Compartmentation of alpha 2-adrenergic receptors in human erythroleukemia (HEL) cells

We have identified alpha 2-adrenergic receptors on human erythroleukemia (HEL) cells, a suspension-grown cell line related to human platelets. properties of receptors were assessed in intact cells by binding of the antagonist [3H]yohimbine and by inhibition of cAMP accumulation. [3H]Yohimbine labeled 5900 +/- 2100 receptors/cell with a Kd of 3.6 +/- 0.9 nM (n = 7). alpha 2-Adrenergic receptors were potently coupled to inhibition of adenylate cyclase, with EC50 values for epinephrine, UK-14,304, and p-aminoclonidine in the low nM range. Treatment of cells with pertussis toxin abolished this response. In radioligand binding studies with membrane preparations [3H]yohimbine and [3H]UK-14,304 bound to the same number of sites (71 versus 69 fmol/mg of protein), and epinephrine competed for [3H]yohimbine binding in a biphasic manner. After addition of GTP, no high affinity [3H]UK-14,304 binding was detected, and epinephrine competed for [3H]yohimbine binding with lower affinity at both 4 degrees and 37 degrees. In studies with intact cells, we detected no specific binding of [3H]UK-14,304 at either 37 degrees or 4 degrees. At 37 degrees, epinephrine competed for all [3H]yohimbine binding sites with a low apparent affinity (Ki = 21 microM), whereas at 4 degrees epinephrine (up to 1 mM) was able to compete for only 59 +/- 13% of [3H]yohimbine-binding sites. The potency of epinephrine in competing for [3H]yohimbine sites in intact cells at 4 degrees was greater than at 37 degrees (Ki = 1 microM) and was similar to that observed with membranes in the presence of GTP. We hypothesize that sites not detectable by epinephrine at 4 degrees are sequestered within the cell. Treatment of HEL cells with pertussis toxin reduced the proportion of receptors on the surface from 51 +/- 12% to 23 +/- 7% (n = 3, p less than 0.05) of the total sites. Treatment of HEL cells with epinephrine (100 microM, 1 hr) reduced the cell surface component to 25 +/- 8% (n = 3) of the total sites. This treatment was not accompanied by significant desensitization of the ability of epinephrine to inhibit cAMP accumulation. We conclude that alpha 2-adrenergic receptors exist in more than one compartment in HEL cells and that interaction of receptors with a guanine nucleotide-binding protein or with agonist may regulate this compartmentation. These cells provide a new model system for the study of expression and metabolism of alpha 2-adrenergic receptors.