Regulation of cAMP-stimulated ion current by intracellular pH, Ca²⁺, and calmodulin blockers

1. Iontophoretic injection of adenosine 3',5'-cyclic monophosphate (cAMP) into identified neurons elicited a slow transient Na⁺ current whose amplitude and duration were sensitive to altered intracellular pH (pH(i)), calmodulin blocking drugs, depolarization, and manipulations of internal and external Ca²⁺. 2. Intracellular acidification between resting pH(i) to several tenths of a pH unit increased the amplitude of the cAMP-stimulated current and prolonged its duration. 3. Intracellular alkalinization of similar magnitude also increased the amplitude and duration of the current response. The effects of alkalinization were somewhat labile. In cells alkalinized by NH₄⁺-containing salines, washout of NH₄⁺ with normal saline caused acidification and further enhanced the cAMP current response. The immediacy of the increase and the dual acid/basic sensitivity of the response suggest an accommodative process whereby the responsiveness of the cell to cAMP adapts to a maintained pH(i). 4. The calmodulin blockers trifluoperazine and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide increased the amplitude and duration of the current response. Phorbol ester activators of Ca²⁺/phospholipid-dependent kinase had no effect on the current. 5. Periods of depolarization preceding tests significantly reduced current response amplitude. This effect was dependent on saline Ca²⁺ and was blocked by Co²⁺. 6. Intracellular injection of the Ca²⁺ chelator ethylene glycol-bis(β-aminoethyl ether)N,N,N',N',-tetraacetic acid also augmented the amplitude and duration of the current response. 7. The above effects are consistent with a possible common site of action on cAMP degradation. This interpretation is consistent with previous evidence for pH-sensitive and Ca²⁺/calmodulin-dependent cAMP phosphodiesterase activity in Pleurobranchaea nervous tissue.