Thermogenic responses to adrenoceptor agonists and brown fat adrenoceptors in overfed rats

Abstract

Rats fed a cafeteria diet to produce hyperphagia showed increases in the maximal thermogenic responses (rise in oxygen consumption) to isoprenaline (mixed β-agonist), prenalterol ( β 2-selective agonist) and clenbuterol ( β 2-agonist), and left-shifts in the dose-response curves to the latter two. The maximal response to phenylephrine (α-agonist) was similar for control and cafeteria rats. Ligand binding studies revealed increases in β-adrenoceptor density of 33–38% in brown fat cells and isolated membranes from cafeteria-fed rats, but a 30% reduction in β-receptors in heart membranes. Cold-adaptation caused a 22% reduction in β-receptor density in brown fat membranes, but no change in heart. The ratio of β 1 β 2 - receptors in brown fat was reduced from 59 45 in control to 47 54 in cafeteria-fed rats, but was not significantly altered in heart ( 58 44 ) or in brown fat from cold-adapted animals ( 64 30 ). α-Adrenoceptor density was increased above control values by 69 and 25% in brown adipose tissue from cafeteria and cold-adapted rats, respectively.