Pharmacokinetic implications of slow equilibration of terbutaline between plasma and erythrocytes

The erythrocyte: plasma concentration ratio of terbutaline was measured in 3 healthy volunteers during and after a steady-state dosing interval. The dose regimen was 3 X 5 mg. The ratios varied only moderately with time. Average ratios of 2.0, 2.2 and 2.6 were found in the 3 subjects. The uptake rate of terbutaline into erythrocytes was studied in vitro. Freshly drawn human blood samples had terbutaline added at a concentration of about 3 ng/mL and were then maintained at room temperature and physiological pH and pCO2. The plasma concentration fell and the erythrocyte concentration rose continuously during the following 8 h, the ratio reaching unity after 6 h. Assuming a monoexponential uptake and a final ratio in vitro similar to that in vivo (about 2.3), we would estimate an uptake half-time between 3.5 and 8.5 h. The two optical isomers of terbutaline did not differ in their uptake rates. In vivo data suggest a much faster uptake. Nevertheless, it may be slow enough to affect the kinetics of terbutaline. The fact that the equilibration process continues in vitro in a withdrawn blood sample also calls for rapid separation of plasma from erythrocytes if plasma levels of terbutaline are to be accurately measured.