RENAL-FUNCTION IN THE CHRONICALLY CANNULATED FETAL LLAMA - COMPARISONWITH STUDIES IN THE OVINE FETUS

Samples of maternal and fetal plasma, fetal urine, and amniotic fluid were collected from 8 chronically cannulated pregnant Ilamas, in the l ast third of gestation. The samples were obtained for up to 18 days po st-surgery. Osmolality, sodium (Na), potassium (K), chloride (CI), and urea were measured on 40 samples collected on days 1, 2, 3, 4-5, 6-7, 8-9, and 10-19. The osmolalities of maternal and fetal plasma, fetal urine and amniotic fluid, averaged over these 7 time periods, were, re spectively, 312+/-2, 311+/-1, 484+/-14, and 317+/-1 mosmol kg(-1). Val ues are given as mean+/-s.e. The major differences from fetal fluid va lues in the ovine fetus (from previously published values) were the hi gher osmolality and urea concentration of llama fetal urine. Urine flo w rate measured in 6 fetuses, 4.5-6.5 kg body weight, was 5.8+/-0.4 mL h(-1); urea clearance rate was 55.5+/-11.8 mL h(-1). Glomerular filtr ation rate (GFR), measured with Cr-51-EDTA in 5 fetuses on 1-4 occasio ns, was 111.4+/-23.3 mL h(-1). Fractional reabsorptions (FR) of Na, K and Cl were 97.9+/-1, 75.9+/-13.5 and 97.7+/-0.4% respectively. The GF R (25 mL kg(-1) h(-1)) and urine flow rate (1 mL kg(-1) h(-1)) were le ss than half and about one-tenth the respective values in ovine fetuse s. As Na reabsorption is the major oxygen-consuming activity of the ki dney, the llama fetal kidney requires only half the oxygen needed by t he ovine fetal kidney to reabsorb the filtered sodium load. The reason for the formation of hypertonic, rather than hypotonic, urine in the fetal llama may be due to both greater morphological maturity of the k idney and the excretion of as yet unidentified osmotically active orga nic substances.