HISTAMINE-ACTIVATED, NONSELECTIVE CATION CURRENTS AND CA2-CELLS FROM HUMAN UMBILICAL VEIN( TRANSIENTS IN ENDOTHELIAL)

Permeation properties and modulation of an ionic current gated by hist amine were measured in single endothelial cells from human umbilical c ord veins by use of the patch-clamp technique in the ruptured-whole-ce ll mode or using perforated patches. We combined these current measure ments with a microfluorimetric method to measure concomitantly free in tracellular calcium concentration (Ca2+!i). Application of histamine induced an intracellular calcium transient and an ionic current that r eversed near 0 mV. The amplitude of the current ranged from -0.2 to -2 nA at -100 mV. The tonic rise in Ca2+!i and the ionic current are pa rtly due to Ca2+ influx. This Ca2+ entry pathway is also permeable for Ba2+ and Mn2+. The amplitude of the histamine-activated current was a lso closely correlated with the amplitude of the concomitant Ca2+ tran sient, suggesting that the latter is at least partially due to Ca2+ in flux through histamine-activated channels. The reversal potential of t he histamine-induced current was 7.6 +/- 4.1 mV (n = 14) when the calc ium concentration in the bath solution (Ca2+!o) was 1.5 mmol/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 t he current in Ca2+-free solution was enhanced compared to that in 10 m mol/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 monov alent cations. The latency between the application of histamine and th e appearance of the current was voltage dependent and was much smaller at more negative potentials. This effect is unlikely to be due to des ensitization, but may suggest a voltage-dependent step in the signal t ransduction chain. Similar histamine-induced Ca2+ signals were observe d if the currents were measured in patches perforated with nystatin. T he onset of the agonist-activated current was, however, much more dela yed and its amplitude significantly lower than in ruptured patches. Th e histamine-induced currents and intracellular Ca2+-transients were la rgely reduced after incubation of endothelial cells with the phorbol e ster TPA. H7, a blocker of protein kinase C, induced membrane currents and Ca2+ signals in the absence of an agonist. It is concluded that t he agonist-activated Ca2+-entry in endothelial cells occurs through no n-selective cation channels which can be down-regulated by protein kin ase C activation.