The effects of mercuric compounds on histamine uptake and binding to uptake carrier in cultured rat astroglial and cerebral endothelial cells were investigated. Experimental results showed that mercuric compounds produced strong stimulation of glial and cerebroendothelial histamine uptake over a concentration range of 25-500 mM. The stimulated histamine uptake showed characteristics similar to those described for basal uptake in terms of sensitivity to inhibitory agents (e.g., impromidine) and the requirement of external Na 1 . Mercury-induced stimulation of histamine uptake could be abolished by sulfhydryl agents, dithiotreitol and cysteamine, indicating a complete reversal of, and not simply a protection from, the action of mercury.
Basal and stimulated uptake of histamine represent bindings to uptake carrier with high and closely equal affinities but markedly higher capacities for stimulated uptake. In controls, the mean value of apparent K D (derived from saturation kinetics at equilibrium) was obtained as 26.7 6 3.9 nM for astroglial cells; and 100 mM mercuric chloride did not modify it significantly. In contrast, the apparent B max values differed markedly; found as 0.63 6 0.10 pmol/mg protein and 3.32 6 0.47 pmol/mg protein in the absence and the presence of 100 mM mercuric chloride respectively.
For the cerebral endothelial cell line, RBE4, the apparent K D was calculated as 22.5 6 3.2 nM and was comparable to that obtained for astroglial cells in control and mercury-stimulated conditions. The apparent B max values were less, but markedly different in these conditions, obtained as 0.18 6 0.03 pmol/mg protein and 1.2 6 0.36 pmol/mg protein in the absence and the presence of mercuric ion respectively. In both cells, impromidine, the potent inhibitor of basal and stimulated histamine uptake, decreased the enhanced capacities of histamine binding (B max ) (without affecting the dissociation constant, K D ) in micromolar range, comparable to its inhibiting potency.
Results confirmed that mercuric ion might enhance the binding capacity of histamine carrier and protein sulfhydryls might play a role in this effect. The observed stimulations by mercuric compounds suggest close similarities in the mechanism of histamine uptake and the structure of histamine carrier in astroglial and cerebral endothelial cells.