Pulsatile LH Release During Periods of Low Level LH Secretion in the Rat Estrous Cycle1

Abstract

The present experiments were carried out to characterize LH release during periods of low level LH secretion in the rat 4-day estrous cycle. Unanesthetized animals with jugular cannulae were bled on the morning (0930-1230 h) or afternoon (1330-1630 h) of either estrus, diestrus 1 (i.e., metestrus), or diestrus 2, or on proestrus morning (75 µl whole blood/6 min). The percent coefficients of variation obtained for alterations in blood LH levels during the morning or afternoon of each of the four stages of the estrous cycle, or when morning and afternoon values for each cycle stage were combined, were all significantly greater than intra-assay variation, indicating that LH release was pulsatile during each stage of the cycle. No significant morning vs afternoon differences in mean blood LH levels, LH pulse amplitude, or pulse frequency existed in any cycle stage. As the cycle progressed, changes occurred in mean blood LH levels which were due to changes in the characteristics of pulsatile LH secretion on each day of the cycle. On estrus, blood LH levels are the lowest of the entire cycle due to the occurrence of the slowest frequency of pulsatile release in the cycle (1 pulse/2 h). On the next day, diestrus 1, blood LH levels are elevated, and are the highest of all the periods of low level LH release in the cycle. This increase in blood LH levels from estrus to diestrus 1 is due to a marked increase in LH pulse amplitude (15 vs 38 ng/ml) as well as a shortening of the LH interpulse interval to 60 min. While this circhoral rhythm remained the same through diestrus 2 (55 min) and proestrus morning (63 min), the LH pulse amplitude decreased on these days to 20 and 16 ng/ml, respectively (compared with 38 ng/ml on diestrus 1), and therefore the mean blood level of LH on both days was significantly lower than on diestrus 1, although still higher than on estrus because of different pulse frequencies. Therefore, LH release is pulsatile during periods of low level LH secretion in the rat estrous cycle, and changes in the pulsatile characteristics of this release occur with different stages of the cycle.