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Radioimmunoassay of Plasma Hormones: Review of Plasma Insulin in Ruminants¹

Department of Animal Science, Iowa State University of Science and Technology, Ames 50010

ABSTRACT

Radioimmunoassay procedures have sufficient sensitivity and specificity for assaying minute quantities of peptide hormones in biological fluids. In combination with other biochemical techniques, radioimmunoassay procedures can help characterize the nature of hormones in plasma. Various radioimrnunoassay procedures differ primarily in the methods to separate the free portion of labeled hormone from the portion bound to antibody. When properly standardized, most of the methods of separating free from bound hormone are reliable and show reasonable agreement in measuring concentrations of hormones in plasma. In selecting a method, it is advantageous to use one that will give some measure of immunoreactivity and extent of plasma or incubation damage of the labeled hormone. Plasma levels of many of the peptide hormones have been measured in ruminants by radioimmunoassay. This review, however, has been limited to those studies of plasma insulin in ruminants. In sheep, immunoreactive insulin was distributed in a volume equal to about 14% of the body weight and disappeared from circulation, with a half-life of 12 to 13 min. No changes were observed in either of these measures of insulin metabolism with feeding or fasting. Plasma insulin in ruminants, as measured by radioimmunoassay, varies between 5 to 40 µU/ml. Postprandially, plasma insulin reaches 20 to 40 µU/ml within 4 hr and then gradually declines to less than 10 µU/ml at 48 to 72 hr. Intravenous infusions of glucose, sodium propionate, sodium butyrate, arginine, lysine, secretin, or pancreozymin have stimulated insulin secretion. It is suggested that secretin and pancreozymin are involved in bringing about the rise in plasma insulin in cattle within 1 hr after feeding and that glucose and, possibly, short-chain fatty acids produced in the rumen are involved in maintaining elevated plasma insulin for about 6 hr after feeding. It seems unlikely that amino acids are important in regulation of plasma insulin in ruminants.

¹ Journal Paper J-6966 of the Iowa Agriculture and Home Economics Experiment Station, Arnes. Project 1790. These studies were supported in part by a grant from the Iowa State University Research Foundation.