PARENT SESSION

(59) PRESENCE OF RELAXIN IN THE MILK OF LACTATING SOWS AND ITS TRANSMISSION TO NEONATAL PIGS VIA SUCKLING.

Yan, Wenbo¹, Lasano, Sally², Steinetz, Bernard ², Bartol, Frank³, Bagnell, Carol¹, ¹ Rutgers University, New Brunswick, NJ² New York University School of Medicine, Tuxedo, NY³ Auburn University, Auburn, AL

ABSTRACT- The existence of growth factors in pig milk is well established. Growth factors, including relaxin (RLX), that are transmitted into the neonatal circulation via colostrum/milk may play functional roles in neonatal development. In the pig, uterine RLX receptor (LGR7) expression is detectable from birth (postnatal day = PND 0) through PND 14, and RLX is uterotrophic during this critical period for porcine uterine organization. However, whether milk is a source of RLX in the neonatal pig is unknown. Objectives of this study were to determine if: (1) RLX is present in porcine colostrum/milk and/or the neonatal circulation; (2) RLX concentrations in colostrum/milk and/or neonatal serum change during the first two weeks of lactation; and (3) substitution of a milk replacer for natural colostrum/milk from birth affects circulating levels of RLX in neonatal serum. Colostrum/milk was collected daily from lactating sows (n = 7) for 14 days beginning on the day of parturition. After birth, piglets were allowed to either suckle from the sow (n = 8 piglets / 4 litters) or were fed a RLX-free, artificial milk replacer (n = 6 piglets). Blood was collected from all piglets daily by external iliac venipuncture. Porcine RLX in colostrum/milk and neonatal serum samples was detected by homologous radioimmunoassay (RIA). To validate the RIA for detecting RLX in whole milk, recovery, parallelism and absence of cross-reactivity with other growth factors were demonstrated. RLX was detected in all colostrum/milk samples. Concentrations of RLX were highest (14.0 ± 1.2 ng/ml) in the first 48 h of lactation and declined to 1.7 ± 0.3 ng/ ml by PND 14. Concentrations of RLX in neonatal serum were highest (183.3 ± 11.9 pg/ml) on PND 0 and 1, prior to the estimated time of gut closure, and dropped below RIA sensitivity (32 pg/ml) thereafter. Serum RLX was undetectable on PND 0 in piglets removed from the sow and fed milk replacer for 12 h. Data indicate that RLX is present in the milk of lactating sows and may be transmitted, via suckling, into the circulation of nursing piglets on the first day or two of neonatal life. The physiological importance of RLX in pig milk is unknown. However, given that the neonatal porcine uterus is LGR7-positive at birth, milk-borne RLX may be beneficial to uterine development in neonatal gilts. (Support USDA-NRICGP 2003-35203-13572)

KEY WORDS: relaxin, milk, serum, neonatal pigs