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A Priming Effect of Luteinizing Hormone Releasing Hormone on Bovine Pituitary Cells In Vitro

Michigan State University, East Lansing 48824

Abstract

The capacity of pituitary glands to release luteinizing hormone (LH) in response to LH-releasing hormone (LHRH) in vivo may be increased by previous frequent exposures to small quantities of LHRH. This phenomenon, called LHRH self-priming, may play an important role in the control of LH secretion. Our initial objective was to determine whether LHRH self-priming occurs via a direct effect on bovine pituitary cells. Subsequently, we studied the effects of interval between LHRH exposures and source of pituitary glands (steers versus cows) on LHRH self-priming. Cells from bovine anterior pituitary glands were dispersed and mixed with Sephadex-G25, and the mixture was poured into glass columns. Columns were superfused and effluent medium was assayed for LH. Challenges were .1 ng LHRH/ml or 0 ng LHRH/ml (sham) each for 10 minutes. In one experiment, quantity of LH released by a second exposure to LHRH averaged 124 ± 6 ng/ml, which was greater than that released in response to an LHRH challenge (90 ± 4) given 40 min earlier. In a second experiment, variables were LHRH sequence (sham-LHRH versus LHRH-LHRH) and interval between challenges (40, 80, 160 and 320 min). Quantities of LH released in response to LHRH following the sham challenge or the first of two LHRH challenges were not different. However, LH released by a second LHRH challenge was 48, 39 and 48% greater (P<.01) than that released by the first when the interval between challenges was 40, 80 and 160 min, respectively. But when the interval was 320 min, no LHRH self-priming occurred. In a third experiment, variables were LHRH sequence (sham-LHRH versus LHRH-LHRH) and sex of the pituitary gland donor (cows versus steers). Interval between challenges was 50 minutes. Quantity of LH released after the second LHRH challenge was 50% greater (P<.01) than that released after the first LHRH challenge when cells were from pituitary glands of cows. Self-priming did not occur when cells were from pituitary glands of steers. However, this experiment compared a pool of pituitary cells from steers with one from cows, so there was no estimate of individual animal variation. Thus, our data do not prove that this difference exists but merely raises that possibility. We conclude that LHRH can self-prime bovine pituitary cells and that the degree to which priming occurs may be dependent upon the interval between exposures to LHRH.

¹ Animal Reproduction Lab., Dept. of Anim. Sci.

² Published with the approval of the director of Michigan Agr. Exp. Sta. and Journal Article No. 9550. The authors acknowledge Houghton Beef packers for bovine pituitaries and Ms. J. Gruber for secretarial assistance.