Na+/H+ Exchange Blockers Reveal the Existence of a Skeletal Muscle Ca2+/H+ Exchanger, which is Altered in Malignant Hyperthermia Muscle Cells

Abstract

In resting skeletal muscle fibers intracellular pH (pHi) is kept constant at a relatively alkaline level. The transporters involved in maintaining muscle pHi at rest are the Na+/H+ exchange system (NHE), and to a lesser extent the Na+- and Cl--dependent bicarbonate dependent transport systems. Many studies conducted in nerve and smooth cells have suggested a link between changes in intracellular free Ca2+ ([Ca2+]i) and changes in intracellular pH (pHi) suggesting a Ca2+/H+ exchange (Daugirdas et al, 1995). Malignant hyperthermia (MH) is a pharmacogenetic disorder of skeletal muscle triggered by inhalational anesthetics and depolarizing neuromuscular blocking agents. We have measured [Ca2+]i and pHi simultaneously in Wt and heterozygous R163C myotubes using double barreled Ca2+-selective and single barreled pH+-selective microelectrodes. MH cells have a higher [Ca2+]i and a lower pHi than Wt cells. Treatment of both MH and Wt cells with the NHE inhibitors dimethylamiloride (DMA) or cariporide, caused an increase in [Ca2+]i and a decrease in pHi in a dose dependent manner. These effects were more prominent in MH than Wt myotubes. YM-244769, a high affinity reverse mode NCX3 blocker, did not modify [Ca2+]i overload or the drop in pHi elicited by DMA or cariporide in either cell type, suggesting that the change in [Ca2+]i was not the consequence to an activation of the reverse form of the exchanger. Gd3+ and dantrolene respectively were able to partially inhibit or fully reverse the DMA or cariporide-mediated elevation of [Ca2+]i and acidification in both types of cells. These results suggest the existence of a Ca2+/H+ exchange in skeletal myotubes, which appears to be altered in MH muscle cells.