M. H. Laughlin

Effects of posture on shear rates in human brachial and superficial femoral arteries

Shear rate is significantly lower in the superficial femoral compared with the brachial artery in the supine posture. The relative shear rates in these arteries of subjects in the upright posture (seated and/or standing) are unknown. The purpose of this investigation was to test the hypothesis that upright posture (seated and/or standing) would produce greater shear rates in the superficial femoral compared with the brachial artery. To test this hypothesis, Doppler ultrasound was used to measure mean blood velocity (MBV) and diameter in the brachial and superficial femoral arteries of 21 healthy subjects after being in the supine, seated, and standing postures for 10 min. MBV was significantly higher in the brachial compared with the superficial femoral artery during upright postures. Superficial femoral artery diameter was significantly larger than brachial artery diameter. However, posture had no significant effect on either brachial or superficial femoral artery diameter. The calculated shear rate was significantly greater in the brachial (73 +/- 5, 91 +/- 11, and 97 +/- 13 s(-1)) compared with the superficial femoral (53 +/- 4, 39 +/- 77, and 44 +/- 5 s(-1)) artery in the supine, seated, and standing postures, respectively. Contrary to our hypothesis, our current findings indicate that mean shear rate is lower in the superficial femoral compared with the brachial artery in the supine, seated, and standing postures. These findings of lower shear rates in the superficial femoral artery may be one mechanism for the higher propensity for atherosclerosis in the arteries of the leg than of the arm.

Exercise training increases L-type calcium current density in coronary smooth muscle

Exercise training produces numerous adaptations in the coronary circulation, including an increase in coronary tone, both in conduit and resistance arteries. On the basis of the importance of voltage-gated Ca2+ channels (VGCC) in regulation of vascular tone, we hypothesized that exercise training would increase VGCC current density in coronary smooth muscle. To test this hypothesis, VGCC current was compared in smooth muscle from conduit arteries (>1.0 mm), small arteries (200-250 μm), and large arterioles (75-150 μm) from endurance-trained (Ex) or sedentary miniature swine (Sed). After 16-20 wk of treadmill training, VGCC current was determined using whole cell voltage-clamp techniques. In both Ex and Sed, VGCC current density was inversely related to arterial diameter, i.e., large arterioles > small arteries > conduit arteries. Exercise training increased peak inward currents approximately twofold in smooth muscle from all arterial sizes compared with those from Sed (large arteriole, -12.52 ± 2.05 vs. -5.74 ± 0.99 pA/pF; small artery, -6.20 ± 0.97 vs. -3.18 ± 0.44 pA/pF; and conduit arteries, -4.22 ± 0.30 vs. -2.41 ± 0.55 pA/pF; 10 mM Ba2+ external). Dihydropyridine sensitivity, voltage dependence, and inactivation kinetics identified this Ca2+ current to be L-type current in all arterial sizes from both Sed and Ex. Furthermore, peak VGCC current density was correlated with treadmill endurance in all arterial sizes. We conclude that smooth muscle L-type Ca2+ current density is increased within the coronary arterial bed by endurance exercise training. This increased VGCC density may provide an important mechanistic link between functional and cellular adaptations in the coronary circulation to exercise training.

Heterogeneity of L-type calcium current density in coronary smooth muscle

Heterogeneity of vascular responses to physiological and pharmacological stimuli has been demonstrated throughout the coronary circulation. Typically, this heterogeneity is based on vessel size. Although the cellular mechanisms for this heterogeneity are unknown, one plausible factor may be heterogeneous distribution of ion channels important in regulation of vascular tone. Because of the importance of voltage-gated Ca2+ channels in regulation of vascular tone, we hypothesized that these channels would be unequally distributed throughout the coronary arterial bed. To test this hypothesis, voltage-gated Ca2+current was measured in smooth muscle from conduit arteries (>1.0 mm), small arteries (200-250 μm), and large arterioles (75-125 μm) of miniature swine using whole cell voltage-clamp techniques. With 2 mM Ca2+ or 10 mM Ba2+ as charge carrier, voltage-gated Ca2+ current density was inversely related to arterial diameter, i.e., large arterioles > small arteries > conduit. Peak inward currents (10 mM Ba2+) were increased ∼2.5- and ∼1.5-fold in large arterioles and small arteries, respectively, compared with conduit arteries (-5.58 ± 0.53, -3.54 ± 0.34, and -2.26 ± 0.31 pA/pF, respectively). In physiological Ca2+ (2 mM), small arteries demonstrated increased inward current at membrane potentials within the physiological range for vascular smooth muscle (as negative as -40 mV) compared with conduit arteries. In addition, cells from large arterioles showed a negative shift in the membrane potential for half-maximal activation compared with small and conduit arteries (-13.23 ± 0.88, -6.22 ± 1.35, and -8.62 ± 0.81 mV, respectively; P < 0.05). Voltage characteristics and dihydropyridine sensitivity identified this Ca2+ current as predominantly L-type current in all arterial sizes. We conclude that L-type Ca2+ current density is inversely related to arterial diameter within the coronary arterial vasculature. This heterogeneity of Ca2+ current density may provide, in part, the basis for functional heterogeneity within the coronary circulation.

