Janet L. Parker

Effects of exercise training on regulation of tone in coronary arteries and arterioles

A large number of studies now support the concept that exercise training alters functional control of the coronary circulation. Recent work has approached this area using ex vivo coronary arterial preparations (proximal coronary arteries, near-resistance arteries, resistance arterioles) isolated from exercise-trained animals and contracting independently of confounding in vivo influences. The combined results of these studies indicate that training-induced alterations in vascular control mechanisms do not occur uniformly throughout the coronary vascular tree. Proximal epicardial coronary arteries (approximately 2.0 mm diameter) isolated from exercise-trained pigs exhibited significantly reduced contractile responsiveness to the alpha-adrenergic receptor agonist, norepinephrine, but unaltered contractile responsiveness to K+, acetylcholine, and endothelin. Also, proximal arteries from exercise-trained animals demonstrated enhanced sensitivity to the vasodilator effects of adenosine. At the other end of the vascular spectrum, in resistance arterioles (< 150 microns diameter) the relaxation responses to adenosine were unaffected by exercise training, but bradykinin-induced vasodilation (endothelium-dependent) was significantly enhanced. In near-resistance arteries (150-240 microns diameter) responses to both bradykinin and adenosine were enhanced by exercise training. Thus, exercise training is associated with intrinsic vessel size-dependent alterations in coronary smooth muscle and endothelium-mediated regulatory mechanisms.

Exercise Training Enhances Vasodilation Responses to Vascular Endothelial Growth Factor in Porcine Coronary Arterioles Exposed to Chronic Coronary Occlusion

Background—Chronic coronary occlusion (CCO) impairs endothelial function of distal collateral–dependent microvasculature; however, long-term exercise training (EX) seems to improve endothelial dysfunction. We hypothesized that EX enhances vasodilation responses to vascular endothelial growth factor (VEGF165), mediated via nitric oxide (NO), in arterioles exposed to CCO. Methods and Results—The proximal left circumflex coronary artery (LCx) of female Yucatan miniswine was surgically instrumented with an ameroid occluder to induce CCO; 8 weeks after surgery, animals were randomized into 14-week sedentary (SED) or EX (treadmill; 5 d/wk) protocols. Coronary arterioles (≈100 &mgr;m in diameter) were isolated from collateral-dependent (LCx) and nonoccluded (left anterior descending; LAD) perfused myocardium of SED and EX animals. Vasodilation was assessed by videomicroscopy and MacLab data acquisition. Responses to VEGF165 were unaffected by EX in nonoccluded LAD arterioles; in contrast, EX markedly enhanced VEGF165-induced vasodilation of collateral-dependent LCx arterioles (P <0.05; EX versus SED). Furthermore, VEGF165-induced vasodilation of EX LCx arterioles exceeded that of EX or SED LAD arterioles (P <0.05). Enhanced vasodilation of EX LCx arterioles was abolished by inhibition of NO synthase and tyrosine kinase activity. Combined inhibition of NO synthase and cyclooxygenase decreased VEGF165-induced vasodilation of all vessels. Conclusions—EX enhances VEGF165-induced vasodilation in arterioles distal to CCO; EX effects seem to be mediated through increases in NO.

Effects of cytokines tumor necrosis factor alpha and interleukin 1 beta on endotoxin-mediated inhibition of endothelium-derived relaxing factor bioactivity and nitric oxide production in vascular endothelium.

Endotoxemia results in the release of cytokines that exert complex effects on the cardiovascular system. The purpose of this study was to 1) determine if interleukin 1 β (IL1 β) and tumor necrosis factor α (TNFα) elicit the release of endothelium-derived relaxing factor (EDRF) and nitric oxide derived from the constitutive nitric oxide synthase present in vascular endothelium, and 2) determine if these cytokines alter endotoxin-mediated decreases in EDRF bioactivity and nitric oxide production. Cultured bovine aortic endothelial cells were directly exposed to endotoxin, human recombinant TNFα, interleukin 1 β, or a combination of endotoxin and cytokine for 1 h, followed by a second hour without endotoxin. Subsequently, both basal as well as agonist-stimulated (bradykinin) EDRF bioactivity and nitric oxide (NO) content of the effluent were quantitated. In additional experiments, endothelial cells were exposed acutely over a 30-min assay period to either endotoxin alone, cytokine alone, or endotoxin and cytokine. Following the 2-h incubation, endotoxin alone markedly reduced basal EDRF bioactivity and NO production (44 ± 13% control, 66 ± 13% control, respectively) and decreased bradykinin-stimulated EDRF bioactivity and NO production (58 ± 5% control, 55 ± 4% control, respectively). TNFα and IL1 β did not stimulate EDRF release or NO production either acutely or after prolonged exposure, nor did they alter agonist-stimulated EDRF bioactivity and NO production. Similarly, co-incubation of endotoxin with TNFα or IL1 β failed to significantly alter the inhibitory effects of endotoxin on EDRF bioactivity and NO production. These results demonstrate that 1) the specific cytokines TNFα and IL1 β do not elicit the release of constitutively derived EDRF from endothelial cells, 2) endotoxin at low concentrations directly inhibits both EDRF bioactivity and NO production, and 3) the cytokines TNFα and IL1 β fail to alter the inhibitory effects of endotoxin on EDRF/NO synthesis from the constitutive NO synthase of vascular endothelial cells.

