Regulation of chloride in quiescent sheep-heart Purkinje fibres studied using intracellular chloride and pH-sensitive micro-electrodes.

1. The intracellular Cl activity, alpha iCl was measured inside quiescent sheep cardiac Purkinje fibres, bathed in normal Tyrode at pH 7.40, buffered with approximately 22 mM-bicarbonate/approximately 5% CO2 + 95% O2. The measurements were made using liquid ion-exchanger Cl-sensitive micro-electrodes. 2. After internal Cl levels had been depleted by prolonged exposure to Cl-free media (glururonate-substituted) when external Cl was restored, there was a rapid re-accumulation of Cl inside the fibres to levels that were much higher than those expected for a passive Cl distribution. Such a process can be conveniently defined as an active inward Cl pump. 3. The inward-pumping was noticeably temperature-sensitive (Q10 approximately 2.6), its rate was reduced about eighteenfold in the nominal absence of external bicarbonate/CO2 and it was substantially inhibited by the drug SITS (4-acetamido-4'-isothiocyanato-stilbene-2,2'-disulphonic acid). 4. The fall of alpha iCl in Cl-free solution was slow and was also equally temperature-sensitive and substantially inhibited by SITS, but was only slightly impaired in the nominal absence of external bicarbonate/CO2. 5. pHi was measured using recessed-tip pH-sensitive micro-electrodes, and in some experiments both pHi and alpha iCl were monitored simultaneously. When alpha iCl slowly declined in Cl-free solution then pHi slowly became alkaline. Upon restoring external Cl, then there was, as usual, a rapid recovery of a high alpha iCl and this was accompanied by a rapid re-acidification of pHi. Both the recovery of alpha iCl and pHi were exponential with virtually the same time constant. 6. Both the slow alkalinization of pHi in Cl-free solution and the rapid re-acidification upon restoring external Cl were substantially inhibited by the drug SITS. 7. When [k]O was raised to 45 mM or more (by removing equivalent amounts of [Na]O), there was a large depolarization of Em and a slow rise of alpha iCl, which was not accompanied by a large change of pHi. The rise of alpha iCl appeared to be unaffected by SITS. 8. It is suggested that a Cl/CHO-3 exchange mechanism can operate reversibly across the membrane of quiescent Purkinje fibres, and that it can account, at least in part, for the high levels of alpha iCl measured in the resting state. It is also concluded that Cl can cross the membrane in other ways, especially in high-K solution possibly by moving passively through conductance channels that are open under these conditions.