The regulation of intracellular calcium by cholinergic agonists was investigated in the human neuroblastoma SH-SY5Y, loaded with fura-2. The resting free Ca2+ concentration in this cell line was 199 +/- 14 nM (mean +/- SEM, n = 19). At 1 mM extracellular Ca2+, high concentrations of carbachol and acetylcholine evoked a biphasic change in intracellular Ca2+ concentration, consisting of a transient initial peak followed by a decline to a plateau that was significantly higher than the basal level. Carbachol (0.5 mM) and acetylcholine (10 microM) caused a maximal increase in the intracellular Ca2+ concentration, reaching a peak of 465 +/- 52 (mean +/- SEM, n = 12) and 422 +/- 48 nM (mean +/- SEM, n = 7), respectively, in less than 4 s. This initial calcium transient declined to a plateau of 268 +/- 36 and 240 +/- 27 nM for carbachol and acetylcholine, respectively, in approximately 40 s. The plateau persisted until the agonist was displaced by the addition of antagonist. Atropine, hexahydrosiladifenidol (HHSD), pirenzepine, and methoctramine inhibited the carbachol-evoked initial calcium transient with Ki values of 0.85 +/- 0.05, 8.3 +/- 1.6, 411 +/- 36, and 240 +/- 46 nM (mean +/- SEM, n = 3), respectively, and the acetylcholine-induced initial calcium transient with Ki values of 0.48 +/- 0.18, 13.5 +/- 8.5, 192 +/- 32, and 414 +/- 25 nM (mean +/- SEM of two experiments), respectively, results suggesting that an M3 muscarinic receptor was predominantly mediating these effects.(ABSTRACT TRUNCATED AT 250 WORDS)