Rapid desensitization and slow recovery of the cyclic AMP response mediated by histamine H-2 receptors in the U937 cell line

The present study focused on the desensitization process of the H-2 recepto r in U937 cells and the recovery of the cyclic AMP (cAMP) response. Treatme nt of U937 leukemic cells with the H-2 histamine receptor agonists (+/-)-N- 1-[3-(3,4-difluorophenyl)-3-(pyridin-2-yl)propyl]-N-2-[3-(1H-imidazol-4-yl) propyl]guanidine (BU-E-75) and amthamine produced a rapid desensitization c haracterized by decreased cAMP production (T-1/2 = 20 min). Pretreatment wi th 10 mu M BU-E 75 did not induce modifications in the responses to prostag landin E-2, isoproterenol, or forskolin. H-2 receptor desensitization was n ot affected by protein kinase A and C inhibitors, but was reduced drastical ly by Zn2+ and heparin, known to act as inhibitors of G protein-coupled rec eptor kinases. Recovery studies of the cAMP response showed that cAMP level s reached 50% of the initial values within 5 hr. Furthermore, desensitizati on produced an important decrease in the basal level of this cyclic nucleot ide. The minimal value was observed 12 hr later, and corresponded to approx imately 1.3% of the initial basal level (7.5 vs 0.1 pmol/10(6) cells). This result could be explained by an increase in phosphodiesterase activity fol lowing 10 mu M BU-E-75 treatment. When cells were exposed for 2 hr to an H- 2 agonist, binding assays showed no modification in the number of H-2 recep tors; internalization began just after 8 hr. Although the initial desensiti zation seems to involve G protein-coupled receptor kinases, results indicat e that additional mechanisms of regulation were triggered by the H-2 agonis ts. (C) 2000 Elsevier Science Inc.