Growing evidence indicates that microRNAs (miRNAs or miRs) are involved in basic cell functions and oncogenesis. Here we report that miR-133 has a critical role in determining cardiomyocyte hypertrophy. We observed decreased expression of both miR-133 and miR-1, which belong to the same transcriptional unit, in mouse and human models of cardiac hypertrophy. In vitro overexpression of miR-133 or miR-1 inhibited cardiac hypertrophy. In contrast, suppression of miR-133 by 'decoy' sequences induced hypertrophy, which was more pronounced than that after stimulation with conventional inducers of hypertrophy. In vivo inhibition of miR-133 by a single infusion of an antagomir caused marked and sustained cardiac hypertrophy. We identified specific targets of miR-133: RhoA, a GDP-GTP exchange protein regulating cardiac hypertrophy; Cdc42, a signal transduction kinase implicated in hypertrophy; and Nelf-A/WHSC2, a nuclear factor involved in cardiogenesis. Our data show that miR-133, and possibly miR-1, are key regulators of cardiac hypertrophy, suggesting their therapeutic application in heart disease.

Valid and reliable experimental models are essential to gain insight into the cellular and molecular mechanisms underlying the beneficial effects of exercise in prevention, treatment, and rehabilitation of lifestyle-related diseases. Studies with large changes, low variation, and reproducible training outcome require individualized training intensity, controlled by direct measurements of maximal oxygen uptake or heart rate. As this approach is expensive and time consuming, we discuss whether maximal treadmill running speed in a gradually increasing ramp protocol might be sufficient to control intensity without losing accuracy. Combined data from six studies of rats and mice from our lab demonstrated a close correlation between running speed and oxygen uptake. This relationship changed towards a steeper linear slope after endurance training, indicating improved work economy, that is, less oxygen was consumed at fixed submaximal running speeds. Maximal oxygen uptake increased 40-70% after high-intensity aerobic interval training in mice and rats. The speed at which oxygen uptake reached a plateau, increased in parallel with the change in maximal oxygen uptake during the training period. Although this suggests that running speed can be used to assess training intensity throughout a training program, the problem is to determine the exact relative intensity related to maximal oxygen uptake from running speed alone. We therefore suggest that directly measured oxygen uptake should be used to assess exercise intensity and optimize endurance training in rats and mice. Running speed may serve as a supplement to ensure this intensity.

Rationale: In the present study we explored the mechanisms behind excitation-contraction (EC) coupling defects in cardiomyocytes from mice with type-2 diabetes (db/db 1 This has severe implications, because cardiovascular mortality is Ϸ2-to 4-fold higher in diabetic compared to nondiabetic patients 2 and accounts for Ϸ80% of the mortality in type 2 diabetes, 3 of which Ϸ50% die of sudden cardiac death. 4 Furthermore, diabetics are 2.5 times more likely to develop congestive heart failure compared to nondiabetics. 5 The db/db diabetic mouse model develops cardiomyopathy in a similar manner as type 2 diabetes in humans, 6 and presents with reduced whole-heart 7 and isolated cardiomyocyte 8 11,12 In contrast, exercise training in healthy mice increases the level of phosphorylated cytosolic CaMKII␦ and in so doing increases CaMKII␦ activity. Under these circumstances, increased phosphorylation of CaMKII␦ was associated with an increased cardiac performance. 13 Alongside increased SR Ca 2ϩ leak, reduced transverse (T)-tubule structure leading to less synchronous SR Ca 2ϩ release contributes further to the depressed EC coupling in models of cardiac dysfunction. 14 The mechanism for increased SR Ca 2ϩ leak, and whether T-tubule structure in diabetic cardiomyopathy is conserved, has currently not been studied.In the present study, we explored the mechanisms behind the impaired cardiomyocyte function and increased SR Ca 2ϩ leak in db/db cardiomyocytes, and then reexamined the same parameters in the db/db mice after an aerobic interval exercise training program. Because the activities of both CaMKII␦ and PKA are associated with both pathological and physiological remodeling, we also investigated the contributions of CaMKII␦ and PKA for the observed exercise training-induced changes. MethodsFor a detailed description, see the data supplement (available online at http://circres.ahajournals.org). Mouse Model of Diabetes and Exercise TrainingThe db/db mice model has been proven to be a suitable model to study the consequences of diabetes on the heart. Here we studied the male diabetic (BKS.Cg-m ϩ/ϩ Lepdb/Bom Tac; 20 exercised and 20 sedentary mice) and sedentary (nϭ23) and exercise trained (nϭ6) nondiabetic healthy heterozygote (BKS.Cg-m ϩ/ϩ Lepdb/ϩ lean); all age-matched (7 weeks at study start). To determine maximal oxygen uptake (VO 2max ), mice ran until exhaustion on a customized treadmill in a metabolic chamber, and high-intensity aerobic interval training was performed as uphill running, alternating between 4 minutes at 85% to 90% of VO 2max and 2 minutes at 50% of VO 2max for 80 minutes/day, 5 days/wk, for 13 weeks. 15 Cardiomyocyte Isolation and Ca 2؉ MeasurementsLeft ventricular myocytes were isolated as previously described. 15 Fura-2/AM-loaded cardiomyocytes were stimulated by bipolar electric pulses for Ca 2ϩ handling measurements including SR Ca 2ϩ leak. CaMKII inhibitor and PKA inhibitor were used to determine the influence of the 2 kinases. Contractility was recorded by video-based sarcomere spacing. ...