No change in myonuclear number during muscle unloading and reloading

Muscle fibers are the cells in the body with the largest volume, and they have multiple nuclei serving different domains of cytoplasm. A large body of previous literature has suggested that atrophy induced by hindlimb suspension leads to a loss of "excessive" myonuclei by apoptosis. We demonstrate here that atrophy induced by hindlimb suspension does not lead to loss of myonuclei despite a strong increase in apoptotic activity of other types of nuclei within the muscle tissue. Thus hindlimb suspension turns out to be similar to other atrophy models such as denervation, nerve impulse block, and antagonist ablation. We discuss how the different outcome of various studies can be attributed to difficulties in separating myonuclei from other nuclei, and to systematic differences in passive properties between normal and unloaded muscles. During reload, after hindlimb suspension, a radial regrowth is observed, which has been believed to be accompanied by recruitment of new myonuclei from satellite cells. The lack of nuclear loss during unloading, however, puts these findings into question. We observed that reload led to an increase in cross sectional area of 59%, and fiber size was completely restored to the presuspension levels. Despite this notable growth there was no increase in the number of myonuclei. Thus radial regrowth seems to differ from de novo hypertrophy in that nuclei are only added during the latter. We speculate that the number of myonuclei might reflect the largest size the muscle fibers have had in its previous history.

The effects of cold water immersion and active recovery on inflammation and cell stress responses in human skeletal muscle after resistance exercise

KEY POINTS

Cold water immersion and active recovery are common post-exercise recovery treatments. A key assumption about the benefits of cold water immersion is that it reduces inflammation in skeletal muscle. However, no data are available from humans to support this notion. We compared the effects of cold water immersion and active recovery on inflammatory and cellular stress responses in skeletal muscle from exercise-trained men 2, 24 and 48 h during recovery after acute resistance exercise. Exercise led to the infiltration of inflammatory cells, with increased mRNA expression of pro-inflammatory cytokines and neurotrophins, and the subcellular translocation of heat shock proteins in muscle. These responses did not differ significantly between cold water immersion and active recovery. Our results suggest that cold water immersion is no more effective than active recovery for minimizing the inflammatory and stress responses in muscle after resistance exercise.

ABSTRACT

Cold water immersion and active recovery are common post-exercise recovery treatments. However, little is known about whether these treatments influence inflammation and cellular stress in human skeletal muscle after exercise. We compared the effects of cold water immersion versus active recovery on inflammatory cells, pro-inflammatory cytokines, neurotrophins and heat shock proteins (HSPs) in skeletal muscle after intense resistance exercise. Nine active men performed unilateral lower-body resistance exercise on separate days, at least 1 week apart. On one day, they immersed their lower body in cold water (10°C) for 10 min after exercise. On the other day, they cycled at a low intensity for 10 min after exercise. Muscle biopsies were collected from the exercised leg before, 2, 24 and 48 h after exercise in both trials. Exercise increased intramuscular neutrophil and macrophage counts, MAC1 and CD163 mRNA expression (P < 0.05). Exercise also increased IL1β, TNF, IL6, CCL2, CCL4, CXCL2, IL8 and LIF mRNA expression (P < 0.05). As evidence of hyperalgesia, the expression of NGF and GDNF mRNA increased after exercise (P < 0.05). The cytosolic protein content of αB-crystallin and HSP70 decreased after exercise (P < 0.05). This response was accompanied by increases in the cytoskeletal protein content of αB-crystallin and the percentage of type II fibres stained for αB-crystallin. Changes in inflammatory cells, cytokines, neurotrophins and HSPs did not differ significantly between the recovery treatments. These findings indicate that cold water immersion is no more effective than active recovery for reducing inflammation or cellular stress in muscle after a bout of resistance exercise.

Stimulated release of a hyperpolarizing factor (ADHF) from mesenteric artery perivascular adipose tissue: involvement of myocyte BKCa channels and adiponectin

BACKGROUND AND PURPOSE

Perivascular adipose tissue (PVAT) releases adipocyte-derived hyperpolarizing factors (ADHFs) that may partly act by opening myocyte K(+) channels. The present study in rat and mouse mesenteric arteries aimed to identify the myocyte K(+) channel activated by PVAT and to determine whether adiponectin contributed to the hyperpolarizing effects of PVAT.

EXPERIMENTAL APPROACH

Myocyte membrane potential was recorded from de-endothelialized, non-contracted rat and mouse mesenteric arteries in the presence and absence of PVAT.

KEY RESULTS

The β3 -adrenoceptor agonist, CL-316,243 (10 μM), generated PVAT-dependent, iberiotoxin-sensitive myocyte hyperpolarizations resulting from BKCa channel opening and which were partially blocked by L-NMMA (100 μM). Adiponectin (5 μg·mL(-1) ) also produced iberiotoxin-sensitive hyperpolarizations in PVAT-denuded arterioles. Activation of myocyte AMP-activated protein kinase (AMPK) using 5 μM A-769662 also induced BKCa -mediated hyperpolarizations. Dorsomorphin abolished hyperpolarizations to CL-316,243, adiponectin and A-769662. In vessels from Adipo(-/-) mice, hyperpolarizations to CL-316,243 were absent whereas those to A-769662 and adiponectin were normal. In rat vessels, adipocyte-dependent hyperpolarizations were blocked by glibenclamide and clotrimazole but those to NS1619 (33 μM) were unaltered.

