Effects of season and stage of gestation on luteinizing hormone release in gilts.

This study was designed to examine the effects of two seasons and stage of gestation on luteinizing hormone (LH) release in the gilt. Eleven Yorkshire-Landrace crossbred gilts were each fitted with an indwelling vena caval cannula. Blood samples were collected at 6 h intervals for six days during early (day 39 to 44) or mid-gestation (day 69 to 74). Serum progesterone, estradiol-17 beta and LH concentrations were determined in samples collected at 6 h intervals. Early and mid-gestation occurred during August and September in group 1 (n = 6) and during January and February in group 2 gilts (n = 5). To characterize pulsatile LH release, samples were collected at 15 min intervals for 8 h on day 40, 43, 70 and 73 of gestation. Following each 8 h sampling period, gilts were treated intravenously with 0.5 micrograms gonadotropin-releasing hormone (GnRH)/kg body weight and blood collected at 10 min intervals for 3 h. Progesterone concentrations decreased (p less than 0.01) from 22.1 +/- 0.4 ng/mL during early gestation to 18.2 +/- 0.4 ng/mL during mid-gestation. Estradiol-17 beta concentrations increased (p less than 0.01) from early to mid-gestation (13.5 +/- 0.8 versus 28.4 +/- 0.7 pg/mL). Frequency of LH pulses and LH pulse amplitude were higher (p less than 0.05) in pregnant gilts during January and February compared to August and September.(ABSTRACT TRUNCATED AT 250 WORDS)