β1- and β3-adrenoceptors mediate relaxation in ovine trachealis smooth muscle

1 Isoprenaline (non-selective) and noradrenaline (β₁-selective) concentration-dependently relaxed ovine tracheal strips precontracted with carbachol. The pD₂ values were 7.07 ± 0.08 and 6.13 ± 0.10 for isoprenaline and noradrenaline, respectively. In the same preparation, salbutamol either produced weak relaxation or in some cases, contractile responses indicating the presence of very little or no β₂-adrenoceptors in this preparation. 2 Isoprenaline-and noradrenaline-induced relaxations were antagonized by propranolol and atenolol with pA₂ values in the range reported in the literature for an action on β₁-adrenoceptors. ICI 118551 also antagonized isoprenaline- and noradrenaline-induced relaxation but at concentrations much higher than are required to block β₂-adrenoceptors, confirming that β₂-adrenoceptors do not contribute significantly to these responses. 3 The selective β₃-adrenoceptor agonist, BRL 37344A produced concentration-dependent relaxation of tracheal strips. BRL 37344A was a full agonist producing 100% relaxation of carbachol-induced tone. BRL 37344A-induced relaxation was weakly antagonized by propranolol confirming an action, mainly, on β₃-adrenoceptors. Cyanopindolol antagonized isoprenaline-induced relaxation (in the presence of propranolol, 10^??7 m ) with a pA₂ value of 8.06 ± 0.24. 4 It was therefore concluded that β₁- and β₃-adrenoceptors mediated agonist-induced relaxation in sheep tracheal strips.