Effects of Acute Glucose Overload on Histamine H₂ Receptor–Mediated Ca²⁺ Mobilization in Bovine Cerebral Endothelial Cells

[Ca²⁺]_i and whole-cell membrane current were measured in microvascular endothelial cells from bovine brain. The effects of histamine on [Ca²⁺]_i were examined, and the acute effect of changing extracellular glucose concentration on Ca²⁺ homeostasis was investigated. Application of 10 μmol/l histamine evoked an initially transient and then sustained increase in [Ca²⁺]_i in normal Krebs solution, but only the transient component in Ca²⁺-free solution, thereby indicating that histamine mobilizes Ca²⁺ both from intracellular store sites and extracellular space. The effects of histamine on [Ca²⁺]_i were inhibited by the H₂ antagonists, ranitidine and cimetidine, but not by the H₁ antagonist, pyrilamine. Incubation of the cells for 2 h in solutions containing low (1.1 and 2.3 mmol/l) or high (23 mmol/l) concentrations of glucose did not influence the resting level of [Ca²⁺]_i. Treatment with low concentrations of glucose did not impair histamine-induced Ca²⁺ mobilization. On the other hand, when histamine was applied to the cells pretreated with 23 mmol/l glucose, it failed to mobilize Ca²⁺ from both intracellular store sites and extracellular space. The effect of histamine was mimicked by dibutyryl cyclic AMP, but glucose overload failed to inhibit this, suggesting that glucose overload inhibits H₂ receptor-mediated cyclic AMP production. Glucose overload-induced impairment of histamine action was reversed by pretreatment with staurosporine and calphostin C and mimicked by phorbol- 12,13-dibutyrate, thereby suggesting the involvement of protein kinase C in the high glucose-induced inhibition of Ca²⁺ mobilization. Whole-cell membrane current measurement showed that there was no difference in the membrane currents between control and high glucose- treated cells. These results indicate that 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 H₂ receptors. Glucose overload acutely inhibits histamine-induced Ca²⁺ mobilization by the activation of protein kinase C.