Does tyrosine act as a nutritional signal mediating the effects of increased feed intake on luteinizing hormone patterns in growth-restricted lambs?

• Published in: Biology of reproduction Vol. 46; no. 4; pp. 573 - 579
• Main Authors: Hall, J.B, Schillo, K.K, Hileman, S.M, Boling, J.A
• Format: Journal Article
• Language: English
• Published: United States Society for the Study of Reproduction 01.04.1992
• Subjects: animal nutrition; Animals; Dose-Response Relationship, Drug; Eating - drug effects; Eating - physiology; ewes; feed intake; Female; Growth Disorders - blood; Growth Disorders - physiopathology; hormone secretion; Hypothalamus - chemistry; Hypothalamus - drug effects; lambs; luteinizing hormone; Luteinizing Hormone - blood; ovariectomized females; Ovariectomy; Radioimmunoassay; Sheep - physiology; tyrosine; Tyrosine - analysis; Tyrosine - blood; Tyrosine - pharmacology
• ISSN: 0006-3363; 1529-7268
• DOI: 10.1095/biolreprod46.4.573

We tested the hypothesis that the increase in LH secretion associated with elevated feed intake in nutritionally growth-restricted lambs is due to increased availability of tyrosine (TYR). Ovariectomized ewe lambs with abomasal cannulae were fed a complete diet in amounts (0.6 Mcal of net energy/day) sufficient to maintain a body weight of 21.7 +/- 1.0 kg between 10 and 32 wk of age. At 32 wk of age, lambs were assigned at random to one of three treatments: 1) maintenance feeding + abomasal infusion of water (M; n = 6); 2) maintenance feeding + abomasal infusion of TYR (M + TYR; n = 7); or 3) twice maintenance feeding + abomasal infusion of water (TM; n = 6). TYR (1.25 g/100 ml water) or water (100 ml) was delivered as a bolus injection at 0600, 1200, 1800, and 2400 h daily for 23 days. Plasma concentrations of TYR 10 and 22 days after initiation of treatments were higher (p < 0.001) in the TM and M + TYR groups compared to the M group and were elevated (p < 0.001) in the M + TYR group compared to the TM group. Concentrations of TYR in hypothalami were higher (p < 0.01) in the M + TYR and TM groups than in the M group, and greater (p < 0.005) in the M + TYR group compared to the TM group. Overall, there was a linear (p < 0.001) correlation between plasma and hypothalamic concentrations of TYR. Frequencies of LH pulses were similar (p > 0.1) for the three groups 3 days prior to initiation of treatments (1.26 +/- 0.35 pulses/6 h). At 10 and 22 days after the onset of treatments, LH pulse frequency was elevated (p < 0.05) in TM (4.08 +/- 0.59 pulses/6 h) and in M + TYR (2.71 +/- 0.51 pulses/6 h) lambs compared to M lambs (2.08 +/- 0.45 pulses/6 h). There was a tendency (p < 0.10) for LH pulse frequency in the TM group to be greater than in the M + TYR group. On the basis of chi-square analysis it was apparent that treatments influenced the number of lambs exhibiting an increase in LH pulse frequency between Day -3 and Day 22. All of the M + TYR lambs exhibited an increase, whereas only 50% of the M lambs showed an increase in LH pulse frequency (p < 0.05). Eighty-three percent of the TM lambs exhibited an increase in LH pulse frequency, but pulse frequency was not significantly different from that in either M or M + TYR lambs. There were no significant differences in mean concentrations of LH or LH pulse amplitudes among treatment groups at any of the sampling periods. We conclude that TYR treatment causes an increase in LH pulse frequency and therefore may be involved with mediating the effects of increased feed intake on LH secretion in growth-restricted lambs.