Aerobic training prevents dexamethasone-induced peripheral insulin resistance

This study investigated how proteins of the insulin signaling cascade could modulate insulin resistance after dexamethasone (Dexa) treatment and aerobic training. Rats were distributed into 4 groups: sedentary control (SC), sedentary+Dexa (SD), trained control (TC), and trained+Dexa (TD), and underwent aerobic training for 70 days or remained sedentary. Dexa was administered during the last 10 days (1 mg · kg(-1) per day i. p.). After 70 days, an intraperitoneal glucose tolerance test (ipGTT) was performed. Protein levels of IRS-1, AKT, and PKC-α in the tibialis anterior (TA) muscle were identified using Western blots. Dexa treatment increased blood glucose and the area under the curve (AUC) of ipGTT. Training attenuated the hyperglycemia and the AUC induced by Dexa. Dexa reduced IRS-1 (- 16%) and AKT (- 43%) protein level with no changes in PKC-α levels. Moreover, these effects on IRS-1 and AKT protein level were prevented in trained animals. These results show for the first time that aerobic exercise prevented reductions of IRS-1 and AKT level induced by Dexa in the TA muscle, suggesting that aerobic exercise is a good strategy to prevent Dexa-induced peripheral insulin resistance.

Angiotensin II and VEGF are involved in angiogenesis induced by short-term exercise training

Results from our laboratory have suggested a pathway involving angiotensin II type 1 (AT(1)) receptors and vascular endothelial growth factor (VEGF) in angiogenesis induced by electrical stimulation. The present study investigated if similar mechanisms underlie the angiogenesis induced by short-term exercise training. Seven days before training and throughout the training period, male Sprague-Dawley rats received either captopril or losartan in their drinking water. Rats underwent a 3-day treadmill training protocol. The tibialis anterior and gastrocnemius muscles were harvested under anesthesia and lightly fixed in formalin (vessel density) or frozen in liquid nitrogen (VEGF expression). In controls, treadmill training resulted in a significant increase in vessel density in all muscles studied. However, the angiogenesis induced by exercise was completely blocked by either losartan or captopril. Western blot analysis showed that VEGF expression was increased in the exercised control group, and both losartan and captopril blocked this increase. The role of VEGF was directly confirmed using a VEGF-neutralizing antibody. These results confirm the role of angiotensin II and VEGF in angiogenesis induced by exercise.

Exercise training causes skeletal muscle venular growth and alters hemodynamic responses in spontaneously hypertensive rats

OBJECTIVE

To investigate whether training changes skeletal muscle venular profile and hemodynamic responses to exercise we studied spontanesouly hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats submitted to training programme (T = 50-60% of VO2max).

DESIGN

Training (T) was performed on a treadmill over a period of 13 weeks. Age-matched control groups were kept sedentary (S). T and S rats were chronically instrumented for hindlimb flow (HLF) and arterial pressure (AP) measurements at rest, during dynamic exercise and recovery in two different situations: control and after extensive intravenous blockade (hexamethonium + losartan + Nomega-nitro-L-arginine methyl ester + hydralazine). For morphometric analysis, skeletal muscle samples (gracilis) were obtained after transcardiac perfusion with fixative.

RESULTS

T caused a significant reduction of resting mean arterial pressure (MAP) (-11%) only in the SHR group without changing basal HLF. In the sedentary SHR (SHRs), basal relative hindlimb resistance was increased by 45%, but was significantly reduced after T (P < 0.05). During dynamic exercise, MAP increased similarly (10-20 mmHg) in all groups. HLF increases were similar for the four groups up to 0.8 km/h; at higher workloads, HLF was higher in trained SHR (SHRT) versus trained WKY (WKYT) (3.9- versus 2.9-fold increase over basal HLF, respectively). After blockade (and pressure correction with IV phenylephrine infusion), steady-state exercise was performed with similar hindlimb vasodilation in all groups and was accompanied by MAP reduction (-17 +/- 8 mmHg) only in SHRT group. Skeletal muscle venular profile (density, diameter and lumen cross-sectional area) was similar in WKY(T), WKY(S) and SHR(S), but significantly increased in SHR(T). In this group the two-fold increase in venule density was correlated with both the reduction in baseline MAP and the increase in HLF during dynamic exercise.

CONCLUSIONS

The results suggest that increased venule density is a specific adaptation of SHR skeletal muscle to training. Venular growth may contribute to both the pressure-lowering effect and the large HLF at high exercise intensities observed in the trained SHR.

Renin gene transfer restores angiogenesis and vascular endothelial growth factor expression in Dahl S rats

In a previous study, we demonstrated that Dahl S rats (SS group) have low plasma renin activity, whereas transfer of a region of chromosome 13 containing the renin gene from Dahl R onto a congenic strain of Dahl SS/Jr/Hsd/MCW rats (S/ren(RR) group) restores renin secretory responses. In the present study, we compared the angiogenic responses to electrical stimulation in the SS and S/ren(RR) groups to explore the hypotheses that the renin-angiotensin system is involved in vascular endothelial growth factor (VEGF) expression and angiogenesis in skeletal muscle. Congenic SS and S/ren(RR) rats fed a 0.4% or 4% salt diet were surgically prepared by chronic implantation of an electrical stimulator. Another group of S/ren(RR) rats was treated with lisinopril 2 days before the surgery and throughout the stimulation protocol. The right tibialis anterior (TA) and extensor digitorum longus (EDL) were stimulated for 8 hours per day for 7 days. The contralateral muscles served as controls. Western blot analysis was performed to identify VEGF protein expression in these muscles. Electrical stimulation produced no change in vessel density of the SS group fed a 0.4% salt diet (change 5.50% and 8.14% for EDL and TA, respectively). Transfer of a region containing the renin gene restored the angiogenic response (change 16% and 30% for EDL and TA, respectively) despite a significantly higher blood pressure. Blockade of the renin-angiotensin system by lisinopril or high salt restored the responses observed in the SS group fed a low salt diet. In addition, increases in VEGF expression to electrical stimulation were observed only in the S/ren(RR) group fed a low salt diet. These results suggest that renin gene transfer restores angiogenesis and VEGF expression in the skeletal muscle of Dahl S rats.

