GH but not IGF-I or insulin increases lipoprotein lipase activity in muscle tissues of hypophysectomised rats

Abstract

Changes in GH secretion are associated with changes in serum lipoproteins, utilisation of fuels and body composition. Since lipoprotein lipase (LPL) is a key enzyme in the regulation of lipid and lipoprotein metabolism, changes in LPL activity may contribute to these effects of GH. The present study was undertaken to investigate the role of GH and the GH-dependent growth factor, IGF-I, in the regulation of LPL in heart, skeletal muscle and adipose tissue. Female rats were hypophysectomised at 50 days of age. One week later, hormonal therapy was commenced. All hypophysectomised rats received -thyroxine and cortisol. Adipose tissue, the heart, soleus and gastrocnemius muscles were excised after 1 week of hormonal therapy. The effect of insulin injections on adipose tissue and heart LPL activity was also studied. In separate experiments, LPL activity in post-heparin plasma was measured. Hypophysectomy had no effect on adipose tissue LPL activity, whereas activity was reduced in heart, soleus and gastrocnemius muscle tissues. GH treatment had no significant effect on LPL activity in adipose tissue or soleus muscle, but increased the LPL activity in heart and gastrocnemius muscle. GH treatment increased post-heparin plasma LPL activity. Recombinant human IGF-I treatment (1·25 mg/kg per day) markedly reduced LPL activity in adipose tissue, but had no effect in muscle tissues. The effect of IGF-I treatment on adipose tissue LPL was not reflected by a decrease in post-heparin plasma LPL activity. Daily injections of insulin for 7 days increased LPL activity in adipose tissue but had no effect on heart LPL activity. In adipose tissue, LPL mRNA levels tended to decrease as a result of IGF-I treatment. In the muscle tissues, no significant effects of hypophysectomy, GH or IGF-I treatment on LPL mRNA levels were observed.It is concluded that GH increases heart and skeletal muscle tissue LPL activity, which probably contributes to an increased post-heparin plasma LPL activity. The effect of GH on muscle LPL activity is probably not mediated by IGF-I or insulin. Insulin and IGF-I have opposite effects on LPL activity in adipose tissue.

Gestational and hormonal regulation of human placental lipoprotein lipase

“…Estrogen has a biphasic effect on LPL in adipose tissue, with high doses inhibiting and low doses stimulating LPL (78). IGF-1, cortisol, and TNF-a are known to have inhibitory effects on LPL in adipose tissue (49)(50)(51). In our experimental model, we found no change in LPL activity after 1 or 3 h of incubation with insulin alone, PPARg ligand, TNF-a, cortisol, epinephrine, or IGF-1.…”

Different modes of GH administration influence gene expression in the male rat brain

“…Recombinant bGH, donated from American Cyanamide Co. (Princeton, NJ, USA), was diluted in a 0.05 M phosphate buffer of pH 8.6 with 1.6% glycerol and 0.02% sodium azide and administered as a s.c. infusion (0.7 mg/kg per day) for 7 days through miniosmotic pumps (model 2004, Alzet, Cupertino, CA, USA) implanted subcutaneously in the neck (henceforth GHi) or as injections (henceforth GHx2, i.e. 0.35 mg/kg, twice daily, equaling a total of 0.7 mg/kg per 24 h) (Oscarsson et al 1999). All animals were weighed every 2 or 3 days to monitor the biological response in weight gain (Table 3).…”

Interaction between growth hormone and insulin in the regulation of lipoprotein metabolism in the rat

“…apolipoprotein B; apolipoprotein E; apoB mRNA editing; triglyceride secretion; sterol regulatory element-binding protein-1; fatty-acid synthase; stearoyl-CoA desaturase; liver; adipose tissue IT IS WELL KNOWN THAT GROWTH HORMONE (GH) has marked effects on lipid and lipoprotein metabolism (1, 30). GH also increases secretion (27, 32) and plasma levels of insulin in humans and rats (31,32,37,44). Moreover, GH increases DNA synthesis and proliferation of ␤-cells and insulin secretion in vitro (27), showing that GH enhances ␤-cell function independently of its insulin-antagonistic action (34,35,45).…”

“…Moreover, GH increases DNA synthesis and proliferation of ␤-cells and insulin secretion in vitro (27), showing that GH enhances ␤-cell function independently of its insulin-antagonistic action (34,35,45). The increased serum insulin levels and the insulin-antagonistic effect of GH may be of importance for several effects of GH in vivo, but few studies have addressed this question (31,36). Treatment of normal rats with the combination of insulin and GH results in an additive effect on body weight gain.…”