The uptake of 45Ca2+ and secretion of catecholamines by primary cultures of adrenal medulla cells were studied. Nicotine, veratridine, potassium, and Ionomycin stimulate both the accumulation of 45Ca2+ and the secretion of catecholamines. Nicotinic antagonists block 45Ca2+ uptake induced by nicotine, tetrodotoxin blocks 45Ca2+ uptake induced by veratridine, and D600 or secretion induced by Ionomycin. The EC50 for nicotine is 3 microM for catecholamine secretion and 10 microM for 45Ca2+ uptake, while the EC50s for veratridine-stimulated uptake and secretion are approximately the same (75 microM). Kinetic studies show that the uptake of Ca2+ is rapid and appears to precede the secretion of catecholamines, and that the rate of uptake declines rapidly. The 50 mM-K+ show saturation kinetics with respect to external calcium concentrations at about 2 mM. On the other hand, the uptake of 45Ca2+ stimulated by nicotine does not become saturated at external calcium concentrations of 10 mM although the secretion of catecholamines reaches a maximum at external calcium concentrations of 2 mM. The data suggest that depolarizing agents such as veratridine and 50 mM-K+ stimulate 45Ca2+ entry through voltage-sensitive calcium channels, while nicotinic agonists stimulate calcium entry through the acetylcholine receptor ion channels as well as through voltage-sensitive calcium channels.
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