Differential α-adrenergic modulation of rapid onset vasodilatation along resistance networks of skeletal muscle in old versus young mice

Rapid onset vasodilation during baroreceptor loading and unloading

The purpose of these experiments was to determine if the increase in vascular conductance following a single muscle contraction (50% of maximal voluntary contraction) (6 male and 6 female subjects) was altered during baroceptor loading and unloading. Rapid onset vasodilation (ROV) was determined by measuring brachial artery blood flow (Doppler ultrasound) and blood pressure (Finapress monitor). Brachial artery vascular conductance was calculated by dividing blood flow by mean arterial pressure. ROV was described by the area under the Δvascular conductance (VC)-time curve during the 30 s following muscle contraction. ROV was determined using chamber pressures of +20, +10, 0, -10, -20, and -40 mmHg (lower body positive and negative pressure, LBPP, and LBNP). We tested the hypothesis that the impact of baroreceptor loading and unloading produces a proportion change in ROV. The level of ROV following each contraction was proportional to the peak force (r2 = 0.393, P = 0.0001). Peak force was therefore used as a covariate in further analysis. ROV during application of -40 mmHg LBNP (0.345 ± 0.229 mL·mmHg-1) was lower than that observed at Control (0.532 ± 0.284 mL·mmHg-1, P = 0.034) and +20 mmHg LBPP (0.658 ± 0.364 mL·mmHg-1, P = 0.0008). ROV was linearly related to chamber pressure from -40 to +20 mmHg chamber pressure (r2 = 0.512, P = 0.022, n = 69) and from -20 to +10 mmHg chamber pressure (r2= 0.973, P < 0.0425, n = 45), Overall, vasoconstrictor tone altered with physiologically relevant baroreceptor loading and unloading resulted in a proportion change in ROV.NEW & NOTEWORTHY Rapid onset vasodilation (ROV) was linearly related to the peak force of each single 1-s muscle contraction. In addition, ROV is reduced by baroreceptor unloading (LBNP: -10, -120, and -40 mmHg) and increased by baroreceptor loading (LBPP: +10 and +20 mmHg). Without accounting for peak force and the level of baroreceptor engagement makes comparison of ROV in subjects of differing muscle size or strength untenable.

TRPV1 in arteries enables a rapid myogenic tone

KEY POINTS

We explored the physiological role of TRPV1 in vascular smooth muscle. TRPV1 antagonists dilated skeletal muscle arterioles both ex vivo and in vivo, increased coronary perfusion and decreased systemic blood pressure. Stretch of arteriolar myocytes and increases in intraluminal pressure in arteries triggered rapid Ca²⁺ signaling and vasoconstriction respectively. Pharmacologic and/or genetic disruption of TRPV1 significantly inhibited the magnitude and rate of these responses. Furthermore, disrupting TRPV1 blunted the rapid vasodilation evoked by arterial constriction. Pharmacological experiments identified key roles for phospholipase C and protein kinase C, combined with temperature, in TRPV1-dependent arterial tone. These results show that TRPV1 in arteriolar myocytes dynamically regulates myogenic tone and blood flow in the heart and skeletal muscle.

ABSTRACT

Arterioles maintain blow flow by adjusting their diameter in response to changes in local blood pressure. In this process called the myogenic response, a vascular smooth muscle mechanosensor controls tone predominantly through altering the membrane potential. In general, myogenic responses occur slowly (minutes). In the heart and skeletal muscle, however, tone is activated rapidly (tens of seconds) and terminated by brief (100 ms) arterial constrictions. Previously, we identified extensive expression of TRPV1 in the smooth muscle of arterioles supplying skeletal muscle, heart and fat. Here we reveal a critical role for TRPV1 in the rapid myogenic tone of these tissues. TRPV1 antagonists dilated skeletal muscle arterioles in vitro and in vivo, increased coronary flow in isolated hearts, and transiently decreased blood pressure. All of these pharmacologic effects were abolished by genetic disruption of TRPV1. Stretch of isolated vascular smooth muscle cells or raised intravascular pressure in arteries triggered Ca²⁺ signaling and vasoconstriction. The majority of these stretch-responses were TRPV1-mediated, with the remaining tone being inhibited by the TRPM4 antagonist, 9-phenantrol. Notably, tone developed more quickly in arteries from wild-type compared with TRPV1-null mice. Furthermore, the immediate vasodilation following brief constriction of arterioles depended on TRPV1, consistent with a rapid deactivation of TRPV1. Pharmacologic experiments revealed that membrane stretch activates phospholipase C/protein kinase C signaling combined with heat to activate TRPV1, and in turn, L-type Ca²⁺ channels. These results suggest a critical role, for TRPV1 in the dynamic regulation of myogenic tone and blood flow in the heart and skeletal muscle. This article is protected by copyright. All rights reserved.

