Mode of stimulation by adenosine 3':5'-cyclic monophosphate of the sodium efflux in barnacle muscle fibres.

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Abstract
1. Giant fibres of the barnacle Balanus nubilus have been used as a preparation for studying the mode of action of cAMP on sodium transport. 2. It is shown that a concentration of cAMP as low as 10(‐6)M, when micro‐injected, causes a sharp rise in the radio‐Na efflux. Ouabain fails to reverse the cAMP effect. 3. The magnitude of the response of the Na efflux to cAMP is markedly reduced by pre‐injecting 100 or 500 mM‐EGTA solutions or by omitting Ca2+ from the bathing medium. Both together fail to bring about a greater reduction in the response. 4. The response to cAMP is greatly reduced by pre‐injecting the protein inhibitor of Walsh and practically abolished by pre‐injecting 500 mM‐EGTA and soaking in Ca‐free artificial sea water, ASW. 5. The Ca2+‐independent component of the Na efflux which is also stimulated by cAMP is shown to involve Na for H exchange. The magnitude of this exchange is governed by external pH. 6. The Na efflux into Ca2+‐free, Li+‐ASW is shown to be markedly stimulated by injecting cAMP, an effect which is enhanced by reducing external pH. 7. The Na efflux at 0 degrees C is stimulated by injecting cAMP. This is shown to be related to activation of the protein kinase by cAMP and to depend on the presence of external Ca2+. 8 (i) Ethacrynic acid when injected reduces the ouabain‐insensitive Na efflux into HEPES‐Ca2+‐free ASW at pH 6‐3. These same fibres show a marked response to cAMP. (II) The ouabain‐insensitive Na efflux into HCO3‐, Ca2+‐free ASW from fibres pre‐treated with ethacrynic acid fails to respond to external acidification. This is interpreted as indicating that ethacrynic acid inactivates the CO2‐sensitive adenyl cyclase system. These same fibres when injected with cAMP show a marked response. (iii) Stimulation of the ouabain‐insensitive Na efflux into HCO‐3, Ca2+‐free ASW by external acidification is reversed by injecting ethacrynic acid. These fibres when injected with cAMP show a reduced response. 9. It is concluded that: (i) stimulation of the Na efflux by injected cAMP is mainly due to activation of cAMP‐dependent protein kinase; (ii) the underlying exchange mechanism consists of Na:Ca and Na:H exchange. Interaction of Ca2+ with a phosphorylated membrane, thereby modifying permeability remains as a real possibility; (iii) the site of action of CO2 and ethacrynic acid is the adenyl cyclase system. 10. The implications of activation of the adenyl cyclase system by CO2 and Na:H exchange are briefly touched upon.