Prolonged inhibition of human myometrial contractility by intermittent isoproterenol.

Regular, spontaneous contractions of human myometrial strips obtained at the time of elective cesarean section were recorded in a tissue bath in five experiments. Administration of the beta-adrenergic agonist isoproterenol (1 mumol/L) resulted in the rapid onset of myometrial relaxation. In the continuous presence of the beta-agonist, myometrial contractions of amplitude equal to or greater than that of baseline resumed after 10 to 40 minutes (mean +/- SEM = 29.7 +/- 7.3), indicating the development of desensitization of the tissue to the tocolytic effects of isoproterenol. In contrast, intermittent exposure of the myometrial strips to isoproterenol prevented the onset of desensitization and resulted in prolonged inhibition of myometrial contractions. These findings suggest that modification of the present regimen of administration of beta-adrenergic agonists from continuous to intermittent infusion may improve the success of these agents in the prevention of preterm births.