Calcium‐inhibitable current in cultured embryonic chick cardiac myocytes: possibly via a novel chloride channel

The role of extracellular Ca2+ (Ca(2+)o) in the modulation of cardiac Cl‐ currents (I(Cl)) such as those activated by cAMP or swelling is uncertain. The effects of Ca(2+)o and extracellular cadmium (Cd(2+)o) on Cl‐ currents in cultured chick cardiac myocytes were investigated in Na+− and K+−free internal and external solutions using the whole‐cell patch‐clamp technique. In the absence of Na+ and K+ internally and externally, the whole‐cell current was predominantly I(Cl). In the absence of cAMP, removal of Ca(2+)o (+ 1 mM EGTA) resulted in an increase in the current that was suppressed by reduction of Cl(o)‐ with a rightward shift of the zero‐current potential towards the CI‐ reversal potential. We designated this current as a Ca2+−inhibitable I(Cl). Addition of 0.5 mM Cd(2+)o with or without removal of Ca(2+)o also resulted in a 1.5− to 2.0‐fold increase in I(Cl) that was attenuated by 1 mM DIDS (4,4'−diisothiocyanatostilbene−2,2'−disulphonic acid). Under similar conditions, I(Cl) activated by Cd(2+)o (in 1 mM Ca(2+)o solution) was not further increased by subsequent removal of Ca(2+)o, suggesting that addition of Cd(2+)o and removal of Ca(2+)o activated the same I(Cl). In contrast, exposure to 1 microM forskolin further enhanced I(Cl) in the presence of Cd(2+)o. With 10 microM cAMP in the pipette solution, Ca2+−inhibitable I(Cl) could be activated in myocytes that do not possess cAMP‐activated Cl‐ channels, indicating that activation of Ca2+−inhibitable I(Cl) does not require cAMP. In the presence of cAMP, in cells that display the cAMP‐activated I(Cl), removal of Ca(2+)o resulted in a further increase in I(Cl) comparable to the Ca2+−inhibitable I(Cl). The Ca2+−inhibitable I(Cl) was minimized when pipette solutions contained 1.5 microM Ca2+. These results suggest that removal of Ca(2+)o or application of Cd(2+)o activates a Ca2+−inhibitable I(Cl) that is distinct from the cAMP‐activated I(Cl).