Cytosolic Calcium in Isolated Type I Cells of the Adult Rabbit Carotid Body: Effects of Hypoxia, Cyanide and Changes in Intracellular pH

Abstract

It has been suggested (Bernon et al., 1983; Biscoe et al., 1989; Roumy & Leitner, 1977) that cytosolic calcium concentration ([Ca²⁺]_i) in type I cells controls the rate of release of a sensory transmitter and thus spike frequency in chemoafferent units. As the neurotransmitter has not yet been identified the question of its excitatory or inhibitory nature remains unsettled. Nevertheless, there must exist a unique relationship between changes in type I cells [Ca²⁺]_i and variations of chemoafferent activity during stimulation if cytosolic calcium is to play such a critical role. I therefore measured [Ca²⁺]_i of isolated type I cell clusters during hypoxia (PO₂ down to 10 torr). However, as carotid body cells consume oxygen, there is, in vivo, a gradient of PO₂ between blood an tissue, which magnitude is still a subject of debate, that might significantly differ from that between medium and isolated cells. Consequently, I also used cyanide which behaves as an inert gas at physiological pH, and thus equilibrates between blood and carotid body tissue or medium and cell clusters within a few tenths of a second with no concentration gradient (Forster, 1968). It should be remembered that cyanide already excites the chemoafferent units at 10µM concentration (Krilov & Anichkov, 1968 ; Leitner & Roumy, unpublished results). Finally the effects of changes in intracellular pH (pH_i) thought to be involved in the chemoafferent response to CO₂, were also studied.