Single cell analysis of free cytosolic calcium changes in human lung mast cells--II. The relationship between desensitization and the cellular regulation of calcium changes.

Stimulation of mast cells results in two opposing reactions, activation events that cause degranulation and desensitization events that inhibit mediator release. Previous studies of human lung mast cells and murine mast cells have suggested that desensitization resulted from events that negatively regulated free cytosolic calcium ([Ca2+]i) levels; the current studies suggest otherwise. Stimulation of purified human lung mast cells with anti-IgE demonstrated that histamine release had reached a maximum at a time (5 mins) when [Ca2+]i levels were still near their maximum elevation. While there was a slow return of [Ca2+]i levels to baseline (T1/2 = 7.8 min), this rate of return could not clearly account for the cessation of histamine release. The heterogeneity in this decay parameter was also calculated to be insufficient to account for the heterogeneity in the peak calcium response while heterogeneity in the cell surface IgE density could adequately account for the heterogeneity in calcium responses. Preincubation of mast cells with anti-IgE antibody without extracellular calcium did lead to a progressive loss of the subsequent [Ca2+]i response when calcium was added back to the reaction, but the rate of desensitization determined by this measure, T1/2 of 8 min, was slower than the rate determined by measuring the progressive inhibition of histamine release (T1/2 of 4.5 min). In addition, no correlation existed for the rate of desensitization as measured by histamine release and that measured by the peak calcium response. These data suggested that the extent of histamine release was not strictly controlled by regulation of free cytosolic calcium and that desensitization events measured by the progressive loss in histamine release and calcium response were also not strictly related.