HISTAMINE INDUCES K- CURRENTS IN HUMAN VASCULAR ENDOTHELIAL-CELLS - ROLE OF IONIC CURRENTS IN STIMULATION OF NITRIC-OXIDE BIOSYNTHESIS(, CA2+, AND CL)

The nature of the membrane currents mediating agonist-induced Ca2+ ent ry and enhanced nitric oxide (NO) production in endothelial cells is s till unclear. Using both perforated-patch and conventional whole-cell clamp technique, we have studied the membrane response associated with histamine stimulation of human vascular endothelial cells. In perfora ted-patch experiments, the initial histamine (10 mu mol/L)-induced cur rent reversed close to the K+ equilibrium potential and was blocked by tetrabutylammonium ions (TBA, 10 mmol/L). In addition, a TBA-insensit ive current that developed slowly in the presence of histamine was rec orded. This delayed histamine-induced current reversed close to neutra l potential and was inhibited by SK&F 96365 (25 mu mol/L), a putative blocker of receptor-operated Ca2+ channels. Similar histamine effects were observed in conventional whole-cell experiments using pipette sol utions with low Ca2+ buffering capacity. Strong buffering of intracell ular free Ca2+ suppressed the initial, but not the delayed, current re sponse. The delayed component of histamine-induced current was substan tially inhibited by the Cl- channel blocker N-phenylanthranilic acid ( NPA, 100 mu mol/L), and an eightfold change in the Cl- gradient shifte d the reversal potential of this current by 30 mV. In CI--free solutio ns, histamine induced an SK&F 96365-sensitive NPA-resistant current, w hich, according to reversal potential measurements in 20 mmol/L extrac ellular Ca2+, corresponded to a cation conductance with 13- to 25-fold selectivity for Ca2+ over K+, Both SK&F 96365 and TBA strongly suppre ssed histamine-induced rises in intracellular free Ca2+ and cellular c GMP levels, whereas NPA did not. Our results provide the first demonst ration that three distinct ionic conductances contribute to the histam ine-induced membrane response of endothelial cells. It is suggested th at histamine induces a Cl- conductance that is apparently not involved in Ca2+ homeostasis and regulation of NO biosynthesis, while, in para llel, joint activation of a rapidly induced K+ permeability and a slow ly developing cation permeability mediate Ca2+ entry and stimulation o f endothelial NO production.