The influence of low pH, high K and microinjected CaCl 2 on the ouabain-insensitive component of sodium efflux in barnacle muscle fibers

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)

Concerning stimulation by high external potassium and calcium injection of the ouabain-insensitive sodium efflux in barnacle muscle fibers

The behavior of the ouabain-insensitive Na efflux in barnacle muscle fibers toward external high K and injection of Ca2+ has been further investigated. Raising Ke to 100 mM after the injection of 0.25 M or 0.1 M GTPNa2 results in a biphasic stimulatory response: the initial response is prompt in onset and small but transitory, whereas the delayed response is large and sustained. This second stimulatory phase is reduced markedly by injecting EGTA but not by PKI. Raising Ke to 100 mM in the presence of the 2 xanthine derivatives, viz. PMX and IAX, leads to a sustained stimulatory response of the ouabain-insensitive Na efflux which is halved by injecting PKI but unaffected by injecting EGTA. Injection of 0.1 M or 0.5 M CaCl2 in the presence of PMX and IAX leads to a sustained stimulatory response, which is almost completely abolished by injecting PKI but unaffected by injecting EGTA. These results confirm the earlier finding that the response of the ouabain-insensitive Na efflux to high Ke in fibers preinjected with GTPNa2 is biphasic and that the delayed second stimulatory phase is sustained rather than transitory. The ability of injected EGTA to only partially reverse the delayed response suggests that a fall in myoplasmic pCa is not the sole factor governing the kinetic picture. The experiments with PMX and IAX strongly suggest that cAMP is involved in the termination of the Ca2+ message.

Stimulation by proctolin of the ouabain-insensitive sodium efflux in single barnacle muscle fibers

A study has been made of the action of the neuropeptide proctolin on the radiosodium efflux from single barnacle muscle fibers. Proctolin (10(-8) M) when applied externally causes stimulation of the Na efflux in unpoisoned and ouabain-poisoned fibers. The response is prompt in onset, reaches a peak in 15 min and decays slowly. The response of the ouabain-insensitive Na efflux to external proctolin is dose-dependent, the concentration threshold being less than 10(-10) M. The response to proctolin is dependent on external Ca2+ but not on Na+. (i) The response to proctolin is abolished by high external Mg2+, as well as by verapamil, Co2+, Cd2+ and WB-4101. (ii) The response is also abolished by preinjecting 0.5 M MgCl2 or 0.1 M EGTA. The calmodulin antagonists trifluoperazine and imipramine are without effect on the response to proctolin. (i) Adenylate cyclase agonists, e.g. forskolin, fail to augment the response to proctolin. (ii) Prior injection of the phosphodiesterase inhibitors 1-propyl-3-methyl-7-(5-hydroxyhexyl)-xanthine (PMX) or 1-isoamyl-3-isobutylxanthine (IAX) fails to augment the response to proctolin. (iii) Prior injection of protein kinase inhibitor is ineffective. The response to proctolin is significantly reduced in the presence of tyramine. Taken together, these results support the view that proctolin stimulates the ouabain-insensitive Na efflux by activating Ca2+ channels and that the cAMP-protein kinase system is not involved in this response.

Stimulation by octopamine of the sodium efflux in barnacle muscle fibres

The behaviour of the ouabain-insensitive Na efflux in barnacle muscle fibres towards DL-octopamine has been investigated. It is found that these fibres are quite often sensitive to external application of octopamine. A concentration as low as 10(-9) M is effective. The kinetic results indicate that the stimulatory response develops within 5-10 min of exposure of the fibre to octopamine and is transitory in nature. The response to octopamine is greater in size in the presence of 1-isoamyl-3-isobutylxanthine (IAX) than in the presence of 1-propyl-3-methyl-7-(5-hydroxyhexyl)-xanthine (PMX). But neither IAX nor PMX stops the response from decaying. The response to octopamine is highly dependent on the presence of external Ca2+. The provisional conclusion is that the barnacle muscle fibre may be a useful preparation for studying the hormonal function of octopamine.

