Intracellular Cl activity, Cl binding, and 36Cl efflux in rabbit papillary muscle

Chloride liquid ion-exchange microelectrodes were used to measure intracellular chloride activity (aCl)i or rabbit papillary muscle in vitro. In HCO3- -- CO2 buffered Ringer (aCl)i was 15.2 +/- 0.6 mM (n = 40) at pH 7.3 and 15.4 +/- 0.3 mM (n = 29) at pH 6.8. In HEPES-buffered Ringer (aCl)i was 24.1 +/- 0.6 mM (n = 21) at pH 7.3 and 20.0 +/- 0.3 mM (n = 17) at pH 6.9. If Cl was distributed passively across the membrane (aCl)i should be about 3 mM. From total muscle Cl analysis and [14C]sorbitol extracellular space measurements we calculate an apparent intracellular Cl concentration of 71 mM. As this estimate is three times larger than the measured free Cl in the cytoplasm we conclude that a significant fraction of muscle Cl must be bound or compartmentalized. Analysis of 36Cl efflux from the papillary muscle indicates the presence of two compartments. The compartment with the slower efflux rate (2.0 X 10(-2) min-1) contains about 25% of the intracellular Cl, which compares with the estimation (21%) of the free intracellular CL fraction obtained from the Cl microelectrode measurements. Our data permit us to conclude that about 37% of the muscle Cl is bound and that it is probably located extracellularly.