CONCLUSIONS AND IMPLICATIONS

Under basal, non-contracted conditions, β3 -adrenoceptor stimulation of PVAT releases an ADHF, which is probably adiponectin. This activates AMPK to open myocyte BKCa channels indirectly and additionally liberates NO, which also contributes to the observed PVAT-dependent myocyte hyperpolarizations. Clotrimazole and glibenclamide each reversed hyperpolarizations to adiponectin and A-769662, suggesting the involvement of myocyte TRPM4 channels in the ADHF-induced myocyte electrical changes mediated via the opening of BKCa channels.

Inflammatory markers CD11b, CD16, CD66b, CD68, myeloperoxidase and neutrophil elastase in eccentric exercised human skeletal muscles

The aim of the present study was to investigate leucocyte markers, CD11b, CD16, CD66b, CD68, myeloperoxidase and neutrophil elastase on skeletal muscle biopsies from biceps brachii after unaccustomed eccentric exercise followed by the second bout of exercise 3 weeks later. The subjects (10 subjects received COX-2 inhibitor (Celecoxib) and 13 subjects received placebo) were divided into three categories: mild, moderate and severe effect of eccentric exercise, according to the reduction and recovery of muscle force-generating capacity after performing 70 maximal eccentric actions with elbow flexors on an isokinetic dynamometer. The results showed that the CD66b antibody was applicable for localization of neutrophils in human skeletal muscle, whereas the other studied neutrophil markers recognized also other leucocytes than neutrophils. The number of CD66b positive cells in skeletal muscle was very low and was not affected by the exercise. The macrophage marker CD68 showed reactivity also against satellite cells and fibroblast-like cells in skeletal muscle and therefore cannot be applied as a quantitative value for inflammatory cells. Skeletal muscle fibre injury, shown as dystrophin negative fibres, was observed approximately in half of the biopsies at 4 and 7 days after the first exercise bout in the categories moderate and severe effect of eccentric exercise. These subjects represent the most prominent loss in muscle force-generating capacity both at the category and the individual levels. Furthermore, deformed skeletal muscle fibres were observed in five subjects in these categories after the second bout of exercise. The present results suggest that neutrophils are not involved in skeletal muscle fibre injury and the reduction in muscle force-generating capacity after a single bout of eccentric exercise is a good indirect indicator of muscle damage in humans. Furthermore, prolonged regeneration process could be one of the reasons for impaired peripheral muscle function after high-force eccentric exercise.

The vascular extracellular calcium-sensing receptor: an update

The extracellular calcium-sensing receptor (CaR) was first described in the parathyroid gland. Recent studies have shown that the CaR is also expressed in blood vessels, especially in the endothelial and adventitial layers but its physiological function is still not clear. However, an understanding of its possible role(s) in the vasculature (perivascular-neurones, heart and blood vessels) is important because of the use of synthetic positive allosteric CaR modulators in hyperparathyroidism and the potential importance of negative modulators in the treatment of osteoporosis. In this review, the effects of CaR activation and inhibition are detailed and the possible role of the CaR as both an amplifier and attenuator of myo-endothelial coupling in the vasculature is described.

Deficient SARS-CoV-2 hybrid immunity during inflammatory bowel disease

Patients with Inflammatory Bowel Disease (IBD) undergoing immunosuppressive therapies face heightened susceptibility to severe COVID-19. An in-depth understanding of systemic inflammation and cellular immune responses after SARS-CoV-2 vaccination and breakthrough infections (BTI) is required for optimizing vaccine strategies in this population. While the prevalence of high serological responders post- third COVID-19 vaccine dose was lower, and the antibody waning was higher in IBD patients than in healthy donors (HD), IBD patients showed an increase in anti-RBD Wild Type IgG levels and cross-reactive Spike -specific memory B cells following BTI. However, there was no significant enhancement in cellular immune responses against anti-SARS-CoV-2 post-BTI, with responses instead characterized by activation of SARS-CoV-2 specific and also bystander CD8 T cells. These results suggest a complex interaction between chronic inflammation in IBD and the generation of new immune responses, highlighting the need for tailored vaccine regimens and anti-inflammatory therapies to boost cellular immunity against SARS-CoV-2.

Vaccine responses and hybrid immunity in people living with HIV after SARS-CoV-2 breakthrough infections

The COVID-19 pandemic posed a challenge for people living with HIV (PLWH), particularly immune non-responders (INR) with compromised CD4 T-cell reconstitution following antiretroviral therapy (CD4 count <350 cells per mm3). Their diminished vaccine responses raised concerns about their vulnerability to SARS-CoV-2 breakthrough infections (BTI). Our in-depth study here revealed chronic inflammation in PLWH and a limited anti-Spike IgG response after vaccination in INR. Nevertheless, the imprinting of Spike-specific B cells by vaccination significantly enhanced the humoral responses after BTI. Notably, the magnitude of cellular CD4 response in all PLWH was comparable to that in healthy donors (HD). However, the polyfunctionality and phenotype of Spike-specific CD8 T cells in INR differed from controls. The findings highlight the need for additional boosters with variant vaccines, and for monitoring ART adherence and the durability of both humoral and cellular anti-SARS-CoV-2 immunity in INR.