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

We have addressed the problem of whether the agonist concentration sen sed by endothelial cells is encoded by the sustained rise of the intra cellular Ca2+ concentration ([Ca2+](i)) or by the frequency of intrace llular Ca2+ oscillations. Single or confluent endothelial cells from u mbilical 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 mo l/l resulted always in a fast spike of [Ca2+](i), followed by a slow d ecline to a sustained plateau level, which depends on the presence of extracellular Ca2+. At the same time of the development of this platea u phase, quenching of the fura-2/AM signal occurred during agonist sti mulation in a Ca2+-free, 1.5 mmol/l Mn2+ containing solution, indicati ng influx of divalent cations during this time. From 48 cells in 1.5 m mol/l [Ca2+](c) we obtained a close relation between histamine concent ration 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 cl ustered 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. C a2+ oscillations were only observed in a concentration window between 0.1 and 1 mu mol/l histamine, both in single and in clustered endothel ial cells. Our results indicate that coding of the agonist concentrati on in endothelial cells is not related to the frequency of Ca2+ oscill ations, but is closely correlated with the plateau level of intracellu lar Ca2+.