Histamine (HIST), an autacoid-increasing stimulus in endothelial cells, leads to a rise of intracellular free Ca2+ (Ca2+i). We analysed whether gap junctions (GJ) are involved in the overall response to HIST of Ca2+i (Fura 2) of cultured endothelial cells (HUVEC). Initially HIST (5mM -250mM) led to a Ca 2+i rise only in 5±0.2% (n=13) of all cells which were randomly dispersed over the area. From these "pacemaker cells" the Ca2+i increase spread with a short time delay into neighbouring cells resulting in a Ca2+i rise in 98±1% (n=10 culture wells) of all cells of the monolayer. Spreading was blocked and Ca2+i increase was mostly restricted to the pacemaker cells after inhibition of GJ coupling. cGMP was increased after treatment with HIST (con: 0.2±0.1 nmol/ml; HIST 3.8&plusminus1.6 nmol/ml and was blocked after inhibition of GJ coupling (0.4±0.2 nmol/ml, n=7). Immunhistochemistry using antibodies against the H1 receptor revealed a non-homogeneous distribution with many only weakly stained cells. Inhibition of the ryanodine receptor did not block the Ca2+ i rise (n=4). Conclusion: The endothelial Ca2+i increase due to HIST is only partly a direct receptor induced event and restricted to a few cells acting as "pacemakers". Secondary signalling via GJ leads to the homogenous Ca2+i response, which is considered physiological. This mechanism may be important for coordinated cell responses and could lead to an impaired endothelial autacoid release when GJ communication is reduced.