Voluntary Wheel Running Selectively Augments Insulin-Stimulated Vasodilation in Arterioles from White Skeletal Muscle of Insulin-Resistant Rats

Exercise (RUN) prevents declines in insulin‐mediated vasodilation, an important component of insulin‐mediated glucose disposal, in rats prone to obesity and insulin resistance.

Acute impact of intermittent pneumatic leg compression frequency on limb hemodynamics, vascular function, and skeletal muscle gene expression in humans

The mechanisms by which intermittent pneumatic leg compression (IPC) treatment effectively treats symptoms associated with peripheral artery disease remain speculative. With the aim of gaining mechanistic insight into IPC treatment, the purpose of this study was to investigate the effect of IPC frequency on limb hemodynamics, vascular function, and skeletal muscle gene expression. In this two study investigation, healthy male subjects underwent an hour of either high-frequency (HF; 2-s inflation/3-s deflation) or low-frequency (LF; 4-s inflation/16-s deflation) IPC treatment of the foot and calf. In study 1 (n = 11; 23.5 ± 4.7 yr), subjects underwent both HF and LF treatment on separate days. Doppler/ultrasonography was used to measure popliteal artery diameter and blood velocity at baseline and during IPC treatment. Flow-mediated dilation (FMD) and peak reactive hyperemia blood flow (RHBF) were determined before and after IPC treatment. In study 2 (n = 19; 22.0 ± 4.6 yr), skeletal muscle biopsies were taken from the lateral gastrocnemius of the treated and control limb at baseline and at 30- and 150-min posttreatment. Quantitative PCR was used to assess mRNA concentrations of genes associated with inflammation and vascular remodeling. No treatment effect on vascular function was observed. Cuff deflation resulted in increased blood flow (BF) and shear rate (SR) in both treatments at the onset of treatment compared with baseline (P < 0.01). BF and SR significantly diminished by 45 min of HF treatment only (P < 0.01). Both treatments reduced BF and SR and elevated oscillatory shear index compared with baseline (P < 0.01) during cuff inflation. IPC decreased the mRNA expression of cysteine-rich protein 61 from baseline and controls (P <0 .01) and connective tissue growth factor from baseline (P < 0.05) in a frequency-dependent manner. In conclusion, a single session of IPC acutely impacts limb hemodynamics and skeletal muscle gene expression in a frequency-dependent manner but does not impact vascular function.

Intermittent pneumatic leg compressions enhance muscle performance and blood flow in a model of peripheral arterial insufficiency

Despite the escalating prevalence in the aging population, few therapeutic options exist to treat patients with peripheral arterial disease. Application of intermittent pneumatic leg compressions (IPC) is regarded as a promising noninvasive approach to treat this condition, but the clinical efficacy, as well the mechanistic basis of action of this therapy, remain poorly defined. We tested the hypothesis that 2 wk of daily application of IPC enhances exercise tolerance by improving blood flow and promoting angiogenesis in skeletal muscle in a model of peripheral arterial insufficiency. Male Sprague-Dawley rats were subjected to bilateral ligation of the femoral artery and randomly allocated to treatment or sham groups. Animals were anesthetized daily and exposed to 1-h sessions of bilateral IPC or sham treatment for 14-16 consecutive days. A third group of nonligated rats was also studied. Marked increases in treadmill exercise tolerance (∼33%, P < 0.05) and improved muscle performance in situ (∼10%, P < 0.05) were observed in IPC-treated animals. Compared with sham-treated controls, blood flow measured with isotope-labeled microspheres during in situ contractions tended to be higher in IPC-treated animals in muscles composed of predominantly fast-twitch white fibers, such as the plantaris (∼93%, P = 0.02). Capillary contacts per fiber and citrate synthase activity were not significantly altered by IPC treatment. Collectively, these data indicate that IPC improves exercise tolerance in a model of peripheral arterial insufficiency in part by enhancing blood flow to collateral-dependent tissues.

