Response of intracellular pH to acute anoxia in individual neurons from chemosensitive and nonchemosensitive regions of the medulla.

The effect of acute (10 minutes) exposure to anoxia on intracellular pH (pHi) in individual brainstem neurons, in slices from neonatal (P7 to P11) rats, was studied using a fluorescence microscopy imaging technique. Neurons from 4 regions of the medulla were studied, two of which contained chemosensitive neurons (nucleus tractus solitarius, NTS, and ventrolateral medulla, VLM) and two regions which did not contain chemosensitive neurons (hypoglossal, Hyp, and inferior olivary, IO). Acute anoxia caused a rapid and maintained acidification of 0.1-0.3 pH unit that was not different in neurons from chemosensitive vs. nonchemosensitive regions. Blocking the contribution of Na+/H+ exchange (NHE) to pHi regulation by exposing neurons to acute anoxia in the presence of the exchange inhibitor amiloride (1 mM) did not affect the degree of acidification seen in neurons from the NTS and VLM region, but significantly increased acidification (to about 0.35 pH unit) in Hyp and IO neurons. In summary, anoxia-induced intracellular acidification is not different between neurons from chemosensitive and nonchemosensitive regions, but NHE activity blunts acidification in neurons from the latter regions. These data suggest that neurons from chemosensitive areas might have a smaller acid load in response to anoxia than neurons from nonchemosensitive regions of the brainstem.