P-O2-P-CO2 stimulus interaction in [Ca2+](i) and CSN activity in the adultrat carotid body

Since glomus cell intracellular calcium ([Ca2+](i)) plays a key role in gen erating carotid sinus nerve (CSN) discharge, we hypothesized that glomus ce ll [Ca2+](i) would correspond to CSN discharge rates during P-O2-P-CO2 stim ulus interaction in adult rat carotid body (CB). Accordingly, we measured s teady state P-O2-P-CO2 interaction in CSN discharge rates during hypocapnia (P-CO2 = 8-10 Torr), normocapnia (P-CO2 = 33-35 Torr) and hypercapnia (P-C O2 = 68-70 Torr) in normoxia (P-O2 similar to 130 Torr) and hypoxia (P-O2 s imilar to 36 Torr). The results showed P-O2-P-CO2 stimulus interaction in C SN responses. [Ca2+](i) levels were measured in isolated type I cells (2-3 cells/field), using Ca2+ sensitive fluoroprobe indo-1AM. The [Ca2+](i) resp onses increased with increasing P-CO2 normoxia. In hypoxia, [Ca2+](i) did n ot increase during hypocapnia but increased during normocapnia, showing P-O 2-P-CO2 interaction. However, CSN response during hypoxia was far greater t han that for [Ca2+](i) response, particularly during hypocapnic hypoxia. Th us, the [Ca2+](i) interaction cannot account for the whole CSN interaction. The origin of this CSN P-O2-P-CO2 interaction must have occurred in part b eyond cellular [Ca2+](i) interaction. Interactions at both sites (glomus ce ll membrane and sinus nerve endings) are reminiscent of reversible O-2-home protein reaction with a Bohr effect. (C) 2000 Elsevier Science B.V. All ri ghts reserved.