The affinity of histamine for Gq protein-coupled histamine H 1-receptors is predominantly regulated by their internalization in human astrocytoma cells

We examined the regulatory mechanisms of the affinity of gq protein-coupled histamine h1-receptors for histamine after histamine pretreatment in intact human u373 mg astrocytoma cells. in control cells,the displacement curves for histamine against the binding of 5 nm [3h] mepyramine,a radioligand for h1-receptors,showed the presence of two binding sites for histamine,that is,high and low affinity sites. pretreatment with 0.1 mm histamine for 30 min at 37°c induced a significant reduction in the percentage of high affinity sites for histamine and a concomitant increase in the percentage of low affinity sites with no change in their pic50 values. these histamine-induced changes were insensitive to 30 μm kn-62,an inhibitor of ca2+/calmodulin-dependent protein kinase ii,but they were completely inhibited either by 0.4 mm zncl2,an inhibitor of g protein-coupled receptor kinases (grks),or under hypertonic conditions,where clathrin-mediated endocytosis is known to be inhibited. these results suggest that histamine-induced conversion of high to low affinity sites for histamine is predominantly regulated by grk/clathrin-mediated internalization of h 1-receptors in human astrocytoma cells. © the japanese pharmacological society.