Effects of hypophysectomy and growth hormone on cultured islets of Langerhans of the rat

Abstract

Summary. The effects on islet function of addition to the culture medium of rat growth hormone was studied in 4-day cultured islets of Langerhans from normal and hypophysectomised rats. In islets from hypophysectomised rats, rates of insulin release were 34% lower than in control rat islets; rates of insulin plus proinsulin and total protein biosynthesis were also lower by 48% and 16% respectively. The rates of glucose oxidation and the islet content of cyclic AMP were unchanged in islets from hypophysectomised rats but the islet content of calmodulin was decreased by 68%. The presence of rat growth hormone during the culture period restored the secretory response of hypophysectomised rat islets to that seen in control islets cultured without growth hormone but had only a marginal effect on the rate of insulin plus proinsulin biosynthesis, and no significant effect on islet calmodulin content. Glucose oxidation was increased by the presence of growth hormone during the culture period in both control (73% increase) and hypophysectomised (38% increase) rat islets. Addition of growth hormone to the culture medium also enhanced rates of insulin release and biosynthesis in control islets by 116% and 20% respectively. It is suggested that these changes arise primarily from modification of the synthesis of specific islet proteins.Key words: Insulin release, insulin biosynthesis, growth hormone, calmodulin, cyclic AMP, islet glucose metabolism, hypophysectomy, cultured islets.Insulin secretion is subject to acute regulation by nutrients such as glucose and amino-acids and by hormones such as glucagon. In addition B-cell secretory * Present address: Department of Physiology, Vargas School of Medicine, Central University of Venezuela, Caracas, Venezuela function is influenced chronically by experimental manipulations such as starvation [1] or hypophysectomy [2][3][4][5], and in pregnancy [6]. We have shown that the impaired insulin secretory response of islets from hypophysectomised rats persists following 4-day culture of the islets and may be reversed by the addition of growth hormone to the culture medium [2]. To investigate the mechanisms involved, we have measured other parameters of islet function in normal and hypophysectomised rat islets cultured in the absence or presence of rat growth hormone. Materials and MethodsIslets were obtained by collagenase digestion [7] from the pancreases of male Wistar rats fed on regular rat diet (PRM, Dixon, Ware, Hefts, UK). Hypophysectomised rats were purchased from Charles River Laboratories and used after 4-5weeks from hypophysectomy. Islets were cultured for 4 days as described previously [2]. The culture medium was RPM1 1640 [8] containing glucose (6mmol/1), penicillin (0.1 mg/ml), streptomycin (0.1 mg/ml) and 10% (v/v) inactivated calf serum (Wellcome, Beckenham, Kent, UK) and was supplemented, where stated, with rat growth hormone 1 .ug/ml (National Institutes of Health, Bethesda, USA, lot GH-B-6); the pH was buffered at 7.4 with N-2-hydroxyethylpiperazine-N'-2-ethane sulph...

Growth and Function of the Pancreatic β cell in vitro

“…In vitro experiments should therefore be extended to at least several hours in order to expect any detect¬ able effects. Similarly, increased glucose-induced insulin release and biosynthesis were evident after keeping islets from both normal and hypophysectomized rats for 4 days in culture with 10 µg/ml bovine growth hormone (Pierluissi et al 1980(Pierluissi et al , 1982. In another study there was no effect of 1 µg/ml rat growth hormone on insulin release after 30 min exposure while a significant increase in both insulin release and biosynthesis was observed after preincubation with rat growth hormone for 16 h (Whittaker & Taylor 1980).…”

Lack of Evidence for Acute Effects of Growth Hormone-Releasing Hormone on Serum Insulin and Glucose Levels in Normal and Hypophysectomized Rats

“…This probably ex plains the low body weight and blood sugar levels. In islets isolated from hypox rats, both basal and glucose-stimulat ed insulin release were lower than in control animals [16,34], This means that the lack of effect of GHRH in hypox rats could be due to a reduced ability of the islets to respond. However, GHRH also had no effect on serum insulin when tested in sham-operated animals.…”

“…The secretory defect in islets from hypox rats is proba bly caused by lack of GH because culture of these islets in the presence of GH restores the response [15,16,34]. In islets from normal rats, glucose-induced insulin release is higher after prolonged exposure to GH in tissue culture [35], GH can also stimulate rat insulin release in acute experiments in vitro [14] and in vivo [15].…”

“…GH exerts most of its metabolic effects on peripheral tissues but can also stimulate (i-cell insulin release [14,15]. Insulin release is reduced in pancreatic islets from hypophysectomized rats, and can be reversed by culturing in the presence of GH [ 16].…”

The Metabolic Actions of Growth Hormone

“…When added to fetal or postnatal rat pancreatic islets in vitro (432, 657,658) or to insulin-secreting tumor cells (602), human GH not only increased insulin synthesis and secretion (601,605,657,658,807,809,885) but also increased beta cell growth and cell division (103,197,603,605,666,(781)(782)(783)(784)808). Islets from hypophysectomized rats store less insulin and release less in response to glucose; and these changes are reversed by GH (516, 638, 639).…”