The role of vascular function on exercise capacity in health and disease

Three sentinel parameters of aerobic performance are the maximal oxygen uptake ( V ̇ O 2 max ), critical power (CP) and speed of the V ̇ O 2 kinetics following exercise onset. Of these, the latter is, perhaps, the cardinal test of integrated function along the O2 transport pathway from lungs to skeletal muscle mitochondria. Fast V ̇ O 2 kinetics demands that the cardiovascular system distributes exercise-induced blood flow elevations among and within those vascular beds subserving the contracting muscle(s). Ideally, this process must occur at least as rapidly as mitochondrial metabolism elevates V ̇ O 2 . Chronic disease and ageing create an O2 delivery (i.e. blood flow × arterial [O2 ], Q ̇ O 2 ) dependency that slows V ̇ O 2 kinetics, decreasing CP and V ̇ O 2 max , increasing the O2 deficit and sowing the seeds of exercise intolerance. Exercise training, in contrast, does the opposite. Within the context of these three parameters (see Graphical Abstract), this brief review examines the training-induced plasticity of key elements in the O2 transport pathway. It asks how structural and functional vascular adaptations accelerate and redistribute muscle Q ̇ O 2 and thus defend microvascular O2 partial pressures and capillary blood-myocyte O2 diffusion across a ∼100-fold range of muscle V ̇ O 2 values. Recent discoveries, especially in the muscle microcirculation and Q ̇ O 2 -to- V ̇ O 2 heterogeneity, are integrated with the O2 transport pathway to appreciate how local and systemic vascular control helps defend V ̇ O 2 kinetics and determine CP and V ̇ O 2 max in health and how vascular dysfunction in disease predicates exercise intolerance. Finally, the latest evidence that nitrate supplementation improves vascular and therefore aerobic function in health and disease is presented.

Conceptualizing Conduction as a Pliant Vasomotor response: Impact of Ca2+ fluxes and Ca2+ Sensitization

Coordinating blood flow to active tissue requires vasomotor responses to conduct among resistance arteries. Vasomotor spread is governed by the electrical and mechanical properties of vessels; the latter being linked to the sigmoid relations between membrane potential (VM), [Ca2+], and smooth muscle contractility. Proteins guiding electrical-to-tone translation are subject to regulation; thus, vasomotor conduction could be viewed as "pliant" to the current regulatory state. Using simple in silico approaches, we explored vasomotor pliancy and how the regulation of contractility impacts conduction along a skeletal muscle artery and a branching cerebrovascular network. Initial simulations revealed how limited electromechanical linearity affects the translation of electrical spread into arterial tone. Subtle changes to the VM-[Ca2+] or [Ca2+]-diameter relationship, akin to regulatory alterations in Ca2+ influx and Ca2+ sensitivity, modified the distance and amplitude of the conducted vasomotor response. Simultaneous changes to both relationships, consistent with agonist stimulation, augmented conduction although the effect varied with stimulus strength and polarity (depolarization vs hyperpolarization). Final simulations using our cerebrovascular network revealed how localized changes to the VM-[Ca2+] or [Ca2+]-diameter relationships could regionally shape conduction without interfering with the electrical spread. We conclude that regulatory changes to key effector proteins (e.g. L-type Ca2+ channels, myosin light chain phosphatase), integral to voltage translation, not only impact conducted vasomotor tone but likely blood flow delivery to active tissues.

