Concentrations of luteinizing hormone and oestradiol in plasma and response to injection of gonadotrophin-releasing hormone analogue at selected stages of anoestrus in domestic bitches.

Concentration of plasma luteinizing hormone (LH) and oestradiol concentrations and responses to a standard challenge with a gonadotrophin-releasing hormone (GnRH) analogue were measured at certain stages of anoestrus during consecutive cycles in five beagle bitches. Blood samples were collected every 20 min for 6h followed immediately by injection of GnRH analogue (0.16 micrograms i.v.) and collection of further samples after 10, 20, 40 and 60 min. Five, 10, 17 and three such sampling sequences were obtained during the luteal phase, transition to anoestrus, anoestrus and pro-oestrus respectively (i.e. 154-71, 114-44, 85-11 and 7-1 days before the preovulatory LH peak, respectively). Pulsatile LH secretion occurred spontaneously at all stages of the luteal phase and anoestrus and there was no effect of cycle stage on mean LH concentration or variability. In contrast, oestradiol could not be detected in most samples from early and mid-anoestrus until approximately one month before the preovulatory LH peak, after which average oestradiol concentration and between sample variability appeared to increase. Mean (+/- SEM) oestradiol concentration for all samples collected from 100-75, 74-50, 49-25, 24-10 and 9-1 days before LH peak was 1.4 +/- 0.1, 1.3 +/- 0.1, 2.4 +/- 0.3, 11.0 +/- 1.4 and 36.0 +/- 3.2 pg ml-1, respectively. Plasma LH concentration increased in all bitches after GnRH analogue injection (2.7 +/- 0.7 ng ml-1 at t = 0, 12.5 +/- 1.0 ng ml-1 at t = 10 min, mean +/- SEM, n = 35) regardless of cycle stage.(ABSTRACT TRUNCATED AT 250 WORDS)