The Effects of Resistance Exercise Training on Skeletal Muscle Metabolism and Insulin Resistance Development in Female Rodents with Type 1 Diabetes.

The etiology of insulin resistance (IR) development in type 1 diabetes mellitus (T1DM) remains unclear; however, impaired skeletal muscle metabolism may play a role. While IR development has been established in male T1DM rodents, female rodents have yet to be examined in this context. Resistance exercise training (RT) has been shown to improve IR and is associated with a lower risk of hypoglycemia onset in T1DM compared to aerobic exercise. The purpose of this study was to investigate the effects of RT on IR development in female T1DM rodents. Forty Sprague Dawley eight-week-old female rats were divided into four groups: control sedentary (CS; n = 10), control trained (CT; n = 10), T1DM sedentary (DS; n = 10), and T1DM trained (DT; n = 10). Multiple low-dose streptozotocin injections were used to induce T1DM. Blood glucose levels were maintained in the 4-9 mmol/l range with intensive insulin therapy. CT and DT underwent weighted ladder climbing 5 days/week for six weeks. Intravenous glucose tolerance tests (IVGTT) were conducted on all animals following the six-week period. Results demonstrate that DS animals exhibited significantly increased weekly blood glucose measures compared to all groups including DT (p < 0.0001), despite similar insulin dosage levels. This was concomitant with a significant increase in insulin-adjusted area under the curve following IVGTT in DS (p < 0.05), indicative of a reduction in insulin sensitivity. Both DT and DS exhibited greater serum insulin concentrations compared to CT and CS (p < 0.05). DS animals also exhibited significantly greater glycogen content in white gastrocnemius muscle compared to CS and DT (p < 0.05), whereas DT and DS animals exhibited greater p-Akt: Akt ratio in the white vastus lateralis muscle and citrate synthase activity in the red vastus lateralis muscle compared to CS and CT (p < 0.05). These results indicate that female rodents with T1DM develop poor glycemic control and IR which can be attenuated with RT, possibly related to differences in intramyocellular glycogen content.

A chronic physical activity treatment in obese rats normalizes the contributions of ET-1 and NO to insulin-mediated posterior cerebral artery vasodilation.

This study tested the hypotheses that obesity-induced decrements in insulin-stimulated cerebrovascular vasodilation would be normalized with acute endothelin-1a receptor antagonism and that treatment with a physical activity intervention restores vasoreactivity to insulin through augmented nitric oxide synthase (NOS)-dependent dilation. Otsuka Long-Evans Tokushima Fatty rats were divided into the following groups: 20 wk old food controlled (CON-20); 20 wk old free food access (model of obesity, OB-20); 40 wk old food controlled (CON-40); 40 wk old free food access (OB-40); and 40 wk old free food access+RUN (RUN-40; wheel-running access from 20 to 40 wk). Rats underwent Barnes maze testing and a euglycemic hyperinsulinemic clamp (EHC). In the 40-wk cohort, cerebellum and hippocampus blood flow (BF) were examined (microsphere infusion). Vasomotor responses (pressurized myography) to insulin were assessed in untreated, endothelin-1a receptor antagonism, and NOS inhibition conditions in posterior cerebral arteries. Insulin-stimulated vasodilation was attenuated in the OB vs. CON and RUN groups (P ≤ 0.04). Dilation to insulin was normalized with endothelin-1a receptor antagonism in the OB groups (between groups, P ≥ 0.56), and insulin-stimulated NOS-mediated dilation was greater in the RUN-40 vs. OB-40 group (P < 0.01). At 40 wk of age, cerebellum BF decreased during EHC in the OB-40 group (P = 0.02) but not CON or RUN groups (P ≥ 0.36). Barnes maze testing revealed increased entry errors and latencies in the RUN-40 vs. CON and OB groups (P < 0.01). These findings indicate that obesity-induced impairments in vasoreactivity to insulin involve increased endothelin-1 and decreased nitric oxide signaling. Chronic spontaneous physical activity, initiated after disease onset, reversed impaired vasodilation to insulin and decreased Barnes maze performance, possibly because of increased exploratory behavior.NEW & NOTEWORTHY The new and noteworthy findings are that 1) in rodents, obesity-related deficits in insulin-mediated vasodilation are associated with increased influence of insulin-stimulated ET-1 and depressed influence of insulin-stimulated NOS and 2) a physical activity intervention, initiated after the onset of disease, restores insulin-mediated vasodilation, likely by normalizing insulin-stimulated ET-1 and NOS balance. These data demonstrate that the treatment effects of chronic exercise on insulin-mediated vasodilation extend beyond active skeletal muscle vasculature and include the cerebrovasculature.

Exercise-mediated regulation of Hsp70 expression following aerobic exercise training.

