ACETYLCHOLINE INCREASES INTRACELLULAR CALCIUM OF ARTERIAL CHEMORECEPTOR CELLS OF ADULT CATS

Several neurotransmitters have been reported to play important roles i n the chemoreception of the carotid body. Among them acetylcholine (AC h) appears to be involved in excitatory processes in the cat carotid b ody. 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 intr acellular calcium concentration ([Ca2+](i)) of cultured carotid body c ells. The carotid body from adult cats was dissociated and cultured fo r up to 2 wk. [Ca2+](i) was measured from clusters of cells with a mic rofluorometric technique using Indo-1 AM. Experiments were performed a t 37 degrees C, and cells were continuously superfused with modified K rebs solutions equilibrated with 5% CO2-16% O-2-79% N-2. ACh (100 mu M ) caused a marked increase in [Ca2+](i) in similar to 70% of clusters, and the responses to 1-300 mu M of ACh were concentration dependent. The magnitude and kinetics of the ACh response were mimicked by the ap plication of nicotine, whereas muscarinic agonists, pilocarpine, and m uscarine failed to evoke a similar response. ACh-induced increase in [ Ca2+](i) was dependent on extracellular Ca2+: it was greatly reduced o r completely abolished by a transient removal of extracellular Ca2+. T he response was consistently but only partially reduced by caffeine (5 mM) or nifedipine (10 mu M). The effect of mecamylamine (100 mu M) wa s inhibitory but small. Moreover, the increase in [Ca2+](i) in respons e to ACh was also observed in some clusters that did not respond to hi gh 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, p erhaps secondary to membrane depolarization, and Ca2+ release from int racellular stores may participate in increasing [Ca2+](i) in response to ACh. Muscarinic receptors appear to play only a small role, if any.