Plasma leptin concentrations correlate with luteinizing hormone secretion in early postpartum Holstein cows

We tested the hypothesis that hypothalamic-pituitary activity, bioassayed by LH pulse frequency, in dairy cattle during early lactation is related to measures of energy status and to circulating profiles of free fatty acids (FFA), insulin, insulin-like growth factor-I (IGF-I), leptin, and growth hormone (GH). On d 14 postpartum, before first ovulation and during the period of negative energy balance (-23.4 +/- 2.4 Mcal/d of metabolizable energy), blood plasma was sampled from 18 multiparous cows at 10-min intervals for 8 h. All samples were assayed for LH and GH and hourly samples were assayed for FFA, insulin, IGF-I, and leptin. Milk yield and composition, body condition score, and energy balance were also measured. Frequency of LH pulses was correlated positively with energy balance (r = 0.51) and plasma leptin concentrations (r = 0.73), and negatively with milk fat content (r = -0.52). Amplitude of the LH pulses was correlated only with leptin (r = 0.53). Frequency of GH pulses was not correlated with any measure of LH secretion, but was correlated negatively with plasma concentrations of insulin (r = -0.62) and IGF-I (r = -0.61). First ovulation was observed 34 +/- 4 d after parturition. These observations reveal an important linkage between pulsatile LH secretion and blood leptin concentrations during the early postpartum period in dairy cows, when their energy balance is negative, and may explain the delay in ovulation.