ALL-OR-NONE LIKE RESPONSES IN INCREMENT OF CYTOPLASMIC FREE CALCIUM CONCENTRATIONS TO HISTAMINE IN SINGLE SMOOTH-MUSCLE CELLS OF GUINEA-PIGTRACHEA

Single smooth muscle cells were isolated from guinea pig trachea and c ultured for 48 hours. The cells were then loaded with a fluorescent-fr ee calcium indicator, fluo-3. Concentrations of cytoplasmic-free calci um ([Ca2+](i)) were determined by using a confocal laser scanning fluo rescence imaging system. Cells exposed to histamine at concentrations less than 1.0 mu M did not indicate any changes of the fluorescence in tensities, but stayed at the basal level of [Ca2+](i). Cells exposed t o histamine at a concentration in the range of 1.5 to 10 mu M indicate d marked increases in [Ca2+](i). A further increase in [Ca2+](i) by ap plication of higher concentrations of histamine was not observed. The threshold concentration of histamine to increase [Ca2+](i) and peak [C a2+](i) varied from cell to cell. In addition, half-maximal response t ime was shortened with increasing concentrations of histamine from 1.5 to 100 mu M. These results indicate that [Ca2+](i) responses to hista mine is an all-or-none type in each cell. The heterogeneity in the req uired threshold concentration of histamine to increase in [Ca2+](i) an d the concentration dependency in half-maximal response time of histam ine-induced [Ca2+](i) increase may be related to the graded responses of histamine-induced contractions in preparations of the tracheal tiss ue.