Neurohypophyseal hormone release in the human: III. Experimental study during labor

Abstract

Uterine or “oxytocic,” milk-ejecting, and antidiuretic activities were studied during spontaneous labor in 35 healthy women. The study was carried out in three groups: (1) in the dorsal position maintained throughout labor and delivery; (2) in the lateral position; and (3) in the dorsal position and with continuous spinal block. Uterine activity was determined by continuously recording intra-amniotic pressure. Milk-ejecting activity was studied by recording intraductal mammary pressure in subjects showing response to threshold amounts of exogenous oxytocin. Antidiuretic activity was estimated by measuring urinary osmolality and free water clearance and controlled by the simultaneous determination of creatinine and osmolal clearances. Both uterine and antidiuretic activities were found to coexist during the last hours of labor. However, antidiuretic activity was significantly decreased in the group with continuous spinal block, probably due to absence of pain. There was no significant milk-ejecting activity during labor. The physiologic significance of these findings is discussed.