Changes in concentrations of cytokines and markers of bone turnover in dairy cows during different stages of a production cycle

Changes in bone metabolism occur to meet the varying demands for calcium during pregnancy and lactation. These changes are regulated locally and systemically by a variety of factors, including hormones, cytokines, chemokines, and growth factors. The goal of the present experiment was to document the dynamics of bone turnover and metabolic state during a complete lactation cycle in dairy cows and to monitor the plasma profile of 10 cytokines previously known to play roles in bone turnover during lactation. Blood samples were collected sequentially from Holstein-Friesian cows during late pregnancy (1-9 wk prepartum; n = 30), early lactation (4-8 wk postpartum [pp]; n = 30), mid lactation (18-22 wk pp; n = 26), and late lactation (35-39 wk pp; n = 21). Due to reproductive failures, 8 of the cows went through extended lactation and samples for these cows were collected at a fifth time point, extended lactation (51-71 wk pp; n = 8). Twelve nonpregnant, nonlactating cows were also sampled. These cows included 4 heifers and 8 nonpregnant (empty) cows. Samples were assayed for calcium, phosphate, the bone resorption marker CTX-I, the bone formation marker, osteocalcin, and 3 metabolic markers, nonesterified fatty acids (NEFA), BHB, and glucose. The plasma levels of IL-1β, IL-4, IL-6, IL-8, IL-10, IFN-γ, IL-17A, TNF-α, MIP-1α, and VEGF-A were also measured using a multiplex cytokine assay. Plasma calcium and phosphate concentrations were found to be within the normal range across all lactation stages. Plasma CTX-I and osteocalcin concentrations corresponded to the stage of production, showing higher levels of CTX-I in late pregnancy and early lactation, and higher levels of osteocalcin in mid lactation, than at other stages of the production cycle. Analysis of metabolic markers, NEFA, and BHB, indicated that fat metabolism was increased to meet high energy demands during specific reproductive stages. The results showed that although glucose concentrations and IL-10, TNF-α, and MIP-1α were not detectably different at any of the stages of production, IL-4, IL-6, IL-8, IFN-γ, and VEGF-A were affected by stage of production. Most of the changes observed in cytokine levels occurred during late pregnancy and late lactation. Plasma IL-6 levels were high during late pregnancy, whereas plasma levels of IFN-γ and IL-4 were elevated during late lactation. In conclusion, these data demonstrate that plasma concentrations of a biomarker bone resorption, but not bone formation, and the cytokines IL-4, IL-6, and IFN-g are related to the stage of production in dairy cattle. Furthermore, the timing of changes in plasma concentrations of cytokines differ, with plasma IL-6 concentration elevated in parallel with CTX-I concentrations and plasma IL-4 and IFN-g concentrations elevated later in lactation.