Effects of Chronic Nitric Oxide Synthase Inhibition on Endothelium-Dependent and -Independent Relaxation in Arteries that Perfuse Skeletal Muscle of Swine

The purpose of this investigation was to test the hypothesis that chronic N(G)-nitro-l-arginine methyl ester (l-NAME) treatment produces differential effects on conduit artery and resistance arteriole relaxation responses to endothelium-dependent and -independent vasodilators in arteries that perfuse skeletal muscle of swine. To test this hypothesis, conduit skeletal muscle arteries and second-order skeletal muscle (2A) arterioles were harvested from 14 Yucatan swine that were chronically administered l-NAME and from 16 controls. In vitro assessments of vasorelaxation to increasing doses of acetylcholine (ACH), bradykinin (BK), and sodium nitroprusside (SNP) were performed in both conduit and 2A arterioles. l-NAME treatment produced a significant reduction in both BK and ACH relaxation responses in the conduit arteries. In contrast, the relaxation response and/or sensitivity to SNP were significantly greater in the intact, but not denuded, conduit arterial rings from chronically l-NAME-treated swine. There were no significant effects of chronic l-NAME treatment on vasodilation of skeletal muscle arterioles. These findings suggest (1) that unlike arterioles, skeletal muscle conduit arteries do not functionally compensate for a lack of NO through the upregulation of alternative vasodilator pathways; (2) that the greater relaxation response in conduit arteries of chronically l-NAME-treated swine to SNP can be explained by alterations to the endothelium.

Exercise Training Improves Vasoreactivity in the Knee Artery

Physical activity has been shown to enhance endothelial function of central and peripheral vascular beds. The primary purpose of the present study was to test the hypothesis that a short-term exercise training program would result in enhanced endothelium-dependent vasorelaxation of a major artery supplying blood flow to the knee joint, the middle genicular artery. Female Yucatan miniature swine were randomly assigned into exercise trained (n=7) or sedentary (n=7) groups. Exercise trained pigs underwent a daily exercise training program on treadmills for 7 days. In vitro assessment of vasorelaxation was determined in a dose response manner by administrating increasing doses of 3 different dilators; adenosine diphosphate, bradykinin, and sodium nitroprusside. The role of nitric oxide synthase and cyclooxygenase pathways in vasomotor responses was evaluated with specific inhibitors using nitro-L-arginine methyl ester and indomethacin incubation, respectively. The results of this investigation indicate that adenosine and bradykinin-induced endothelium-dependent vasorelaxation were significantly enhanced in middle genicular artery from exercise trained pigs (p<0.05). Endothelium-independent vasorelaxation was not altered with exercise training as determined by the response to sodium nitroprusside. The findings of the present investigation indicate that short-term exercise training enhances endothelial function of middle genicular artery through adaptations in the nitric oxide synthase and by non-nitric oxide synthase, non-cyclooxygenase pathways.

Impact of a single session of intermittent pneumatic leg compressions on skeletal muscle and isolated artery gene expression in rats.

Intermittent pneumatic leg compressions (IPC) have proven to be an effective noninvasive approach for treatment of patients with claudication, but the mechanisms underlying the clinical benefits remain elusive. In the present study, a rodent model of claudication produced by bilateral ligation of the femoral artery was used to investigate the acute impact of a single session of IPC (150 min) on hemodynamics, skeletal muscle (tibialis anterior), and isolated collateral artery (perforating artery) expression of a subset of genes associated with inflammation and vascular remodeling. In addition, the effect of compression frequency (15 vs. 3 compressions/min) on the expression of these factors was studied. In ligated animals, IPC evoked an increase of monocyte chemoattractant protein-1 (MCP-1) and cytokine-induced neutrophil chemoattractant 1 (CXCL1) mRNA (P < 0.01) and immunostaining (P < 0.05), as well as a minor increase in VEGF immunostaining in the muscle endomysium 150 min postintervention. Further, collateral arteries from these animals showed an increased expression of MCP-1 (approximately twofold, P = 0.02). These effects were most evident in the group exposed to the high-frequency protocol (15 compressions/min). In contrast, IPC in sham-operated control animals evoked a modest initial upregulation of VEGF (P = 0.01), MCP-1 (P = 0.02), and CXCL1 (P = 0.03) mRNA in the muscle without concomitant changes in protein levels. No changes in gene expression were observed in arteries isolated from sham animals. In conclusion, IPC acutely up-regulates the expression of important factors involved in vascular remodeling in the compressed muscle and collateral arteries in a model of hindlimb ischemia. These effects appear to be dependent on the compression frequency, such that a high compression frequency (15 compressions/min) evokes more consistent and robust effects compared with the frequency commonly employed clinically to treat patients with claudication (3 compressions/min).

Metformin does not Enhance Insulin-Stimulated Vasodilation in Skeletal Muscle Resistance Arteries of the OLETF Rat

To test the hypothesis that chronic metformin treatment enhances insulin‐induced vasodilation in skeletal muscle resistance arteries and arterioles.