Exercise training elicits a number of adaptive changes in skeletal muscle that result in an improved metabolic efficiency. The molecular mechanisms mediating the cellular adaptations to exercise training in human skeletal muscle are unknown. To test the hypothesis that recovery from exercise is associated with transcriptional activation of specific genes, six untrained male subjects completed 60-90 min of exhaustive one-legged knee extensor exercise for five consecutive days. On day 5, nuclei were isolated from biopsies of the vastus lateralis muscle of the untrained and the trained leg before exercise and from the trained leg immediately after exercise and after 15 min, 1 h, 2 h, and 4 h of recovery. Transcriptional activity of the uncoupling protein 3 (UCP3), pyruvate dehydrogenase kinase 4 (PDK4), and heme oxygenase-1 (HO-1) genes (relative to beta-actin) increased by three- to sevenfold in response to exercise, peaking after 1-2 h of recovery. Increases in mRNA levels followed changes in transcription, peaking between 2 and 4 h after exercise. Lipoprotein lipase and carnitine pamitoyltransferase I gene transcription and mRNA levels showed similar but less dramatic induction patterns, with increases ranging from two- to threefold. In a separate study, a single 4-h bout of cycling exercise (n = 4) elicited from 5 to >20-fold increases in UCP3, PDK4, and HO-1 transcription, suggesting that activation of these genes may be related to the duration or intensity of exercise. These data demonstrate that exercise induces transient increases in transcription of metabolic genes in human skeletal muscle. Moreover, the findings suggest that the cumulative effects of transient increases in transcription during recovery from consecutive bouts of exercise may represent the underlying kinetic basis for the cellular adaptations associated with exercise training.

SUMMARY Myoglobin is a cytoplasmic hemoprotein, expressed solely in cardiac myocytes and oxidative skeletal muscle fibers, that reversibly binds O2 by its heme residue, a porphyrin ring:iron ion complex. Since the initial discovery of its structure over 40 years ago, wide-ranging work by many investigators has added importantly to our understanding of its function and regulation. Functionally, myoglobin is well accepted as an O2-storage protein in muscle, capable of releasing O2during periods of hypoxia or anoxia. Myoglobin is also thought to buffer intracellular O2 concentration when muscle activity increases and to facilitate intracellular O2 diffusion by providing a parallel path that augments simple diffusion of dissolved O2. The use of gene targeting and other molecular biological techniques has revealed important new insights into the developmental and environmental regulation of myoglobin and provided additional functions for this hemoprotein such as scavenging nitric oxide and reactive O2 species. These recent findings, coupled with additional emerging technologies and the discovery of other tissue globins, provide a framework for addressing new questions about myoglobin and readdressing old ones.

The norepinephrine transporter (NET) is a membrane protein responsible for termination of the action of synaptic norepinephrine and is a site of action of many drugs used to treat major depression. The present study determined whether the binding of [ 3 H]nisoxetine to the NET is altered in the locus coeruleus (LC) in major depression, using brain tissue collected postmortem from subjects diagnosed with major depression and from age-matched normal control subjects. Thirteen of the 15 major depressive subjects studied died by suicide. The distribution of [ 3 H]nisoxetine binding along the rostro-caudal axis of the nucleus was uneven and was paralleled by a similar uneven distribution of neuromelanin-containing cells in both major depressives and psychiatrically normal control subjects. The binding of [ 3 H]nisoxetine to NETs in the midcaudal portion of the LC from major depressive subjects was significantly lower than that from age-matched, normal control subjects. The binding of [ 3 H]nisoxetine to NETs in other regions of the LC was similar in major depressives and control subjects. In contrast to reductions in binding to NETs, there were no significant differences in the number of noradrenergic cells at any particular level of the LC between major depressives and normal control subjects. The decreased binding of [ 3 H]nisoxetine to NETs in the LC in major depression may reflect a compensatory downregulation of this transporter protein in response to an insufficient availability of its substrate (norepinephrine) at the synapse.

