The time course of intracellular calcium movements in single human umbilical vein smooth muscle cells.

Intracellular Ca2+ ([Ca2+]i) was measured in single isolated human umbilical vein smooth muscle cells. Stimulation with histamine, in the absence of external Ca2+, mobilised Ca2+ from intracellular stores. When repeated brief applications of agonist were used, the time to onset, amplitude and rate of rise of the Ca2+ transients were found to change. Two components could often be discerned in the rising phase of the transients, an initial slow "pacemaker" and a second faster and larger component. Following the first histamine-activated transient the basal level of [Ca2+]i was invariably lower than that prior to stimulation. This lower value was maintained whilst the cell remained in Ca(2+)-free solution, but could be returned to a higher level if the cell was exposed to external Ca2+. When the mobilisation of the intracellular store was reduced to undetectable levels, re-exposure to Ca(2+)-containing medium reactivated responses. In the absence of external Ca2+, continuous application of histamine activated a series of transient increases in intracellular Ca2+, which decreased progressively in amplitude and rate of rise. The interval between transients also increased. These findings are discussed in terms of the activation of inositol trisphosphate-sensitive intracellular Ca2+ stores and their sensitivity to cytoplasmic Ca2+ and intrasarcoplasmic reticulum Ca2+.