Insulin-related material extractable from brain and other tissues of rat: Possible biologic and methodologic...

Insulin, the amount and its source, in brain and other extrapancreatic tissues remains a subject of disagreement. One major obstacle in understanding the source of the disagreement is the wide range of variations in methods used in the extraction and processing of the tissue insulin. In the present study we showed that glass homogenizers, in our hands, are superior to the Waring blender; that storage of the acid ethanol supernatants (at ?80 degrees C) before processing or evaporation to dryness during early stages may be deleterious; gel filtration of extracts results in significant losses. Losses with hydrophobic (C(18)) chromatography were less severe than we had noted earlier with this method. With paired samples from controlled hypoinsulinemic (fasted), and hyperinsulinemic (fasted-refed) animals, we showed that insulin concentrations in brain, lung and probably liver are unaffected by changes in plasma insulin, whereas kidney insulin is directly related to plasma insulin concentrations. These findings confirm the essential data of previous studies. The unresponsiveness of brain insulin and most other tissue insulin compartments to fluctuations in plasma insulin, whether due to local synthesis or transport from extracellular sources followed by intracellular storage, needs to be incorporated into any overall conceptualization of tissue insulin.