Plasma insulin patterns in the unanesthetized rat during intracardial infusion and spontaneous ingestion of graded loads of glucose.

Rats were provided with double permanent heart catheters, allowing simultaneous infusion and rapid blood sampling in the freely moving animals. Intracardial glucose infusion (75, 150, and 300 mg) over 15-min periods induced biphasic plasma insulin responses, their onset and magnitude being correlated with the blood glucose increments. The insulin-ogenic index decreased at increasing infused loads. After spontaneous oral ingestion of 75, 150, 300, and 750 mg glucose over 2 min or less, plasma insulin increased rapidly during the initial 4 to 8 min. At the highest leads this was followed by a gradual further increase until 15 min. The rapid insulin response increased with the ingested load. About half of this response had occurred already at 2 min, i.e., prior to the first rise of blood glucose at 3 min. Maximum blood glucose levels (125-135 mg/dl) occurred between 8 and 15 min and did not correlate with the ingested loads. The insulinogenic index increased at higher oral loads. It is suggested that the plasma insulin response to glucose ingestion results from successive and cumulative operation of anticipatory nervously triggered insulin secretion, anticipatory loaded-dependent potentiation of secretory stimulation by rising blood glucose, and further adjustment of the secretion rate until blood glucose declines. The possible mechanisms are discussed.