The pancreatic insulin-, glucagon-, and somatostatin-positive cell populations were quantitated in normal and alloxan-diabetic rats. The method of quantitation (linear scanning) allowed an estimation of absolute changes in these cell populations through 14 months of diabetes. The changes in cell masses were correlated with changes in plasma and pancreatic immunoreactive insulin and glucagon. A marked reduction in the insulin-positive beta cells was demonstrated within seven days after alloxan treatment. No significant change in the glucagon-positive alpha cell population was noted in the diabetic rats when compared with normoglycemic controls. A statistically significant increase in the pancreatic somatostatin-positive delta cell population was demonstrable only after 14 months of alloxan diabetes. The results would suggest that the hyperglucagonemia of insulin-deficient diabetes is not a consequence of an increased pancreatic alpha cell population. In addition, since the increase in the pancreatic delta cell mass was found only late in the course of alloxan diabetes in the rat, the increase in delta cells is probably not of significance in the pathophysiology of diabetes in this experimental model.

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