Several neurotransmitters have been reported to play important roles in the chemoreception of the carotid body. Among them acetylcholine (ACh) appears to be involved in excitatory processes in the cat carotid body. As one of the steps to elucidate possible roles of ACh in carotid body chemoreception in the cat, we examined the effect of ACh on intracellular calcium concentration ([Ca2+](i)) of cultured carotid body cells. The carotid body from adult cats was dissociated and cultured for up to 2 wk. [Ca2+](i) was measured from clusters of cells with a microfluorometric technique using Indo-1 AM. Experiments were performed at 37°C, and cells were continuously superfused with modified Krebs solutions equilibrated with 5% CO2-16% O2-79% N2. ACh (100 μM) caused a marked increase in [Ca2+](i) in ~70% of clusters, and the responses to 1-300 μM of ACh were concentration dependent. The magnitude and kinetics of the ACh response were mimicked by the application of nicotine, whereas muscrinic agonists, pilocarpine, and muscarine failed to evoke a similar response. ACh-induced increase in [Ca2+](i) was dependent on extracellular Ca2+: it was greatly reduced or completely abolished by a transient removal of extracellular Ca2+. The response was consistently but only partially reduced by caffeine (5 mM) or infedipine (10 μM). The effect of mecamylamine (100 μM) was inhibitory but small. Moreover, the increase in [Ca2+](i) in response to ACh was also observed in some clusters that did not respond to high K (100 mM) Krebs. These results suggest that ACh increases [Ca2+](i) of cultured carotid body cells by activating neuronal nicotinic ACh receptors, leading to Ca2+ influx via nicotinic channels. In addition, other pathways such as Ca2+ influx through L-type calcium channels, perhaps secondary to membrane depolarizatin, and Ca2+ release from intracellular stores may participate in increasing [Ca2+](i) in response to ACh. Muscarinic receptors appear to play only a small role, if any.
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