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Biomedical subjects

D J Flint

Publications and source records attributed to D J Flint.

At least 109 records · Page 6Linked to original sources
PubMed1985

Insulin processing and action in adipocytes: evidence for generation of insulin-containing vesicles by leupeptin and monensin.

Incubation of adipocytes with 125I-insulin plus leupeptin or monensin, but not chloroquine, resulted in the appearance of a novel peak of 125I-insulin (modal density about 1.20 g/ml) on density gradient centrifugation; the appearance of the peak depended on the presence of specific insulin receptors on the cell surface. The fractions comprising this peak contained vesicles, probably originating from the Golgi apparatus, and dit not appear to be contaminated with lysosomes, mitochondria or plasma membrane. Entrapment of insulin in these vesicles per se did not prevent the activation of glucose transport, acetyl-CoA carboxylase or pyruvate dehydrogenase by insulin.

Acetyl-CoA Carboxylase↗
PubMed1985

Adaptations of adipose tissue metabolism and number of insulin receptors in pregnant sheep.

The endocrine control of adipose tissue metabolism during pregnancy in sheep has been investigated. The number of insulin receptors of sheep adipocytes was increased during pregnancy. There was no apparent change in the concentration of serum insulin during pregnancy in sheep while the rise in serum progesterone concentration was smaller and more gradual than in rats. Net lipid deposition in adipocytes occurred during the first 55 days of pregnancy, probably due primarily to increased lipoprotein lipase activity. Net deposition of lipid had ceased by mid-pregnancy while by 125 days of pregnancy, the rate of fatty acid synthesis in adipose tissue was decreased and the serum fatty acid concentration had risen, suggesting the onset of net lipid mobilization in the tissue. Results are compared with those from other studies with rats; it would appear that different mechanisms regulate lipid deposition during pregnancy in sheep and rats.

Adaptation, Physiological↗
PubMed1985

Insulin binding and action on bovine adipocytes.

The present study was undertaken to determine whether bovine adipocytes could bind and respond to insulin. Adipose tissue from young bulls showed little response to physiological and higher concentrations of insulin over a 3-h incubation. In contrast, insulin did stimulate glucose uptake during prolonged incubations and also increased the activity of pyruvate kinase; in each case, half-maximum stimulation was observed with concentrations of about 0.5 ng/ml. Specific binding of [125I] iodoinsulin to bovine adipocytes was observed; the binding capacity was greater than that of young female rats determined under the same conditions. Bovine adipocytes internalized insulin. Incubation of bovine adipocytes with insulin resulted in a concentration-dependent loss of insulin receptors from the adipocyte surface. In summary, bovine adipocytes possess the capacity to both bind and respond to insulin; these findings are discussed in relation to a recent report to the contrary.

Adipose Tissue↗
PubMed1985

Effects of cortisol and progesterone on insulin binding and lipogenesis in adipocytes from normal and diabetic rats.

Cortisol implants in normal and diabetic rats reduced body weight, adiposity, insulin receptor concentration and both basal and insulin-stimulated rates of lipogenesis in isolated adipocytes, whilst insulin sensitivity was unchanged. In normal but not diabetic rats these changes were accompanied by increased serum glucose and insulin concentrations. In contrast, progesterone implants in normal and diabetic rats increased body weight gain, adiposity, insulin receptor concentration and both basal and insulin-stimulated rates of lipogenesis in adipose tissue, again without affecting insulin sensitivity. Progesterone did not affect serum insulin concentrations in normal or diabetic rats but accelerated the decline in serum glucose concentrations which occurred during an overnight fast in diabetic rats. The results suggest that cortisol inhibits lipogenesis in adipose tissue without affecting insulin sensitivity, cortisol reduces insulin binding in adipose tissue without a requirement for hyperinsulinaemia, which might itself indirectly lead to down-regulation of the insulin receptor, and in diabetic rats progesterone stimulates lipogenesis in adipose tissue without any increase in food intake or serum insulin concentrations suggesting that progesterone may have a direct anabolic role in adipose tissue.

Adipose Tissue↗
PubMed1984

Effects of cell volume on insulin binding, internalization and degradation in rat adipocytes.

