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.