Differential homologous desensitization of the human histamine H receptors of 445 and 365 amino acids expressed in CHO-K1 cells.

Histamine H receptors (HRs) signal through Gα proteins and are found in neuronal cells as auto- and hetero-receptors. Alternative splicing of the human HR (hHR) originates 20 isoforms, and the mRNAs of two receptors of 445 and 365 amino acids (hHR and hHR) are widely expressed in the human brain. We previously showed that the hHR stably expressed in CHO-K1 cells experiences homologous desensitization. The hHR lacks 80 residues in the third intracellular loop, and in this work we therefore studied whether this isoform also experiences homologous desensitization and the possible differences with the hHR. In clones of CHO-K1 cells stably expressing similar receptor levels (211 ± 12 and 199 ± 16 fmol/mg protein for hHR and hHR, respectively), there were no differences in receptor affinity for selective HR ligands or for agonist-induced [S]-GTPγS binding to membranes and inhibition of forskolin-stimulated cAMP accumulation in intact cells. For both cell clones, pre-incubation with the HR agonist RAMH (1 μM) resulted in functional receptor desensitization, as indicated by cAMP accumulation assays, and loss of receptors from the cell surface and reduced affinity for the agonist immepip in cell membranes, evaluated by radioligand binding. However, functional desensitization differed in the maximal extent (96 ± 15% and 58 ± 8% for hHR and hHR, respectively) and the length of pre-exposure required to reach the maximum desensitization (60 and 30 min, respectively). Furthermore, the isoforms differed in their recovery from desensitization. These results indicate that the hHR experiences homologous desensitization, but that the process differs between the isoforms in time-course, magnitude and re-sensitization.