RESPONSE OF CIRCULATING SOMATOSTATIN, INSULIN, GASTRIN AND GIP, TO INTRADUODENAL INFUSION OF NUTRIENTS IN NORMAL MAN

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Abstract
The effect of direct infusion of nutrients into the duodenum of normal subjects on circulating plasma somatostatin, insulin, gastrin and gastric inhibitory polypeptide (GIP) levels was studied. Six normal subjects were given on 4 separate occasions 150 ml of isotonic solutions containing 100 calories of carbohydrate, protein or fat, and a control solution of saline by infusion into the 2nd part of the duodenum. Plasma somatostatin rose slightly after carbohydrate, mean basal 30 .+-. 3 pg/ml, peak 46 .+-. 16 pg/ml at 15 min, and more markedly after protein, peak 57 .+-. 9 pg/ml at 30 min. Fat was the most potent intraduodenal stimulus to plasma somatostatin release into circulation, peak 101 .+-. 11 pg/ml at 30 min. The plasma insulin rise was greatest after carbohydrate, peak 68 .+-. 10 IU, but there was a significant rise after protein also, peak 34 .+-. 6 IU. Plasma gastrin rose significantly after protein only, peak 70 .+-. 22 pg/ml. Plasma GIP rose markedly after carbohydrate, basal 506 .+-. 50 pg/ml, peak 1480 .+-. 120 pg/ml. Protein was also a potent stimulus of circulating plasma GIP release, peak 1200 .+-. 190 pg/ml, while fat was the least potent, peak 730 .+-. 190 pg/ml. Calorie for calorie, fat is the most potent intraduodenal nutrient stimulus of circulating somatostatin. Somatostatin may be an enterogastrone, a circulating hormone released by intraduodenal fat which inhibits gastric acid secretion. Fat is the least potent intraduodenal nutrient stimulus of circulating GIP release. This is evidence against the hypothesis that circulating GIP acts as an enterogastrone.