Involvement of hormone processing in insulin-activated glucose transport by isolated cardiac myocytes.

Isolated muscle cells from adult rat heart were used to study the relationship between myocardial insulin processing and insulin action on 3-O-methylglucose transport at 37 degrees C. Internalization of the hormone as measured by determination of the non-dissociable fraction of cell-bound insulin increased linearly up to 10 min, reaching a plateau by 30-60 min at 3 nM-insulin. At this hormone concentration the onset of insulin action was found to be biphasic, with a rapid phase up to 8 min, followed by a much slower phase, reaching maximal insulin action by 30-60 min. Insulin internalization was totally blocked by phenylarsine oxide, whereas dansylcadaverine had no effect on this process. Initial insulin action (5 min) on glucose transport was not affected by chloroquine and dansylcadaverine, but was completely abolished by treatment of cardiocytes with phenylarsine oxide. This drug effect was partly prevented by the presence of 2,3-dimercaptopropanol. Under steady-state conditions (60 min), the stimulatory action of insulin was decreased by about 60% by both chloroquine and dansylcadaverine. This study, demonstrates that insulin action on cardiac glucose transport is mediated by processing of the hormone. The data suggest dual pathways of insulin action involving initial processing of hormone-receptor complexes and lysosomal degradation.