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

Intracellular Cl activity (aiCl), 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. Removal of external Cl (glucuronate substituted) produced a fall in aiCl 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 aiCl. The dependence of steady-state aiCl upon external Cl activity, aoCl, was roughly hyperbolic with 50% recovery occurring at an aoCl 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. Exposure of a Cl-depleted fibre to various levels of external Cl produced an exponential rise with time in aiCl. The initial rate-of-rise in aiCl was estimated to be a saturating function of aoCl, with a half-maximal effect occurring at an aoCl 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. Steady-state aiCl was essentially insensitive to changes in external HCO3 concentration, HCO3o, if these changes were made at a constant external pH, pHo, i.e. when a reduction in HCO3o was accompanied by a simultaneous reduction in the partial pressure of CO2, PCO2. In contrast, if PCO2 was maintained constant, then a change in HCO3o (thus producing a change in pHo) resulted in an inverse change in aiCl. This change in aiCl was also accompanied by a change in pHi: when aiCl increased, pHi decreased and vice versa. The anion-exchange inhibitor, DIDS (4,4-diisothiocyanato-stilbene disulphonic acid) abolished the effect on aiCl of changes in HCO3o and pHo (at constant PCO2). Furthermore DIDS reduced the influence of pHo upon pHi. Both the fall of aiCl in Cl-free solution and the subsequent reuptake of Cl following re-exposure to Cl-containing solution were slowed by a reduction in HCO3o (constant pHo, reduced PCO2). Both reuptake and wash-out of Cl were saturating functions of HCO3o with half-maximal effect occurring at an HCO3o of 1-1.3 mmol/l. The reuptake of Cl was little affected by removal of external Na (bis,2-hydroxy ethyl, dimethyl ammonium substituted). The reuptake of Cl was unaffected by amiloride (1 mmol/l) but slowed by piretanide (1 mmol/l).(ABSTRACT TRUNCATED AT 400 WORDS)