Angiogenesis induced by electrical stimulation is mediated by angiotensin II and VEGF

OBJECTIVE

Physiological angiogenesis in skeletal muscle is an adaptive response to physical training and electrical stimulation. This study investigated the role of angiotensin II (Ang II) in regulating both angiogenesis and vascular endothelial growth factor (VEGF) protein expression induced by electrical stimulation.

METHODS

The right tibialis anterior (TA) and extensor digitorum longus (EDL) muscles of Sprague-Dawley rats were stimulated for 8 hours per day for 7 days. The contralateral muscles served as controls. Two days before the surgery and throughout the stimulation protocol, the rats received either lisinopril or losartan in their drinking water. Rats without any drug treatment were used as control. Immunohistochemistry and Western blot analysis were performed to identify the source and quantify the VEGF protein expression in these muscles. The relationship between angiogenesis and VEGF expression was explored using a VEGF-neutralizing antibody.

RESULTS

Chronic electrical stimulation of the skeletal muscles led to significant increases in vessel density (14% and 30% for EDL and TA, respectively) within 7 days. In addition, stimulation increased VEGF protein levels in the stimulated muscles. Both lisinopril and losartan blocked elevation in VEGF expression and inhibited the angiogenesis induced by stimulation. VEGF neutralization also inhibited angiogenesis, confirming the relationship between Ang II, VEGF, and vessel growth.

CONCLUSION

The current study suggests a pathway involving angiotensin II receptors (AT1) and VEGF in electrically stimulated angiogenesis.

Exercise training normalizes wall-to-lumen ratio of the gracilis muscle arterioles and reduces pressure in spontaneously hypertensive rats

OBJECTIVE

To investigate mechanisms underlying the training-induced blood pressure-lowering effect we analyzed the hemodynamic responses and morphometric changes of the skeletal muscle microcirculation of spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats during an exercise training program. DESIGN TRAINING: (50-60% VO2 max) was performed on a treadmill for 13 weeks and control groups were kept sedentary over the same period of time. Trained and sedentary rats were chronically instrumented for hindlimb flow and arterial pressure (AP) recordings under conscious unrestrained conditions. Gracilis and myocardial muscle samples were obtained for morphometric analysis after transcardiac perfusion of fixative.

RESULTS

SHR, when compared to WKY presented an elevated blood pressure, an increased relative hindlimb vascular resistance, capillary rarefaction in both gracilis and myocardium and an increased wall-to-lumen ratio of gracilis arterioles. Training increased significantly both capillary density and capillary/fiber ratio in the gracilis and myocardium of WKY and SHR groups, causing a complete reversal of capillary rarefaction in trained SHR. In SHR, training also reduced resting blood pressure and caused normalization of both relative hindlimb vascular resistance and gracilis arterioles wall-to-lumen ratio. Regression analysis revealed strong positive correlation between hindlimb vascular resistance and mean AP (MAP) and between arterioles wall-to-lumen ratio and MAP.

CONCLUSIONS

The results suggest that low-intensity training can significantly reduce pressure in SHR while normalizing both the arteriole morphology and the resistance of the skeletal muscle microcirculation.

Validation of transit-time flowmetry for chronic measurements of regional blood flow in resting and exercising rats

The objective of the present study was to validate the transit-time technique for long-term measurements of iliac and renal blood flow in rats. Flow measured with ultrasonic probes was confirmed ex vivo using excised arteries perfused at varying flow rates. An implanted 1-mm probe reproduced with accuracy different patterns of flow relative to pressure in freely moving rats and accurately quantitated the resting iliac flow value (on average 10.43 +/- 0.99 ml/min or 2.78 +/- 0.3 ml min-1 100 g body weight-1). The measurements were stable over an experimental period of one week but were affected by probe size (resting flows were underestimated by 57% with a 2-mm probe when compared with a 1-mm probe) and by anesthesia (in the same rats, iliac flow was reduced by 50-60% when compared to the conscious state). Instantaneous changes of iliac and renal flow during exercise and recovery were accurately measured by the transit-time technique. Iliac flow increased instantaneously at the beginning of mild exercise (from 12.03 +/- 1.06 to 25.55 +/- 3.89 ml/min at 15 s) and showed a smaller increase when exercise intensity increased further, reaching a plateau of 38.43 +/- 1.92 ml/min at the 4th min of moderate exercise intensity. In contrast, exercise-induced reduction of renal flow was smaller and slower, with 18% and 25% decreases at mild and moderate exercise intensities. Our data indicate that transit-time flowmetry is a reliable method for long-term and continuous measurements of regional blood flow at rest and can be used to quantitate the dynamic flow changes that characterize exercise and recovery.