Myoglobin, an intracellular haemoprotein expressed in the heart and oxidative skeletal myofibres of vertebrates, binds molecular oxygen and may facilitate oxygen transport from erythrocytes to mitochondria, thereby maintaining cellular respiration during periods of high physiological demand. Here we show, however, that mice without myoglobin, generated by gene-knockout technology, are fertile and exhibit normal exercise capacity and a normal ventilatory response to low oxygen levels (hypoxia). Heart and soleus muscles from these animals are depigmented, but function normally in standard assays of muscle performance in vitro across a range of work conditions and oxygen availability. These data show that myoglobin is not required to meet the metabolic requirements of pregnancy or exercise in a terrestrial mammal, and raise new questions about oxygen transport and metabolic regulation in working muscles.

Static contraction of the hindlimb muscles of cats reflexly increases cardiovascular function, an effect that is potentiated by occlusion of the arterial supply to the working muscles. Although group III and IV afferents are known to be stimulated by and to cause the reflex cardiovascular responses to static muscular contraction, little is known about the responses of these afferents to static contraction when the arterial supply to a working muscle is occluded. We therefore recorded the impulse activity of 24 group III afferents and 30 group IV afferents with endings in the triceps surae while we statically contracted this muscle group, both when the abdominal aorta was occluded and when it was patent. A chi 2 analysis revealed that ischemia increased the responses to static contractions of a significantly higher percentage of group IV afferents than group III afferents (46.7% vs. 12.5%, respectively; P less than 0.02). In addition, two patterns of responses to ischemic contraction were observed. The first pattern was displayed by afferents (n = 10) that were stimulated by nonischemic contraction but were stimulated more by ischemic contraction. The second pattern was displayed by afferents (n = 7) that were not stimulated by nonischemic contraction but were stimulated by ischemic contraction. We conclude that afferents displaying both patterns are likely to contribute to the reflex cardiovascular responses to ischemic contraction.

Although both static and rhythmic twitch contractions of the hindlimb muscles of anaesthetised cats have been shown to reflexly evoke pressor responses, the increase in arterial pressure evoked by the former type of contraction has been shown to be substantially larger than that evoked by the latter. We have therefore recorded the impulse activity of single group III and IV muscle afferents, whose activation reflexly increases arterial pressure, while we both statically and rhythmically twitch-contracted the triceps surae muscles of anaesthetised cats. We found that group III afferents (n = 17) discharged significantly more impulses in response to static contraction than in response to rhythmic contraction. By contrast, group IV afferents (n = 18) fired approximately the same number of impulses in response to the two types of contraction. In addition, we found that many of the group III but only a few of the group IV afferents displayed discharge properties suggestive that these afferents were mechanoreceptors. We conclude that the discharge of group III afferents are likely to be responsible for the difference in the magnitudes of the reflex pressor responses evoked by static and rhythmic contraction.

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Continuous contractile activity of skeletal muscle elicits an early and dramatic increase in ribosomal RNA, suggesting that translational efficiency and/or capacity is enhanced during the adaptive response to increased metabolic demand. In view of the important role heat shock or stress proteins (HSPs) play as molecular chaperones during protein synthesis, we examined whether expression of the inducible 70-kDa HSP (HSP70) and/or mitochondrial 60-kDa HSP (HSP60) is altered in rabbit tibialis anterior muscle during continuous low-frequency motor nerve stimulation. Induction of the HSP70 gene was evident within 24 h after the onset of stimulation as reflected by increases in HSP70 transcription (> 20-fold) and mRNA (> 50-fold). HSP70 protein levels were significantly elevated (10- to 12-fold) after 14 and 21 days of stimulation. Mitochondrial HSP60 mRNA and protein also increased during stimulation (> 18- and > 5-fold after 21 days, respectively). In situ hybridization and immunohistochemistry coupled with myosin ATPase staining revealed that expression of HSP70 was restricted to oxidative type I and IIa fibers during the first 3 days of stimulation but shifted to primarily type II fibers after 21 days of stimulation. These findings demonstrate that induction of HSP70 during the adaptive response to chronic motor nerve stimulation proceeds from type I/IIa to type IId(x)/b fibers, suggesting that the expression of HSPs may be required to support the folding and compartmentalization of nascent proteins during the transformation process.