α₁‐Adrenoceptor subtype substitution in knockout mice

Abstract

Since the discovery of a 1 -adrenoceptor subtypes, there has been continued interest in the identification of their functional roles. This has been facilitated by the availability of increasingly selective antagonists for individual subtypes, especially for the a 1A and a 1D , as well as knockout (KO) mice lacking either a 1A , a 1B or a 1D adrenoceptors (Hague et al., 2003).In some tissues, such as heart or blood vessels, multiple a 1 subtypes are present and produce the same functional response, although the dominant subtype can vary between species or particular blood vessels (Hrometz et al., 1999). In other cases, such as in most urogenital smooth muscles, the response is mediated primarily by a single subtype (a 1A ) independent of species (Ruffolo & Hieble, 1999). Depending on the species, the dominant a 1 -adrenoceptor mediating metabolic responses in the liver is either a 1A (human, cat, dog, rabbit) or a 1B (rat, mouse, hamster). In the monkey, hepatic a 1A and a 1B adrenoceptors both contribute (GarciaSainz et al., 1996). In this issue of British Journal of Pharmacology, Deighan et al. (2004) show that, in a 1B KO mice, the function of the hepatic a 1B adrenoceptor can be assumed by the a 1A .Radioligand-binding assays show RS100329, a selective a 1A antagonist, to have 30-fold greater affinity for a 1 -adrenoceptors in hepatocytes from the KO animals (pK i ¼ 9.3) than in WT animals (pK i ¼ 7.8). In contrast, the selective a 1D antagonist BMY 7378 had low affinity in both WT and KO mice (pK i ¼ 6.3 and 6.2, respectively).At 4 months of age, the liver from a 1B KO mice had a substantial a 1 -receptor binding (30 fmol mg À1 protein), although less than WT mice of the same age (50 fmol mg À1 protein). However, 3-month-old KO mice had almost no hepatic a 1 -adrenoceptor binding. This would indicate that replacement of the missing a 1B adrenoceptor by the a 1A is a relatively slow process, and suggest that, in general, comparisons of receptor density between WT and KO mice should be made at multiple time points.Based on multiple literature reports, it is now clear that a 1 -adrenoceptors are not confined to the cell membrane, and that the subcellular localization can differ between subtypes (Mackenzie et al., 2000;Stanasila et al., 2003;Hague et al., 2004). Deighan et al. (2004) show that the distribution of a 1A adrenoceptors in KO mice is identical to that of the a 1B in WT mice, consistent with the ability of the two subtypes to mediate the same physiological function.This