Development of concentration thresholds for fetal swallowing and arginine vasopressin stimulation in the preterm ovine fetus.

We previously demonstrated that fetal swallowing was stimulated by intracarotid saline injections in the near term (130 +/- 1 days gestation) ovine fetus. In order to examine the ontogeny of the regulation of thirst-mediated swallowing behavior and arginine vasopressin (AVP) secretion, the present study examined fetal swallowing and AVP secretory responses to hypertonic NaCl and sucrose in preterm (115 days gestation) ovine fetuses. Five fetuses were chronically prepared with thyrohyoid, nuchal and thoracic esophagus, and diaphragm electromyogram (EMG) electrodes, an esophageal flow probe, and vascular catheters. Fetuses received intracarotid injections (0.15 ml/kg) of increasing concentrations of NaCl (0.15-2.55 M), administered at 2-min intervals. The threshold saline concentration for swallowing was defined as the minimum NaCl dose eliciting swallow responses (within 20 s) following 4 of 5 injections at each dose. In 2 fetuses, the threshold dose was 0.75 M (confirmed by 4 of 5 injections resulting in swallows at the next higher dose). In the other 3 animals, no swallowing threshold was observed (i.e., no dose through 2.55 M elicited swallows to 4 of 5 injections). There were no significant increases in AVP secretion. On a subsequent day, equimolar sucrose injections did not affect fetal swallowing activity or AVP secretion. Together with previous reports, these studies indicate that plasma osmolality dipsogenic mechanisms become functional in some ovine fetuses by 115 days gestation, while all fetuses exhibit intact osmotic dipsogenic responses at 130 days gestation. These results further suggest an acute process of development of dipsogenic responsiveness, rather than by gradually increasing sensitivity to osmotic stimuli.