Chronic systemic administration of salmeterol to rats promotes pulmonary β₂‐adrenoceptor desensitization and down‐regulation of G_sα

The aim of the present study was to examine the effects of chronic infusion of the long‐acting agonist salmeterol on pulmonary β 2 ‐adrenoceptor function in Sprague‐Dawley rats in vivo and to elucidate the molecular basis of any altered state. Systemic administration of rats with salmeterol for 7 days compromised the ability of salmeterol and prostaglandin E 2 (PGE 2 ), given acutely by the intravenous route, to protect against ACh‐induced bronchoconstriction when compared to rats treated identically with vehicle. β 1 ‐ and β 2 ‐adrenoceptor density was significantly reduced in lung membranes harvested from salmeterol‐treated animals, which was associated with impaired salmeterol‐ and PGE 2 ‐induced cyclic AMP accumulation ex vivo . Three variants of G sα that migrated as 42, 44 and 52 kDa peptides on SDS polyacrylamide gels were detected in lung membranes prepared from both groups of rats but the intensity of each isoform was markedly reduced in rats that received salmeterol. The activity of cytosolic, but not membrane‐associated, G‐protein receptor‐coupled kinase was elevated in the lung of salmeterol‐treated rats when compared to vehicle‐treated animals. The ability of salmeterol, administered systemically, to protect the airways of untreated rats against ACh‐induced bronchoconstriction was short‐acting (t off ∼45 min), which contrasts with its long‐acting nature when given to asthmatic subjects by inhalation. These results indicate that chronic treatment of rats with salmeterol results in heterologous desensitization of pulmonary G s ‐coupled receptors. In light of previous data obtained in rats treated chronically with salbutamol, we propose that a primary mechanism responsible for this effect is a reduction in membrane‐associated G sα . The short‐acting nature of salmeterol, when administered systemically, and the reduction in β‐adrenoceptor number may be due to metabolism to a biologically‐active, short‐acting and non‐selective β‐adrenoceptor agonist. British Journal of Pharmacology (2001) 132 , 1261–1270; doi: 10.1038/sj.bjp.0703946