Enhanced L-type Ca2+ channel current density in coronary smooth muscle of exercise-trained pigs is compensated to limit myoplasmic free Ca2+ accumulation.

1 We hypothesized that enhanced voltage‐gated Ca2+ channel current (VGCC) density in coronary smooth muscle cells of exercise‐trained miniature Yucatan pigs is compensated by other cellular Ca2+ regulatory mechanisms to limit net myoplasmic free Ca2+ accumulation. 2 Whole‐cell voltage clamp experiments demonstrated enhanced VGCC density in smooth muscle cells freshly dispersed from coronary arteries of exercise‐trained vs. sedentary animals. 3 In separate experiments using fura‐2 microfluorometry, we measured depolarization‐induced (80 mm KCl) accumulation of myoplasmic free Ba2+ and free Ca2+. Both maximal rate and net accumulation of free Ba2+ in response to membrane depolarization were increased in smooth muscle cells isolated from exercise‐trained pigs, consistent with an increased VGCC density. Depolarization also produced an enhanced maximal rate of free Ca2+ accumulation in cells of exercise‐trained pigs; however, net accumulation of free Ca2+ was not significantly increased suggesting enhanced Ca2+ influx was compensated to limit net free Ca2+ accumulation. 4 Inhibition of sarco‐endoplasmic reticulum Ca2+‐transporting ATPase (SERCA; 10 μm cyclopiazonic acid) and/or sarcolemmal Na+‐Ca2+ exchange (low extracellular Na+) suggested neither mechanism compensated the enhanced VGCC in cells of exercise‐trained animals. 5 Local Ca2+‐dependent inactivation of VGCC, assessed by buffering myoplasmic Ca2+ with EGTA in the pipette and using Ca2+ and Ba2+ as charge carriers, was not different between cells of sedentary and exercise‐trained animals. 6 Our findings indicate that increased VGCC density is compensated by other cellular Ca2+ regulatory mechanisms to limit net myoplasmic free Ca2+ accumulation in smooth muscle cells of exercise‐trained animals. Further, SERCA, Na+‐Ca2+ exchange and local Ca2+‐dependent inactivation of VGCC do not appear to function as compensatory mechanisms. Additional potential compensatory mechanisms include Ca2+ extrusion via plasma membrane Ca2+‐ATPase, mitochondrial uptake, myoplasmic Ca2+‐binding proteins and other sources of VGCC inactivation.

Endotoxin-induced myocardial dysfunction: is there a role for nitric oxide?

Nitric oxide has been implicated in the regulation of cardiac contractile function as well as the depressed myocardial contractility associated with sepsis and endotoxemia. We examined the effects of nitric oxide synthase (NOS) inhibitors and a nitric oxide generator on contractile responses of left atrial preparations and ventricular myocytes isolated from endotoxemic guinea pigs, which exhibit depressed myocardial contractile function. The NOS inhibitor L-NAME had no effect on contractile tension developed by control atria or atria isolated from guinea pigs 4 or 16 h after an intraperitoneal injection of endotoxin. Similarly, contraction of ventricular myocytes isolated from control or endotoxemic guinea pigs (4 h after endotoxin injection) was unchanged by exposure to several NOS inhibitors. In addition, neither Ca(2+)-dependent nor Ca(2+)-independent ventricular NOS activity was affected by endotoxemia. These data suggest that nitric oxide alone is not responsible for the cardiac contractile dysfunction of endotoxemic guinea pigs.

