To study the influence of insulin on its own secretion, collagenase-isolated islets of rat pancreas were prelabelled with [3H]leucine for 2 h. After washing the islets, (pro-)insulin release was stimulated by glucose in the presence or absence of exogenous insulin (up to 2-5 mu./ml). Hormone release was unchanged by the presence of exogenous insulin as judged by determination of both immunoreactive insulin and radioactivity incorporated into the proinsulin and insulin fractions of the medium. No direct feedback mechanism for insulin secretion was apparent from this study.
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Schatz et al. (1977) studied this question.