title: Regulation of intracellular pH in sheep cardiac Purkinje fibre: interactions among Na+, H+, and Ca2+1.
abstract: Experiments were performed on sheep cardiac Purkinje fibres using pH- and sodium-selective microelectrodes, while simultaneously measuring tension, to determine if the fall in intracellular pH (pHi) following a rise in intracellular Na+ activity (aiNa) is caused by inhibition or reversal of acid extrusion on Na+-H+ exchange. A rise in aiNa was induced either by using the cardioactive steroid strophanthidin to inhibit the sarcolemmal Na+-K+ pump or by increasing the frequency of stimulation (0-4 Hz). Both of these manoeuvres led to an increase in aiNa and a decrease in pHi. Following exposure to strophanthidin, amiloride (an inhibitor of sarcolemmal Na+-H+ exchange) produced a decrease in both pHi and aiNa. These effects of amiloride increased with decreasing pHi, indicating that acid extrusion on Na+-H+ exchange is stimulated by the fall in pHi. The changes in intracellular Na+ and H+ caused by amiloride were quantitatively consistent with an electroneutral stoichiometry. The fall in pHi during strophanthidin exposure is therefore not caused by inhibition or reversal of acid extrusion Na+-H+ exchange. It is likely that the fall in pHi during a rate increase is also independent of Na+-H+ exchange. This is because (i) it has been shown previously to occur in the presence of amiloride and (ii) the calcium antagonist D600 completely abolished the stimulation-dependent fall in pHi. It is concluded that the intracellular acidosis following inhibition of the sarcolemmal Na+-K+ pump or following an increase in the rate of stimulation is secondary to a rise in intracellular calcium.
mesh_terms: D000584:Amiloride / Q000494:pharmacology; D000643:Ammonium Chloride; D000818:Animals; D001826:Body Fluids / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D004558:Electric Stimulation; D006329:Heart Conduction System / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007424:Intracellular Fluid / Q000378:metabolism*; D008564:Membrane Potentials / Q000187:drug effects; D008839:Microelectrodes; D011690:Purkinje Fibers / Q000378:metabolism*; D012756:Sheep; D012964:Sodium / Q000378:metabolism*; D013327:Strophanthidin / Q000494:pharmacology
chemical_list: D000643:Ammonium Chloride; D013327:Strophanthidin; D000584:Amiloride; D012964:Sodium; D002118:Calcium