Calcium Signaling in Individual BC3H1 Cells: Speed of Calcium Mobilization and Heterogeneity

Receptor/ligand binding on a cell surface may activate the calcium signal transduction cascade, resulting in the release of calcium from intracellular stores into the cytosol. Changes in intracellular free calcium, [Ca2+]i, following ligand stimulation have been linked to a variety of cell responses, from muscle contraction to hormone secretion. We have monitored changes in [Ca2+]i in single smooth muscle‐like BC3H1 cells following stimulation by the vasoconstrictor phenylephrine, using the fluorescent calcium probe, fura‐2, in a digital fluorescence imaging system. We find that not all cells respond to ligand stimulation with changes in [Ca2+]i. In addition, cells which respond to ligand stimulation exhibit considerable heterogeneity in the speed of calcium mobilization for a given ligand concentration. Both the population‐averaged speed for calcium mobilization and the fraction of cells which respond to ligand stimulation are increasing functions of the ligand concentration. In contrast, the magnitude of the ligand‐stimulated increase in [Ca2+]i from basal to peak levels in responding cells is independent of ligand concentration. We postulate that the heterogeneity seen in the ligand‐induced mobilization of calcium among single cells is a function of distinct differences between cells, such as number of receptors, size of the intracellular calcium store, or phospholipase C activity. We have developed a mathematical model, based on the calcium signal transduction cascade, to predict single‐cell calcium responses to ligand stimulation. We have systematically incorporated cell‐to‐cell parameter heterogeneity into the model by randomly selecting single‐cell parameter values from a Gaussian distribution. Model simulations predict both single‐cell and population‐averaged trends that we have observed experimentally. The results of this work suggest that increases in a population response may be the result of increased participation in the response as opposed to increases in the magnitudes of individual cell responses.