Recovery of blood flow regulation in microvascular resistance networks during regeneration of mouse gluteus maximus muscle

KEY POINTS

Skeletal muscle regenerates after injury, however the recovery of its microvascular supply is poorly understood. We injured the gluteus maximus muscle in mice aiming to investigate the recovery of blood flow regulation in microvascular resistance networks. We hypothesized that blood flow regulation recovers in concert with myofibre regeneration. Microvascular perfusion ceased within 1 day post injury and was restored at 5 days coincident with the appearance of new myofibres; however, the resistance network was dilated and unresponsive to vasoactive agents. Spontaneous vasomotor tone, endothelium-dependent dilatation and adrenergic vasoconstriction increased at 10 days in concert with myofibre regeneration. Vasomotor control recovered at 21 days, when regenerated myofibres matured and active force production stabilized. Functional vasodilatation in response to muscle contraction recovered at 35 days. Physiological integrity of microvascular smooth muscle and endothelium recovers in parallel with myofibre regeneration. Additional time is required to restore the efficacy of signalling between myofibres and microvascular networks controlling their oxygen supply.

ABSTRACT

Myofibre regeneration after skeletal muscle injury is well-studied, although little is known about how microvascular perfusion is restored. The present study aimed to evaluate the recovery of blood flow regulation during skeletal muscle regeneration. In anaesthetized male C57BL/6J mice (aged 4 months), the gluteus maximus muscle (GM) was injured by local injection of barium chloride solution (1.2%, 75 μL). Functional integrity of the resistance network was evaluated at 5, 10, 21 and 35 days post-injury vs. Control by measuring internal diameter of feed arteries, first-, second- and third-order arterioles supplying the GM using intravital microscopy. The resting diameters of all branch orders were significantly greater (P < 0.05) than Control at 5 and 10 days and recovered to Control by 21 days, as did spontaneous vasomotor tone. Vasodilatation to ACh and vasoconstriction to phenylephrine (10^-9 to 10^-5  m) were absent at 5 days, increased at 10 days and recovered to Control by 21 days; reactivity improved in a distal-to-proximal gradient. Across branch orders, functional vasodilatation to single tetanic contraction (100 Hz, 500 ms) and to rhythmic twitch contractions (4 Hz, 30 s) was impaired at 5 days, improved through 21 days and was not different from Control at 35 days. Peak force development (g) was 60% of Control at 10 days and recovered by 21 days. Diminished vasomotor tone during the initial stages of regeneration promotes tissue perfusion as myofibre recovery begins. Recovery of tone and vasomotor responses to agonists occur in concert with myofibre regeneration. Delayed recovery of functional vasodilatation indicates that additional time is required to restore signalling between contracting myofibres and their vascular supply.

Microvascular mechanisms limiting skeletal muscle blood flow with advancing age

Effective oxygen delivery to active muscle fibers requires that vasodilation initiated in distal arterioles, which control flow distribution and capillary perfusion, ascends the resistance network into proximal arterioles and feed arteries, which govern total blood flow into the muscle. With exercise onset, ascending vasodilation reflects initiation and conduction of hyperpolarization along endothelium from arterioles into feed arteries. Electrical coupling of endothelial cells to smooth muscle cells evokes the rapid component of ascending vasodilation, which is sustained by ensuing release of nitric oxide during elevated luminal shear stress. Concomitant sympathetic neural activation inhibits ascending vasodilation by stimulating α-adrenoreceptors on smooth muscle cells to constrict the resistance vasculature. We hypothesized that compromised muscle blood flow in advanced age reflects impaired ascending vasodilation through actions on both cell layers of the resistance network. In the gluteus maximus muscle of old (24 mo) vs. young (4 mo) male mice (corresponding to mid-60s vs. early 20s in humans) inhibition of α-adrenoreceptors in old mice restored ascending vasodilation, whereas even minimal activation of α-adrenoreceptors in young mice attenuated ascending vasodilation in the manner seen with aging. Conduction of hyperpolarization along the endothelium is impaired in old vs. young mice because of "leaky" membranes resulting from the activation of potassium channels by hydrogen peroxide released from endothelial cells. Exposing the endothelium of young mice to hydrogen peroxide recapitulates this effect of aging. Thus enhanced α-adrenoreceptor activation of smooth muscle in concert with electrically leaky endothelium restricts muscle blood flow by impairing ascending vasodilation in advanced age.

