An investigation of chloride‐bicarbonate exchange in the sheep cardiac Purkinje fibre.

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Abstract
1. Intracellular Cl activity .**GRAPHIC**. and intracellular pH (pHi) were measured in isolated sheep cardiac Purkinje fibres using a liquid ion exchanger Cl-selective micro-electrode and a glass recessed-tip, pH-selective micro-electrode. 2. Removal of external Cl (glucuronate substituted) produced a fall in .**GRAPHIC**. from about 20 to about 4 mmol/l: the residual level is probably caused by intracellular interference on the Cl-sensitive electrode. Re-exposure of the fibre to increased levels of external Cl produced, in the steady state, increased levels of .**GRAPHIC**. The dependence of steady-state .**GRAPHIC**. upon external Cl activity, .**GRAPHIC**. was roughly hyperbolic with 50% recovery occurring at an .**GRAPHIC**. of about 9.5 mmol/l. At all levels of external Cl tested, Cl was accumulated to a level much higher than that predicted for passive electrochemical equilibrium. 3. Exposure of a Cl-depleted fibre to varius levels of external Cl produced an exponential rise with time in .**GRAPHIC**. The initial rate-of-rise in .**GRAPHIC**. was estimated to be a saturating function of .**GRAPHIC**. with a half-maximal effect occurring at an .**GRAPHIC**. of about 33 mmol/l. The rate-of-rise was about 10-fold greater than that predicted from constant-field theory using published values for PCl, the Cl permeability coefficient. 4. Steady-state .**GRAPHIC**. was essentially insensitive to changes in external HCO3 concentration, [HCO3]0, if these changes were made at a constant external pH, PH0, i.e. when a reduction in [HCO3]0 was accompanied by a simultaneous reduction in the partial pressure of CO2, PCO2. 5. In contrast, if PCO2 was maintained constant, then a change in [HCO3]0 (thus producing a change in pH0) resulted in an inverse change in .**GRAPHIC**. This change in .**GRAPHIC**. was also accompanied by a change in pHi: when .**GRAPHIC**. increased, pHi decreased and vice versa. 6. The anion-exchange inhibitor, DIDS (4,4-diisothiocyanato-stilbene disulphonic acid) abolished the effect on .**GRAPHIC**. of changes in [HCO3]0 and pH0 (at constant PCO2). Furthermore DIDS reduced the influence of pH0 upon pHi. 7. Both the fall of .**GRAPHIC**. in Cl-free solution and the subsequent reuptake of Cl following re-expose to Cl-containing solution were slowly by a reduction in [HCO3]0 (constant pH0, reduced PCO2). Both reuptake and wash-out of Cl were saturating functions of [HCO3]0 of 1-1.3 mmol/l. 8. The reuptake of Cl was little affected by removal of external Na (bis,2-hydroxy ethyl, dimethyl ammonium substituted). 9. The reuptake of Cl was unaffected by amiloride (1 mmol/l) but slowed by piretanide (1 mmol/l). 10. The results are discussed in terms of a saturable and reversible Cl-HCO3 exchange carrier int he sarcolemma with competition between Cl and HCO3 ions for binding and transport. It is proposed that such a mechanism can account quantitatively for the high .**GRAPHIC**. measured in the steady state.