Duration of improved muscle glucose uptake after acute exercise in obese Zucker rats

Skeletal muscle is insulin resistant in the obese Zucker rat. Endurance training reduces muscle insulin resistance, but the effects of a single acute exercise session on muscle insulin resistance in the obese Zucker rat are unknown. Therefore, insulin responsiveness of muscle glucose uptake was measured in 15-week-old obese rats either 1, 48, or 72 hours after two hours of intermittent exercise (30:30 min; work:rest). Hindlimbs of sedentary lean (LS) and obese (OS) rats and exercised obese (OE) rats were perfused after a 10-hour fast under both basal (0 mU x ml(-1)) and maximal (20 mU x ml(-1)) insulin concentrations to measure net glucose uptake. Insulin responsiveness of net glucose uptake was significantly reduced in OS compared to LS (8.5 +/- 1.6 vs 15.3 +/- 2.0 micromol x g(-1) x h(-1), respectively). Compared to OS, insulin responsiveness of net glucose uptake was significantly increased by 56% and 80% at 1 hour and 48 hours after acute exercise. However, 72 hours after acute exercise, the increased insulin responsiveness of net glucose uptake was no longer evident. These results indicate that improved responsiveness of muscle glucose uptake persists for at least 48 hours after two hours of acute intermittent exercise in 15-week-old obese Zucker rats.     

Aversion instead of preference learning indicated by nicotine place conditioning in rats

Although nicotine is a drug of abuse for millions of smokers, it has been difficult to demonstrate clearly the motivational properties of nicotine with rats using the conditioned place preference (CPP) paradigm. The first experiment attempted to replicate CPPs reported by other researchers using nicotine doses of 0.4, 0.8, and 1.2 mg/kg. There was a trend for all three doses to produce aversions, but it was significant only for the 0.8 mg/kg dose. Exposures to the CS alone extinguished aversions, but a priming dose (0.2 mg/kg) of nicotine given after extinction produced aversions only in animals exposed to 1.2 mg/kg. Experiment 2 tested whether preexposure to morphine or nicotine would sensitize animals to nicotine's reinforcing effects. In this experiment, rats were exposed to either six nicotine (0.6 mg/kg) or morphine (1.0 mg/kg) dosings prior to preference conditioning. Neither preferences nor aversions were observed in any group following subsequent conditioning with 0.6 mg/kg nicotine. The results suggest that previous observations of preference effects may have been due to specific procedural factors or may have depended on negative reinforcement due to stress reduction.     

The efficacy of carbohydrate supplementation and chronic high- carbohydrate diets for improving endurance performance

Carbohydrate (CHO) is the body's most limited fuel and the most heavily metabolized during moderate-intensity exercise. For this reason it is recommended that endurance athletes consume a high-CHO diet (8-10 g CHO . kg body weight-1 . day-1) to enhance training and performance. A review of the literature supports the benefits of CHO supplementation on endurance performance. The benefits of chronic high-CHO diets on endurance performance are not as clear. Recent evidence suggests that a high-CHO diet may be necessary for optimal adaptations to training. However, the paucity of date in this area precludes any concrete conclusions. The practicality of high-CHO diets is not well understood. The available evidence would indicate that a high-CHO diet is the best dietary recommendation for endurance athletes.     

Heterogeneous cardiac sympathetic denervation and decreased myocardial nerve growth factor in streptozotocin-induced diabetic rats: implications for cardiac sympathetic dysinnervation complicating diabetes

Heterogeneous myocardial sympathetic denervation complicating diabetes has been invoked as a factor contributing to sudden unexplained cardiac death. In subjects with diabetic autonomic neuropathy (DAN), distal left ventricular (LV) denervation contrasts with preservation of islands of proximal innervation, which exhibit impaired vascular responsiveness. The aims of this study were to determine whether this heterogeneous pattern of myocardial sympathetic denervation occurs in a rat model of diabetes and to explore a potential association with regional fluctuations in myocardial nerve growth factor (NGF) protein. Myocardial sympathetic denervation was characterized scintigraphically using the sympathetic neurotransmitter analog C-11 hydroxyephedrine ([11C]HED) and compared with regional changes in myocardial NGF protein abundance and norepinephrine content after 6 and 9 months in nondiabetic (ND) and streptozotocin-induced diabetic (STZ-D) rats. In ND rats, no difference in [11C]HED retention or norepinephrine content was detected in the proximal versus distal myocardium. After 6 months, compared with ND rats, myocardial [11C]HED retention had declined in the proximal segments of STZ-D rats by only 9% (NS) compared with a 33% decrease in the distal myocardium (P < 0.05). Myocardial norepinephrine content was similar in both ND and STZ-D rats. At 6 months, LV myocardial NGF protein content in STZ-D rats decreased by 52% in the proximal myocardial segments (P < 0.01 vs. ND rats) and by 82% distally (P < 0.01 vs. ND rats, P < 0.05 vs. proximal segments). By 9 months, [11C]HED retention had declined in both the proximal and distal myocardial segments of the STZ-D rats by 42% (P < 0.01 vs. ND rats), and LV norepinephrine content and NGF protein were decreased in parallel. Therefore, 6 months of STZ-induced diabetes results in heterogeneous cardiac sympathetic denervation in the rat, with maximal denervation occurring distally, and is associated with a proximal-to-distal gradient of LV NGF protein depletion. It is tempting to speculate that regional fluctuations of NGF protein in the diabetic myocardium contribute to heterogeneous cardiac sympathetic denervation complicating diabetes.     

The influence of ocular pulsatility on scanning laser Doppler flowmetry.

PURPOSE: To determine the effect of the cardiac cycle on scanning laser Doppler flowmeter measurements of retinal capillary blood flow in rhesus monkeys and humans. METHODS: Multiple scanning laser Doppler flowmetry images of rhesus monkey and human retinal capillary blood flow over a range of heart rates were obtained. Average flow values were determined for the 64 scan lines that compose the two-dimensional flow map. Cutaneous blood flow was measured simultaneously with a laser Doppler flowmeter. The temporal relationships between retinal capillary blood flow, peripheral arterial pulse, and cutaneous blood flow were determined. In addition, human retinal capillary blood flow in a 10 x 10-pixel area during different phases of the cardiac cycle was compared. RESULTS: Regular oscillations in human and rhesus monkey retinal capillary blood flow are evident as alternating bright and dark horizontal bands in scanning laser Doppler flowmetry images. These fluctuations are temporally correlated with cutaneous blood flow. Linear regression of actual vs predicted heart rate based on peaks in retinal capillary flow yielded r = 0.999 in a rhesus monkey and 0.938 in a human. Retinal capillary blood flow in a 10 x 10-pixel area fluctuated as much as 50% depending on the phase of the cardiac cycle. CONCLUSIONS: The alternating bright and dark banding pattern observed in scanning laser Doppler flowmetry scans of retinal capillary blood flow is related to the cardiac pulse. The errors introduced by pulse-related fluctuations in retinal capillary blood flow are significant and must be minimized or corrected for accurate and reproducible measurements of ocular hemodynamics.