Adipocytes from old rats (greater than 450 g) were separated into 2 populations with mean cell volumes of 201 +/- 14 and 813 +/- 41 pl (mean +/- SEM, 20 observations) by filtering through nylon mesh (64 microns diameter) and compared with adipocytes from young rats (less than 150 g) with a mean adipocyte volume of 154 +/- 20 pl (14 observations). Large adipocytes had more insulin receptors per cell but less per unit of surface area. They internalized greater amounts of insulin than small cells in the presence or absence of bacitracin and chloroquine, although the proportion of bound hormone which was internalized was similar in all 3 groups. Down-regulation of the insulin receptor was evident in large and small adipocytes after incubation in the presence of 10(-7)M insulin. Large cells degraded insulin (extracellularly and intracellularly) at significantly greater rates than small cells whether expressed per cell or per unit of surface area. Small cells from old rats had essentially identical properties to small cells from young rats in all parameters examined. The results suggest that the decreased surface density of insulin receptors observed in large adipocytes from old rats is due to size rather than age and that the decreased insulin sensitivity of large adipocytes is not due to an inability to internalize insulin or down-regulate its receptors but may be due to increased rates of insulin degradation.

Adipose Tissue↗
PubMed1984

Biphasic effect of PMSG on lactation in the rat.

A single injection of PMSG to rats on Day 10 of lactation produced a biphasic inhibitory effect on litter growth. The initial effect (24-72 h) occurred in conjunction with a rise in the concentration of oestradiol-17 beta and LH and a decline in serum prolactin, whereas the secondary effect (96 h onward) occurred in conjunction with an increase in oestradiol-17 beta and prolactin concentrations. Progesterone implants prevented the initial, but not the secondary, effect of PMSG on lactation without producing any effect on serum oestradiol-17 beta concentrations. The results suggest that the initial effect of PMSG on litter growth may be produced by changes in pituitary prolactin and gonadotrophin secretion rather than by an effect of oestrogens acting on the mammary gland.

Animals↗
PubMed1984

Effects of prolactin, progesterone and ovariectomy on metabolic activities and insulin receptors in the mammary gland and adipose tissue during extended lactation in the rat.

Prolactin implants prevented the decline in milk yield and the resumption of oestrous cycles which occurred between days 22 and 28 in untreated lactating rats. Ovariectomy and progesterone implants only partially prevented the decline in milk yield despite preventing the occurrence of oestrous cycles. All three treatments increased total RNA content of the mammary gland compared with controls. In untreated rats there were no changes in mammary DNA content or the number of insulin receptors whereas lipoprotein lipase (LPL) activity decreased significantly during the declining phase of lactation. In contrast, the number of insulin receptors, LPL activity and glucose incorporation into lipid increased in adipose tissue. Prolactin prevented the increase in insulin receptors and lipid synthesis and significantly decreased LPL activity in adipose tissue. Progesterone stimulated LPL activity in the mammary gland and also prevented the increase in lipid synthesis and insulin receptors in adipose tissue but was without effect on LPL activity whereas ovariectomy stimulated LPL activity in the mammary gland but prevented only the increase in the number of insulin receptors in adipose tissue. The results show that raising the serum prolactin concentration can prevent the decline in milk yield during extended lactation and whilst part of this effect may be due to a direct effect on the mammary gland and an indirect effect due to inhibition of oestrous cycles, prolactin may also produce part of its effect on milk synthesis by inhibiting competitive metabolic processes in tissues such as adipose tissue.

Adipose Tissue↗
PubMed1984

Stimulation of milk secretion with inhibition of milk ejection by corticosteroids during extended lactation in the rat.

Milk yield declined significantly between days 22 and 28 of lactation in rats, when lactation was extended by frequent replacement of older litters with younger ones. Corticosterone implants but not cortisol injections or implants prevented this decline. Cortisol, however, appeared to inhibit milk ejection since the mammary glands became engorged with milk and milk yield was improved dramatically by oxytocin injections. In both cases corticosteroid concentrations increased approximately threefold above basal concentrations. Both corticosteroids increased total mammary gland RNA content and lipoprotein lipase (LPL) activity of the mammary gland but were without effect on insulin binding. They also decreased LPL activity, lipogenesis and the number of insulin receptors on adipose tissue. Serum prolactin and insulin concentrations were unaffected by any of the treatments. The results suggest that corticosteroids inhibit milk ejection under certain conditions, may be circulating in lower concentrations, which thereby limit milk production, during prolonged lactation and may improve milk yield during extended lactation in part by suppressing anabolic activity in adipose tissue.

Adipose Tissue↗
PubMed1983

Adipose tissue metabolism during early pregnancy in the rat: temporal relationships of changes in the metabolic activity, number of insulin receptors, and serum hormone concentrations.

