Clinical pharmacology of beta 3-adrenoceptors

1. An atypical non beta 1/beta 2-adrenoceptor (AR) subtype (beta 3-AR) has been identified which is selectively stimulated by a group of ligands which mediate lipolytic and thermic responses in brown and white adipose tissue. 2. Molecular studies have shown that beta 3-AR in man are mainly expressed in visceral adipocytes, and to a lesser extent in gall-bladder and colon. In vitro studies with beta 3-AR agonists have shown activity at other sites including skeletal muscle and myocardium. 3. Regulation of beta 3-AR may differ from beta 1/beta 2-AR subtypes in that continuous agonist exposure does not result in receptor down-regulation. 4. A polymorphism of the human beta 3-AR gene (Trp64Arg) has been identified which is associated with obesity, insulin resistance and an earlier onset of non-insulin-dependent diabetes mellitus (NIDDM). Studies are required to establish whether expression of the mutant gene results in altered metabolic responses to beta 3-AR stimulation in man. 5. There is accumulating evidence to support a therapeutic role of beta 3-AR agonists in NIDDM because of anti-obesity and anti-diabetic activity, as a consequence of thermogenic effects as well as increased insulin sensitivity and glucose tolerance. 6. Selectivity studies with BRL35135 and isoprenaline in humans have demonstrated a beta 3-AR mediated component to thermogenesis which is dissociated from beta 1/beta 2-mediated effects on carbohydrate and fat metabolism. Similar studies have suggested a functional beta 3-AR mediating cardiac but not airway responses in humans. An evaluation of beta 3-AR agonists in irritable bowel syndrome may be warranted in view of colonic antimotility properties in vitro.