'Hypoxic Ventilatory Decline' in the Intracellular Ca2+ Response to Sustained Isocapnic Hypoxia in Carotid Body Glomus Cells

In humans the ventilatory response to sustained isocapnic hypoxia is biphasic: after an initial rapid rise there follows a steady decline of the next 20-30 mm termed hypoxic ventilatory decline (HVD). It is not known whether this secondary phase resides in a reducing activity of the peripheral or the central chemoreflex. We wished to assess if the Ca(2+) transient that occurs in glomus cells in response to hypoxia exhibits a form of HVD with sustained hypoxia that parallels the human ventilatory response, or if it exhibits a different response. Glomus cells enzymatically isolated from rat pups were exposed to 10 min sustained hypoxia (5% CO(2) in N(2)), asphyxia (20% CO(2) in N(2)), hypercapnia (20% CO(2) in air), 100 mM K(+) and 2 mM Ba(2+). Intracellular Ca(2+) transients [Ca(2+)]i were measured using indo-1 dye. Hypoxia elicited rapid increase in [Ca(2+)]i followed by a gradual persistent decline over 10 min to 50% of the peak value. Asphyxia also elicited a biphasic response, with the acute response twice as great as that for hypoxia and the subsequent decline also twice as large occurring over a similar time course. Hypercapnia- and hyperkalaemia-evoked [Ca(2+)]i responses displayed a more rapid initial decline (within 2- min) but then stabilised. Exposure to Ba(2+) evoked characteristic spiking activity in the [Ca(2+)]i signal. Although the glomus cell shows some adaptation of response to a variety of stimuli, its response to hypoxia is characterized by a biphasic response with continued secondary decline in [Ca(2+)]i in a manner akin to HVD.