The hypothalamic origin of an insulin secretion promoting factor present in the plasma of normal rats

Histopathological changes in the pituitary glands of female hamsters infected with the 139H strain of scrapie

Previous studies in hamsters showed that the 139H strain of scrapie injected intracerebrally caused a generalized endocrinopathy and marked hypoglycaemia and hyperinsulinaemia. The low scrapie infectivity levels in the pancreas suggested that the changes noted in that organ were of neuroendocrine origin. In the current study, female weanling Syrian hamsters were inoculated intracerebrally with scrapie strain 139H or 263K, or with homogenate of normal hamster brain. Coronal sections of the pituitary gland were stained with haematoxylin and eosin, Gomori's one-step trichrome, Congo red, thioflavin-S, and antibodies specific for several pituitary hormones. Sections were examined by light microscopy. The hamsters inoculated with scrapie strain 139H showed extensive pituitary vacuolization. Most vacuoles were located in the ventral or ventrolateral parts of the pars distalis. The pituitary glands of 139H-infected hamsters also showed cellular changes, namely, hypertrophy, atrophy and cytoplasmic vesicles. Nuclear changes such as swelling, vesicle formation, chromatin increase, pyknosis, karyorrhexis and karyolysis also occurred. The cellular and nuclear changes were most pronounced in the regions with vacuolation. Hamsters infected with the 263K strain did not show these changes. Immunocytochemical examination suggested that parenchymal cell types which produce different hormones were affected in areas of vacuolation. The changes produced by 139H were not seen in hamsters infected with strain 263K. This study provides the first evidence of cytopathological changes in the pituitary glands of scrapie-infected animals and suggests a relationship between the pituitary changes and the pathological findings in the pancreas and other endocrine organs of 139H-infected hamsters.

Physiological concentrations of oxytocin powerfully stimulate insulin secretionin vitro

The effect of natural oxytocin on insulin secretion was investigated by using isolated, perfused rat pancreases. Oxytocin produced a dose-dependent stimulation of insulin secretion starting with a concentration as low as 2.3PM: and with a maximal effect obtained at 66PM: . Specific oxytocin antagonist, desGly-NH(2) (9), d(CH(2))(5) [Tyr (Me)(2), Thr(4)] OVT, reduced by 70% the stimulatory effect of 66PM: natural oxytocin. A specific oxytocin receptor agonist OH(Thr(4), Gly(7))OT showed an insulinotropic action similar to equivalent amounts of oxytocin. Replacement or modifications of Q(4), L(8) or the NH(2) terminal group in the oxytocin molecule reduced or abolished the biological activity. This study demonstrated that: (1) in normal rat pancreas, oxytocin stimulates insulin secretion at concentrations similar to those present in the plasma; (2) oxytocin exerts this secretagogue action in presence of basal physiological glucose levels (5 mM); (3) oxytocin stimulates insulin secretion by interacting with its own receptor. A potential role for oxytocin as an insulin-releasing hormone is thus conceivable.

Neuro-humoral control of insulin secretion

Hypothalamic extract from normal rats was shown to contain a principle able to stimulate insulin secretion both in vivo and in vitro. This substance, whose peptidic nature was demonstrated by enzymatic digestion, has a molecular weight of about 1000 Daltons. An active release of this peptide was obtained when incubating hypothalamic fragments under appropriate stimulatory conditions (i.e. KCl 50 mmol l-1 in the incubation medium) (i.e. molecular weight and insulin secretion stimulating activity) similar to the hypothalamic peptide was evidentiated in the plasma of normal rats. Since manipulations of hte hypothalamus (i.e. electrical lesions or stimulations) could respectively decrease or enhance the plasma concentration of this substance, the hypothalamic origin of the insulin secretion stimulating principle present in the plasma was strongly suggested. The possible participation of this hypothalamic peptide to the neural control of insulin secretion is suggested.