In humans, the strong statistical association between fitness and survival suggests a link between impaired oxygen metabolism and disease. We hypothesized that artificial selection of rats based on low and high intrinsic exercise capacity would yield models that also contrast for disease risk. After 11 generations, rats with low aerobic capacity scored high on cardiovascular risk factors that constitute the metabolic syndrome. The decrease in aerobic capacity was associated with decreases in the amounts of transcription factors required for mitochondrial biogenesis and in the amounts of oxidative enzymes in skeletal muscle. Impairment of mitochondrial function may link reduced fitness to cardiovascular and metabolic disease.

Cachexia is a syndrome characterized by wasting of skeletal muscle and contributes to nearly one-third of all cancer deaths. Cytokines and tumor factors mediate wasting by suppressing muscle gene products, but exactly which products are targeted by these cachectic factors is not well understood. Because of their functional relevance to muscle architecture, such targets are presumed to represent myofibrillar proteins, but whether these proteins are regulated in a general or a selective manner is also unclear. Here we demonstrate, using in vitro and in vivo models of muscle wasting, that cachectic factors are remarkably selective in targeting myosin heavy chain. In myotubes and mouse muscles, TNF-α plus IFN-γ strongly reduced myosin expression through an RNA-dependent mechanism. Likewise, colon-26 tumors in mice caused the selective reduction of this myofibrillar protein, and this reduction correlated with wasting. Under these conditions, however, loss of myosin was associated with the ubiquitin-dependent proteasome pathway, which suggests that mechanisms used to regulate the expression of muscle proteins may be cachectic factor specific. These results shed new light on cancer cachexia by revealing that wasting does not result from a general downregulation of muscle proteins but rather is highly selective as to which proteins are targeted during the wasting state.

Cachexia is a syndrome characterized by wasting of skeletal muscle and contributes to nearly one-third of all cancer deaths. Cytokines and tumor factors mediate wasting by suppressing muscle gene products, but exactly which products are targeted by these cachectic factors is not well understood. Because of their functional relevance to muscle architecture, such targets are presumed to represent myofibrillar proteins, but whether these proteins are regulated in a general or a selective manner is also unclear. Here we demonstrate, using in vitro and in vivo models of muscle wasting, that cachectic factors are remarkably selective in targeting myosin heavy chain. In myotubes and mouse muscles, TNF-α plus IFN-γ strongly reduced myosin expression through an RNA-dependent mechanism. Likewise, colon-26 tumors in mice caused the selective reduction of this myofibrillar protein, and this reduction correlated with wasting. Under these conditions, however, loss of myosin was associated with the ubiquitin-dependent proteasome pathway, which suggests that mechanisms used to regulate the expression of muscle proteins may be cachectic factor specific. These results shed new light on cancer cachexia by revealing that wasting does not result from a general downregulation of muscle proteins but rather is highly selective as to which proteins are targeted during the wasting state.

To test for a role of the calcineurin-NFAT (nuclear factor of activated T cells) pathway in the regulation of fiber type-specific gene expression, slow and fast muscle-specific promoters were examined in C2C12 myotubes and in slow and fast muscle in the presence of calcineurin or NFAT2 expression plasmids. Overexpression of active calcineurin in myotubes induced both fast and slow muscle-specific promoters but not non-muscle-specific reporters. Overexpression of NFAT2 in myotubes did not activate muscle-specific promoters, although it strongly activated an NFAT reporter. Thus overexpression of active calcineurin activates transcription of muscle-specific promoters in vitro but likely not via the NFAT2 transcription factor. Slow myosin light chain 2 (MLC2) and fast sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA1) reporter genes injected into rat soleus (slow) and extensor digitorum longus (EDL) (fast) muscles were not activated by coinjection of activated calcineurin or NFAT2 expression plasmids. However, an NFAT reporter was strongly activated by overexpression of NFAT2 in both muscle types. Calcineurin and NFAT protein expression and binding activity to NFAT oligonucleotides were different in slow vs. fast muscle. Taken together, these results indicate that neither calcineurin nor NFAT appear to have dominant roles in the induction and/or maintenance of slow or fast fiber type in adult skeletal muscle. Furthermore, different pathways may be involved in muscle-specific gene expression in vitro vs. in vivo.

