Free Fatty Acids Suppress Growth Hormone, but not Luteinizing Hormone, Secretion in Sheep*

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Abstract
An experiment was conducted to determine the effects of exogenously administered FFA on GH and LH secretion in sheep. Ovariectomized ewes received iv infusions of a mixture of FFA (166 mg/min; n = 5) or 0.9% saline (n = 4) for 10 h. Jugular blood was sampled every 15 min for 14 h, beginning 4 h before initiation of infusion. After 8 h of FFA or saline treatment, each ewe received a pituitary challenge of 10 .mu.g GRF and 1 .mu.g GnRH, administered together as an iv bolus. Lipid infusion increased (P < 0.01) serum FFA concentrations to levels characteristic of those in fasted sheep [23.0 .+-. 0.8 mg/100 ml (mean .+-. SE)]. Frequency of GH pulses (P < 0.01) and the GH response to GRF (P < 0.0001) were suppressed by FFA treatment. Mean serum GH concentrations increased gradually (P < 0.01) during the 10-h infusion period in saline-treated but not lipid-treated, ewes. This finding may reflect diurnal changes in somatotrope secretory activity that are blocked by FFA. Mean serum LH concentrations, LH pulse frequency and amplitude, and the LH secretory response to GnRH were unaffected by FFA or saline infusion. In agreement with previous work in sheep and other species, these results provide evidence for an inhibitory effect of FFA on GH release. The exact mechanism responsible for this action, however, remains to be elucidated. Finally, acutely elevated FFA levels do not appear to influence LH secretion in the ovariectomized ewe.

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