1. In view of conflicting reports on the source of Ca2+ needed to trigger the secretory response to muscarinic stimulation of chromaffin cells, we have reinvestigated this problem in the cat adrenal gland perfused with oxygenated Krebs solution at 37-degrees-C. Above a basal rate of secretion of 60 ng/30 s of total catecholamines, 5 s pulses of 100-mu-M-methacholine evoked 10-fold increases of secretion. This response was entirely mediated by muscarinic receptors, since it was blocked by submicromolar concentrations of atropine but not by d-tubocurarine. 2. Delayed application of methacholine pulses after Ca2+ removal from the Krebs solution led to a progressive decline of the secretory response with a t1/2 of 15 s. Secretion was blocked by 85% after a 60 s period of Ca2+ deprivation; extension of the external Ca2+ (Ca(o)2+) wash-out period up to 5 min did not further reduce the secretory response. 3. When EGTA (1 mM) was present in the 0 Ca2+ solution, the rate of decline of methacholine responses, as a function of the time of exposure to 1 mM-EGTA, was similar to that obtained with 0 Ca2+. Again, about 15-20% of the secretory response was resistant even to prolonged periods of washing out with the 0 Ca2+-EGTA solution. 4. The Ca2+ ionophore ionomycin (1-mu-M) first decreased and then accelerated the rate of decline of methacholine responses upon Ca(o)2+ wash-out. Particularly relevant is the complete blockade of secretion when the Ca(o)2+ wash-out is performed in the presence of this ionophore. This suggests the existence of a small intracellular functional Ca2+ store sensitive to ionomycin. 5. After abolition of the secretory response through 60 s periods of wash-out with a 0 Ca2+-EGTA-ionomycin solution, followed by delayed 5 s methacholine pulses after Ca(o)2+ reintroduction, the glands instantly recovered their normal muscarinic-mediated secretory response. This suggests that upon muscarinic stimulation, Ca2+ required by the secretory machinery to trigger such response immediately comes from extracellular sources. How Ca(o)2+ gains the cell interior so fast upon muscarinic stimulation is unknown; we have previously suggested that the muscarinic receptor in the cat chromaffin cell could be coupled to an ionophore channel which might be chemically activated by muscarinic agonists. 6. Secretory responses to 5 s pulses with 35 or 100 mM-K+ declined faster (t1/2 of 3 and 6 s, respectively) upon Ca(o)2+ wash-out than those of methacholine. It seems, therefore, that K+-evoked secretion depends on Ca(o)2+ entry in a manner more astringent than the muscarinic-mediated secretory response. 7. Secretion evoked by the selective nicotinic receptor agonist 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP, 5 s pulses of 100-mu-M) declined with the time of Ca(o)2+ removal with a t1/2 of 8-9 s. In the case of acetylcholine (100-mu-M for 5 s), the decline of secretion upon Ca(o)2+ removal exhibited a t1/2 of 12 s. 8. In conclusion, the Ca2+ required by the secretory machinery in cat chromaffin cells stimulated with acetylcholine, DMPP, methacholine or high K+ seems to come mostly from the extracellular milieu. However, an intracellular Ca2+ pool probably located in the smooth endoplasmic reticulum seems to contribute a small proportion to the secretory response triggered by muscarinic-cholinergic receptor stimulation. The mobilization of the intracellular Ca2+ pool seems to be unable by itself to trigger a substantial secretory response in the absence of extracellular Ca2+.
