Effects of hexoses on insulin biosynthesis, content, and release in channel catfish islets

Abstract

Catfish islets were incubated in the presence of glucose, mannose, fructose, or galactose at concentrations of 10, 40, or 200 mg/100 ml. Insulin biosynthesis was measured by the rate of incorporation of radiolabeled isoleucine into proinsulin and insulin; islet insulin content and secretion from islets were monitored by radioimmunoassay. The presence of 10 mg/100 ml of mannose, fructose, or galactose or the absence of hexose caused a significant decrease in incorporation when compared to that in control islets incubated in the presence of 40 mg/100 ml of glucose. This is the first demonstration that glucose can maintain insulin biosynthesis in piscine islets. Galactose at 40 or 200 mg/100 ml was also found to decrease incorporation. This sugar at all three concentrations evoked a significant increase of insulin release, which is the first demonstration that galactose can act as an insulin secretogogue. The only hexose to affect insulin content of islets over a 24-hr period was fructose, which caused a decrease of insulin retained in the islet tissue at all concentrations tested.