In situ ileal absorption of insulin in rats:: Effects of hyaluronidase pretreatment diminishing the mucous/glycocalyx layers

Purpose. To test the hypothesis that the ileal mucous/glycocalyx layers can be removed by hyaluronidase and that their significant roles in insulin absorption can thereby be identified. Methods. Rat ileal segments were pretreated with various concentrations of hyaluronidase by "perfusion" or "exposure", and the absorption of insulin and 4.4-, 20-, and 40-kDa fluorescein isothiocyanate-labeled dextrans (FDs) were studied in the in situ ileal loop system. Diminished mucous/glycocalyx layers following the hyaluronidase pretreatment was confirmed by transmission electron microscopy (TEM), whereas intra- and intercellular integrity and/or damage was examined by light microscopy, lactate dehydrogenase (LDH) leakage, and membrane electrical resistance (R-m). Results. Hyaluronidase "perfusion" pretreatment at concentrations greater than or equal to160 U/ml for 30 min significantly increased the hypoglycemic responses following in situ administration of insulin at 50 IU/kg. This enhancing effect was found to be dependent on hyaluronidase concentration and "exposure" periods and accompanied by the TEM observation of diminished mucous/glycocalyx layers from the hyaluronidase pretreatment, yet causing undetectable histological damage. In contrast, the absorption of FDs and the values for LDH leakage and R-m were unaffected by the hyaluronidase pretreatment, suggesting that the layers functioned insignificantly to diffusive absorption. Conclusions. Hyaluronidase pretreatment increased ileal absorption of insulin, but not FDs, by virtue of the diminished mucous/glycocalyx layers without causing detectable cellular damage.