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The pituitary glands from mice rendered obese by gold thioglucose treatment and by dietary manipulation, and pituitary glands from lean mice after a high food intake or a glucose load, were shown to stimulate insulin secretion from isolated pancreatic islets. The insulin releasing activity of pituitary glands from obese (ob/ob) mice was reduced by fasting for 24 and 48 h. Results obtained with pituitary glands from ob/ob and from lean ob/ + and + /+ mice suggest that the insulin releasing property manifests a gene dosage effect. Pituitary glands from 3-week-old (young) ob/ob mice stimulated insulin secretion to the same extent as pituitary glands from 3-month-old (adult) ob/ob mice. The pancreatic islets of young ob/ob mice were shown to be somewhat more responsive to stimulation by the pituitary factor than were lean ob/ + or + / + islets from this age group. The concept that high insulin level, partly under pituitary control, and high caloric intake may be interlinked and may, in combination, be a major factor in producing obesity is discussed. Furthermore, it is suggested that the pituitary insulin releasing factor may play a role in the early development of obesity in the animal models studied.
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The influence of plasma from genetically obese (ob/ob) and lean (+/+) mice on insulin secretion has been studied by perifusion of collagenase-prepared pancreatic islets maintained for 48 h in culture. Insulin secretion was measured at 2-min intervals and plasma from the ob/ob mice not from the +/+ mice rapidly stimulated insulin release, reaching a maximum in 2–4 min and falling to basal levels in about 10 min. Experimental evidence is given indicating that the plasma insulin secretagogue is identical to β-cell-trophin, a peptide of the pituitary pars intermedia which stimulates insulin secretion. The evidence is based on (1) the antigenic properties of the peptides (both cross-react with a -COOH terminal ACTH antiserum raised to the 17–39 moiety of ACTH), (2) identical chromatographic separation on Biogel columns and on reverse-phase high pressure liquid chromatography and (3) the similarity of their insulin releasing action from perifused islets.
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ABSTRACT
The sympathetic nervous system is believed to play a part in the control of insulin release from the pancreatic islets of Langerhans. Stimulation of α-adrenoceptors is thought to inhibit the release of insulin whereas stimulation of β-adrenoceptors enhances insulin release. The present experiments were conducted to establish the existence of β-adrenergic receptors on guinea-pig and rat islet cells and to quantify them using the selective β-adrenergic ligands [3H]dihydroalprenolol (DHA) and [125I]cyanoiodopindolol (CYP).
Guinea-pig islets had 62 fmol β-adrenoceptors/mg protein using [3H]DHA, corresponding to 43 700 binding sites/cell and 25 fmol β-adrenoceptors/mg protein using [125I]CYP, corresponding to 17 400 sites/cell. Rat islet cells were found to have 4·6 fmol β-adrenoceptors/mg protein using [125I]CYP, corresponding to 7200 sites/cell. Adenylate cyclase activation exhibited a positive dose–response relationship when exposed to the β-adrenoceptor agonist isoprenaline, with a maximum response (190 ± 21% above basal) at 10 μmol isoprenaline/l. This response was abolished with 1 μmol/l of the β-adrenergic antagonist 1-alprenolol. Insulin secretion in the presence of 10 mmol glucose/l, but in the absence of the α-adrenoceptor blocker phentolamine, was not affected by 10 μmol isoprenaline/l. However, perifusion experiments showed that secretion of insulin from isolated rat islets in the presence of 10 mmol glucose/l was significantly increased (332%) by 10 μmol isoprenaline/l in the presence of 10 μmol phentolamine/l.
These results suggest that binding of selective radio-labelled ligands occurs to β-adrenergic receptors on the B cell surface of the islets of Langerhans, and that these receptors are functionally coupled to insulin secretion through modulation of adenylate cyclase activity.
J. Endocr. (1986) 111, 263–270
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Abstract
The mechanisms which initiate and maintain the energy partitioning into maternal tissues during pregnancy are unknown. The present study shows that in each of the weeks prior to pregnancy in the rat, plasma β-cell tropin (βCT) concentrations (nmol/l) were 0·68±0·10, 0·61 ±0·14 and 0·73±0·11 (n=11, mean±s.e.m.). During early (1-3 days) pregnancy the concentration rose to 1·32 ±0·26 and by early-mid (7–10 days) pregnancy they had increased to 1·96 ±0·41. By mid-late (14–17 days) pregnancy plasma βCT concentration had declined to the prepregnancy concentrations (0·67 ± 0·16). This mid-term increase in the circultating concentration of the lipogenic hormone βCT may contribute to the deposition of lipid associated with the early period of gestation. The increased circulating βCT could be derived from the pituitary gland neurointermediate lobe or by secretion from the placenta. It should be emphasised that the measurements in the present study represent a 'snap-shot' at discrete intervals and do not provide information about the dynamic hormonal interplay which occurs physiologically.
Journal of Endocrinology (1995) 146, 177–182
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ABSTRACT
The neurointermediate pituitary peptide β-cell tropin (BCT) has potent insulin-releasing and lipogenic properties and is elevated in obesity and type-2 diabetes. The effects of BCT and glucose on the release of insulin and amylin from the perfused pancreas of obese 'fatty' (fa/fa) rats and lean (Fa/?) controls were measured. Pancreata were perfused, sequentially, with buffer containing: 5·6 mmol glucose/l (basal); basal glucose±0·5 nmol BCT/l; 16·7 mmol glucose/l (high). Insulin and amylin release during basal glucose treatment was eight to nine times greater from pancreata from fatty than from lean rats. BCT induced a fivefold greater monophasic insulin and amylin release from fatty compared with lean pancreata. When not preceded by BCT there was a twofold greater high glucose-induced amylin release from fatty pancreata but no difference in insulin secretion. When preceded by BCT stimulation, high glucose induced twofold greater insulin and fourfold larger amylin release from fatty compared with lean pancreata. Molar secretion ratios of insulin: amylin varied between 30:1 and 50:1. In view of the elevated levels of BCT found in the fatty rat and in the light of the above findings, it is concluded that the peptide may have a role in the development of hyperinsulinaemia, hyperamylinaemia and insulin resistance in this animal model of obesity and diabetes.
Journal of Endocrinology (1993) 137, 375–381