An issue central to understanding the biological benefits associated with regular exercise training is to elucidate the intracellular mechanisms governing exercise-conferred cardioprotection. Heat shock proteins (HSPs), most notably the inducible 70-kDa HSP family member Hsp70, are believed to participate in the protection of the myocardium during cardiovascular stress. Following acute exercise, activation of PKA mediates the suppression of an intermediary protein kinase, ERK1/2, which phosphorylates and suppresses the activation of the heat shock transcription factor 1 (HSF1). However, following exercise training, ERK1/2 has been reported to regulate the transcriptional activation of several genes involved in cell growth and proliferation and has been shown to be associated with training-mediated myocardial hypertrophy. The present project examined the transcriptional activation of hsp70 gene expression in acutely exercised (60 min at 30 m/min) naïve sedentary and aerobically trained (8 wk, low intensity) male Sprague-Dawley rats. Following acute exercise stress, no significant differences were demonstrated in the expression of myocardial Hsp70 mRNA and activation of PKA between sedentary and trained animals. However, trained animals elicited expression of the hsp70 gene (P < 0.05) in the presence of elevated ERK1/2 activation. Given the association of ERK1/2 and the suppression of hsp70 gene expression following acute exercise in naïve sedentary rats, these results suggest that training results in adaptations that allow for the simultaneous initiation of both proliferative and protective responses. While it is unclear what factors are associated with this training-related shift, increases in HSF1 DNA binding affinity (P < 0.05) and posttranscriptional modifications of the Hsp70 transcript are suggested.

PKA-mediated ERK1/2 inactivation and hsp70 gene expression following exercise.

Exercise induces the expression of the cardioprotective protein, Hsp70, through the activation of its transcription factor HSF1. Recently, we reported that administration of a protein kinase A (PKA) inhibitor suppressed exercise-induced hsp70 gene expression, suggesting a role for PKA in the regulation of HSF1 activation in vivo. While the mechanism by which PKA regulates HSF1 is unclear, studies in vitro have reported that HSF1 is phosphorylated on two serine residues by mitogen activated protein kinases (MAPKs); ERK1/2 (ser307) and JNK/SAPK (ser363). As PKA is a regulator of these protein kinases, the current study examined the role of PKA in their activation and subsequent regulation of exercise-induced hsp70 gene expression. Following treadmill-running exercise (60 min at 30 m/min; 2% grade), both ERK1/2 and JNK/SAPK demonstrated distinct phosphorylation profiles. Increased phosphorylation of ERK1/2 was observed immediately post-exercise, whereas JNK/SAPK phosphorylation was not significantly elevated until 30 min post-exercise. Administration of the PKA inhibitor (H89; 0.360 mg/kg) maintained ERK1/2 phosphorylation to at least 30 min post exercise (n = 5; P < 0.05) while JNK/SAPK phosphorylation was unaltered. Inhibition of this PKA-mediated increase in ERK1/2 phosphorylation through the simultaneous administration of an ERK1/2 inhibitor (UB-1026; 0.25 mg/kg) restored exercise-induced hsp70 mRNA levels in PKA-inhibited rats that previously demonstrated a suppressed response (P < 0.05). Given that ERK1/2 has been shown to be a negative regulator of HSF1 in vitro, these results suggest a role for ERK1/2 in the PKA-mediated regulation of HSF1 activation following exercise.

Regulation of myocardial heat shock protein 70 gene expression following exercise.

Post-exercise induction of myocardial heat shock protein (Hsp70) gene expression involves the activation of the heat shock transcription factor (HSF1). While the exact mechanisms governing the regulation of HSF1 are unclear, activation is believed to occur subsequent to hyperphosphorylation of specific serine residues. As two important serine kinases, protein kinase A (PKA) and protein kinase C (PKC), have been implicated in many phosphorylative events in myocardial cells, we examined the role of these kinases in the activation of Hsp70 gene expression following exercise. In this report, we show that prior administration of a PKA inhibitor, N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide ?2HCl (H-89; 0.36 mg/kg), significantly suppressed the elevation in Hsp70 mRNA (P < 0.05) and protein synthesis (P < 0.05) in male Sprague-Dawley rats following a single bout of exercise. In contrast, this post-exercise elevation in Hsp70 mRNA and protein synthesis was not suppressed following the administration of a PKC inhibitor chelerythrine chloride (CHEL; 5 mg/kg) (P < 0.05). Of note, inhibition of PKA did not alter the nuclear localization and binding affinity of HSF1 to the promotor region of the Hsp70 gene. These data indicate that PKA, and not PKC, plays a necessary role in the early exercise-induced regulation of Hsp70 gene expression, downstream of DNA-binding acquisition. However, the current study does not support previous observations regarding major changes in HSF1 phosphorylation and suggests that other PKA-related mechanisms mediate the activation of Hsp70 gene expression following exercise.