Relationships Between Basal Gonadotropin Secretion Rates and Serum Gonadotropin Concentrations in Proestrous Rats1

We have previously demonstrated that the elevation in serum follicle-stimulating hormone (FSH) concentration which is present during the late evening of proestrus in the rat is accompanied by an increase in the basal FSH release rate (anterior pituitary gland FSH release independent of the immediate presence of any hormones of nonanterior pituitary gland origin) which occurs during the late evening of proestrus. In the present study, a variety of treatments previously shown to block or block and restore FSH release during the late evening of proestrus were employed to determine if these experimentally induced changes in serum FSH levels could be explained on the basis of changes in the basal FSH release rate. Rats were decapitated at 1200 or 2400 h of proestrus and hemianterior pituitary glands were preincubated in medium 199 for 30 min. This was followed by a 2-h incubation and assay of the medium to determine the basal release rates of FSH and luteinizing hormone (LH). The FSH and LH concentration were determined in the other half of the gland and in the serum collected from trunk blood. Serum FSH concentrations and the basal FSH release rate increased from 1200 to 2400 h of proestrus in controls. Injection of phenobarbital at 1230 h of proestrus, of porcine follicular fluid during the morning and afternoon or the afternoon and the early evening of proestrus, or of anti-luteinizing hormone releasing hormone (LHRH) serum at 1100 and 1400 h of proestrus suppressed these rises between 1200 and 2400 h of proestrus. In contrast, injection of anti-LHRH serum at 2000 and 2300 h of proestrus was without effect on these rises between 1200 and 2400 h of proestrus. Injection of LHRH or of rat FSH at 1500 h of proestrus in phenobarbital-blocked rats, treatments previously shown to stimulate endogenous FSH release during the late evening of proestrus, restored elevated levels of FSH in the serum and increased the basal FSH release rate by 2400 h of proestrus. Serum FSH levels and the basal FSH release rate correlated (r=0.73 with 102 degrees of freedom; P<0.001), but there was no relationship between serum FSH levels or the basal FSH release rate and gland FSH stores. Serum LH concentrations were low in all groups when compared with levels observed in other rats at the peak of the preovulatory LH surge on the afternoon of proestrus. However, serum LH levels at 2400 h in controls were still higher (about 2-fold) than the presurge LH levels seen at 1200 h of proestrus. Serum LH levels in some of the treatment groups at 2400 h of proestrus were lower than those seen in controls at 2400 h or at both 2400 and 1200 h of proestrus. In untreated controls, gland LH concentration and the basal LH release rate decreased between 1200 and 2400 h of proestrus. A decrease in gland LH concentration and/or the basal LH release rate between 1200 and 2400 h of proestrus was observed in some of the treatment groups. Analyses of all the data showed the basal LH release rate to correlate with both serum LH levels (r=−0.30; P<0.01) and gland LH concentration (r=0.51; P<0.001). The results 1) support our view that the increase in the basal FSH release rate during the late evening of proestrus is responsible for causing the elevations in serum FSH levels at this time in the cyclic rat, 2) suggest that the rise in serum FSH concentrations during the afternoon of proestrus is at least one factor that can mediate this increase in the basal FSH release rate, and 3) support our view that the increase of LH in serum during proestrus does not involve an increase in the basal LH release rate.