All-or-none like responses in increment of cytoplasmic free calcium concentrations to histamine in single smooth muscle cells of guinea pig trachea.

Single smooth muscle cells were isolated from guinea pig trachea and cultured for 48 hours. The cells were then loaded with a fluorescent-free calcium indicator, fluo-3. Concentrations of cytoplasmic-free calcium ([Ca2+]i) were determined by using a confocal laser scanning fluorescence imaging system. Cells exposed to histamine at concentrations less than 1.0 microM did not indicate any changes of the fluorescence intensities, but stayed at the basal level of [Ca2+]i. Cells exposed to histamine at a concentration in the range of 1.5 to 10 microM indicated marked increases in [Ca2+]i. A further increase in [Ca2+]i by application of higher concentrations of histamine was not observed. The threshold concentration of histamine to increase [Ca2+]i and peak [Ca2+]i varied from cell to cell. In addition, half-maximal response time was shortened with increasing concentrations of histamine from 1.5 to 100 microM. These results indicate that [Ca2+]i responses to histamine is an all-or-none type in each cell. The heterogeneity in the required threshold concentration of histamine to increase in [Ca2+]i and the concentration dependency in half-maximal response time of histamine-induced [Ca2+]i increase may be related to the graded responses of histamine-induced contractions in preparations of the tracheal tissue.