Stimulation history affects vasomotor responses in rat mesenteric arterioles

Resistance vessels regulate blood flow by continuously adjusting activity of the wall smooth muscle cells. These cells integrate a variety of stimuli from blood, endothelium, autonomic nerves, and surrounding tissues. Each stimulus elicits an intracellular signaling cascade that eventually influences activation of the contractile machinery. The characteristic time scale of each cascade and the sharing of specific reactions between cascades provide for complex behavior when a vessel receives multiple stimuli. Here, we apply sequential stimulation with invariant concentrations of vasoconstrictor (norepinephrine/methoxamine) and vasodilator (SNAP/carbacol) to rat mesenteric vessels in the wire myograph to show that (1) time elapsed between addition of two vasoactive drugs and (2) the sequence of addition may significantly affect final force development. Furthermore, force oscillations (vasomotion) often appear upon norepinephrine administration. Using computational modeling in combination with nitric oxide (NO) inhibition/NO addition experiments, we show that (3) amplitude and number of oscillating vessels increase over time, (4) the ability of NO to induce vasomotion depends on whether it is applied before or after norepinephrine, and (5) emergence of vasomotion depends on the prior dynamical state of the system; in simulations, this phenomenon appears as "hysteresis." These findings underscore the time-dependent nature of vascular tone generation which must be considered when evaluating the vasomotor effects of multiple, simultaneous stimuli in vitro or in vivo.

Calcium and electrical signaling in arterial endothelial tubes: New insights into cellular physiology and cardiovascular function

The integral role of the endothelium during the coordination of blood flow throughout vascular resistance networks has been recognized for several decades now. Early examination of the distinct anatomy and physiology of the endothelium as a signaling conduit along the vascular wall has prompted development and application of an intact endothelial "tube" study model isolated from rodent skeletal muscle resistance arteries. Vasodilatory signals such as increased endothelial cell (EC) Ca²⁺ ([Ca²⁺ ]_i ) and hyperpolarization take place in single ECs while shared between electrically coupled ECs through gap junctions up to distances of millimeters (≥2 mm). The small- and intermediate-conductance Ca²⁺ activated K⁺ (SK_Ca /IK_Ca or K_Ca 2.3/K_Ca 3.1) channels function at the interface of Ca²⁺ signaling and hyperpolarization; a bidirectional relationship whereby increases in [Ca²⁺ ]_i activate SK_Ca /IK_Ca channels to produce hyperpolarization and vice versa. Further, the spatial domain of hyperpolarization among electrically coupled ECs can be finely tuned via incremental modulation of SK_Ca /IK_Ca channels to balance the strength of local and conducted electrical signals underlying vasomotor activity. Multifunctional properties of the voltage-insensitive SK_Ca /IK_Ca channels of resistance artery endothelium may be employed for therapy during the aging process and development of vascular disease.

Rapid versus slow ascending vasodilatation: intercellular conduction versus flow-mediated signalling with tetanic versus rhythmic muscle contractions

KEY POINTS

In response to exercise, vasodilatation ascends from downstream arterioles into upstream feed arteries (FAs). We hypothesized that the signalling events underlying ascending vasodilatation variy with the intensity and duration of skeletal muscle contraction. In the gluteus maximus muscle of C57BL/6 mice, brief tetanic contraction evoked rapid onset vasodilatation (ROV) (<1 s) throughout the resistance network. Selective damage to endothelium midway between FAs and primary arterioles eliminated ROV only in FAs. Blocking SK_Ca and IK_Ca channels attenuated ROV, implicating hyperpolarization as the underlying signal. During rhythmic twitch contractions, slow onset vasodilatation (10-15 s) in FAs remained intact following loss of ROV and was eliminated following nitric oxide synthase inhibition. Tetanic contraction initiates hyperpolarization that conducts along endothelium into FAs. Rhythmic twitch contractions stimulate FA endothelium to release nitric oxide in response to elevated shear stress secondary to metabolic dilatation of arterioles. Complementary endothelial signalling pathways for ascending vasodilatation ensure increased oxygen delivery to active skeletal muscle.

