Effects of partial agonists on beta-adrenergic receptors

Nanette Cocero Cordero, University of Pennsylvania


A useful approach for investigating the mechanism by which B-adrenergic receptors function as biochemical transducers has been to compare the interactions of partial or atypical agonists with full agonists using cell lines that express $\beta$-adrenergic receptors. The use of clonal cell lines avoids the complications inherent in the use of solid tissues, which are generally comprised of multiple types of cells. Cell lines expressing $\beta\sb2$-adrenergic receptors are widely used for studies of the properties and regulation of receptor expression. However, cell lines selectively expressing $\beta\sb1$-adrenergic receptors have not been readily available. Mouse L cells were transfected with a gene cloned from a genomic library prepared from Balb/c mouse liver. The pharmacologic profile of the gene product expressed in the new cell line called L$\beta\sb1$ cells is that of a $\beta\sb1$-adrenergic receptor. In these studies, xamoterol, denopamine and ractopamine were compared to the full agonist isoproterenol in their ability to stimulate the activity of adenylyl cyclase, their ability to decrease the density of $\beta$-adrenergic receptors; and their susceptibility to effects of GTP. Cell lines that express $\beta\sb1$-adrenergic receptors (L$\beta\sb1$ cells), $\beta\sb2$-adrenergic receptors (L$\sb6$ myoblasts and S49 lymphoma cells) and both $\beta\sb1$- and $\beta\sb2$-adrenergic receptors (C$\sb6$ glioma cells) were used. Computer-assisted analysis of dose-response curves resulting from studies of the inhibition of the binding of $\sp{125}$I-iodocyanopindolol ($\sp{125}$I-ICYP) showed that xamoterol has 500-fold selectivity for $\beta\sb1$-adrenergic receptors whereas denopamine has 21-fold selectivity for $\beta\sb1$-adrenergic receptors and ractopamine has 3-fold selectivity. Addition of GTP to binding assays shifted the dose-response curves for inhibition of the binding of $\sp{125}$I-ICYP by isoproterenol, denopamine and ractopamine to the right; this effect was not observed with xamoterol. Studies of agonist stimulated cyclic AMP accumulation revealed that denopamine is a partial agonist with approximately 30% efficacy compared to isoproterenol, whereas xamoterol had no noticeable effect on the conversion of $\sp3$H-adenine to $\sp3$H-cyclic AMP. These results suggest that denopamine is a $\beta\sb1$-selective partial agonist while xamoterol is an atypical agonist. Incubation of L$\beta\sb1$ cells and L$\sb6$ cells or S49 lymphoma cells with isoproterenol, xamoterol, denopamine and ractopamine resulted in decreases in the density of both subtypes of $\beta$-adrenergic receptors. Studies of agonist stimulated cyclic AMP accumulation revealed that ractopamine is a partial agonist at $\beta\sb2$-adrenergic receptors with approximately 50% efficacy compared to isoproterenol. Studies of the effect of ractopamine on the stimulation of adenylyl cyclase using membranes prepared from cells incubated with increasing concentrations of isoproterenol were carried out. In these experiments, ractopamine became a weak partial agonist or an antagonist.

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Recommended Citation

Cocero Cordero, Nanette, "Effects of partial agonists on beta-adrenergic receptors" (1993). Dissertations available from ProQuest. AAI9321374.