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Concurrent Pulsatile Secretion of Luteinizing Hormone and Follicle-Stimulating Hormone during Different Phases of the Estrous Cycle and Anestrus in Beagle Bitches

^a Department of Clinical Sciences of Companion Animals and ^b Department of Herd Health and Reproduction,Faculty of Veterinary Medicine, Utrecht University, NL-3508 TD Utrecht, The Netherlands ^c Research Institute for Endocrinology, Reproduction and Metabolism, University Hospital, Vrije Universiteit,Amsterdam, The Netherlands

The 6-h secretory profiles of LH and FSH and the possible concordance between the episodic release of LH and FSH were studied in 6 beagle bitches during early, mid-, and late anestrus and during the follicular and luteal phases of the estrous cycle. Plasma samples were obtained at 10-min intervals via jugular venipuncture. In all stages of anestrus and in the luteal phase, FSH and LH secretion was pulsatile. All FSH pulses coincided with LH pulses. However, the mean duration of the FSH pulse (115 min) was significantly longer than that of the LH pulse (72 min). The basal plasma LH concentration was low compared with the maximum peak levels, whereas FSH pulses were characterized by relatively low peaks compared with the basal levels. In contrast to the basal plasma LH levels and the area under the curve (AUC) for LH, the basal plasma FSH levels and the AUC for FSH increased significantly as anestrus progressed. During the follicular phase, the secretory pattern of LH was characterized by frequent increases of short duration. During this phase, the basal plasma FSH concentration was relatively low, whereas the basal plasma LH level was high in comparison with that in the other phases of the estrous cycle. The luteal phase was characterized by an increased frequency of LH pulses, a shorter duration of the LH peaks, and a tendency to a lower amplitude of both LH and FSH peaks compared with values observed during anestrus. It is concluded that in the bitch, FSH and LH pulses are released in concordance and that progression from early to late anestrus is associated with an increase in basal plasma FSH concentration without a concomitant rise in basal plasma LH concentrations. The latter suggests that in the bitch an increase in circulating FSH should be considered to be a critical event required for the initiation of ovarian folliculogenesis and consequently for the termination of anestrus.

¹ Correspondence: H.S. Kooistra, Department of Clinical Sciences of Companion Animals, Faculty of Veterinary Medicine, Utrecht University, PO Box 80.154, NL-3508 TD Utrecht, The Netherlands. FAX: 31 30 2518126; h.s.kooistra{at}vet.uu.nl

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