Inhibition Of Endothelium-dependent Vasodilation By Escherichia Coli Endotoxemia

To test the hypothesis that release of endothelium-derived relaxing factor/nitric oxide is inhibited by Gram-negative lipopolysaccharide (LPS; endotoxin), we examined endothelium-independent and endothelium-dependent vasodilator agents in aortic vascular smooth muscle isolated from guinea pigs 4 h after injection of saline (controls) or induction of Escherichia coli endotoxemia. LPS significantly inhibited vasodilator responses to the endothelium-dependent agonists acetylcholine (ACh; 10 10-10 5M) and ADP (10 8-10 5M). However, LPS did not affect vasodilator responses to the endothelium-independent agonist nitroprusside (10 10-10 4M). The nitric oxide synthase (NOS) inhibitor Nγ-nitro-L-arginine methyl ester (L-NAME) inhibited the vasodilator response to ACh; whereas, the cyclooxygenase inhibitor indomethacin (INDO) did not reduce vasodilator effects of ACh. Neither L-NAME nor INDO affected the vasodilator effects of nitroprusside in LPS or control vessels. In contrast, L-NAME converted the vasodilator action of ADP to a vasoconstrictor response that was blocked individually by INDO and the thromboxane synthase inhibitor dazoxiben, suggesting that ADP releases NO and also the vasoconstrictor and platelet aggregating eicosanoid thromboxane A2. These findings suggest that acute (4 h) endotoxemia inhibits function of the constitutive isoform of NOS in vascular endothelial cells. Since L-NAME unmasked a vasoconstrictor action of the endogenous purinoceptor agonist ADP, pharmacologic agents that inhibit NOS may exacerbate LPS-induced inhibition of endothelial NOS; this series of events could lead to diminution of vasodilator reserves and perhaps to augmentation of platelet aggregation during Gram-negative sepsis.

Effects of exercise training on coronary collateralization and control of collateral resistance

Coronary collateral vessels serve as a natural protective mechanism to provide coronary flow to ischemic myocardium secondary to critical coronary artery stenosis. The innate collateral circulation of the normal human heart is typically minimal and considerable variability occurs in extent of collateralization in coronary artery disease patients. A well-developed collateral circulation has been documented to exert protective effects upon myocardial perfusion, contractile function, infarct size, and electrocardiographic abnormalities. Thus therapeutic augmentation of collateral vessel development and/or functional adaptations in collateral and collateral-dependent arteries to reduce resistance into the ischemic myocardium represent a desirable goal in the management of coronary artery disease. Tremendous evidence has provided documentation for the therapeutic benefits of exercise training programs in patients with coronary artery disease (and collateralization); mechanisms that underlie these benefits are numerous and multifaceted, and currently under investigation in multiple laboratories worldwide. The role of enhanced collateralization as a major beneficial contributor has not been fully resolved. This topical review highlights literature that examines the effects of exercise training on collateralization in the diseased heart, as well as effects of exercise training on vascular endothelial and smooth muscle control of regional coronary tone in the collateralized heart. Future directions for research in this area involve further delineation of cellular/molecular mechanisms involved in effects of exercise training on collateralized myocardium, as well as development of novel therapies based on emerging concepts regarding exercise training and coronary artery disease.

Contractile dysfunction of ventricular myocytes isolated from endotoxemic guinea pigs.

Mechanisms responsible for the decline in cardiac function following sepsis or endotoxemia are unclear but may result from indirect effects of cardiodynamic readjustments to diminishing venous return or to direct effects of endogenous factors on myocardial function. We examined contractile properties of ventricular myocytes isolated from endotoxemic guinea pig hearts to 1) verify and characterize inotropic dysfunction in the absence of immediate influences from extrinsic neurohumoral agents, and 2) assess the ability of β-adrenergic receptor activation to modulate contractility. Myocytes were isolated by enzymatic dispersion from hearts 4 h following an intraperitoneal injection of Escherichia coli endotoxin. Contractility was assessed using a computer-driven image analysis system. Inotropic responsiveness of endotoxemic myocytes to changes in frequency of stimulation (.2–2.0 Hz) or increases in extracellular calcium ([Ca2+]o, 1.8–8.0 mM) was significantly less than control myocytes, even with maximally effective frequencies or [Ca2+]o. These data demonstrate that the endotoxin-induced dysfunction is intrinsic to ex vivo cardiac myocytes and independent of immediate influence from extracardiac factors by 4 h in vivo exposure to endotoxemia. Inotropic responsiveness to β-adrenergic receptor activation remained intact in endotoxemic myocytes; maximally effective concentrations (>10 nM) reversed the endotoxin-induced contractile dysfunction. These data confirm that E. coli endotoxemia incorporates intrinsic contractile dysfunction of myocardial cells, while sparing their ability to respond to inotropic mechanisms activated by β-adrenoceptor agonists.

