b 3 -Adrenoceptor agonists have recently been introduced for the treatment of overactive urinary bladder syndrome. Their target, the b 3 -adrenoceptor, was discovered much later than b 1 -and b 2 -adrenoceptors and exhibits unique properties which make extrapolation of findings from the other two subtypes difficult and the b 3 -adrenoceptor a less-understood subtype. This article discusses three aspects of b 3 -adrenoceptor pharmacology. First, the ligand-recognition profile of b 3 -adrenoceptors differs considerably from that of the other two subtypes, i.e., many antagonists considered as nonselective actually are b 3 -sparing, including propranolol or nadolol. Many agonists and antagonists classically considered as being b 3 -selective actually are not, including BRL 37,344 ( (6) . Moreover, the binding pocket apparently differs between the human and rodent b 3 -adrenoceptor, yielding considerable species differences in potency. Second, the expression pattern of b 3 -adrenoceptors is more restricted than that of other subtypes, particularly in humans; this makes extrapolation of rodent findings to the human situation difficult, but it may result in a smaller potential for side effects. The role of b 3 -adrenoceptor gene polymorphisms has insufficiently been explored and may differ even between primate species. Third, b 3 -adrenoceptors lack the phosphorylation sites involved in agonist-induced desensitization of the other two subtypes. Thus, they exhibit downregulation and/or desensitization in some, but not other, cell types and tissues. When desensitization occurs, it most often is at the level of mRNA or signaling molecule expression. All three of these factors have implications for future studies to better understand the b 3 -adrenoceptor as a novel pharmacological target.

HighlightsMuscarinic receptors increase smooth muscle tone in airways and urinary bladder.β-Adrenoceptors relax smooth muscle tone and oppose muscarinic contraction.Opposition involves transmitter release, signal transduction and receptor expression.This supports the combined use of muscarinic antagonists and β-adrenoceptor agonists.