Metabolic effects and secretory properties of a radiation-induced transplantable rat insulinoma

A guide to understanding polycystic ovary syndrome (PCOS)

Polycystic ovary syndrome (PCOS) is the commonest endocrine disorder affecting women of reproductive age. Some 20% of women will have polycystic ovaries on an ultrasound scan and around 7% of women have the additional clinical or biochemical features of PCOS. As a complex multisystem disorder its background can be confusing to understand. They key feature, however, is an increased production of androgen by the ovaries. This review uses ovarian biology to describe a strategy to aid understanding and explanation of PCOS. This framework can be also be used to teach about PCOS and to inform different approaches to its management.

Insulin modulates glucose-dependent insulinotropic polypeptide (GIP) secretion from enteroendocrine K cells in rats

Effects of insulin excess and deficiency on glucose-dependent insulinotropic polypeptide (GIP) was examined in rats following insulinoma transplantation or streptozotocin (STZ) administration. Over 14 days, food intake was increased (p < 0.001) in both groups of rats, with decreased body weight (p < 0.01) in STZ rats. Non-fasting plasma glucose levels were decreased (p < 0.01) and plasma insulin levels increased (p < 0.001) in insulinoma-bearing rats, whereas STZ treatment elevated glucose (p < 0.001) and decreased insulin (p < 0.01). Circulating GIP concentrations were elevated (p < 0.01) in both animal models. At 14 days, oral glucose resulted in a decreased glycaemic excursion (p < 0.05) with concomitant elevations in insulin release (p < 0.001) in insulinoma-bearing rats, whereas STZ-treated rats displayed similar glucose-lowering effects but reduced insulin levels (p < 0.01). GIP concentrations were augmented in STZ rats (p < 0.05) following oral glucose. Plasma glucose and insulin concentrations were not affected by oral fat, but fat-induced GIP secretion was particularly (p < 0.05) increased in insulinoma-bearing rats. Exogenous GIP enhanced (p < 0.05) glucose-lowering in all groups of rats accompanied by insulin releasing (p < 0.001) effects in insulinoma-bearing and control rats. Both rat models exhibited increased (p < 0.001) intestinal weight but decreased intestinal GIP concentrations. These data suggest that circulating insulin has direct and indirect effects on the synthesis and secretion of GIP.

Effects of amino acids, hormones and drugs on insulin release and 45Ca uptake by transplantable rat insulinoma cells maintained in tissue culture

1. Acute effects of amino acids, hormones and drugs on transplantable rat insulinoma cells were examined after 2-3 days culture in RPMI-1640 (11.1 mM glucose) to eliminate necrotic cells and counter prior hypoglycaemia. 2. At 2.6 mM Ca2+, rat insulinoma cells (greater than 95% viability) released 48-97 ng insulin/10(6) cells during 60 min incubations with uptake of 1.0-1.8 nmol 45Ca/10(6) cells. 3. Insulin release and 45Ca uptake by rat insulinoma cells were not modified by arginine, leucine, 2-ketoisocaproate, tolbutamide, glibenclamide, somatostatin, adrenaline, noradrenaline, diazoxide or cyproheptadiene. 4. Responsiveness to acetylcholine (stimulation of insulin release and 45Ca uptake) and to GIP (stimulation of insulin release) was demonstrated. Thiol reagents (CMBS, CPDS and DTNB) and agents affecting microtubules-microfilaments (colchicine, vinblastine and cytochalasin B) enhanced insulin release. 5. The results suggest that rat insulinoma cells exhibit a generalized defect in the regulation of insulin release by nutrients, hormones and drugs which act in pancreatic B-cells by alteration of cellular Ca2+. Responsiveness to agents affecting insulin release through alternative mechanisms appears to be retained.

Effects of cationic modification on 45Ca uptake and insulin release by transplantable rat insulinoma cells maintained in tissue culture

1. Acute effects of cations on 45Ca uptake and insulin release by transplantable rat insulinoma cells were examined after 2-3 days culture in RPMI-1640 containing 11.1 mM glucose. 2. At 2.6 mM Ca2+, rat insulinoma cells (greater than 95% viability) released 78-158 ng insulin/10(6) cells during 60 min incubation with uptake at 2.19-3.24 nmol 45Ca/10(6) cells. 3. Addition of 2 mM La3+, Co2+, Mn2+, Zn2+ or Ba2+ did not affect 45Ca uptake. Insulin release was also unaffected by these cations with the exception of 87% inhibition in the presence of La3+ or Zn2+. 4. Omission of 5.9 mM K+, 1.2 mM Mg2+, 115 mM Na+ or H+ (pH 8.5) did not affect 45Ca uptake or insulin release, irrespective of osmotic compensation using choline chloride or sucrose. Rat insulinoma cells were similarly unresponsive to addition of 30.9 mM K+, 12 mM Mg2+ or H+ (pH 6.3). 5. Omission of 2.6 mM Ca2+ (with or without addition of 1 mM EGTA) or addition of 20.5 mM Ca2+ did not affect insulin release. 6. The results indicate that rat insulinoma cells are little affected by cationic modifications which have profound effects on Ca2+ handling and insulin release by pancreatic beta-cells. Dysregulation of insulin release by insulinoma cells is associated with marked irregularities in the control of transmembrane Ca2+ fluxes and sensitivity to extracellular Ca2+.

Effects of cytotoxic drugs and inhibitors of insulin secretion on a serially transplantable rat insulinoma and cultured rat insulinoma cells

The effects of cytotoxic drugs and inhibitors of insulin secretion were examined in vivo in rats with a radiation-induced transplantable insulinoma, and in vitro using cultured rat insulinoma cells and the derived RINm5F insulin-secreting cell line. Administration of diazoxide to insulinoma-bearing rats resulted in a transient decrease of plasma insulin with a temporary rise of glucose concentrations. Mannoheptulose and somatostatin failed to affect the marked hyperinsulinaemia and hypoglycaemia. Streptozotocin produced a rapid and sustained decrease of insulin concentrations in insulinoma-bearing rats, accompanied by a progressive elevation of plasma glucose. Administration of alloxan failed to affect circulating insulin or glucose concentrations. In vitro, streptozotocin and alloxan exerted approximately equipotent time-dependent and concentration-dependent cytotoxic effects on insulinoma cells and RINm5F cells as established by cell staining with trypan blue. The cytotoxic actions of both drugs were decreased by agents believed to scavenge free radicals or to act as inhibitors of poly(ADP-ribose) synthetase. The results suggest that the cytotoxic actions of streptozotocin and alloxan on rat insulinoma cells and RINm5F cells are mediated by the generation of hydroxyl free radicals and DNA strand breaks. The ineffectiveness of alloxan in insulinoma-bearing rats probably reflects the high rate of decomposition of the drug in vivo.