OVINE FETAL SWALLOWING RESPONSE TO INTRACEREBROVENTRICULAR HYPERTONICSALINE

In the near-term ovine fetus, systemic hyperosmolality stimulates dips ogenic responses. Putative systemic dipsogens (hypertonicity, angioten sin II) may initiate responses by stimulation of select cerebral circu mventricular nuclei lacking a blood-brain barrier. To investigate whet her central osmotic-dipsogenic mechanisms are functional in utero, fet al swallowing responses to intracerebroventricular (icv) hypertonic sa line were examined. Five pregnant ewes with singleton fetuses (128 +/- 1 days gestation) were prepared with fetal lateral cerebral ventricle and vascular catheters, electrocortical (ECoG) electrodes, and electr omyogram wires on the fetal thyrohyoid muscle, nuchal and thoracic eso phagus, and diaphragm and studied for a minimum of 5 days postoperativ ely. After a 2-h basal period, fetuses received an icv infusion of art ificial cerebrospinal fluid followed by successive 30-min infusions of hypertonic NaCl in artificial cerebrospinal fluid (500 and 700 mosmol /kgH(2)O). In response to the icv hypertonic NaCl infusions, fetal swa llowing significantly increased (1.4 +/- 0.4 to 3.9 +/- 1.4 and 2.9 +/ - 0.5 swallows/min low-voltage ECoG, respectively). Plasma arginine va sopressin levels increased, although the change was not statistically significant (9.1 to 24.2 pg/ml; P = 0.07), and there was no change in fetal plasma osmolality, sodium concentration, or ECoG activity. Toget her with previous studies, these results indicate that both central an d systemic osmotic dipsogenic mechanisms are functional in utero.