PROTEIN-LINKED RECEPTORS LABELED BY [H-3] HISTAMINE IN GUINEA-PIG CEREBRAL-CORTEX .1. PHARMACOLOGICAL CHARACTERIZATION

Binding of histamine to washed membranes from guinea pig cerebral cort ex can be described empirically as two classes of distinct and indepen dent sites (log I(P1) = -8.45 +/- 0.02, R1,t = 98 +/- 6 pmol/g of prot ein; log K(P2) = -6.34+/- 0.22, R2,t = 990 +/- 60 pmol/g of protein). At 1.4 nm [H-3]histamine, the kinetics of association and dissociation are biexponential. The values of k(-Pj)/k(+Pj) calculated for paralle l one-step processes agree well with the corresponding values of K(Pj) . Both k(-P1) and k(-P2) are increased by 0.1 mm guanylylimidodiphosph ate; apparent capacity at equilibrium is reduced for both classes of s ites, with little or no change in K(P1) or K(P2). Twenty-six H-2 and H -3 agonists and antagonists block access of [H-3]histamine to the same sites, and the binding patterns reveal either one or two hyperbolic t erms [i.e., SIGMA(j=1)n F'(j)K(j)/(K(j) + [L])]. Two terms are require d for six agonists and six antagonists, and F'2 varies widely from lig and to ligand. Also, the quantity log (K2/K1) is correlated with F'1 a mong agonists but with F'2 among antagonists (K1 < K2). The pharmacolo gical selectivity is suggestive of both H-2 and H-3 receptors. An H-2 specificity emerges from the appropriate values of K(j) for 12 H-2 ago nists (i.e., K1 when n = 1 and K2 when n = 2; p = 0.00045), although a specificity distinct from that of H-2 receptors is found with H-2 ant agonists. An H-3 specificity emerges from the inhibitory potencies (IC 50) of eight H-3 agonists (p = 0.00025) and eight H-3 antagonists (p = 0.001 9); also, the sites labeled by [H-3]histamine resemble H-3 rece ptors reportedly labeled by N(alpha)-[H-3]methylhistamine and (R)-alph a-[H-3]methylhistamine. Ligand-dependent differences in F'2 are incons istent with the notion of distinct and independent sites, and the tend ency of antagonists to promote the sites of weaker affinity (F'2) argu es against a ligand-regulated equilibrium between two states. The phys ical significance of the binding parameters is therefore unclear. The failure to identify an unambiguous pharmacological specificity may ref lect the failure to assess binding in the correct mechanistic context.