Differential Activation of Dual Signaling Responses by Human H₁ and H₂ Histamine Receptors

Abstract

Stimulation of human H₁ and H₂‐histamine receptors (HRs) primarily activates signaling pathways to increase intracellular calcium [Ca²⁺]_i and cyclic AMP (cAMP), respectively. Activation of H₂‐HR in human embryonic kidney (HEK) cells by histamine and dimaprit increases both cAMP formation and [Ca²⁺]_i, as determined by cAMP‐scintillation proximity assays and fluorescence imaging plate reader (FLIPR) assays. In HEK cells expressing relatively high levels of H₂‐HR (B_max = 26 pmol/mg protein), histamine and dimaprit are full agonists in eliciting cAMP responses with pEC₅₀ values of 9.30 and 7.72 that are 1000‐fold more potent than their respective pEC₅₀ values of 6.13 and 4.91 for increasing [Ca²⁺]_i. The agonist potencies decrease for both responses at lower H₂‐HR density (5 pmol/mg protein) and dimaprit exhibits partial agonist behavior for the [Ca²⁺]_i response. The inverse agonists ranitidine and cimetidine more potently inhibit cAMP production in the higher expressing H₂‐HR line. Histamine also activated both signaling pathways via human H₁‐HRs highly expressed (B_max = 17 pmol/mg protein) in HEK cells, with a 1000‐fold greater potency for [Ca²⁺]_i vs. cAMP responses (pEC₅₀ = 7.86 and 4.82, respectively). These studies demonstrate a markedly different potency for activation of multiple signaling pathways by H₁‐ and H₂‐HRs that may contribute to the selectivity of histamine responses in vivo.

Key Words: :

• Histamine
• GPCR
• Constitutive activity
• Intracellular calcium
• FLIPR
• Cyclic AMP