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Isolated peritoneal mast cells from rats were observed, by phase contrast microscopy, to extrude secretory granules when exposed to 48/80 or the ionophores A-23187 and X-537A, which are known to facilitate transmembrane fluxes of calcium. These effects were abolished when the cells were treated with EDTA and suspended in a Ca-free environment. Ca-deprived cells exposed to any one of the three drugs promptly extruded granules when calcium, but not magnesium, was added to the incubation medium. Such Ca-evoked or Ca-dependent responses persisted when Na was omitted from the incubation medium and replaced with sucrose, choline, or K. The responses thus seem independent of possible shifts in the alkali metal ions. The results are considered support for the view that Ca influx mediates stimulussecretion coupling and does so by initiating exocytosis.When various observations on the medullary chromaffin cell led Douglas and Rubin (1) to propose the calcium-influx hypothesis of stimulus-secretion coupling for the medullary chromaffin cell, they suggested that a similar mechanism might operate in the mast cell, whose secretory response in anaphylaxis was known at the time to be Ca-dependent (2). Later, when exocytosis was established beyond all doubt as the mode of medullary secretion, and when the coexistence of exocytosis and calcium-dependence was demonstrated in other cells, the behavior of the mast cell was suggested to be but one example of a seemingly general secretory mechanism, namely calcium-activated, or dependent, exocytosis (2). On this hypothesis, various secretagogues (substances eliciting secretion) act by promoting Ca-influx or by mobilizing cellular Ca, and it is the appearance of an excess of free Ca ions someplace in the cell that then initiates this secretory response. More recently the list of cells appearing to fit this pattern has been greatly extended (3).The mast cell offers an interesting model system 'for studying stimulus-secretion coupling, since the large size of its secretory granules allows light microscopic observation of granule extrusion in the living cell (e.g., 4) and there is extensive electron microscopic evidence confirming that such extrusion occurs by exocytosis (4-6). Doubts about the validity of the isolated mast cell as a model system for studying exocytosis based upon evidence that responses to 48/80 (the classical mast cell secretagogue or histaminereleasing substance) can be obtained in Ca-free media (7) have recently been dispelled by the evidence that the response can utilize tightly bound Ca. Thus, histamine release promoted by 48/80 was abolished by incubation with EDTA and reappeared after reintroduction of Ca (7). Our main purpose in the present experiments is to present evidence that the simple introduction of calcium induces massive granule extrusion from mast cells maintained in a Cafree environment and primed by 48/80 or by the ionophores X-537A and A-23187, which are known to facilitate fluxes of Ca ions through various membranes (8). The results encourag...

SUMMARY1. Neurotensin stimulated histamine release and granule extrusion when applied to isolated rat peritoneal mast cells.2. This secretary response was prevented by the removal of calcium or energy and was not accompanied by the release of lactic dehydrogenase.3. The secretary response produced by neurotensin was prevented by prior treatment of mast cells with cromoglycate.4. The intravenous injection of neurotensin into anaesthetized rats produced a rapid and significant increase in the level of blood histamine that was dependent upon the dose of neurotensin.5. Treatment of rats with compound 48/80, 24 hr before neurotensin, abolished the elevation in blood histamine caused by neurotensin. The intravenous injection of cromoglyeate 1-2 min before neurotensin greatly reduced the response to neurotensin.6. The intradermal injection of neurotensin (0-03-30 p-mole) increased capillary permeability in rats pre-treated intravenously with Evans Blue. This response was abolished by the antihistamine, diphenhydramine. Increasing the dose of neurotensin to 300 p-mole partially overcame this inhibition by diphenhydramine.7. Our results demonstrate that neurotensin can elicit an exocytotic secretary response from isolated rat peritoneal mast cells and elevate histamine levels in blood.

This paper concerns the role of nitric oxide (NO) in controlling metamorphosis in the marine gastropod Crepidula fornicata. Metamorphosis was stimulated by the nitric oxide synthase (NOS) inhibitors AGH (aminoguanidine hemisulfate) and SMIS (S-methylisothiourea sulfate) at concentrations of about 100-1000 micromol l(-1) and 50-200 micromol l(-1), respectively. Metamorphosis was not, however, induced by the NOS inhibitor l-NAME (l-N(G)-nitroarginine methyl ester) at even the highest concentration tested, 500 micromol l(-1). Moreover, pre-incubation with l-NAME at 20 and 80 micromol l(-1) did not increase the sensitivity of competent larvae to excess K(+), a potent inducer of metamorphosis in this species; we suggest that either l-NAME is ineffective in suppressing NO production in larvae of C. fornicata, or that it works only on the constitutive isoform of the enzyme. In contrast, metamorphosis was potentiated by the guanylate cyclase inhibitor ODQ (1H-[1,2,4]oxadiazolo[4,3, -a]quinoxalin-1-one) in response to a natural metamorphic inducer derived from conspecific adults. Because NO typically stimulates cGMP production through the activation of soluble guanylate cyclase, this result supports the hypothesis that NO acts as an endogenous inhibitor of metamorphosis in C. fornicata. The expression of NOS, shown by immunohistochemical techniques, was detected in the apical ganglion of young larvae but not in older larvae, further supporting the hypothesis that metamorphosis in C. fornicata is made possible by declines in the endogenous concentration of NO during development.