Expression and Regulation of Glucokinase in Rat Islet β- andα -Cells during Development.

Glucokinase (GK) is the rate-limiting enzyme in the glycolytic pathway of the b-cell and, even in the rat fetus at 22-days gestation, immediately before birth, acts as a sensor of glucose influencing the rate of glucose utilization. However, when GK first appears in islets during b-cell development is unknown. Whether GK is expressed in fetal glucagon-producing cells is also unknown. To determine this information, fetal rat islets were examined at 16-, 18-, and 22-days gestation. GK was identified immunocytochemically in both band a-cells at all these ages, with the number of GK immunoreactive cells positively correlated to the fetal age from 16–22 days. Western blot analysis of islet protein extracts demonstrated the presence of GK, at 52 kDa, at 16 days and thereafter. To determine whether glucose had any effect on regulation of GK biosynthesis, fetal islets were cultured in medium containing a wide range of concentrations of glucose for 7 days. The amount of GK protein was significantly decreased in low concentrations of glucose and augmented at high concentrations. In conclusion, GK was expressed in both band a-cells in fetal rat islets during development. GK is an integral part of the function of both of these cells at all stages in the development of the fetal islet. (Endocrinology 140: 3762–3766, 1999) I THE MATURE pancreatic b-cell and hepatocyte, glucokinase (GK) plays a dominant role in the regulation of glucose homeostasis by catalyzing the rate-limiting biochemical reaction of glycolysis. It regulates glucose-induced insulin secretion from the b-cell and is responsible for maintaining a gradient for glucose transport into hepatocytes (1–4). This enzyme first appears in the rat hepatocyte, 16 days after birth (5, 6), at a time when the diet changes from milk [which is high in fat (7)] to solids [which are rich in carbohydrate (8)]. In contrast, GK is present in the rat islet at 20–21 days of fetal life (9), but exactly when it first appears in islets is unknown. It was believed that GK was present in the mature pancreas only in b-cells. Heimberg et al. (10) have recently shown that this enzyme is expressed also in mature glucagon-producing a-cells. This has been disputed by others (11, 12). Which cells in the fetal rat islet produce GK are unknown, although it is presumed that b-cells do, because culturing islets in high concentrations of glucose up-regulates activity of the enzyme (9). To answer this question, we have examined fetal rat islets immunocytochemically at 16-, 18-, and 22-days gestation for GK, as well as insulin and glucagon. Furthermore, regulatory expression of GK protein by glucose in these developing islet cells was studied. These experiments shed light on the role of GK in fetal islet band a-cells during development. Materials and Methods