Abstract

The H1-histamine receptor antagonist [3H]mepyramine bound with high affinity (Kd = 3-5 nM) to membranes derived from 1321N1 human astrocytoma cells. The H1-receptor antagonists triprolidine and diphenhydramine inhibited [3H]mepyramine binding with Kj values of 1-5 nM, whereas the Kj of the H2-histamine receptor antagonist cimetidine was greater than 100 microM. Histamine also inhibited [3H]mepyramine binding to 1321N1 cell membranes, and the histamine inhibition curve was shifted to the right and steepened in the presence of 1 microM guanosine 5'-O-(3-thiotriphosphate). Treatment of 1321N1 cells with pertussis toxin had no effect on the capacity of histamine to inhibit [3H]mepyramine binding either in the absence or presence of guanosine 5'-O-(3-thiotriphosphate). Therefore, agonist-occupied histamine receptors in these cells apparently interact with a guanine nucleotide regulatory protein that is not the inhibitory guanine nucleotide regulatory protein of adenylate cyclase. Although adenylate cyclase activity was not affected by histamine in a cell-free preparation, incubation of 1321N1 cells with histamine resulted in an attenuation of cyclic AMP accumulation. Analysis of cyclic AMP degradation in the presence of histamine indicated that the effects of histamine on cyclic AMP accumulation are mediated through activation of phosphodiesterase. This idea was supported by the fact that the phosphodiesterase inhibitor 1-isobutyl 3-methylxanthine blocked attenuation of cyclic AMP accumulation by histamine in a noncompetitive manner. Histamine also markedly increased phosphoinositide breakdown and 45Ca2+ efflux in 1321N1 cells. These histamine-induced effects apparently are mediated through H1-receptors, since triprolidine, but not cimetidine, potently inhibited histamine action. As for histamine interaction with its receptor, pertussis toxin had no effect on histamine-induced phosphoinositide breakdown, 45Ca2+ efflux, or attenuation of cyclic AMP accumulation. Taken together, these data indicate that 1321N1 human astrocytoma cells are a useful model system for the study of H1-histamine receptors and the biochemical responses mediated through these receptors.