AMPLITUDE-MODULATION OF CA2+ SIGNALS INDUCED BY HISTAMINE IN HUMAN ENDOTHELIAL-CELLS

We have addressed the problem of whether the agonist concentration sensed by endothelial cells is encoded by the sustained rise of the intracellular Ca2+ concentration ([Ca2+](i)) or by the frequency of intracellular Ca2+ oscillations. Single or confluent endothelial cells from umbilical veins were stimulated for 15 min with histamine (0.03 to 100 mu mol/l), and the concomitant changes in [Ca2+](i) were measured with fura-2/AM. Application of histamine at concentrations above 0.1 mu mol/l resulted always in a fast spike of [Ca2+](i), followed by a slow decline to a sustained plateau level, which depends on the presence of extracellular Ca2+. At the same time of the development of this plateau phase, quenching of the fura-2/AM signal occurred during agonist stimulation in a Ca2+-free, 1.5 mmol/l Mn2+ containing solution, indicating influx of divalent cations during this time. From 48 cells in 1.5 mmol/l [Ca2+](c) we obtained a close relation between histamine concentration and time integral of [Ca2+](i) taken over the 15 min recording of the plateau [Ca2+](i). The half-maximal increase in the integral of [Ca2+](i) was at 0.7 mu mol/l for solitary cells, 1.2 mu mol/l for clustered cells and 1.2 mu mol/l for the plateau Ca2+ level. Repetitive Ca2+ spikes or Ca2+ oscillations appeared only in 16 out of 48 cells, but their frequency was not correlated to the agonist concentration. Ca2+ oscillations were only observed in a concentration window between 0.1 and 1 mu mol/l histamine, both in single and in clustered endothelial cells. Our results indicate that coding of the agonist concentration in endothelial cells is not related to the frequency of Ca2+ oscillations, but is closely correlated with the plateau level of intracellular Ca2+.