existing in the literature for the giant axon of the squid, frog muscle and barnacle muscle have been analyzed from the standpoint of the theory of membrane potential due to Kobatake and co-workers. The average values derived for the effective charge density phi chi (where phi is a constant, 0 less than phi less than 1, and represents the fraction of counterions that are free, and chi is the stoichiometric charge density in the membrane) present on the different biomembranes existing in their normal ionic environment are 0.3, 0.325 and 0.17 M for the squid axon, frog and barnacle muscles, respectively. On the assumption that the values of phi are 0.4 and 0.2 for nerve and muscle membranes, respectively, values of 0.75, 1.62 and 0.85 M have been derived for the stoichiometric charge density (chi) present in the respective biological membranes. These correspond to 1 negative charge per 222, 103 and 195 A2 of the membrane area of the squid axon, frog and barnacle muscles, respectively.

Effects of pH and ionic strength on the potassium system in the internally perfused giant barnacle muscle fibre

Membrane potentials in single barnacle muscle fibres of Megabalanus psittacus, internally perfused with 200 mM K-acetate (KAc) solution of pH 7.5 were in the range --53 to --60 mV. These were followed as a function of pH of the external chloride saline. Decrease of pH of the Cl-saline from 8.5 to 3.5 hyperpolarized the membrane reversibly by about 8 mV. On further decrease of pH to 3.0, a transient hyperpolarization (from --60 to --65 mV) followed by a sudden and irreversible drop of potential to --40 mV was observed. Replacement of the external Cl-saline by acetate saline in the pH range 8.5 to 6.5 had no effect on the membrane potential. Further decrease of pH to 5.0 brought about an irreversible reduction of membrane potential. For fibres bathing in Cl- or Ac-saline at pH 7.5 when the internal pH was changed to 5.5, a transient hyperpolarization in Cl-saline and a sustained hyperpolarization in Ac-saline were observed. Further studies of membrane potential changing the concentration of external K, keeping the concentration of Cl or Ac constant, and changing the concentration of Cl or Ac, keeping the concentration of K constant, at different internal and external pHs showed that as the pH was reduced the membrane became more permeable to anions, Cl being more permeable than Ac. Experiments in which the membrane potential was controlled showed that when the internal pH of 200 mM KAc solution was reduced from 7.5 to 6.5 or raised to 8.5, the changes in outward K currents at various depolarizations were negligible. However, when the pH was reduced to 5.5 or 5.0, there was a progressive decrease in the outward K currents. The leakage currents in all these cases were relatively small. Use of high ionic strength solution of 580 mM KAc internally protected the K system in that when the pH was lowered the depression of the outward K current was smaller than that observed at a corresponding pH when the internal solution was 200 mM KAc. Use of low ionic strength solution of 50 mM KAc had the opposite effect. The results have been explained in terms of the ion exchange theory by postulating that the membrane has fixed amphoteric groups.

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.