The influence of injected cyclic AMP protein kinase catalytic subunit on the sodium efflux in barnacle muscle fibres

Steroid and peptide control mechanisms in membrane of Xenopus laevis oocytes resuming meiotic division

Stage 5-6 Xenopus laevis oocytes are arrested in prophase of the first meiotic division, and can be studied in vitro after removal from their follicle cell environment. While they do not mature spontaneously, they demonstrate germinal vesicle (nucleus) breakdown (GVBD) if exposed to approximately 1 microM-progesterone (the hormone released in vivo at the time of ovulation and maturation). The oocytes' then become eggs ready to be fertilized. The progesterone-oocyte interaction, contrary to what is observed in all endocrine steroid target organs so far studied, takes place at the surface membrane level and is not narrowly progesterone-specific, since other hormones such as cortisol or testosterone can also cause resumption of meiosis in vitro. This is the first description of such a paracrine steroid system, which depends however on a receptor mechanism, as indicated by physicochemical experiments, studies with antagonistic (competitive) steroids, and cell-free specific inhibitory effects on membrane-bound adenylate cyclase. It was found that insulin and related growth factors (mitosis-stimulating activity, MSA; insulin-like growth factor, IGF) are also reinitiators of meiosis. Insulin also potentiates the effects of low progesterone concentration (approximately 1 nM) in completely denuded oocytes (free of the vitelline membrane). From these observations it is suggested that there may be a physiological, cooperative involvement of a steroid (progesterone) and an insulin-like peptide factor within the ovaries which promotes oocyte maturation in vivo. The molecular mechanisms of the hormone-dependent changes in cyclic AMP and Ca2+ remain to be elucidated in detail, as well as the respective roles of these two sets of metabolic events.

ATP as a positive effector of the sodium efflux in single barnacle muscle fibers

A study has been made of the mechanism by which the injection of ATPNa2 stimulates the ouabain-insensitive Na efflux in fibers from the barnacle, Balanus nubilus. The results of this study are as follows: ATPNa2 is found to be a more potent effector of the Na efflux in unpoisoned fibers than ATPMg on an equimolar basis, but not more potent than ADPNa2. In ouabain-poisoned fibers ATPNa2 and ATPMg are equipotent but the former is more potent than ADPNa2. The magnitude of the response to ATPNa2 injection into ouabain-poisoned fibers depends on: (i) the ouabain concentration used; (ii) the concentration of ATPNa2 injected, and (iii) the external Ca2+ concentration. Ouabain is without effect when it is applied at the time of ATPNa2 injection. Responsiveness to ouabain, however, is found to return if the glycoside is applied after complete decay of the response to ATP. Under these conditions, the effect of ouabain in fibers injected with ATPNa2 is significantly less than in fibers injected with ATPMg. Preinjection of EGTA in high concentrations fails to reduce the size of the response to ATPNa2 injection. Injection of Mg2+ following peak stimulation by ATP almost completely reverses the response. The response to Mg2+ is concentration-dependent. Ryanodine but not neomycin reduces the response to ATP. ATP gamma S is not as effective as ATPNa2. Nor is AMP-PNP consistently as effective as ATPNa2. Collectively, these results support the hypothesis that the response of the Na efflux to ATPNa2 injection involves the operation of the putative Na(+)-Ca2+ exchanger in the reverse mode and that a raised Cai2+ is not an absolute requirement. They also strongly suggest that two other governing factors are the Na+ gradient across the sarcolemma and the myoplasmic pMg. Mg2+ seems to act as an inhibitor.

Microinjection of synthetic protein kinase inhibitor into single barnacle muscle fibers before and after cyclic AMP

Single muscle fibers from the barnacle Balanus nubilus have been used as a preparation to see if a synthetic 20-residue PKI (5-24)-peptide is able to block or reverse the stimulatory response of the ouabain-insensitive Na efflux to injected cAMP. The results obtained show that this peptide behaves as a powerful inhibitor of the cAMP-mediated response and is also able to partially reverse the sustained stimulation of the Na efflux observed in ouabain-poisoned fibers following the injection of subunit A of cholera toxin.

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.

The modulatory action of 5-hydroxytryptamine on sodium efflux: the barnacle muscle fibre as a model system

A study has been made of the action of 5-hydroxytryptamine (5-HT) on the radio-sodium efflux from single barnacle muscle fibres. (i) Stimulation of the Na efflux by external application of 5-HT is seen in both unpoisoned and ouabain-poisoned fibres. (ii) Concentrations of 5-HT as low as 10(-9)M are effective. (iii) Characteristically, the response to 5-HT is prompt in onset, reaches a peak within 20 min and then decays rather rapidly. Fibres from certain barnacle specimens are sometimes unresponsive to 5-HT. Such fibres, however, can be rendered responsive by preinjecting into them the non-hydrolysable GTP analogue, Gpp(NH)p. The response of the ouabain-insensitive Na efflux to 5-HT depends on external Ca2+ and, to a certain extent, on external Na+. (i) The response to 5-HT is unaffected by prior external application of Ca2+ antagonists, viz. verapamil, Cd2+ and WB-4101. (ii) The calmodulin antagonist, trifluoperazine (10(-5)M), completely abolishes the response to 5-HT, even in fibres preinjected with Gpp(NH)p. (iii) Diphenylhydantoin is less effective than trifluoperazine (TFP). Whereas the receptor antagonist methysergide is ineffective, cyproheptadine is very effective. (i) Prior application of the phosphodiesterase inhibitor 1-propyl-3-methyl-7-(5-hydroxyhexyl)-xanthine (PMX) or the inhibitor 1-isoamyl-3-isobutyl-xanthine (IAX) augments the size of the response to 5-HT, but fails to stop the response from decaying. (ii) Augmentation of the response to 5-HT by IAX is seen despite the presence of 10(-5) M-TFP. Prior injection of Mg2+ or protein kinase inhibitor (PKI) leads to abolition or reduction of the response to 5-HT. These results demonstrate that barnacle fibres are a useful preparation for investigation the natriferic action of 5-HT. They also support the view that the response to 5-HT involves a receptor-adenylate cyclase complex and is the result of activation by newly formed cAMP of cAMP-dependent protein kinase.

