Genetic mechanisms that determine oxidative capacity of striated muscles. Control of gene transcription

Citrate synthase expression and enzyme activity after endurance training in cardiac and skeletal muscles

The present study was designed to examine the acute and chronic effects of endurance treadmill training on citrate synthase (CS) gene expression and enzymatic activity in rat skeletal and cardiac muscles. Adult rats were endurance trained for 8 wk on a treadmill. They were killed 1 h (T(1), n = 8) or 48 h (T(48), n = 8) after their last bout of exercise training. Eight rats were sedentary controls (C) during the training period. CS mRNA levels and enzymatic activities of the soleus and ventricle muscles were determined. Training resulted in higher CS mRNA levels in both the soleus muscles (21% increase in T(1); 18% increase in T(48), P < 0.05) and ventricle muscles (23% increase in T(1); 17% increase in T(48), P < 0.05) when compared with the C group. The CS enzyme activities were 42 (P < 0.01) and 25% (P < 0.01) greater in the soleus muscles of T(1) and T(48) groups, respectively, when compared with that of the C group. Soleus CS enzyme activity was significantly greater in the T(1) vs. T(48) groups (P < 0.05). However, no appreciable alterations in CS enzyme activities were observed in the ventricle muscles in both training groups. These findings suggest differential responses of skeletal and cardiac muscles in CS enzymatic activity but similar responses in CS gene expression at 1 and 48 h after the last session of endurance training. Moreover, our data support the existence of an acute effect of exercise on the training-induced elevation in CS activity in rat soleus but not ventricle muscles.

Invited Review: contractile activity-induced mitochondrial biogenesis in skeletal muscle

Chronic contractile activity produces mitochondrial biogenesis in muscle. This adaptation results in a significant shift in adenine nucleotide metabolism, with attendant improvements in fatigue resistance. The vast majority of mitochondrial proteins are derived from the nuclear genome, necessitating the transcription of genes, the translation of mRNA into protein, the targeting of the protein to a mitochondrial compartment via the import machinery, and the assembly of multisubunit enzyme complexes in the respiratory chain or matrix. Putative signals involved in initiating this pathway of gene expression in response to contractile activity likely arise from combinations of accelerations in ATP turnover or imbalances between mitochondrial ATP synthesis and cellular ATP demand, and Ca(2+) fluxes. These rapid events are followed by the activation of exercise-responsive kinases, which phosphorylate proteins such as transcription factors, which subsequently bind to upstream regulatory regions in DNA, to alter transcription rates. Contractile activity increases the mRNA levels of nuclear-encoded proteins such as cytochrome c and mitochondrial transcription factor A (Tfam) and mRNA levels of upstream transcription factors like c-jun and nuclear respiratory factor-1 (NRF-1). mRNA level changes are often most evident during the postexercise recovery period, and they can occur as a result of contractile activity-induced increases in transcription or mRNA stability. Tfam is imported into mitochondria and controls the expression of mitochondrial DNA (mtDNA). mtDNA contributes only 13 protein products to the respiratory chain, but they are vital for electron transport and ATP synthesis. Contractile activity increases Tfam expression and accelerates its import into mitochondria, resulting in increased mtDNA transcription and replication. The result of this coordinated expression of the nuclear and the mitochondrial genomes, along with poorly understood changes in phospholipid synthesis, is an expansion of the muscle mitochondrial reticulum. Further understanding of 1) regulation of mtDNA expression, 2) upstream activators of NRF-1 and other transcription factors, 3) the identity of mRNA stabilizing proteins, and 4) potential of contractile activity-induced changes in apoptotic signals are warranted.

1H nuclear magnetic resonance studies of sarcoplasmic oxygenation in the red cell-perfused rat heart

The proximal histidine N delta H proton of deoxymyoglobin experiences a large hyperfine shift resulting in its 1H nuclear magnetic resonance (NMR) signal appearing at approximately 76 ppm (at 35 degrees C), downfield of the diamagnetic spectral region. 1H NMR of this proton is used to monitor sarcoplasmic oxygen pressure in isolated perfused rat heart. This method monitors intracellular oxygenation in the whole heart and does not reflect oxygenation in a limited region. The deoxymyoglobin resonance intensity is reduced upon conversion of myoglobin to the ferric form by sodium nitrite. 1H resonances of the N delta H protons of the alpha and beta subunits of bovine deoxyhemoglobin do not interfere with the measurement of myoglobin deoxygenation in blood-perfused rat heart. We find that steady-state myoglobin deoxygenation is increased progressively (and reversibly) as oxygenation of the perfusing medium is decreased in both saline and red blood cell-perfused hearts at constant work output. An eightfold increase in the heart rate of the blood-perfused heart resulted in no change in the deoxymyoglobin signal intensity. Intracellular PO2 of myoglobin-containing cells is maintained remarkably constant in changing work states.

Mitochondrial biogenesis in striated muscle

Mitochondrial biogenesis (synthesis) has been observed to occur in skeletal muscle in response to chronic use. It also occurs in cardiac muscle during growth and hypertrophy, and it may be impaired during the aging process. This review summarizes the literature on the processes of mitochondrial biogenesis at the biochemical and molecular levels, with particular reference to striated muscles. Mitochondrial biogenesis involves the expression of nuclear and mitochondrial genes and the coordination of these two genomes, the synthesis of proteins and phospholipids and their import into the organelle, and the incorporation of these lipids and proteins into their appropriate locations within the matrix, inner or outer membranes. The emphasis is on the regulation of these events, with information derived in part from other cellular systems. Although descriptions of mitochondrial content changes in heart and skeletal muscle during altered physiological states are plentiful, much work is needed at the molecular level to investigate the regulatory processes involved. A knowledge of biochemical and molecular biology techniques is essential for continued progress in the field. This is a promising area, and potential new avenues for future research are suggested.

Radiofrequency catheter ablation for AV nodal reentry: a technique for rapid transection of the slow AV nodal pathway

Selective radiofrequency (RF) catheter ablation of the slow AV nodal pathway has shed new light on the anatomy and physiology of the atrioventricular junction. The recording of "slow pathway potentials" facilitates localization of the slow pathway and has led to a concept of multiple pathway components with atrial insertion sites covering a potentially broad region surrounding the coronary sinus os. The critical area for complete interruption of the slow pathway may be larger than lesion size produced by ablation at a single site, resulting in multiple RF applications with lengthy sessions and prolonged radiation exposure. Information from both old and recent literature suggests that the slow AV nodal pathway is represented by a group of fibers originating from the posteroinferior interatrial septum and coursing anterosuperiorly near the tricuspid annulus before converging upon the compact AV node. Based on this anatomical arrangement, the present study was conducted to evaluate a technique designed to transect the slow pathway by producing a linear RF lesion perpendicular to the orientation of the slow pathway within the mid-portion of Koch's triangle. Using this technique, 30 of 30 patients with common AV nodal reentry were rendered noninducible using 1 to 3 RF applications. Total procedure time averaged 3.4 +/- 1.1 hours and fluoroscopy time averaged 14.8 +/- 4.6 minutes. As a marker of efficacy, episodic nonsustained atrial tachycardia (NSAT) during RF delivery occurred in 28 of 30 (93%) successful applications. Three patients experienced tachycardia recurrence and were successfully ablated by repeat procedure. Conduction characteristics and refractoriness of the fast pathway were unchanged in 23 of 23 patients reevaluated at a mean of 7.2 weeks postablation.(ABSTRACT TRUNCATED AT 250 WORDS)