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

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.

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.