The effects of oral and subcutaneous administration of terbutaline in asthmatic patients

In 8 asthmatic subjects, serum concentrations of terbutaline were correlated with changes in volume of air expelled in the first second of forced expiration (FEV1), other parameters of lung function, c-AMP plasma concentrations and finger tremor values after administration of either 5 mg terbutaline sulphate orally, 0.5 mg terbutaline sulphate subcutaneously or an oral placebo. Prior to the investigation, the patients had not used bronchodilators for 10-14 days. All lung function parameters and tremor values showed a significant correlation with the terbutaline (serum) concentrations in the central compartment and with the c-AMP plasma concentrations. FEV1 was the least variable lung function test. The maximal effect on FEV1 occurred 30-60 min after the subcutaneous dose and 2-4 h after the oral dose, corresponding with the maximal terbutaline serum concentrations and c-AMP plasma levels. In this study, the FEV1 improvement showed a linear relationship with the terbutaline serum concentrations between 1.6 and 6 ng/mL. Three out of 8 asthmatics showed an increase in tremor of more than 6 dB after subcutaneous administration, which corresponded with the awareness of this side-effect. In contrast to theophylline and anticholinergic agents, terbutaline serum concentrations showed a very low interindividual variance. As a consequence, the serum terbutaline concentration was almost predictable. However, the lung function improvement was not predictable since there seems to exist an individual sensitivity for this agonist.