The temporal relationships between changes in rates of fatty acid and acylglycerol glycerol synthesis; the activity of lipoprotein lipase of parametrial adipocytes and their capacity to bind insulin; and the serum concentrations of insulin, progesterone, prolactin, and total lactogenic activity have been examined in rats during the first 15 days of pregnancy. The rate of fatty acid and acylglycerol synthesis showed a transient increase at Days 9 and 12 of pregnancy, whereas there was no change in the activity of lipoprotein lipase activity except for a fall between Days 12 and 15 of pregnancy. The capacity of adipocytes to bind insulin was increased by Day 6 of pregnancy and remained elevated until at least Day 15; no changes in the affinity for insulin were observed. Serum progesterone, insulin, and total lactogenic activity were elevated by Days 3, 9, and 12 of pregnancy, respectively. The results show that progesterone but not placental lactogen could be responsible for the rise in the insulin-binding capacity of rat adipocytes during pregnancy, whereas the fall in lipogenic capacity at about Day 12 of pregnancy coincides with the rise in serum placental lactogen.

Adipose Tissue↗
PubMed1983

Absence of down-regulation of the insulin receptor by insulin. A possible mechanism of insulin resistance in the rat.

Insulin resistance occurs in rat adipocytes during pregnancy and lactation despite increased or normal insulin binding respectively; this suggests that a post-receptor defect exists. The possibility has been examined that, although insulin binding occurs normally, internalization of insulin or its receptor may be impaired in these states. Insulin produced a dose-dependent reduction in the number of insulin receptors on adipocytes from virgin rats maintained in culture medium, probably due to internalization of the hormone-receptor complex. In contrast, adipocytes from pregnant and lactating rats did not exhibit this 'down-regulation' phenomenon. Down regulation was, however, apparent in all groups when the experiments were performed in Tris buffer (where receptor recycling is inhibited), suggesting that in pregnant and lactating rats insulin receptors are rapidly recycled back to the plasma membrane, whereas in virgin rats this recycling process is less effective. Internalization of insulin was also determined by using 125I-labelled insulin. Adipocytes from pregnant and lactating rats appeared to internalize similar amounts of insulin to virgin rats. In the presence of the lysosomal inhibitor chloroquine, adipocytes from pregnant rats internalized more insulin than virgin or lactating rats. These results suggest that adipocytes from pregnant and lactating rats internalize insulin and its receptor normally, whereas intracellular processing of the insulin receptor may differ from that in virgin rats. In addition the rate of lysosomal degradation of insulin may be altered in adipocytes from pregnant rats.

Adipose Tissue↗
PubMed1983

Control of fatty acid synthesis in lactation.

Lactation results not only in an increased rate of fatty acid synthesis in the mammary gland but also in a decreased rate of fatty acid synthesis in adipose tissue and, in the rat at least, an increased rate of hepatic fatty acid synthesis. Progesterone (during pregnancy), prolactin and (in ruminants) GH are implicated in the regulation of the reciprocal changes in fatty acid synthesis in mammary gland and adipose tissue. Progesterone and prolactin, at least, appear to influence the rate of fatty acid synthesis by modulating the insulin-binding capacities of the tissues, but it is clear that steps in the mechanism of action of insulin subsequent to its binding to the receptor are also changed in adipose tissue during lactation.

Adipose Tissue↗
PubMed1982

Insulin binding to rat mammary gland at various stages of cell isolation and purification.

Epithelial cell-rich fractions of rat mammary gland were prepared using percoll gradients after collagenase dispersion. Their insulin-binding characteristics were similar to those of crude acini but superior to unfractionated isolated cells. Optimal binding was obtained after 60 min at 20 degrees C or 16 h at 4 degree C at pH 7.8 Binding at 37 degrees C was lower due probably to an enhanced rate of insulin degradation. 48 h after ovariectomy of 18-day pregnant rats insulin binding to acini doubled due to an increase in the number of insulin receptors. Progesterone but not bromocriptine (which prevented the rise in serum prolactin which occurred after ovariectomy) prevented this increase in insulin binding. These results illustrate that the change in serum progesterone rather than prolactin increases insulin binding to the mammary cell at parturition whilst insulin binding decreases in adipose tissue at the same time (Flint et al., Mol. Cell Endocrinol, 20, 101-111, 1981) enabling coordinated changes in the metabolism of these 2 tissues to take place during the perinatal period.

Animals↗
PubMed1982

Regulation of insulin receptors by prolactin in lactating rat mammary gland.

Bromocriptine treatment of lactating rats, or removal of the litter, led to a decrease in the number of insulin receptors in the mammary gland and an increase in the concentration of insulin in the serum. Bromocriptine also induced a decrease in the concentration of both prolactin and progesterone in the serum, whilst concurrent treatment with the former but not the latter prevented all the effects of bromocriptine for 48 h. Removal of the litter produced a similar decrease in the concentration in the serum of prolactin but not of progesterone. Treatment with prolactin prevented all of the effects of removal of the litter for 24 but not 48 h. This suggests that these effects of prolactin may require a mammary gland actually synthesizing milk since the gland rapidly fills with milk after removing the litter whereas milk removal continues to take place in bromocriptine-treated rats allowed to continue nursing their litters.

