Intracellular pH regulation in neurons from chemosensitive and nonchemosensitive regions of Helix aspersa

We used 2',7'-bis( carboxyethyl)- 5(6)-carboxyflourescein (BCECF), a pH-sen sitive fluorescent dye, to study intracellular pH (pH(i)) regulation in neu rons in CO2 chemoreceptor and nonchemoreceptor regions in the pulmonate, te rrestrial snail, Helix aspersa. We studied pH(i) during hypercapnic acidosi s, after ammonia prepulse, and during isohydric hypercapnia. In all treatme nt conditions, pH(i) fell to similar levels in chemoreceptor and nonchemore ceptor regions. However, pH(i) recovery was consistently slower in chemorec eptor regions compared with nonchemoreceptor regions, and pH(i) recovery wa s slower in all regions when extracellular pH (pH(e)) was also reduced. We also studied the effect of amiloride and DIDS on pH(i) regulation during is ohydric hypercapnia. An amiloride-sensitive mechanism was the dominant pH(i ) regulatory process during acidosis. We conclude that pH(e) modulates and slows pH(i) regulation in chemoreceptor regions to a greater extent than in nonchemoreceptor regions by inhibiting an amiloride-sensitive Na+/H+ excha nger. Although the phylogenetic distance between vertebrates and invertebra tes is large, similar results have been reported in CO2-sensitive regions w ithin the rat brain stem.