Frequency distribution histograms of calcium responses to UTP. Changes in ΔR = F340/F380 were determined from the experiment depicted in Fig. 4 and 5 similar experiments (n = 288 cells). The first column in each histogram represents unresponsive cells (ΔR < 0.2). "UTP + Ca2+": responses obtained in presence of extracellular calcium; "UTP + EGTA": responses obtained in the virtual absence of extracellular calcium. A, leftmost histograms: AD-cells; B, rightmost histograms: NA-cells. A lognormal distribution function was fitted to each histogram, not taking into account unresponsive cells. The horizontal bars represent mean ΔR ± S.D.