Intracellular Acidification Is Associated with Changes in Free Cytosolic Calcium and Inhibition of Action Potentials in Rat Trigeminal Ganglion

The effect of intracellular acidification and subsequent pH recovery in sensory neurons has not been well characterized. We have studied the mechanisms underlying Ca2+-induced acidification and subsequent recovery of intracellular pH (pH(i)) in rat trigeminal ganglion neurons and report their effects on neuronal excitability. Glutamate (500 mu M) and capsaicin (1 mu M) increased intracellular Ca2+ concentration ([Ca2+](i)) with a following decrease in pH(i). The recovery of [Ca2+](i) to the prestimulus level was inhibited by LaCl3 (1 mM) and o-vanadate (10 mM), a plasma membrane Ca2+/ATPase (PMCA) inhibitor. Removal of extracellular Ca2+ also completely inhibited the acidification induced by capsaicin. TRPV1 was expressed only in small and medium sized trigeminal ganglion neurons. mRNAs for Na+/H+ exchanger type 1 (NHE1), pancreatic Na+-HCO3- cotransporter type 1 (pNBC1), NBC3, NBC4, and PMCA types 1-3 were detected by RT-PCR. pH(i) recovery was significantly inhibited by pretreatment with NHE1 or pNBC1 siRNA. We found that the frequency of action potentials (APs) was dependent on pH(i). Application of the NHE1 inhibitor 5'-(N-ethyl-N-isopropyl) amiloride (5 mu M) or the pNBC1 inhibitor 4',4'-di-isothiocyanostilbene-2',2'-sulfonic acid (500 mu M) delayed pH(i) recovery and decreased AP frequency. Simultaneous application of 5'-(N-ethyl-N-isopropyl) amiloride and 4',4'-di-isothiocyanostilbene-2',2'-sulfonic acid almost completely inhibited APs. In summary, our results demonstrate that the rise in [Ca2+](i) in sensory neurons by glutamate and capsaicin causes intracellular acidification by activation of PMCA type 3, that the pH(i) recovery from acidification is mediated by membrane transporters NHE1 and pNBC1 specifically, and that the activity of these transporters has direct consequences for neuronal excitability.