Cyclic AMP prevents activation of a swelling‐induced chloride‐sensitive conductance in chick heart cells.

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Abstract
1. Changes in myocardial cell volume and whole‐cell currents were measured simultaneously during hyposmotically induced cell swelling. In the conventional patch clamp configuration, hyposmotic challenge caused myocytes to swell continuously and was associated with the development of a sustained, swelling‐induced chloride conductance (ICl). In contrast, perforated patch‐clamped myocytes demonstrated regulatory volume decreases (RVD) during hyposmotic challenge, and ICl was not generated. 2. The swelling‐induced ICl in conventionally patch‐clamped myocytes was inhibited by application of forskolin (15 microM) and was prevented when the pipette filling solution contained cAMP (10 microM) and isobutylmethylxanthine (IBMX, 1 mM). ICl could also be prevented by inhibition of protein phosphatase activity, using okadaic acid (100 nM). Conversely, a swelling‐induced current could be generated in myocytes under perforated patch clamp by inhibition of protein kinase A, using the antagonist Rp‐cAMPS (10 microM). These data demonstrate that cAMP‐dependent protein phosphorylation is both necessary and sufficient to prevent development of ICl during cell swelling. 3. Unlike other chloride currents described previously in heart muscle, generation of the novel swelling‐induced ICl requires dephosphorylation of a cAMP‐dependent protein phosphorylation site; hence it can be prevented by stimulation of cAMP‐dependent protein phosphorylation or by inhibition of protein phosphatase activity.