Plasma Growth Hormone and Insulin Levels in the Pig

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Abstract
Plasma levels of GH, measured by radio-immunoassay, increased slightly following the intravenous injection of insulin sufficient to provoke severe hypoglycemia. No change in the basal plasma GH level was observed when hyperglycemia was produced by either oral or intravenous glucose. When the blood glucose level fell very rapidly, as seen after cessation of an intravenous infusion of glucose, plasma GH increased 3-fold. Plasma GH increased from basal levels of 5.8 [plus or minus] 0.8 to a maximum of 12.1 [plus or minus] 1.8 ug/ml after 2 days of starvation and then decreased toward control values with further fasting. Stress and exercise provoked significant increases in plasma GH, but the injection of catecholamines in amounts sufficient to produce hyperglycemia and FFA [free fatty acid] mobilization had no effect on plasma GH. The changes in plasma GH are sluggish in the pig compared to those seen in the normal-weight human. Injection of porcine GH (0.1-1.0 mg/kg) into the pig resulted in an initial decrease in plasma FFA followed by a 3- to 5-fold rise 60-90 min. after the injection. GH disappeared from plasma with a half-time of 20-30 min. and distributed in a volume equal to 15-18% of body weight. It was estimated that the GH secretion in a 50 kg pig would be approximately 2 mg/day. Plasma GH levels in pigs selected for small size for 14 generations were not significantly different from those in normal pigs. Insulin levels in the pig were 11.0 [plus or minus]1.0 u[mu]/ml, decreasing to 5.8 [plus or minus] 0.7 u[mu]/ml after 3 days of fasting. Compared to other species the increase in plasma insulin following either an oral or intravenous glucose load was quite small. Furthermore, the glucose disappearance rates were comparable to those observed in diabetic human subjects. Apparently the obesity of the pig is not a consequence of hyper secretion of insulin.