ABSTRACT

In response to exercise, vasodilatation initiated within the microcirculation of skeletal muscle ascends the resistance network into upstream feed arteries (FAs) located external to the tissue. Ascending vasodilatation (AVD) is essential for reducing FA resistance that otherwise restricts blood flow into the microcirculation. In the present study, we tested the hypothesis that signalling events underlying AVD vary with the intensity and duration of muscle contraction. In the gluteus maximus muscle of anaesthetized male C57BL/6 mice (aged 3-4 months), brief tetanic contraction (100 Hz for 500 ms) evoked rapid onset vasodilatation (ROV) in FAs that peaked within 4 s. By contrast, during rhythmic twitch contractions (4 Hz), slow onset vasodilatation (SOV) of FAs began after ∼10 s and plateaued within 30 s. Selectively damaging the endothelium with light-dye treatment midway between a FA and its primary arteriole eliminated ROV in the FA along with conducted vasodilatation of the FA initiated on the arteriole using ACh microiontophoresis. Superfusion of SK_Ca and IK_Ca channel blockers UCL 1684 + TRAM 34 attenuated ROV, implicating endothelial hyperpolarization as the underlying signal. Nevertheless, the SOV of FAs during rhythmic contractions persisted until inhibition of nitric oxide synthase with N^ω -nitro-l-arginine methyl ester. Thus, ROV of FAs reflects hyperpolarization of downstream arterioles that conducts along the endothelium into proximal FAs. By contrast, SOV of FAs reflects the local production of nitric oxide by the endothelium in response to luminal shear stress, which increases secondary to arteriolar dilatation downstream. Thus, AVD ensures increased oxygen delivery to active muscle fibres by reducing upstream resistance via complementary signalling pathways that reflect the intensity and duration of muscle contraction.

Sympathetic nervous system activation reduces contraction-induced rapid vasodilation in the leg of humans independent of age

Contraction-induced rapid vasodilation is attenuated similarly in the upper and lower limbs of older adults. In the forearm, this attenuation is in part due to a greater sympathetic vasoconstriction. We examined whether the age-related reduction in contraction-induced vasodilation in the leg is also due to a sympathetic vasoconstrictive mechanism. Thirteen young (24 ± 1 yr) and twelve older adults (67 ± 1 yr) performed single-leg knee extension at 20 and 40% of work-rate maximum (WR_max) during control and cold-pressor test (CPT) conditions. Femoral artery diameter and blood velocity were measured using Doppler ultrasound. Vascular conductance (VC; ml·min^-1·mmHg^-1) was calculated using blood flow (ml/min) and mean arterial pressure (mmHg). Peak (ΔVC from baseline) and total VC were blunted in older adults during control conditions across exercise intensities (P < 0.05). Peak and total VC were reduced during CPT in both age groups across exercise intensities (P < 0.05). The relative change (i.e., %reduction; CPT vs. control) in peak (-25 ± 5 vs. -22 ± 4% at 20% WR_max; and -21 ± 6 vs. -27 ± 5% at 40% WR_max; P = 0.42-0.55) and total VC (-28 ± 5 vs. -36 ± 6% at 20% WR_max; and -22 ± 8 vs. -33 ± 5% at 40% WR_max; P = 0.23-0.34) were similar between young and older adults. When matched for absolute workload (~10 W), age differences persisted in peak VC (P < 0.05) under both conditions, with similar relative changes in peak and total VC during CPT. Our data suggest that 1) sympathetic stimulation reduces contraction-induced rapid vasodilation in the leg of young and older adults similarly; and 2) enhanced sympathetic vasoconstriction does not fully explain age-related differences in contraction-induced vasodilation within the leg.NEW & NOTEWORTHY Aging is associated with attenuated contraction-induced rapid onset vasodilation (ROV). Within the forearm, this attenuation is partially due to enhanced sympathetic vasoconstriction. In the current study, we found that sympathetic vasoconstriction reduces contraction-induced ROV within the leg of both young and older adults, with the magnitude of change being similar between age groups. Our current results suggest that age-related attenuations in contraction-induced ROV within the leg are not fully explained by a sympathetic vasoconstrictor mechanism.