beta(1)-Adrenergic and D(1) Dopaminergic Receptors in Human Neurotumor Cells: Differences in Spare Receptors and Desensitization of Adenylyl Cyclase.
Haman SK-N-MC neurotumor cells express both beta(1)-adrenergic and D(1) dopaminergic receptors. Although there were sevenfold more beta(1) than D(1) receptors, maximum stimulation of adenylyl cyclase activity by isoproterenol was only threefold more than that by dopamine. We had shown previously that the pattern of agonist-mediated densensitization was different for the two receptors. To compare the efficiency of the two receptors to couple to the same adenylyl cyclase and to explore the possible role of spare receptors, the cells were exposed to N -ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), which irreversibly alkylates and inactivates catecholamine receptors. Loss of receptor binding activity was dependent on the concentration of EEDQ, D(1) receptors being more sensitive to EEDQ than beta(1) receptors with IC(50) values of 1 and 30 muM, respectively. beta(1) receptors were more sensitive to the irreversible beta-adrenergic antagonist N(8) -bromoacetyl-N(1)-(3'-(4-indolyloxy)-2'-hydroxy-propyl)-[Z]-1,8-diamino-p-menthane (BIM) with an IC(50) of 1 nM. Inactivation of D(1) receptors parallelled the reduction in dopamine-stimulated adenylyl cyclase activity without a shift in K(act), indicating the absence of spare D(1 receptors in SK-N-MC cells. By contrast, there appeared to be spare beta1) receptors as the K(act) shifted before any reduction in isoproterenol-stimulated activity occurred in EEDQ- or BIM-treated cells. Inactivation of 70% of the beta(1) receptors resulted in only a 20% decrease in isoproterenol-stimulated adenylyl cyclase activity, and the remaining beta(1) receptors were as efficient as D(1) receptors in stimulating adenylyl cyclase. In addition, inactivation of the spare beta(1) receptors did not alter the unique pattern of agonist-mediated desensitization. We conclude that human beta(1) and D(1) receptors are equally efficient at stimulating adenylyl cyclase but differ in their mechanisms of agonist-mediated desensitization.
Zhou
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XM
Membrane Biochemistry Section, Laboratory of Molecular and Cellular Neurobiology, National Institute of Neurological Disorders and Stroke, The National Institutes of Health, Bethesda, Maryland 20892.
Feussner
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GK
Fishman
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PH