Bromocriptine-induced premature oestrus is associated with changes in the pulsatile secretion pattern of follicle-stimulating hormone in beagle bitches

The secretory profiles of LH and FSH were investigated before and during th e administration of bromocriptine in six beagle bitches. Plasma samples wer e obtained via jugular venepuncture at 10 min intervals for 6 h every 2 wee ks until the next ovulation. Bromocriptine treatment was started 100 days a fter ovulation. Both before and after bromocriptine treatment, LH and FSH p ulses occurred together. The mean duration of the FSH pulse (120 min) was s ignificantly longer than that of the LH pulse (80 min). The interoestrous i nterval in the bitches treated with bromocriptine was significantly shorter than that of the preceding cycle (160 +/- 3 versus 206 +/- 24 days). The m ean basal plasma FSH concentration (7.4 +/- 0.6 versus 6.1 +/- 0.7 iu l(-1) ) and the mean area under the curve for FSH (46.6 +/- 4.7 versus 40.4 +/- 4 .4 iu l(-1) in 6 h) increased significantly after the start of the bromocri ptine treatment. Ln contrast, the differences in mean basal plasma LH conce ntration (2.1 +/- 0.2 versus 2.0 +/- 0.2 pg l(-1)) and the mean area under the curve for LH (19.0 +/- 3.1 versus 19.5 +/- 2.5 pg l(-1) in 6 h) between the day before and 14 days after the start of the bromocriptine treatment were not significant. Bromocriptine administration also lowered the mean am plitude of the FSH pulse and shortened the mean duration of the FSH pulse, without influencing the LH pulse. In addition to demonstrating the concurre nt pulsatile secretion of LH and FSH, the results of the present study demo nstrate that the bromocriptine-induced shortening of the interoestrous inte rval in the bitch is associated with an increase in plasma FSH concentratio n without a concomitant increase in plasma LH concentration. This finding i ndicates that treatment with the dopamine agonist bromocriptine increases p lasma FSH to a concentration that results in the enhancement of follicle de velopment.