Effect of polyamines and cystic fibrosis serum on glucose transport

The effect of plasma or serum from homozygotes and heterozygotes for the cystic fibrosis (CF) gene on the active uptake of 3-0-¹⁴C methyl D glucose (3-0-¹⁴C MDG) by rat jejunal epithelium was studied. Furthermore, the role of the polyamine, spermidine, and its products of metabolic degradation on glucose transport were investigated, and a relationship to the pathogenesis of membrane dysfunction in cystic fibrosis was postulated. Glucose transport in everted rat jejunal rings was used in the study. Results were expressed as 3-0-¹⁴C MDG concentration ratio between the intracellular (ICF) and the extracellular fluid spaces (ECF) of the jejunal rings at the end of a 60 min incubation period. The mean ratio obtained from incubations of the rat jejunal rings in medium consisting of Krebs Ringer bicarbonate buffer and the labeled sugar was considered as 100% uptake. When plasma or serum, with or without spermidine, was mixed with the medium in a volume ratio of 1:3, a decrease in the active uptake of 3-0-¹⁴C MDG was observed, expressed as percent inhibition. Percent inhibition of 3-0-¹⁴C MDG uptake obtained when the rat jejunal rings were incubated in normal plasma was compared to that obtained with plasma from cystic fibrosis genotypes. It was found that: plasma from 25 homozygous children had greater inhibitory effect on glucose uptake than plasma from 26 normal children; plasma from 9 heterozygous women had greater inhibitory effect than from 6 normal women; the inhibitory effect of plasma from 3 homozygous children was not influenced by dialysis; the inhibitory effects of paired plasma and serum samples from 9 homozygotes were comparable; spermidine added to the incubating electrolyte solution did not affect glucose transport; the addition of spermidine to reaction mixtures containing normal plasma potentiated the inhibitory effect; and mixing and incubation of fresh bovine serum with reaction mixtures containing plasma from homozygotes decreased the inhibitory effect. The predominant inhibitory effect of plasma or serum from homozygotes and heterozygotes for the CF gene appears to be related to a nondialyzeable molecule(s). It does not seem to reflect the presence of high plasma glucose levels in cystic fibrosis nor to be the result of competitive inhibition between sugars. It does not seem to be the result of sodium or other electrolyte differences. A similar inhibitory effect is acquired by normal plasma after the addition of spermidine. On the other hand, plasma from CF homozygotes loses its inhibitory effect after incubation with fresh bovine serum. These findings may indicate that products of metabolic degradation of spermidine are responsible for the inhibitory effect of glucose transport and suggest the possibility of a role in abnormal polyamine metabolism in the pathogenesis of cystic fibrosis.