[H-3] HISTAMINE UPTAKE AND RELEASE BY ASTROCYTES FROM RAT-BRAIN - EFFECTS OF SODIUM DEPRIVATION, HIGH POTASSIUM, AND POTASSIUM CHANNEL BLOCKERS

Histamine transport has been characterized in cultured astroglial cell s of rat brain. The kinetics of [H-3]-histamine uptake yielded a K-m o f 0.19 +/- 0.03 mu M and a V-max of 3.12 +/- 0.75 pmol X mg protein(-1 ) X min(-1). Transport system revealed high affinity for histamine and an approximately ten times higher capacity than that shown in culture d glial cells of chick embryonic brain. Ouabain which interferes with utilization of ATP to generate ion gradients, and the replacement of N a+ with choline inhibited the initial rate of uptake showing a strong Na+-dependency and suggesting the presence of a tightly coupled sodium /histamine symporter. Dissipation of K+-gradient (in > out) by high K or by K+-channee blockers, BaCl2, (100 mu M), quinine (100 mu M) or S parteine (20 mu M) produced also remarkable inhibitions in the uptake of [H-3]-histamine. Impromidine, a structural histamine-analogue could inhibit the uptake non-competitively in a range of concentrations of 1 to 10 mu M with a K-i value of 2.8 mu M, indicating the specificity of the uptake. [H-3]histamine uptake measurements carried out by using a suspension of dissociated hypothalamic cells, of rat brain showed a strong gliotoxin-sensitivity and yielded a Km of 0.33 +/- 0.08 mu M; and a V-max of 2.65 +/- 0.35 pmoles X mg protein(-1) X min(-1). The up take could be reversed by incubating the cells in histamine-free Krebs medium. The [H-3]histamine efflux was sensitive to Na+ omission, ouab ain treatment and high K+ or K+ channel blockers, resulting in marked elevations in the efflux. Data indicate that glial uptake of histamine is a high affinity, Na+-dependent and electrogenic, driven by an inwa rd-oriented sodium ion gradient and an outward-oriented potassium ion gradient and functions as part of histamine inactivation, at least in a shunt mechanism.