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.

Sensitivity to injected cholera toxin of the sodium efflux in single barnacle muscle fibers

A study has been made of the effect of microinjected cholera toxin (CT) on the efflux in single barnacle muscle fibers. Characteristically, injected CT causes sustained stimulation of the ouabain-insensitive Na efflux but only after a lag phase. An effect is seen with as little as a 10(-7) M-solution of CT. Sustained stimulation after a lag phase is also seen following injection of subunit A fragment. Enrichment of fibers with NAD+ fails to enhance the response to CT. Prior injection of GTP or its non-hydrolyzeable analogue, Gpp(NH)p, markedly reduces the response to CT, whilst prior injection of CT reduces the response to guanine nucleotides. Evidence is also brought forward that omission of external Ca2+ reversibly reduces the response to CT and that pre- or postinjection of EGTA markedly reduces the response to CT. In addition, fibers preinjected with CT show increased aequorin light emission. Whereas verapamil and Cd2+ are ineffective, both Mg2+ and trace metals, e.g. Fe and Zn, reverse the response to CT following injection. Prior injection of protein kinase inhibitor reduces the response to CT. As for calmodulin inhibitors, e.g. chlorpromazine, imipramine and mepacrine, they are effective in reducing the response to CT but not calmodulin antibody (IgG). Collectively, the above results are compatible with the view that sustained stimulation of the ouabain-insensitive Na efflux by injected CT is due to persistent activation of adenylate cyclase by the toxin and that a fall in myoplasmic pCa facilitates or augments this activation mechanism.

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.

Concerning the stimulation by injected cyclic AMP of the sodium efflux in barnacle muscle fibres and its transient nature

The efflux of 22Na in ouabain-poisoned barnacle muscle fibres and its transitory response to injected cAMP has been studied by using a new xanthine derivative, 1-propyl-3-methyl-7-(5-hydroxy-hexyl)-xanthine (PMX). Injection of PMX prior to cAMP fails to significantly alter the behaviour of the ouabain-insensitive Na efflux towards the nucleotide. By contrast, injection of PMX following peak stimulation by injected cAMP stops the rate constant for 22Na efflux from falling. This effect of PMX is not mimicked by injected HEPES. (a). Injection of Mg2+ following PMX brings about almost complete reversal of the sustained stimulatory response. (b). Injection of trace metals, e.g. Fe and Zn, following PMX brings about complete reversal of the sustained stimulatory response. (c). Injection of RI or RII subunits following PMX brings about partial reversal of the sustained stimulatory response. Partial reversal is also seen with externally applied imipramine (50 microM). These results support the view that the transitory nature of the response of the ouabain-insensitive Na efflux to injected cAMP is due to high phosphodiesterase activity in these fibres and that the major portion of the response itself is due to activation by cAMP of cAMP-dependent protein kinase.

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.

Some further observations on the stimulation by high external potassium of the sodium efflux in barnacle muscle fibers

A study has been made in single barnacle muscle fibers of the mechanism underlying the stimulatory response of the ouabain-insensitive Na efflux to high K0. Fibers poisoned with ouabain and preinjected with GTPNa2 show a biphasic stimulatory response to 100 mM K0 which is practically abolished by exposure of these fibers to verapamil. Prior or post-injection of cAMP-protein kinase type II regulatory subunits reduces the size of the response to 100 mM K0. Prior injection of cAMP-protein kinase catalytic subunits (CSU) attenuates the response to 100 mM K0 in fibers showing moderate sensitivity to CSU. In fibers showing extreme sensitivity to injected CSU, elevation of K0 partially reverses the sustained stimulatory response to CSU. Prior injection of Mg2+ leads to almost complete abolition of the response to 100 mM K0. External and internal application of F- reduces the response to 100 mM-K0. Injection of KF following decay of the response to high K0 results in a sustained stimulatory response. The drugs, chlorpromazine, trifluoperazine, imipramine and diphenylhydantoin are able to appreciably reduce the response to 100 mM K0. Taken together, these observations support the idea that the stimulatory response to high K0 is primarily due to activation by newly formed cAMP of cAMP-protein kinase, and that the transitory nature of the response and variable sensitivity of these fibers to high K0 is closely linked to the activity of a putative Mg2+-dependent protein phosphatase.