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

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 less than 0.001) in the TM and M + TYR groups compared to the M group and were elevated (p less than 0.001) in the M + TYR group compared to the TM group. Concentrations of TYR in hypothalami were higher (p less than 0.01) in the M + TYR and TM groups than in the M group, and greater (p less than 0.005) in the M + TYR group compared to the TM group. Overall, there was a linear (p less than 0.001) correlation between plasma and hypothalamic concentrations of TYR.(ABSTRACT TRUNCATED AT 250 WORDS)