Endocrinology of anoestrous bitches

Plasma luteinizing hormone (LH), prolactin and oestradiol concentrations and responses to a standard challenge with a gonadotrophin-releasing hormone (GnRH) analogue were measured at defined stages of consecutive oestrous cycles in beagle bitches. In each of 35 sampling sequences blood samples were collected every 20 min for 6 h followed immediately by injection of a GnRH analogue (GnRH-A) and collection of further samples after 10, 20, 40 and 60 min. Cycle stages were based on progesterone profiles and the day of the preovulatory LH peak. Sampling sequences were obtained during the luteal phase (n = 5), transition to anoestrus (n = 10), anoestrus (n = 17) and pro-oestrus (n = 3), which were 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 cycle and there was no effect of cycle stage on mean LH concentration or variability. In contrast, oestradiol was undetectable in most samples from early and mid-anoestrus until approximately 1 month before the preovulatory LH peak, after which average oestradiol concentrations and between-sample variability appeared to increase. Mean (+/- SEM) oestradiol concentrations for all preinjection samples collected from 100-75, 74-50, 49-25, 24-10 and 9-1 days before the 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.(ABSTRACT TRUNCATED AT 250 WORDS)