Self-Limitation of Intravenous Tocolysis with β2-Adrenergic Agonists Is Mediated through Receptor G Protein Uncoupling

Tocolysis with a beta-adrenergic receptor agonist is the most common approach to premature labor management after the 25th wk of pregnancy. However, prolonged treatment is associated with a marked loss of efficacy. The biochemical mechanisms involved remain unclear. This study was undertaken to investigate the effect of fenoterol on beta-adrenergic receptor signal transduction in human myometrium. Myometrial biopsy specimens were obtained from 40 women at cesarean section between the 25th and 34th wk of pregnancy. Nineteen patients had received no tocolysis (controls, group I) and 21 had been treated with fenoterol (<48 h in 10, group II; > or = 48 h in 11, group III). As methods we used membrane preparation, adenylyl cyclase assay and cAMP RIA. Adenylyl cyclase activity was determined by the measurement of cAMP levels to evaluate signal transduction after stimulation of beta-adrenergic receptors with isoproterenol, G protein with GTP, and adenylyl cyclase with forskolin. The functional activity of GTP-binding regulatory proteins (G(s)) and adenylyl cyclase was not altered by fenoterol treatment. In the control group, the increase in adenylyl cyclase activity in response to GTP plus isoproterenol was greater than in response to GTP alone. The increase was reduced by 50% in group II and was insignificant in group III. There was no correlation between gestational age and basal adenylyl cyclase activity. Intravenous tocolysis with the beta2-adrenergic receptor agonist fenoterol leads to complete desensitization of the beta-adrenergic receptor system. In addition to the known reduction in receptor number (down-regulation) as underlying mechanism, uncoupling of the receptor from the stimulatory G protein G(s) was identified.