Increased sensitivity to injected 5'-guanylylimidodiphosphate of the sodium efflux in barnacle muscle fibres preexposed to aldosterone

1. A study has been made of the response to injected Gpp(NH)p of the ouabain-insensitive Na efflux in barnacle muscle fibres preexposed to aldosterone. 2. The response to injected Gpp(NH)p is not only greater in size than in unexposed fibres but also sustained. 3. Injection of MgCl2 following peak stimulation causes a partial reversal of the response. 4. Injection of ATPNa2 (and 5'-App(NH)p) leads to a sustained stimulatory response which is not significantly greater than that seen in unexposed fibres. 5. MgCl2 injection causes complete reversal of this response. 6. The response of preexposed fibres to injected CaCl2 in varying concentration and to injected cholera toxin is not significantly different from that seen in unexposed fibres. 7. This is also true of Gpp(NH)p when it is injected after peak stimulation by cholera toxin. 8. Prior application of verapamil (10(-4)M) drastically reduces the response to injected Gpp(NH)p. 9. The residual response is sustained but markedly reduced by injected Mg2+, Fe or Zn. 10. Injection of PKI following Gpp(NH)p reduces the response, provided PKI is also injected before Gpp(NH)p. By contrast, injection of R11 subunits causes a partial reversal if injected only once. 11. Imipramine and trifluoperazine, when applied externally (5 X 10(-5)M), cause almost complete reversal of the response. 12. The suggestion is made that the response to injected Gpp(NH)p is mainly due to activation of Ca2+-channels resulting in activation of the calmodulin/Ca-dependent form of adenylate cyclase and that the primary site of aldosterone action is at the level of the calmodulin form of adenylate cyclase.

Some aspects of sodium efflux from single barnacle muscle fibres

This mini-review attempts to summarize information about the efflux of 22Na from single barnacle muscle fibres, based on the use of the microinjection technique. The view is put forward that the Na efflux consists of three components: an ouabain-sensitive component, an ouabain-insensitive component (representing secondary active transport), and an Na-Na exchange diffusion component. Evidence is brought forward which supports the view that the ouabain-insensitive Na efflux is divisible operationally into 3 phases: (i) the cyclic nucleotide-sensitive phase, (ii) the Cai-sensitive phase, and (iii) the pHe-sensitive phase. It is shown how the barnacle muscle fibre preparation has yielded information about the validity of the cAMP-protein kinase hypothesis and how it can be used to shed some light on the post-translational mechanism of aldosterone action.

Stimulation by high external potassium of the sodium efflux in barnacle muscle fibers

Single barnacle muscle fibers from Balanus nubilus were used to study the effect of elevated external potassium concentration, [K]o, on Na efflux, membrane potential, and cyclic nucleotide levels. Elevation of [K]o causes a prompt, transient stimulation of the ouabain-insensitive Na efflux. The minimal effective concentrations is mM. The membrane potential of ouabain-treated fibers bathed in 10 mM Ca2+ artificial seawater (ASW) or in Ca2+-free ASW decreases approximately linearly with increasing logarithm of [K]o. The slope of the plot is slightly steeper for fibers bathed in Ca2+-free ASW. The magnitude of the stimulatory response of the ouabain-insensitive Na efflux to 100 mM Ko depends on the external Na+ and Ca2+ concentrations, as well as on external pH, but is independent of external Mg2+ concentration. External application of 10(-4) M verapamil virtually abolishes the response of the Na efflux to subsequent K-depolarization. Stabilization of myoplasmic-free Ca2+ by injection of 250 mM EGTA before exposure of the fiber to 100 mM Ko leads to approximately 60% reduction in the magnitude of the stimulation. Pre-injection of a pure inhibitor of cyclic AMP-dependent protein kinase reduces the response of the Na efflux to 100 mM Ko by 50%. Increasing intracellular ATP, by injection of 0.5 M ATP-Na2 before elevation of [K]o, fails to prolong the duration of the stimulation of the Na efflux. Exposure of ouabain-treated, cannulated fibers to 100 mM Ko for time periods ranging from 30 sec to 10 min causes a small (approximately 60%), but significant, increase in the intracellular content of cyclic AMP with little change in the cyclic GMP level. These results are compatible with the view that the stimulatory response of the ouabain-insensitive Na efflux to high Ko is largely due to a fall in myoplasmic pCa resulting from activation of voltage-dependent Ca2+ channels and that an accompanying rise in internal cAMP accounts for a portion of this response.