of Ͻ6°C or Ͼ29°C have been shown to induce large changes in arterial blood pressure and heart rate in homeotherms. The present study was designed to investigate whether small incremental changes in T a, such as those found in typical laboratory settings, would have an impact on blood pressure and other cardiovascular parameters in mice and rats. We predicted that small decreases in T a would impact the cardiovascular parameters of mice more than rats due to the increased thermogenic demands resulting from a greater surface area-to-volume ratio in mice relative to rats. Cardiovascular parameters were measured with radiotelemetry in mice and rats that were housed in temperaturecontrolled environments. The animals were exposed to different T a every 72 h, beginning at 30°C and incrementally decreasing by 4°C at each time interval to 18°C and then incrementally increasing back up to 30°C. As T a decreased, mean blood pressure, heart rate, and pulse pressure increased significantly for both mice (1.6 mmHg/°C, 14.4 beats ⅐ min Ϫ1 ⅐°C Ϫ1 , and 0.8 mmHg/°C, respectively) and rats (1.2 mmHg/°C, 8.1 beats ⅐ min Ϫ1 ⅐°C Ϫ1 , and 0.8 mmHg/°C, respectively). Thus small changes in T a significantly impact the cardiovascular parameters of both rats and mice, with mice demonstrating a greater sensitivity to these Ta changes. blood pressure; heart rate; standard deviation of the interbeat interval; radiotelemetry THE EXTERNAL ENVIRONMENT can have a substantial impact on the cardiovascular system. Exposure of humans (1,7,18,32), rats (2,3,14,20), and mice (22) to cold ambient temperature (T a ) results in elevated blood pressure and heart rate. It appears as if the tachycardia and hypertension are the indirect result of sympathetic nervous system (SNS) activation of thermoregulatory mechanisms because elevated plasma norepinephrine (NE) levels correlate with elevated blood pressure in the cold (7,14,17,32). Furthermore, propranolol, a ␤-adrenergic blocking agent, can blunt or completely reverse the cardiovascular effects of cold exposure (21). Cold-induced activation of the SNS, in turn, appears to elevate blood pressure through activation of the renin-angiotensin system (RAS). Blockade of the RAS systemically (19), centrally (15,16,22), or genetically in an angiotensinogen knockout mouse model (23) blunts or prevents cold-induced hypertension, suggesting that RAS signaling pathways are necessary for T a -induced effects on blood pressure.Elevation of T a beyond typical housing temperatures also impacts the cardiovascular system. Warming rats (31) and mice (30, 31) from 23°C to 28 -31°C, closer to, if not within, their thermoneutral zone (TNZ) results in a drop in heart rate and blood pressure. Metabolic rate in homeotherms is at its minimum in the TNZ, which is approximately 28 -31°C for rodents (4). Animals with smaller body sizes have higher surface area-to-volume ratios and thus typically exhibit warmer TNZs. At temperatures below the TNZ, nonshivering thermogenesis is stimulated by increased sympathetic activity to offset the...

Swoap SJ, Li C, Wess J, Parsons AD, Williams TD, Overton JM. Vagal tone dominates autonomic control of mouse heart rate at thermoneutrality. Am J Physiol Heart Circ Physiol 294: H1581-H1588, 2008. First published February 1, 2008 doi:10.1152/ajpheart.01000.2007.-It is generally accepted that cardiac sympathetic tone dominates the control of heart rate (HR) in mice. However, we have recently challenged this notion given that HR in the mouse is responsive to ambient temperature (Ta) and that the housing T a is typically 21-23°C, well below the thermoneutral zone (ϳ30°C) of this species. To specifically test the hypothesis that cardiac sympathetic tone is the primary mediator of HR control in the mouse, we first examined the metabolic and cardiovascular responses to rapid changes in Ta to demonstrate the sensitivity of the mouse cardiovascular system to Ta. We then determined HR in 1) mice deficient in cardiac sympathetic tone ("␤-less" mice), 2) mice deficient in cardiac vagal tone [muscarinic M2 receptor (M2R Ϫ/Ϫ ) mice], and 3) littermate controls. At a Ta of 30°C, the HR of ␤-less mice was identical to that of wild-type mice (351 Ϯ 11 and 363 Ϯ 10 beats/min, respectively). However, the HR of M2R Ϫ/Ϫ mice was significantly greater (416 Ϯ 7 beats/min), demonstrating that vagal tone predominates over HR control at this Ta. When these mice were calorically restricted to 70% of normal intake, HR fell equally in wild-type, ␤-less, and M2R Ϫ/Ϫ mice (⌬HR ϭ 73 Ϯ 9, 76 Ϯ 3, and 73 Ϯ 7 beats/min, respectively), suggesting that the fall in intrinsic HR governs bradycardia of calorically restricted mice. Only when the Ta was relatively cool, at 23°C, did ␤-less mice exhibit a HR (442 Ϯ 14 beats/min) that was different from that of littermate controls (604 Ϯ 10 beats/min) and M2R Ϫ/Ϫ mice (602 Ϯ 5 beats/min). These experiments conclusively demonstrate that in the absence of cold stress, regulation of vagal tone and modulation of intrinsic rate are important determinants of HR control in the mouse. telemetry; intrinsic heart rate; cold stress; caloric restriction; sympathetic nervous system THE FREQUENCY OF CARDIAC CONTRACTION in mammals is achieved through modulation of heart rate (HR) around its intrinsic rate (IHR). HR is slowed by parasympathetic nervous system activity via the muscarinic M 2 receptor (M 2 R) (17) and elevated by sympathetic nervous system activity via the ␤ 1 -adrenergic receptor (39). Blockade of these autonomic inputs indicates that murine IHR is typically about 500 beats/min, well below the generally reported resting HR of about 600 beats/min [reviewed in Ref. 27, supporting the concept that cardiac sympathetic tone predominates at rest in the mouse (19,27)]. However, some recent studies indicate that resting mouse HR is lower than 500 beats/min at rest, calling into question the sympathovagal balance controlling HR in mice (6,11,35).Our laboratories (48, 54 -56) and others (8, 52) have demonstrated that mouse HR is markedly reduced when experiments are conducted in thermoneutral conditions [ambient ...