Effects of exercise training on cellular mechanisms of endothelial nitric oxide synthase regulation in coronary arteries after chronic occlusion

Exercise training enhances agonist-mediated relaxation in both control and collateral-dependent coronary arteries of hearts subjected to chronic occlusion, an enhancement that is mediated in part by nitric oxide. The purpose of the present study was to elucidate exercise training-induced adaptations in specific cellular mechanisms involved in the regulation of endothelial nitric oxide synthase (eNOS) in coronary arteries of ischemic hearts. Ameroid constrictors were surgically placed around the proximal left circumflex coronary artery (LCX) of adult female Yucatan miniature swine. Eight weeks postoperatively, animals were randomized into sedentary (pen-confined) or exercise training (treadmill run; 5 days/wk; 14 wk) protocols. Coronary artery segments ( approximately 1.0 mm luminal diameter) were isolated from collateral-dependent (LCX) and control (nonoccluded left anterior descending) arteries 22 wk after ameroid placement. Endothelial cells were enzymatically dissociated, and intracellular Ca(2+) responses (fura 2) to bradykinin stimulation were studied. Immunofluorescence and laser scanning confocal microscopy were used to quantify endothelial cell eNOS and caveolin-1 cellular distribution under basal and bradykinin-stimulated conditions. Immunoblot analysis was used to determine eNOS, phosphorylated (p)-eNOS, protein kinase B (Akt), pAkt, and caveolin-1 protein levels. Bradykinin-stimulated nitrite plus nitrate (NOx; nitric oxide metabolites) levels were assessed via HPLC. Exercise training resulted in significantly enhanced bradykinin-mediated increases in endothelial Ca(2+) levels, NOx levels, and the distribution of eNOS-to-caveolin-1 ratio at the plasma membrane in endothelial cells of control and collateral-dependent arteries. Exercise training also significantly increased total eNOS and phosphorylated levels of eNOS (pSer(1179)) in collateral-dependent arteries. Total eNOS protein levels were also significantly increased in collateral-dependent arteries of sedentary animals. These data provide new insights into exercise training-induced adaptations in cellular mechanisms of nitric oxide regulation in collateral-dependent coronary arteries of chronically occluded hearts that contribute to enhanced nitric oxide production.

Greater gains in strength and power with intraset rest intervals in hypertrophic training.

Abstract Oliver, JM, Jagim, AR, Sanchez, AC, Mardock, MA, Kelly, KA, Meredith, HJ, Smith, GL, Greenwood, M, Parker, JL, Riechman, SE, Fluckey, JD, Crouse, SF, and Kreider, RB. Greater gains in strength and power with intraset rest intervals in hypertrophic training. J Strength Cond Res 27(11): 3116–3131, 2013—We sought to determine if hypertrophic training with intraset rest intervals (ISRs) produced greater gains in power compared with traditional rest (TRD) hypertrophic training. Twenty-two men (age 25 ± 5 years, height 179.71 ± 5.04 cm, weight 82.1 ± 10.6 kg, 6.5 ± 4.5 years of training) matched according to baseline characteristics were assigned to 12 weeks of training using TRD or ISR. Body composition, strength (1-repetition maximum [1RM] bench and squat), and power output (60% 1RM bench and squat, and vertical jump) were assessed at baseline, 4, 8, and 12 weeks. Determination of myosin heavy chain (MHC) percentage from the vastus lateralis was performed pretraining and posttraining. Body composition was analyzed by analysis of variance, whereas performance measures and MHC were analyzed by analysis of covariance with baseline values as the covariate. Data are presented as mean ± SD changes pre to post. The ISR produced greater power output in bench (TRD 32.8 ± 53.4 W; ISR 83.0 ± 49.9 W, p = 0.020) and vertical jump (TRD 91.6 ± 59.8 W; ISR 147.7 ± 52.0 W; p = 0.036) with squat power approaching significance (TRD 204.9 ± 70.2 W; ISR 282.1 ± 104.2 W; p = 0.053) after post hoc analysis (p < 0.10). The ISR produced greater gains in bench (TRD 9.1 ± 3.7 kg; ISR 15.1 ± 8.3 kg; p = 0.010) and squat (TRD 48.5 ± 17.4 kg; ISR 63.8 ± 12.0 kg; p = 0.002) strength. Both protocols produced significant gains in lean mass with no significant differences between groups (1.6 ± 2.1 kg; p = 0.869). The MHCIIx percentage decreased (−31.0 ± 24.5%; p = 0.001), whereas the MHCIIA percentage increased (28.9 ± 28.5%; p = 0.001) with no significant differences between groups. Results indicate that hypertrophy training with ISR produces greater gains in strength and power, with similar gains in lean mass and MHC alterations as TRD. The ISR may be best used in hypertrophic training for strength and power sports.