Mode of Gonadotropin Secretion in Infantile Female Rats and the Role of Estrogen in Feedback Regulation*

Abstract

The temporal aspects of gonadotropin release were studied in infantile (14-day-old) female rats. Blood samples were collected using vena cava cannulae at 10-, 15-, 20-, or 30-min intervals and assayed for either LH alone or for both LH and FSH. Within individual animals, blood LH levels exhibited a degree of variability suggestive of a pulsatile release of the hormone. Wide fluctuations in circulating FSH were also observed, indicating that FSH might also be secreted in a pulsatile manner. Peak values for both gonadotropins were 2- to 5-fold higher than baseline levels. The majority of pups exhibited a nonrhythmic release of hormones. In some animals, LH and FSH appeared to be secreted in a temporally coincident manner. Administration of LHRH to cannulated pups evoked a simultaneous discharge of both gonadotropins. To determine whether endogenous 17 beta-estradiol (E2) plays a physiological role in the control of LH secretion in infantile rats, circulating LH levels were monitored in cannulated pups which had been treated twice daily with anti-E2 serum for 4 days. Passive immunization to E2 did not affect pulsatile LH secretion. Therefore, the results of this study demonstrate that in infantile female rats, LH is secreted in a pulsatile manner which is independent of E2 feedback regulation.