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growth hormone, human growth hormone, growth hormones, IGF 1,
Human Growth Hormone - HGH
growth hormone, human growth hormone, growth hormones, IGF 1,
Growth hormone and Insulin-ike Growth Factor-I

DEPARTMENT OF PHYSIOLOGY GÖTEBORG UNIVERSITY
DISSERATIONS (Ph.D. Thesis):     Abstract
Vikman-Adolfsson, K.


Growth hormone and insulin-like growth factor-I in the regulation of lipoprotein lipase in the rat.

Growth hormone has profound effects on lipid metabolism. Both body fat mass and lipoprotein metabolism are regulated by Growth Hormone. Lipoprotein lipase (LPL) is a key enzyme of triglyceride removal from the circulation and hence supply adipose as well as muscle tissues with fatty acids for storage or as an energy fuel. In the present study, the regulation of Growth Hormone receptors (GHR) and the Growth Hormone dependent growth factor, insulin-like growth factor-I (IGF-I) expression by Growth Hormone in adipose tissue of hypophysectomized rats was investigated. Moreover, the effects of Growth Hormone and IGF-I in the regulation of LPL activity in adipose tissue and muscle tissues were studied in order to get further insight into the mechanisms by which Growth Hormone and IGF-I regulate body fat and lipoprotein metabolism. Hypophysectomized rats were used. They were given hydrocortisone acetate and L-thyroxine as replacement therapy. In addition, human or bovine recombinant Growth Hormone or human recombinant IGF-I were given by subcutaneous injections or as a continuous subcutaneous infusion using osmotic minipumps. After 5-7 days of treatment, the rats were killed and the tissues excised. Messenger ribonucleic acid levels in the selected tissues (GHR, IGF-I and LPL) were quantified by a solution hybridization technique. Lipase activity was measured using assays in which radiolabeled triacylglycerol is hydrolyzed and labeled free fatty acids measured. IGF-I, insulin, glucose and free fatty acid concentrations were quantified using standard methods. Glucose incorporation into lipids in adipose tissue explants or isolated adipocytes as well as into glycogen in the soleus muscle were studied in vitro. Hypophysectomy resulted in decreased GHR mRNA levels in adipose tissue of hypophysectomized rats. After a single injection of Growth Hormone, GHR mRNA was induced within two hours. A Growth Hormone dose-dependent increase in GHR mRNA levels was found. Northern blot analysis revealed two transcripts of 3.6 kb and of 1.2 kb, which are believed to encode the full length Growth Hormone receptor and a Growth Hormone binding protein, respectively. IGF-I mRNA levels decreased in adipose tissue following hypophysectomy. Growth Hormone treatment dose-dependently increased IGF-I mRNA levels. In vitro, addition of Growth Hormone to isolated adipocytes increased IGF-I mRNA and the accumulation of IGF-I into the medium. Post-heparin plasma lipoprotein lipase and hepatic lipase (HL) activities decreased after hypophysectomy of female rats. In these experiments, Growth Hormone was given either as two daily s.c injections or as a continuous s.c infusion. Irrespective of the mode of administration, LPL and HL activities increased by Growth Hormone treatment. In adipose tissue, LPL activity was unchanged after hypophysectomy, but was increased by Growth Hormone treatment. LPL mRNA levels changed in parallel. In the heart and the gastrocnemius muscle, hypophysectomy resulted in decreased LPL activity and Growth Hormone administration increased LPL activity. In the heart LPL activity changed in parallel with LPL mRNA levels, whereas no effect of Growth Hormone treatment on LPL mRNA levels was found in the gastrocnemius muscle. Finally, the effect of IGF-I treatment on LPL activity was studied. In adipose tissue, IGF-I treatment of hypophysectomized rats reduced LPL activity. No effect of IGF-I treatment on LPL activity in the heart or gastrocnemius muscle was observed. Insulin stimulated glucose incorporation into lipids was reduced in adipose tissue. In the soleus muscle, glycogen content and insulin stimulated glycogen synthesis increased after IGF-I treatment. In summary, Growth Hormone interacts and regulates its own receptor in adipose tissue. This interaction results in increased expression and production of IGF-I. Growth Hormone increases both LPL and hepatic lipase activities, effects that may be of importance in the regulation of lipoprotein turn-over by Growth Hormone. IGF-I does not stimulate LPL activities, indicating that the effect of Growth Hormone on lipoprotein turn-over may not be mediated via IGF-I. On the the other hand, IGF-I inhibited basal and insulin stimulated lipogenesis in adipose tissue and increased glycogen storage in skeletal muscle.

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Table of Contents
  1. Human growth hormone.. is it the fountain of youth?
  2. Growth Hormone and Aging.
  3. Growth hormone - proven scientific information of GH.
  4. Symptoms of deficiency in growth hormone.
  5. Signs of deficiency.
  6. Measuring growth hormone in the body.
  7. Amino acids can also stimulate growth hormone.
  8. Hormones that enhance the effects of growth hormone.
  9. IGF-1 and Prostate Cancer: An Insubstantial Link
  10. More Information.

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