The myosin heavy chain (MHC) IIB gene is selectively expressed in skeletal muscles, imparting fast contractile kinetics. Why the MHC IIB gene product is expressed in muscles like the tibialis anterior (TA) and not expressed in muscles like the soleus is currently unclear. It is shown here that the mutation of an E-box within the MHC IIB promoter decreased reporter gene activity in the fast-twitch TA muscle 90-fold as compared with the wild-type promoter. Reporter gene expression within the TA required this E-box for activation of a heterologous construct containing upstream regulatory regions of the MHC IIB promoter linked to the basal 70-kDa heat shock protein TATA promoter. Electrophoretic mobility shift assays demonstrated that mutation of the E-box prevented the binding of both MyoD and myogenin to this element. In cotransfected C2C12myotubes and Hep G2 cells, MyoD preferentially activated the MHC IIB promoter in an E-box-dependent manner, whereas myogenin activated the MHC IIB promoter to a lesser extent, and in an E-box-independent manner. A time course analysis of hindlimb suspension demonstrated that the unweighted soleus muscle activated expression of MyoD mRNA before the de novo expression of MHC IIB mRNA. These data suggest a possible causative role for MyoD in the observed upregulation of MHC IIB in the unweighted soleus muscle.

The laboratory mouse is a facultative daily heterotherm in that it experiences bouts of torpor under caloric restriction. Mice are the most frequently studied laboratory mammal, and often, genetically modified mice are used to investigate many physiological functions related to weight loss and caloric intake. As such, research documenting the cardiovascular changes during fasting-induced torpor in mice is warranted. In the current study, C57BL/6 mice were implanted either with EKG/temperature telemeters or blood pressure telemeters. Upon fasting and exposure to an ambient temperature (T(a)) of 19 degrees C, mice entered torpor bouts as assessed by core body temperature (T(b)). Core T(b) fell from 36.6 +/- 0.2 degrees C to a minimum of 25.9 +/- 0.9 degrees C during the fast, with a concomitant fall in heart rate from 607 +/- 12 beats per minute (bpm) to a minimum of 158 +/- 20 bpm. Below a core T(b) of 31 degrees C, heart rate fell exponentially with T(b), and the Q(10) was 2.61 +/- 0.18. Further, mice implanted with blood pressure telemeters exhibited similar heart rate and activity profiles as those implanted with EKG/temperature telemeters, and the fall in heart rate and core T(b) during entrance into torpor was paralleled by a fall in blood pressure. The minimum systolic, mean, and diastolic blood pressures of torpid mice were 62.3 +/- 10.2, 51.9 +/- 9.2, 41.0 +/- 7.5 mmHg, respectively. Torpid mice had a significantly lower heart rate (25-35%) than when euthermic at mean arterial pressures from 75 to 100 mmHg, suggesting that total peripheral resistance is elevated during torpor. These data provide new and significant insight into the cardiovascular adjustments that occur in torpid mice.