cAMP Stimulates Na⁺-dependent ATP Formation in the Alkalophylic Cyanobacterium Spirulina platensis

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The cellular ATP concentration of Spirulina platensis increased when cAMP was added at a low concentration of 0.02 mM. The increase in ATP accompanied a decrease of ADP and AMP. The presence of a high concentration of Na⁺ (0.22 M) remarkably intensified the effect of cAMP. A Na⁺ channel blocker, amiloride, and a Na⁺ ionophore, monensin, suppressed the cAMP-dependent increase in cellular ATP. A H⁺ ionophore, carbonyl cyanide m-clorophenylhydrazone, which inhibits the activity of F_oF₁ ATP synthase, did not affect the cAMP-dependent rise in ATP. It is suggested that cAMP stimulates the formation of ATP coupled directly with the translocation of Na⁺.