The importance of external magnesium in the sensitivity of the sodium efflux to injected cyclic AMP: The barnacle muscle fibre as a preparation

Abstract

1. 1. The dependence of the response of the Na efflux in ouabain-poisoned barnacle muscle fibres on the external Mg 2+ concentration has been investigated. 2. 2. External Mg 2+ suppresses stimulation by injected cAMP of the ouabain-insensitive Na efflux over a wide concentration range. The response of fibres suspended in 50 mM Mg 2+-ASW is rather small. 3. 3. By contrast, the response of fibres suspended in 50 mM Mg 2+-ASW containing 1-propyl-3-methyl-7-(5-hydroxy-hexyl)-xanthine to the injection of cAMP is quite large. 4. 4. The response of fibres suspended in 50 mM Mg 2+-ASW to the injection of catalytic subunit of cAMP-protein kinase is also large. 5. 5. These results give support to the view that a persistent catalytic reaction is a condition which reduces the suppressive action of external Mg 2+.