Histamine H₁‐Receptor Binding Sites in Guinea Pig Brain Membranes: Regulation of Agonist Interactions by Guanine Nucleotides and Cations

Agonist, but not antagonist, interactions with histamine H 2 ‐receptors labeled by [ 3 H]mepyramine are regulated selectively by sodium, divalent cations, and guanine nucleotides. Sodium decreases the affinity of histamine and the agonist 2‐amino‐ethylpyridine for [ 3 H]mepyramine sites in guinea pig brain membranes up to 10‐fold. The effect of sodium is exerted to a lesser extent by lithium, while potassium and rubidium are much weaker. Guanine nucleotides also decrease the affinity of histamine for H 1 binding sites about twofold. GTP and its nonmetabolized analogue GMP‐PNP as well as GDP exert similar effects, while GMP, ATP, ADP, and AMP are inactive. The effects of GTP and sodium on histamine interactions with H 1 ‐receptors are additive. By contrast, certain divalent cations enhance the potency of histamine at H 1 ‐receptors. Manganese is most potent, while magnesium is almost as active as manganese and calcium is essentially inactive. Sodium, divalent cations, and guanine nucleotides have negligible effects on the interactions of antihistamines with H 1 ‐receptors.