Ca2+/Calmodulin‐Mediated Regulation of the Desensitizing Process in Gq Protein‐Coupled Histamine H1 Receptor‐Mediated Ca2+ Responses in Human U373 MG Astrocytoma Cells

• Published in: Journal of neurochemistry 2000-08, Vol.75 (2), p.772-781
• Main Authors: Hishinuma, Shigeru, Ogura, Kazumi
• Format: Article
• Language: eng ; jpn
• Subjects: Astrocytoma cell; Biological and medical sciences; Ca2+/calmodulin; Desensitization/resensitization; Gq protein‐coupled receptor; Histamine H1 receptor; Medical sciences; Miscellaneous; Neuropharmacology; Neurotransmitters. Neurotransmission. Receptors; Pharmacology. Drug treatments; Protein kinase/phosphatase
• ISSN: 0022-3042; 1471-4159
• DOI: 10.1046/j.1471-4159.2000.0750772.x

We investigated Ca2+/calmodulin (CaM)‐mediated regulation of the desensitizing process of the histamine H1 receptor‐mediated increase in intracellular Ca2+ concentration in human U373 MG astrocytoma cells. The desensitizing process was evaluated by measuring the histamine‐induced Ca2+ responses in cells pretreated with histamine for 15 s‐30 min under various conditions. Under normal physiological conditions, desensitization developed with three successive phases : a fast desensitization within 15 s, a transient resensitization at 45 s, and a prompt and sustained redesensitization from 1 to 30 min. Similar processes of desensitization/resensitization occurred even under hypertonic conditions, where histamine‐mediated internalization of the histamine H1 receptor is inhibited. The transient resensitization phase was selectively prevented by deprivation of extracellular Ca2+ and, even more strikingly, by the presence of W‐7 (a CaM antagonist). FK506 and cyclosporin A, Ca2+/CaM‐dependent protein phosphatase (PP2B) inhibitors, mimicked such effects. In the presence of KN‐62, a Ca2+/CaM‐dependent protein kinase II (CaM kinase II) inhibitor, the early development of desensitization disappeared, allowing a slow and simple development of desensitization. The early processes of desensitization and resensitization were unaffected by W‐5, okadaic acid, and KN‐04 (less potent inhibitors against CaM, PP2B, and CaM kinase II, respectively) or by GF109203X and chelerythrine (protein kinase C inhibitors). The high‐affinity site for histamine was converted to a lower‐affinity site by histamine treatment, which also showed a transient restoration phase at 45 s in a manner sensitive to KN‐62 and FK506. These results provide the first evidence that Ca2+/CaM plays a crucial role in determining the early phase of the desensitizing process via activation of CaM kinase II and PP2B, by regulating agonist affinity for histamine H1 receptors.