Ungulate cardiac Purkinje fibres: the influence of intracellular pH on the electrical cell‐to‐cell coupling

Passive electrical properties of sheep cardiac Purkinje fibres were assessed by performing linear cable analysis. In a separate set of experiments pH(i) was monitored using recessed-tip pH-sensitive micro-electrodes.2. In Tris-buffered Tyrode solution (nominally CO(2)-free), the pH(i) was 7.27, in bicarbonate-buffered solution equilibrated with 6% CO(2), the mean pH(i) was 7.02.3. Application of 15 mM-NH(4)Cl produced a rapid intracellular alkalinization (0.19 pH units), followed by a slower acidification. Removal of NH(4)Cl gave rise to a slow and transient intracellular acidification (0.5 pH units).4. The biphasic and transient shift in pH(i), induced by the NH(4)Cl treatment, was accompanied by a change of the inside longitudinal resistance per unit fibre length, r(i), displaying a similar time course. The increase in pH(i) produced a maximum decrease in r(i) of 16.4%, while the decrease in pH(i) yielded a maximum increase in r(i) of 30.4%.5. Changing from bicarbonate-buffered Tyrode solution equilibrated with 6% CO(2) to Tris-buffered Tyrode solution led to an increase in pH(i) (0.26 pH units). A subsequent change to bicarbonate-buffered Tyrode solution equilibrated with 15% CO(2) produced a decrease in pH(i) (0.48 pH units). Both changes were sustained.6. This CO(2) protocol gave rise to corresponding changes in r(i); the intracellular alkalosis was associated with a decrease in r(i) (21.2%), and the intracellular acidosis was accompanied by an increase in r(i) (30%).7. Based on recent findings showing an interaction between pH(i) and pCa(i) (Hess & Weingart, 1980), it is concluded that the changes in r(i) are directly caused by protons and not indirectly via secondary changes of the Ca(2+)(i).8. The pH(i)-dependent changes in r(i) are likely to reflect alterations of the nexal resistance, r(n), because the cytoplasmic resistance, r(c), has the inverse sensitivity to pH(i).9. Unlike pCa(i), pH(i) would seem to be able to modify the cell-to-cell coupling by increasing or decreasing r(i) over a rather narrow range, without ever producing electrical uncoupling.10. Because of basic differences in the action of calcium and protons on the cell-to-cell coupling (magnitude of the effect, operative concentration range), it is tempting to conclude that there is more than one kind of binding site which controls the nexal conductance.