Increased LH pulse frequency and estrogen secretion associated with termination of anestrus followed by enhancement of uterine estrogen receptor geneexpression in the beagle bitch

The relationships among pulsatile LH secretion pattern, estrogen secretion, and expression of the uterine estrogen receptor gene were examined through out the estrous cycle in beagle bitches. In Experiment 1, blood samples wer e collected from 30 bitches every 10 min for 8 h from a cephalic vein durin g different phases of the estrous cycle. An increase in the mean plasma lev els of LH occurred from mid to late anestrus (P<0.01). The LH pulse frequen cy increased (P<0.01) from late anestrus to proestrus, and was strongly cor related (r=0.96, P<0.001) with the mean plasma level of estradiol-17 beta ( E-2) In Experiment 2, middle uterine samples, including the myometrium and endometrium, from 18 bitches were taken at 6 stages of the estrous cycle. T he total number of estrogen receptors and nuclear estrogen receptor and its mRNA levels in the uterus also increased (P<0.01) from late anestrus to pr oestrus. Mean plasma E-2 level and the number of uterine estrogen receptor were positively correlated (r=0.81, P<0.05). In Experiment 3, nine bitches were ovariectomized in mid anestrus. Two weeks later they received a single injection of 10 or 50 mu g/kg, im,estradiol benzoate. The number of uterin e estrogen receptor and their mRNA levels for ovariectomized bitches were l ow, but increased (P<0.05) after treatment with a low dose of estradiol ben zoate. These results suggest that increases in LH pulse frequency and estro gen secretion are associated with termination of anestrus and that subseque nt enhancement of uterine estrogen receptor expression may be up-regulated by estradiol. (C) 1999 by Elsevier Science Inc.