Slow-release somatotropin reduces plasma leptin in lactating dairy cows

Somatotropin has dramatic effects on adipose tissue and lipid metabolism. Leptin, produced and released primarily by adipose cells, exerts a regulatory control on energy homeostasis. The aims of this study were to determine the effects of bST administration on milk production, plasma leptin and selected plasma metabolites in lactating dairy cows. Forty Holstein cows(90±33 DIM)were randomly divided into 2 groups: Control and bST. The bST group received 640mg/4wk of slow release bST(Posilac) for two cycles. Milk yield and composition were measured at 7 days post-injection of each cycle. Blood samples were collected on the same day before feeding, and analyzed for leptin, NEFA, total protein, α-amino nitrogen, and urea nitrogen. Both milk yield and milk fat percentage were increased(29.9 vs. 35.3 kg/d; P≤.01; 3.59 vs. 3.92%; P≤.05) by bST administration, while milk protein content was unaffected by treatment (3.19 vs. 3.13%. At 7 days post-injection, bST decreased (P<.01) plasma leptin by 33% (4.26 vs. 2.86 µg/l) whereas plasma NEFA was drastically increased (225 vs. 875 µmol/l; P≤.01). Plasma total protein was increased by bST (78.9 vs. 81.4 g/l; P≤.05), while both α-amino nitrogen and urea in plasma of treated cows were reduced (P≤.01) by 20% (2.54 vs. 2.03 mmol/l; 6.55 vs. 5.27 mmol/l, respectively). These data confirmed a galactopoetic effect of bST, which imposed, on peak of response, a higher demand of nutrients sustained by an enhanced lipolysis in adipose tissue. Lower plasma leptin observed in present study could be due to reduced body fat mass as consequence of lipolysis induced by bST. This is in line with higher plasma NEFA concentration observed in bST group. Plasma nitrogen metabolites indicate a higher efficiency in protein metabolism in treated cows. This study show that plasma leptin is linked with the nutritional status of cows, even though other hormones and metabolites are also involved in the signaling and control of body energy store.