EFFECTS OF ACUTE GLUCOSE OVERLOAD ON HISTAMINE H-2 RECEPTOR-MEDIATED CA2-CELLS( MOBILIZATION IN BOVINE CEREBRAL ENDOTHELIAL)

[Ca2+](i) and whole-cell membrane current were measured in microvascul ar endothelial cells from bovine brain. The effects of histamine on [C a2+](i), were examined, and the acute effect of changing extracellular glucose concentration on Ca2+ homeostasis was investigated. Applicati on of 10 mu mol/l histamine evoked an initially transient and then sus tained increase in [Ca2+](i) in normal Krebs solution, but only the tr ansient component in Ca2+-free solution, thereby indicating that hista mine mobilizes Ca2+ both from intracellular store sites and extracellu lar space. The effects of histamine on [Ca2+](i) were inhibited by the H-2 antagonist, ranitidine and cimetidine, but not by the H-1 antagon ists, pyrilamine. Incubation of the cells for 2 h in solutions contain ing low (1.1 and 2.3 mmol/l) or high (23 mmol/l) concentrations of glu cose did not influence the resting level of [Ca2+](i). Treatment with low concentrations of glucose did not impair histamine-induced Ca2+ mo bilization. On the other hand, when histamine was applied to the cells pretreated with 23 mmol/l glucose, it failed to mobilize Ca2+ from bo th intracellular store sites and extracellular space. The effect of hi stamine was mimicked by dibutyryl cyclic AMP, but glucose overload fai led to inhibit this, suggesting that glucose overload inhibits H-2 rec eptor-mediated cyclic AMP production. Glucose overload-induced impairm ent of histamine action was reversed by pretreatment with staurosporin e and calphostin C and mimicked by phorbol-12,13-dibutyrate, thereby s uggesting the involvement of protein kinase C in the high glucose-indu ced inhibition of Ca2+ mobilization. Whole-cell membrane current measu rement showed that there was no difference in the membrane currents be tween control and high glucose-treated cells. These results indicate t hat in bovine brain microvascular endothelial cells, histamine induces Ca2+ release from intracellular store sites and subsequent entry from the extracellular space through the activation of H-2 receptors. Gluc ose overload acutely inhibits histamine-induced Ca2+ mobilization by t he activation of protein kinase C.