CALCIUM-INHIBITABLE CURRENT IN CULTURED EMBRYONIC CHICK CARDIAC MYOCYTES - POSSIBLY VIA A NOVEL CHLORIDE CHANNEL

The role of extracellular Ca2+ (Ca-0(2+)) in the modulation of cardiac Cl- currents (I-Cl) such as those activated by cAMP or swelling is un certain. The effects of Ca-0(2+), and extracellular cadmium (Cd-0(2+)) on Cl- currents in cultured chick cardiac myocytes were investigated in Na+- and K+-free internal and external solutions using the whole-ce ll patch-clamp technique. In the absence of Na+ and K+ internally and externally, the whole-cell current was predominantly I-Cl. In the abse nce of cAMP, removal of Ca-0(2+) (+ 1 mM EGTA) resulted in an increase in the current that was suppressed by reduction of Cl-0(-), with a ri ghtward shift of the zero-current potential towards the Cl- reversal p otential. We designated this current as a Ca2+-inhibitable I-Cl. Addit ion of 0.5 mM Cd-0(2+) with or without removal of Ca-0(2+) also result ed 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 simi lar conditions, I-Cl activated by Cd-0(2+); (in 1 mM Ca-0(2+) solution ) was not further increased by subsequent removal of Ca-0(2+), suggest ing that addition of Cd-0(2+) and removal of Ca-0(2+) activated the sa me I-Cl. In contrast, exposure to 1 mu M forskolin further enhanced I- Cl in the presence of Cd-0(2+). With 10 mu M cAMP in the pipette solut ion, 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-0(2+), resul ted in a further increase in I-Cl, comparable to the Ca2+-inhibitable I-Cl. The Ca2+-inhibitable I-Cl was minimized when pipette solutions c ontained 1.5 mu M Ca2+. These results suggest that removal of Ca-0(2+) or application of Cd-0(2+) activates a Ca2+-inhibitable I-Cl, that is distinct from the cAMP-activated I-Cl.