Terbutaline serum concentrations related to different lung function parameters and beta-receptor function.

In eight asthmatic and six normal subjects serum terbutaline concentrations were correlated with changes in FEV1, MEF50%, Gaw, sGaw, and plasma cAMP concentrations after administration of 0.5 mg terbutaline subcutaneously, 5 mg orally, and an oral placebo. In asthmatics and normals the changes in serum terbutaline levels and plasma cAMP levels were closely correlated and in the same range. No change in lung function parameters was registered in normals, whereas in asthmatics a significant improvement was shown. In asthmatics all lung function parameters closely parallelled the changes in serum terbutaline concentrations and cAMP plasma concentrations, the FEV1 being the least variable lung function parameter. The maximal effect in FEV1 occurred 30-60 min after the subcutaneous dose and 2-4 h after the oral dose. The FEV1 improvement showed a linear relationship with the serum terbutaline concentration between 1.6 and 6 ng/ml. In contrast with theophyllines and anticholinergics, serum terbutaline concentrations showed a very low interindividual variance. Nevertheless it appeared that prediction of a certain improvement in lung function was not possible on the basis of the serum concentration. Since changes in plasma cAMP values after terbutaline therapy were similar in normals and asthmatics, the lung function improvement in the asthmatics suggests that if a beta-adrenergic defect exists, this must be mainly situated in the lung tissue.