Insulinoma, as a typical case of hyperinsulinism, has engendered interest in mechanism and regulation of insulin release. In two patients with insulinoma, plasma insulin responses after administration of various insulin releasers and inhibitors were studied. Ultrastructure of beta cells in tumors were also observed electronmicroscopically. In both cases, the fasting insulin levels in plasma were greater than that observed in any other normal subjects. Excessive rises of plasma insulin were

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... after administration of glucose orally, and of glucose or glucagon intravenously. The rise of insulin level during glucose loading was depressed completely by pulse administration of epinephrine, and propanolol, a beta receptor blocking agent, while not during glucagon loading. The plasma insulin levels in both cases were consistenly high during intravenous administration of glucose for five hours. Under the condition studied here, it seems likely that insulinoma, as a benign tumor, has no automatism in insulin release, but has more exaggerated insulin level in plasma than normal control. Electronmicroscopically, it was shown that in insulinoma 8-granules of 8-cells were generally scanty. With a little variation, the cores of granules were generally round, while there were mixed forms in normal ones; being round, rectangle and triangle. The cell organella such as endoplasmic reticulum, Golgi apparatus ribosome seemed more prominent, indicating an increased protein synthesis. These findings would suggest that in insulinom a, the major part of insulin synthetized in 8-cells can be secreted to the blood stream without being stored as 8-granules. (pp. 733-,-740) Vol.