Modulation of dog atrial swelling‐induced chloride current by cAMP: protein kinase A‐dependent and ‐independent pathways.

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Abstract
1. The modulation of dog atrial swelling‐induced chloride current (I(Cl,swelling)) by cAMP‐elevating agents was studied. Forskolin (10 microM) or isoprenaline (1 microM) exerted multiple effects. Although the pattern between cells was variable, there was, in general, a stimulatory action and a more slowly developing inhibitory effect. 2. In any given cell, the response to forskolin or isoprenaline was qualitatively similar suggesting that all of the responses were dependent on stimulation of adenylyl cyclase. The effects of forskolin or isoprenaline on I(Cl,swelling) were inhibited by intracellular dialysis with a P‐site inhibitor of adenylyl cyclase, 2'‐deoxyadenosine 3'‐monophosphate (300 microM). 3. Intracellular dialysis with a peptide inhibitor of protein kinase A (PKI(6‐22); 100 microM) blocked the inhibitory response to forskolin or isoprenaline and all cells responded with a monophasic stimulation of I(Cl,swelling). 4. After intracellular dialysis of cells with PKI(6‐22) (100 microM) and cAMP (100 microM), current amplitude was not further stimulated by forskolin. 5. After intracellular dialysis with PKI(6‐22) and adenosine 5'‐O‐(3‐thiotriphosphate) (ATPgammaS), forskolin stimulated I(Cl,swelling) and the effect of forskolin subsided after it was washed out. 6. In conclusion, there are dual pathways by which cAMP can modulate dog atrial cell I(Cl,swelling). Inhibition results from protein kinase A (PKA)‐dependent phosphorylation. In addition, a stimulatory pathway exists that is independent of phosphorylation by PKA or other cellular kinases. Although alternative explanations are possible, the stimulatory effect of cAMP may represent a direct modulation of I(Cl,swelling).