The Self-Priming Effect of Gonadotropin-Releasing Hormone on Luteinizing Hormone Release: Observations Using Rat Anterior Pituitary Fragments and Dispersed Cells Continuously Perifused in Parallel*

Abstract

To study in vitro the self-priming effect of GnRH [gonadotropin releasing hormone] on LH [luteinizing hormone] release, rat anterior pituitaries were prepared either as fragments or dispersed cells and continuously perifused in parallel chambers. The experimental groups consisted of rats killed at 0800 h on diestrus day 1, diestrus day 2, proestrus or estrus, or at 1400 h on proestrus. After basal LH release had stabilized, the tissue preparations were exposed to 10 nM GnRH as two 30 min challenges separated by 1 h. LH secretory rates (ng/min per pituitary for fragments; ng per min/10 cells for dispersed cells) were calculated for basal release (during the 20 min period immediately preceding each GnRH challenge) in response to GnRH, and as the sum of basal and GnRH-stimulated release. Comparison of the 2 preparations revealed that basal and GnRH-stimulated LH release by pituitary fragments was more variable than LH release by dispersed cells. While dispersed cells responded promptly to the addition/withdrawal of stimuli, fragments did so more gradually. With respect to GnRH self-priming, the 2nd mean secretory rate for basal LH release by fragments (range, 28.8-46.5) was significantly (0.1 > P > 0.01) higher than the 1st rate (range, 14.4-22.0) on diestrus day 1, diestrus day 2, proestrus at 0800 h, and estrus. With dispersed cells, the 1st and 2nd basal rates were similar to each other on diestrus day 1 and estrus, but on diestrus day 2 and on proestrus at 0800 and 1400 h, the 2nd basal rate (range, 36.8-93) was significantly (P < 0.001) higher than the 1st rate (range, 17.7-31.7). When fragments received GnRH, the 2nd mean secretory rate (range, 35.2-64.2) was significantly (0.1 > P > 0.03) higher than the 1st rate (range, 13.4-34.1) on diestrus day 2 and proestrus at 0800 h. With dispersed cells, the mean secretory rate in response to the 2nd GnRH challenge was higher only on diestrus day 2 (37.0 .+-. 4.1 vs. 60.3 .+-. 3.8; P < 0.05). When considered as the total of basal plus GnRH-stimulated LH release, the 2nd secretory rate by fragments (range, 54.5-110.8) was significantly (0.1 > P > 0.02) higher than the 1st rate (range, 27.9-51.4) on diestrus day 1, diestrus day 2, and proestrus at 0800 h. With dispersed cells, the mean secretory rate (range, 97.1-304.1) in response to the 2nd GnRH challenge was significantly (P < 0.05) higher than the 1st rate (range, 54.7-245.5) on diestrus day 2 and on proestrus at 0800 and 1400 h. When defined as an increase in LH release over baseline in response to a 2nd challenge with GnRH, self-priming occurs in dispersed cells from diestrous day 2 rats as well as in fragments obtained from such rats. However, if the definition of self-priming is broadened to encompass total LH release (basal plus GnRH stimulated), then dispersed cells from proestrous rats killed at 0800 and 1400 h as well as those from diestrous day 2 rats exhibit self-priming. When tested in an identical manner but on separate occasions, dispersed pituitary cells respond to GnRH with less variability in LH release than do pituitary fragments and react more quickly than do fragments to the addition/withdrawal of GnRH. The differing diffusion characteristics of the 2 preparations.