Differential regulation of IGF-I, its receptor and GH receptor mRNAs in the right ventricle and caval vein in volume-loaded genetically hypertensive and normotensive rats

Cardiac changes induced by excess exogenous growth hormone in juvenile miniature poodles

The transient elevated plasma growth hormone (GH) levels that occur at a young age in giant breed dogs may have consequences in adult life. The aim of this study was to investigate whether excess juvenile GH has consequences for cardiac function and morphology. To simulate the naturally occurring juvenile hypersomatotropism in giant breed dogs, elevated plasma GH and insulin-like growth factor-I (IGF-I) concentrations were induced in six miniature poodles (GH dogs) by daily administration of supraphysiological doses of GH starting at 12 weeks of age. Eight miniature poodles of the same age that received vehicle only served as controls. Cardiac anatomy and function were evaluated by echocardiography. After euthanasia at 21 weeks of age, the hearts were examined for weight, myocyte dimensions and collagen fraction. The hearts of the GH dogs had larger atria (+22%), a thicker left ventricular wall (+21%), greater weight (+84%), and their cardiomyocytes were 15% longer, 25% thicker, and 92% greater in volume than those of control dogs. The mean collagen fraction was also higher in the GH dogs (5.6%) than in the controls (3.1%). In conclusion, excess GH in juvenile miniature poodles resulted in myocardial hypertrophy and increased collagen content. These findings are consistent with observations in acromegalic human patients and in rats treated with GH.

Cardiac resistance to growth hormone in uremia

BACKGROUND

Cardiovascular disease is a major cause of death in end-stage renal disease (ESRD). Since growth hormone is required for maintaining normal cardiac structure and function and as growth hormone has a salutary effect on cardiac remodeling in disease, we postulated that if cardiac resistance to growth hormone develops in chronic renal failure (CRF) this may predispose to the cardiomyopathy of uremia. We set out to test whether in CRF there is resistance to the cardiac action of growth hormone and whether this defect might be caused by altered growth hormone signaling.

METHODS

Growth hormone-deficient (dw/dw) rats and growth hormone-intact Sprague-Dawley rats underwent a subtotal nephrectomy or sham operation and pair feeding.

RESULTS

In dw/dw rats treated with growth hormone for 8 days there was a significant increase in insulin-like growth factor-1 (IGF-1) mRNA levels in controls but this response was attenuated in CRF. Next, growth hormone-stimulated Janus kinase-signal transducers and activators of transcription (JAK2-STAT5) signaling was studied 15 minutes after intravenous growth hormone in dw/dw and Sprague-Dawley rats. Growth hormone receptor, JAK2, STAT5a, and STAT5b protein levels were unaltered in CRF. Growth hormone-induced JAK2, growth hormone receptor (GHR), and STAT5 tyrosine phosphorylation was significantly depressed in CRF as was nuclear translocation of phosphorylated STAT5. When rats were treated with pharmacologic dose growth hormone, STAT5 phosphorylation increased similarly in CRF and control rats.

CONCLUSION

Uremic rats develop cardiac resistance to growth hormone caused at least, in part, by a postreceptor defect in growth hormone-induced signaling that is characterized by impaired phosphorylation and nuclear translocation of STAT5. These findings raise the question whether growth hormone resistance contributes to the cardiac changes of uremia.

Short- and long-term swimming exercise training increases myocardial insulin-like growth factor-I gene expression

OBJECTIVES. We investigated the effect of short- and long-term swimming exercise, with or without insulin-like growth factor (IGF)-I administration, on the expression of myocardial IGFs and contractile proteins.

METHODS

Sprague-Dawley male rats (n=36) were subjected to swimming exercise for 2 or 6 weeks. IGF-I (0.5mg/rat) was administered continuously for 1 week, using alzet osmotic pumps. Control groups remained sedentary. IGF-I, IGF-I receptor (IGF-IR), IGF-II, skeletal alpha-actin (sk-actin), and beta myosin heavy chain (beta MHC) mRNAs were measured using Northern blot analysis and RT-PCR.

RESULTS

A significant 2-fold increase in myocardial IGF-I mRNA was found after 2 and 6 weeks of swimming in both IGF-I treated and untreated rats (p<0.001). IGF-IR mRNA was significantly (p<0.05) increased after 6 weeks of training only in the IGF-I treated animals. IGF-II mRNA remained unchanged at all time points. While beta MHC mRNA was significantly decreased (p=0.003) at 2 and 6 weeks, sk-actin mRNA remained unchanged.

CONCLUSIONS

Short- and long-term swimming exercise training increase myocardial expression of IGF-I mRNA. Exogenous administration of IGF-I, during the first week of the exercise session, did not produce any effect on myocardial IGF-I but was associated with increased IGF-IR signal after the long-term exercise training. These data suggest a relationship between IGF-I expression and cardiac adaptation to exercise training.