Animals↗
PubMed1981

Metabolism of sheep adipose tissue during pregnancy and lactation. Adaptation and regulation.

1. Changes in the mean volume, the rate of fatty acid and acylglycerol glycerol synthesis, the activity of lipoprotein lipase and the numbers and affinities of insulin receptors of subcutaneous adipocytes are reported for sheep at different stages of pregnancy and lactation. In addition, the serum concentrations of insulin, progesterone, prolactin, choriomammotropin, somatotropin, glucose, acetate, L-lactate, glycerol and unesterified fatty acids are reported for these sheep. 2. A switch from lipid accumulation to net lipid mobilization accompanied by a decline in the capacity for lipid synthesis, occurred at the onset of the last third of pregnancy. Net lipid mobilization continued during lactation. 3. The changes that occurred in the serum concentrations of the various hormones listed above are discussed in relation to their possible roles in the modulation of adipose tissue metabolism in sheep during pregnancy and lactation. The observations are compared with those from previous studies on the hormonal control of adipose tissue metabolism in the rat during pregnancy and lactation.

Adaptation, Physiological↗
PubMed1981

Aspects of adipose-tissue metabolism in foetal lambs.

1. The mean volume of adipocytes, the rates of fatty acid and acylglycerol glycerol synthesis from various precursors (in vitro), the rates of oxidation of acetate and glucose (in vitro) and the activities of lipoprotein lipase and various lipogenic enzymes were determined for perirenal adipose tissue from foetal lambs during the last month of gestation. 2. The fall in the rate of growth of perirenal adipose tissue during the last month of gestation is associated with a diminished capacity for fatty acid synthesis and lipoprotein lipase activity, but no change in the rate of acylglycerol glycerol synthesis was observed. There was no fall in the activities of cytosolic acetyl-CoA synthetase or the NADP-linked dehydrogenases, suggesting that the decrease in the rate of fatty acid synthesis was due to an impairment at the level of acetyl-CoA carboxylase or fatty acid synthetase. 3. The rate of fatty acid synthesis from acetate was greater than that from glucose. The rate of fatty acid synthesis from glucose per adipocyte of foetal lambs was similar to that of young sheep. The characteristic metabolism of adipose tissue of the adult sheep is thus present in the foetus, despite the relatively large amounts of glucose in the foetal 'diet'.

Adipose Tissue↗
PubMed1981

Prolactin and the regulation of adipose-tissue metabolism during lactation in rats.

Removal of litters from young lactating rats for 24 or 48 h or treatment of lactating rats with bromocriptine increased the rate of fatty acid synthesis and the activities of lipoprotein lipase and fatty acid synthetase in adipose tissue, decreased the lipoprotein lipase and fatty acid synthetase activities of mammary gland and lowered the serum-prolactin concentration. Concurrent injections of prolactin prevented the effects of bromocriptine and 24 h of litter removal on most of these changes in adipose tissue, but did not prevent the effects of 48 h of litter removal. The results suggest that effects of prolactin on adipose-tissue metabolism are dependent on a functional mammary gland. Most of the responses of adipose tissue to litter removal were reduced in older rats.

Adipose Tissue↗
PubMed1980

Changes in the number of insulin receptors of isolated rat hepatocytes during pregnancy and lactation.

The weight of the rat liver increased during pregnancy and lactation due solely to an increase in hepatocyte size. Insulin receptors were identified using 125I-labelled insulin and isolated hepatocytes in vitro. Scatchard analysis was interpreted in terms of high affinity (KD 4 nM) and low affinity (KD 80 nM) receptors for insulin. No change in the affinity of either receptor site was detected during pregnancy or lactation; There was, however, a significant increase in the number of both types of receptor on the hepatocyte by day 12 of pregnancy which was maintained until at least day 20 of pregnancy. During lactation, the number of receptors declined to values similar to those of virgin rats. Serum insulin concentrations, determined by radioimmunoassay, were elevated during pregnancy, returned to values similar to virgin rats during early lactation and were significantly reduced compared with virgin rats by day 15 of lactation. These results illustrate that physiological conditions exist whereby the number of insulin receptors may increase, despite elevated serum insulin concentrations, in apparent conflict with the 'down-receptor' hypothesis.

Animals↗