1. Histamine secretion from rat peritoneal mast cells occurs spontaneously in the absence of an external stimulus. Spontaneous secretion increases as the concentration of Sr in the extracellular medium is raised from 1 to 10 m-mole/l. Ca 0.1-10 m-mole/l. does not increase spontaneous secretion.2. Spontaneous histamine secretion in the presence of Sr occurs slowly compared with evoked histamine secretion, reaching a maximum only after more than 120 min incubation with Sr 10 m-mole/l. at 37° C and pH 7.6. Phosphatidyl serine, 10 μg/ml., increases the rate of spontaneous secretion in the presence of Sr.3. The spontaneous secretion occurring in the presence of Sr is highly dependent on the extracellular H ion concentration. Maximal secretion occurs at pH 8.4 and only a very limited secretion is detected at pH below 7.6. The rate of spontaneous secretion is also greater at higher pH. Inhibition of secretion caused by lowering the pH can be reversed by raising the Sr ion concentration over a limited range.4. Intact glycolytic and oxidative metabolism is required for the spontaneous secretion of histamine in the presence of Sr. Removal of extracellular glucose inhibits the secretion by about 80%, and the further addition of inhibitors of oxidative phosphorylation almost abolishes the secretion.5. Ca, Mg and Mn all inhibit the spontaneous secretion of histamine which occurs in the presence of Sr. The antagonism of the effect of Sr by Mg appears not to be competitive.6. Dibutyryl cyclic AMP, 10 μmole/l. to 3 m-mole/l. and theophylline, 30 μmole/l. inhibit spontaneous secretion in the presence of Sr. Cyclic AMP, AMP, and cyclic GMP 10 m-mole/l. are without effect on the spontaneous secretion. The inhibitory effect of dibutyryl cyclic AMP and of theophylline are dependent on pH: greater inhibition being achieved at lower pH.
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