A novel Cl- conductance in cultured chick cardiac myocytes: role of intracellular Ca2+ and cAMP.

Cl- conductance in cultured embryonic chick cardiac myocytes was characterized using whole-cell patch clamp techniques. Following elimination of cation currents in Na+- and K+-free internal and external solutions, the basal whole-cell current was predominantly a Cl- current. Cl--sensitive current (I-Cl) was defined as the difference between the whole-cell currents recorded in normal and low [Cl-](0) when measured in the same cell. The whole-cell current in the absence or presence of 10 mu M cAMP was time independent, displayed outward rectification with the pipette [Cl-] < 40 mM, and was not saturated with a physiological Cl- gradient. The Cl- current was also activated by 1 mu M forskolin and inhibited by 0.3 mM anthracene-9-carboxylic acid (9-AC). Forskolin was less effective than cAMP (internal dialysis) in activating the Cl- current. The cAMP- or forskolin-activated and basal Cl- current were reasonably fit by the Goldman-Hodgkin-Katz equation. The calculated P-Cl in the presence of cAMP was increased by five- to sixfold over the basal level. In the presence of 5 mM EGTA to decrease free [Ca2+](i), the whole-cell current could not be stimulated by cAMP, forskolin or IBMX (0.1 mM). These data suggest that cultured chick cardiac myocytes have a low basal Cl- conductance, which, as in some mammalian cardiac ventricular myocytes, can be activated by cAMP. However, this study shows that the activation process requires physiological free [Ca2+](i).