“…While inhibition of extraneuronal uptake with normetanephrine (50 gM) only caused a 2 fold increase in the potency of Iso, it is possible that 1-adrenoceptor blockade caused by normetanephrine partially masked sensitization to Iso (Kenakin, 1980;Goldie & Paterson, 1982 In guinea-pig atria where a homogeneous population of 1l-adrenoceptors also exists (O'Donnell & Wanstall, 1979a,b), the pA2 value for atenolol against Iso was also 7.2 (Harms, 1976). In contrast, in the pig lung parenchyma strip where P2-adrenoceptors predominate, the pA2 value for atenolol against Iso was only 5.4 (Goldie et aL, 1982a). Furthermore, in the present study, salbutamol (32-selective) was a very weak antagonist of Iso-induced relaxation of pig bronchus (pA2 = 4.4).…”