Role of β-adrenergic receptor subtypes in Lipolysis

In vitro lipolysis stimulated by low (-)-isoprenaline concentrations (< or =30 nM) in epididymal white adipocytes from Sprague-Dawley rats was inhibited at least 60-80% by the specific beta1-antagonists LK 204-545 and CGP 20712A (1 microM), suggesting that at these low (10 nM) concentrations of (-)-isoprenaline lipolysis was primarily (80%) but not solely mediated via beta1-adrenergic receptors. Low concentrations (100 nM) of (-)-noradrenaline and formoterol also confirmed a role for beta1-adrenergic receptors in mediating lipolysis at low concentrations of these agonists. At higher agonist concentrations, beta3-adrenergic receptors were fully activated and were the dominant beta-adrenergic receptor subtype mediating the maximum lipolytic response, and the maximum response was not affected by the beta1-antagonists, demonstrating that the beta3-receptor is capable of inducing maximum lipolysis on its own. Studies of lipolysis induced by the relatively beta2-selective agonist formoterol in the presence of beta1-blockade (1 microM CGP 20712A) demonstrated the inability of the beta2-selective antagonist ICI 118-551 to inhibit the residual lipolysis at concentrations of ICI 118-551 < or = 1 microM. Higher concentrations of ICI 118-551 inhibited the residual formoterol-induced lipolysis competetively, but with low affinity (approximately 500-fold lower than its beta2-adrenergic receptor pA2, 7.80 +/- 0.21), suggesting that formoterol was not acting via beta2-adrenergic receptors. These data are consistent with beta1-adrenergic receptors playing an important role in lipolysis at physiological but not pharmacological concentrations of catecholamines and that beta2-adrenergic receptors play no obvious direct role in mediating beta-adrenergic receptor agonist-induced lipolysis in vitro. Finally, racemic-SR 59230A, unlike the pure (S, S)-isomer (a beta3-selective antagonist), was found to be a nonselective antagonist at the three beta-adrenergic receptor subtypes, showing that the other enantiomers have different selectivity.