Permeation properties and modulation of an ionic current gated by histamine were measured in single endothelial cells from human umbilical cord veins by use of the patch-clamp technique in the ruptured-whole-cell mode or using perforated patches. We combined these current measurements with a microfluorimetric method to measure concomitantly free intracellular calcium concentration ([Ca2+]i). Application of histamine induced an intracellular calcium transient and an ionic current that reversed near 0 mV. The amplitude of the current ranged from -0.2 to -2nA at -100mV. The tonic rise in [Ca2+]i and the ionic current are partly due to Ca2+ influx. This Ca2+ entry pathway is also permeable for Ba2+ and Mn2+. The amplitude of the histamine-activated current was also closely correlated with the amplitude of the concomitant Ca2+ transient, suggesting that the latter is at least partially due to Ca2+ influx through histamine-activated channels. The reversal potential of the histamine-induced current was 7.6±4.1 mV (n=14) when the calcium concentration in the bath solution ([Ca2+]o) was 1.5mmol/l. With 10 mmol/l [Ca2+]o it was -13.7±4.7 mV and shifted to +13.0±1.5 mV in nominally Ca2+-free solution (n=3 cells). The amplitude of the current in Ca2+-free solution was enhanced compared to that in 10 mmol/l [Ca2+]o. The shift of the reversal potential and the concomitant change of the current amplitude suggest that the channel is permeable for calcium but has a smaller permeability for calcium than for monovalent cations. The latency between the application of histamine and the appearance of the current was voltage dependent and was much smaller at more negative potentials. This effect is unlikely to be due to desensitization, but may suggest a voltage-dependent step in the signal transduction chain. Similar histamine-induced Ca2+ signals were observed if the currents were measured in patches perforated with nystatin. The onset of the agonist-activated current was, however, much more delayed and its amplitude significantly lower than in ruptured patches. The histamine-induced currents and intracellular Ca2+-transients were largely reduced after incubation of endothelial cells with the phorbol ester TPA. H7, a blocker of protein kinase C, induced membrane currents and Ca2+ signals in the absence of an agonist. It is concluded that the agonist-activated Ca2+-entry in endothelial cells occurs through non-selective cation channels which can be down-regulated by protein kinase C activation.
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