Electrodiffusive movements of chloride ion in sodium-free solution: a possible calcium activated chloride conductance in cultured heart cells.
OBJECTIVE
The aim was to demonstrate that removal of extracellular sodium (Na+o), a manoeuvre known to increase intracellular calcium (Cai2+), can activate a chloride ion conductance in cultured embryonic chick heart cells.


METHODS
Intracellular chloride activity (aiCl) and membrane potentials (Em) were simultaneously measured using chloride selective and 3 M KCl filled microelectrodes.


RESULTS
In Na+o-free and K+o-free solutions, a frusemide (0.3 mM) resistant decrease in aiCl of 10 mM was recorded within 10 min, along with a 5 mV hyperpolarisation, and the calculated chloride equilibrium potential (ECl) followed the change of Em, suggesting a possibly enhanced chloride conductance. When cells were exposed to Na+o-free solution, the decrease of aiCl by approximately 10 mM was associated with an initial depolarisation, followed by a hyperpolarisation to approximately -60 mV (more negative than ECl). Subsequent removal of K+o depolarised Em to -39 mV (approximately ECl), and no further loss of aiCl occurred. Restoration of K+o caused a hyperpolarisation of Em (more negative than ECl) and a continuing decline of aiCl. Prevention of K+o induced hyperpolarisation by addition of 1 mM Ba2+ stopped the decline of aiCl (Em approximately ECl), suggesting that following Na+o removal, alteration of the driving force for chloride led to a chloride efflux via an enhanced chloride conductance. When Em depolarised to -50 mV after 3 min exposure to 0.1 mM ouabain, removal of [Cl-]o caused a further depolarisation and readdition of [Cl-]o induced an 18 mV hyperpolarisation. This chloride induced hyperpolarisation was blocked by removal of [Ca2+]o (+1 mM EGTA).


CONCLUSIONS
The increase in chloride conductance observed under conditions that are known to raise Ca2+i supports the presence of a Ca2+i activated chloride conductance in cultured chick heart cells.
Department of Cell Biology, Duke University Medical Center, Durham, North Carolina.
S
S
Liu
M
M
Lieberman
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D002642:Chick Embryo; D002712:Chlorides / Q000378:metabolism*; D004553:Electric Conductivity; D007424:Intracellular Fluid / Q000378:metabolism*; D017136:Ion Transport; D008564:Membrane Potentials / Q000187:drug effects; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D010042:Ouabain / Q000494:pharmacology; D000254:Sodium-Potassium-Exchanging ATPase / Q000187:drug effects
D002712:Chlorides; D010042:Ouabain; D000254:Sodium-Potassium-Exchanging ATPase; D002118:Calcium