Mode of action of theophylline on sodium efflux in barnacle muscle fibers

The response of the Na efflux in unpoisoned barnacle fibers to 10 mM theophylline is biphasic; i.e., inhibition is followed by stimulation. The stimulatory response is unaffected by ouabain. Fibers pretreated with ouabain show no transitory inhibition when 10 mM theophylline is applied, but show prompt stimulation the magnitude of which is comparable to that observed with unpoisoned fibers. The same holds true for lower concentrations of theophylline. Prior injection of 500 mM EGTA completely abolishes the biphasic action of 10 mM theophylline. External application of 10 mM theophylline following removal of external Ca2+ fails to bring about a biphasic effect. Ca2+ restoration, however, results in a moderate rise in the Na efflux. External application of 10 mM theophylline stimulates the Na efflux into Ca2+-free artificial seawater (ASW) when the test fibers are pretreated with ouabain. Injection of the protein inhibitor of Walsh leads to reduced stimulation by 10 mM theophylline of the Na efflux in unpoisoned fibers. Injection of the protein inhibitor of Corbin into unpoisoned fibers leads to reduced stimulation by 10 mM theophylline. Injection of cAMP into ouabain-poisoned fibers, following internal application of Corbin's inhibitor and external application of 10 mM theophylline, fails to cause a marked rise in the ouabain-insensitive Na efflux. Injection of Corbin's inhibitor into ouabain-poisoned fibers, following the onset of peak stimulation by 10 mM theophylline, fails to reduce the Na efflux. Fibers injected with 1 mM and 100 mM EGTA and exposed to 10 mM theophylline show a marked reduction in the response of the ouabain-insensitive Na efflux to injected cAMP when the concentration of theophylline is 10 mM. A poor response to injected cAMP is also seen in fibers bathed in Ca-free ASW containing 10 mM theophylline. Theophylline (10 mM) fails to cause an enhanced stimulation of the ouabain-insensitive Na efflux into Ca-free 3 mM-HEPES ASW or 10 mM-Ca2+ -3mM-HEPES ASW following the addition of protons to the bathing medium. An enhanced response is similarly not observed with injected cAMP following the addition of theophylline to the bathing medium. Injection of 8-fluorotheophylline, 3-isobutyl-1-methylxanthine and doxantrazole leads to a marked reduction in the response of the ouabain-insensitive Na efflux to injected cAMP. Contraction always takes place upon injecting these substances. These results are in keeping with the theory that theophylline acts chiefly by reducing myoplasmic pCa(pCa=-log10[Ca2+]), and that a reduced pCa leads to stimulation of the ouabain-insensitive Na efflux as the result of activation of the cGMP-dependent protein kinase system by newly formed cGMP.

Effects of cardiac aglycones on the sodium efflux in barnacle muscle fibres

1. External application of 10(-4) M digitoxigenin caused an 80% fall in the Na efflux in barnacle muscle fibres. The observed inhibition was irreversible and could be produced by as little as 10(-7) M digitoxigenin. 2. Internal application of 10(-2) M digitoxigenin lowered the Na efflux by 46% but was without effect following addition of external ouabain or K removal. 3. External application of digitoxigenin (or ouabain) following internal application of the aglycone was without effect on the Na efflux. 4. The efflux of [3H]digitoxigenin was fairly rapid, whereas that of [3H]ouabain was slow. 5. Inhibition of the Na efflux by internal application of digitoxigenin is probably the result of an action of the aglycone on the external side of the membrane following its leakage from the fibre interior.

An investigation of sodium transport in barnacle muscle fibres by means of the microsyringe technique

1. The cation composition of single barnacle muscle fibres following damage by axial insertion of a microsyringe has been measured. The Na and Ca contents of these fibres were raised.2. Electronmicroscopic studies of fibres following insertion of a microsyringe indicated that the damage done resulted in tubular obstruction of the T-system.3. Fibres loaded with radiosodium by micro-injection showed that the Na(*) efflux declined exponentially with time, but that in most fibres the slope ratio of d/dt ln [Na(*)](1) to d/dt (ln d[Na(*)](i)/dt) was less than unity. Injections of distilled water deep in the fibre failed to influence the course of the Na(*) efflux.4. K removal reduced the Na efflux by 47%. However, a few fibres displayed very little K-dependence.5. When measured in fibres already soaked in a K-free medium for long periods the sodium efflux consisted of a brief rapid phase, followed by a slow phase of Na loss.6. In the presence of 30 mM-K, there was little or no rise in the Na efflux. Raising the external K to 50 or 100 mM caused a marked rise in the Na efflux. Raising the external K to 30 mM in the absence of external Ca(2+) led to a rise in the Na efflux. A high K solution always caused shortening of these fibres.7. Internal application of 1 M or 1 mM-CaCl(2) often caused a significant rise in the Na efflux.8. Internal application of 2.5 or 5 M saline caused a prompt and large fall in the Na efflux. In the presence of high K saline-loaded fibres failed to contract.9. Internal application of 0.5 M-ATP stimulated the Na efflux. A larger effect was not observed in fibres pre-treated with 2 M-MgCl(2). Internal application of 0.5 M-ArP was without effect.10. The results indicate that the barnacle fibre is a suitable preparation for the study of Na fluxes by means of the micro-injection technique. They also indicate that the mechanism regulating the Na efflux is not quite the same as that found in squid axon or frog muscle.