Hepatotrophic effects of insulin on glucose, glycogen, and adenine nucleotides in hepatocytes isolated from fasted adult rats

Abstract

Previous evidence that portal blood insulin is an hepatotrophic factor led to this study of its effect on hepatocytes, isolated from fasted rats, in suspension culture. Control hepatocytes (C), noninsulin-treated, and those infused continuously at low (LI) and high (HI) levels of insulin were compared concurrently with regard to their survival, glucose transport, and intracellular concentrations of glucose, glycogen, and adenine nucleotides, over a 48-hr period of incubation. Low insulin was adjudged to be comparable to portal insulin concentrations in fasted animals and HI to those in fed animals. All hepatocytes had been depleted of glucose, glycogen, and adenine nucleotides at the start of the study by prior fasting of the rat. For the first 6 hr of culture, there was little difference between C, LI, and HI with reference to the above parameters. In contrast, after 48 hr of incubation, cell survival, as judged by the DNA content, was significantly lower in C compared with LI and HI. The transport of 3-0-[methyl-3H] D-glucose was also significantly lower in C compared with LI and HI. The higher uptakes in both LI and HI were reduced by phloridzin, which had little effect on C. Correspondingly, the intracellular glucose concentrations in C were significantly lower than the extracellular glucose concentrations in contrast to those in LI and HI, which were comparable. For intracellular concentrations of glycogen and adenine nucleotides, the results of LI and HI were amalgamated as they were not significantly different from each other at 48 hr. Upon analysis, glycogen values were significantly higher for insulin-treated cells. Similarly, the total adenine nucleotide pool (ATP + ADP + AMP) also was clearly higher in HI + LI than in C. These results indicate that contrary to findings in studies with the perfused liver that have been of a short-term nature, insulin is necessary in the longer term (>12 hr) for maintaining the transport of glucose into hepatocytes; thereby insulin promotes the maintenance of intracellular glucose, glycogen, and adenine nucleotide concentrations, and also enhances cell survival.