Possible Role of Dexamethasone in Sensitizing the Beta-2-Adrenergic Receptor System in vivo in Calves during Concomitant Treatment with Clenbuterol

Abstract

β₂-Agonists blunt the function of the β-adrenoceptor G-protein adenylate cyclase-signalling system, whereas glucocorticoids reverse the agonist-mediated diminished β-adrenergic responses; however, these effects have not been reported in vivo in calf lymphocytes. In this study, we first investigated the presence of the β₂-adrenergic receptors on calf lymphocytes, and second we tested the effects of either clenbuterol alone or in combination with dexamethasone on receptor expression and function (isoproterenol-induced intracellular adenosine 3′,5′-cyclic monophosphate (cAMP) formation) in vivo. (–)-[¹²⁵I]-Iodocyanopindolol (ICYP) binding to intact calf lymphocytes was rapid, saturable (maximal number of binding sites 987 ± 89 ICYP-binding sites/cell, n = 4) and of high affinity (K_D value 17.23 ± 2.8 pmol/l, n = 4). These binding sites were of the β₂-subtypes of adrenoceptors as indicated by the fact that β-agonists inhibited ICYP binding with an order of potency: (–)-isoproterenol > (–)-adrenaline > (–)-noradrenaline. Furthermore, the selective β₂-adrenoceptor antagonist ICI 118.551 was about >1,500 times more potent in inhibiting ICYP binding than was the β₁-selective adrenoceptor antagonist CGP 20712A. Consequently, calves were treated with clenbuterol (1.0 µg/kg b.i.d., i.v.) for 9 days alone or simultaneously with dexamethasone (0.1 mg/kg, i.v., once a day for 4 days). Clenbuterol decreased the number of lymphocyte β₂-adrenergic receptors by about 40–50% after only 48 h of drug administration. This was accompanied by a decrement in isoproterenol-induced lymphocyte cAMP formation. Upon application of both drugs, dexamethasone restored the clenbuterol-mediated decrease in β₂-adrenoceptors and cAMP production. Dexamethasone elevated the number of β₂-adrenoceptors and cAMP almost 1.5- to 2-fold at 24 h of drug administration, an effect that persisted for up to 24 h following drug withdrawal. Neither clenbuterol nor the combination with dexamethasone had an influence on the affinity of the receptor for the ligand. The present results demonstrate that dexamethasone in vivo upregulates the number and function of calf lymphocyte β₂-adrenoceptors, and thus enhances the sensitivity of the β₂-adrenoceptor signal-transduction pathway for clenbuterol during concomitant treatment with both drugs.