Yuk-Chow Ng

Effects of norepinephrine on expression of IGF-1/IGF-1R and SERCA2 in rat heart

OBJECTIVESnThe effects of norepinephrine on expression of cardiac genes during pathological cardiac growth and heart failure are not fully understood. Tissue insulin-like growth factor 1 (IGF-1) and its receptor (IGF-1R) play an important role in the regulation of the hyperplastic capacity of cardiac myocytes. Sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2), on the other hand, is important in regulating cardiac contractile function. The present study examined the effects of elevated levels of NE on expression of IGF-1/IGF-1R and SERCA2 mRNAs.nnnMETHODSnRats were infused with NE using osmotic minipumps for 3 and 6 days at a rate of 50 micrograms/kg/h and also at a higher dose (130 micrograms/kg/h) for 6 and 14 days. Levels of expression of IGF-1/IGF-1R and SERCA2 mRNAs were determined by ribonuclease protection assay and by Northern blotting, respectively.nnnRESULTSnNE treatment significantly increased IGF-1 mRNA levels in both left- and right-ventricle; however, levels of IGF-1R increased in the left- but not the right-ventricle. By contrast, NE infusion at both the lower dose and the higher dose failed to alter expression of SERCA2 mRNA.nnnCONCLUSIONnOur results suggest that NE treatment differentially regulates expression of IGF-1 and IGF-1R in the ventricles of rat heart and that NE appears not to affect expression of SERCA2 mRNA.

Fiber specific differential phosphorylation of the α1-subunit of the Na+,K+-ATPase in rat skeletal muscle: the effect of aging

In skeletal muscle, the Na+,K+-ATPase maintains the Na+ and K+ gradients and modulates contractile functions. The different fibers of the skeletal muscle possess diverse properties and functions, and thus, the demands for the Na-pump activity might be different. Because phosphorylation of the α1-subunit of the Na+,K+-ATPase appears to serve a regulatory role in the activity of Na+,K+-ATPase, we postulated that a difference in the phosphorylation of the α1-subunit may be found among the fibers. We utilized two well-characterized specific antibodies for the α1-subunit, namely the McK1 and α6F, to determine, by immunofluorescence, if the α1-subunit in rat skeletal muscle fiber is differentially phosphorylated. McK1 has the unique property that its binding to the α1-subunit is greatly reduced when Ser-18 is phosphorylated. Our data show that, in red gastrocnemius muscle, only a small number of the fibers were stained on the sarcolemmal membrane by McK1, while other fibers were almost completely devoid of any staining. By contrast, the staining pattern by McK1 in the white gastrocnemius muscle was mostly uniform. Immunostaining of serial sections using the α6F antibody showed that the α1-subunit is expressed in all fibers. Dephosphorylation of the tissue sections by phosphatase partially restored immunostaining of the α1-subunit by McK1. Fiber typing results showed that, in red gastrocnemius, those fibers stained positive for α1-subunit by McK1 are the Type I fibers, whereas those stained negative are the Type IIA, IID, and IIB fibers. With age, the number of fibers in red gastrocnemius stained positive for McK1 increased markedly in 30-month old rats compared to 6-month old rats. In conclusion, our result suggests that, in rats, the α1-subunit of the Na+,K+-ATPase is differentially phosphorylated in the fibers of the red gastrocnemius muscle. Furthermore, advanced age is associated with an apparent decrease in the phosphorylation of the α1-subunit, in addition to the previously demonstrated increase in the levels of expression of the subunit.

Endurance training is associated with reduced expression of the mRNA of the subunits of Na+ ,K+-ATPase but elevated muscle Na+,K+-ATPase content and maximal activity.

Clinical and experimental research provides evidence that thyroid hormone signalling is downregulated in pathological conditions, such as cardiac hypertrophy, heart failure and after myocardial infarction with potential physiological consequences. The expression and phosphorylation of several molecules related to cardiac contractility, metabolism and response to stress may be regulated by thyroid hormone, including calcium cycling proteins, myosin, and even heat shock proteins. Thus, thyroid hormone is thought of as a potential therapeutic option for treating heart disease and thyroid analogues are now tried in patients. The study by Minatoya et al. in this issue demonstrates that cardiac unloading induced by heterotopic heart transplantation in isogenic rats resulted in slower myocyte relaxation and [Ca]i decay as well as depressed myocyte shortening in response to high [Ca]o with impaired augmentation of peak systolic [Ca]i. These changes were accompanied by lower SERCA2a/PLB ratio and decreased phosphorylation of phospholamban (PLB) at Ser 16 in the unloaded myocardium. T3 at physiological doses normalized the SERCA2a/PLB ratio and the phosphorylated levels of PLB. Consequently, the slower relaxation, delayed [Ca]i decay and depressed contractile reserve in myocytes from unloaded hearts returned to normal. This new experimental evidence may be of clinical and therapeutic relevance. Chronic cardiac unloading is a frequent condition with the use of left ventricular assist devices in treating heart failure.