IDENTIFICATION AND CHARACTERIZATION OF HISTAMINE H1-RECEPTORS AND H2-RECEPTORS IN GUINEA-PIG LEFT ATRIAL MEMBRANES BY [H-3] MEPYRAMINE AND [H-3] TIOTIDINE BINDING

1 Histamine receptors in the membranes prepared from guinea-pig left atria wer 2 The binding of the H-1-antagonist, [H-3]-mepyramine, was saturable and of high affinity with a maximum binding capacity of 307 +/- 27 fmol mg-1 protein (n = 14) and with an equilibrium dissociation constant (K(D)) of 1.5 +/- 0.2 nM (n = 14). The binding was rapid and readily reversible. 3 The competition curve for [H-3]-mepyramine binding by histamine was biphasic and revealed high and low affinity states of binding. The addition of 5'-guanylylimidodiphosphate (GppNHp) (100-mu-M) converted this heterogeneous binding into homogeneous binding of low affinity. 4 The competition curves of H-1-antagonists with [H-3]-mepyramine had Hill coefficients not significantly different from unity, consistent with competition with [H-3]-mepyramine at a single site. GppNHp did not shift the competition curves. 5 Dissociation constants for H-1-antagonists determined from inhibition of [H-3]-mepyramine binding correlated well with the constants derived from inhibition of the positive inotropic response of guinea-pig left atria to histamine. 6 The H-2-antagonist, [H-3]-tiotidine, labelled an apparently homogeneous population of recognition sites with a maximum binding capacity of 41 +/- 8 fmol mg-1 protein (n = 6) and a K(D) of 10.8 +/- 1.2 nM (n = 6). 7 Although histamine competed for [H-3]-tiotidine binding in a concentration-dependent manner, the curve was monophasic and was not shifted by GppNHp. 8 It is concluded that both H-1- and H-2-receptors exist in guinea-pig left atria. H-1-receptors probably couple to intracellular effector(s) through a guanine nucleotide-dependent transducing mechanism. On the other hand, H-2-receptors seem unlikely to be linked to guanine nucleotide regulatory proteins in guinea-pig left atria, which may explain the failure of histamine to cause an increase in cyclic AMP in spite of the presence of H-2-receptors.