Abstract Skeletal muscle mass reflects the balance between muscle protein synthesis (MPS) and muscle protein breakdown (MPB). In healthy humans, a decline in MPS is recognized as the primary driver of muscle disuse‐atrophy, with limited contribution from increased MPB. However, disuse‐induced declines in protein pool size directly influence the calculation of muscle protein turnover. We examined how accounting for changes in protein pool size impacted MPS and MPB rates assessed using deuterium oxide in healthy humans. Fourteen females (21 ± 2 years of age; 24.5 ± 3.0 kg/m 2 ) had one leg subjected to 10 days of unloading, whilst the contralateral leg underwent loading in the form of resistance exercise. Steady‐state and non‐steady‐state modelling approaches were used to calculate MPS and MPB rates. In response to unloading, there was a significant decline in MRI‐derived vastus lateralis volume, and thus non‐steady‐state equations were applied exclusively to this limb. Regardless of whether steady‐state or non‐steady‐state approaches were applied, mixed MPS rates were significantly reduced in response to unloading, whilst mixed MPB rates remained unchanged. Notably, unloading elicited protein fraction‐specific responses, whereby myofibrillar MPS rates declined significantly whilst sarcoplasmic MPS rates were unaltered. We also found that in response to loading, mixed, myofibrillar and sarcoplasmic MPS rates remained unchanged compared with basal rates. Our findings show that unilateral unloading in females induces muscle disuse‐atrophy driven by reductions in MPS rates, rather than by elevations in MPB rates, a response that was protein fraction specific and preserved irrespective of whether steady‐state or non‐steady‐state equations were used. image Key points In humans, short‐term muscle disuse‐atrophy is generally attributed to reduced muscle protein synthesis (MPS) rates, but integrated tracer approaches to measure protein turnover can be influenced by changes in protein pool size. We used deuterated water ( 2 H 2 O) to measure rates of MPS and muscle protein breakdown (MPB) during 10 days of unilateral unloading in young, healthy females. Steady‐state and non‐steady‐state equations were applied to account for changes in the protein pool size that occur with disuse‐atrophy. Unloading reduced muscle size and MPS rates, whereas MPB rates were unchanged. Accounting for the change in the protein pool size using non‐steady‐state equations did not alter this conclusion. These findings refine our interpretation of the mechanisms driving muscle disuse‐atrophy in healthy humans that might differ from more severe clinical or preclinical scenarios of muscle wasting.
Abstract The endocannabinoid system is important for pain modulation, whereas its utilization for analgesia is restricted due to the limited efficacy of cannabinoids and side effects. Cannabinoid receptor 1 (CB 1 ) in the spinal dorsal horn provides presynaptic inhibition of nociceptive neurotransmission. CB 1 regulatory protein, SH3‐containing GRB2‐like protein 3‐interacting protein 1 (SGIP1), controls its functional availability, while its role in pain modulation remains poorly understood. We examined the role of SGIP1 in nociceptive signalling under control and inflammatory conditions, using behavioural tests, patch‐clamp recordings in spinal cord slices and calcium imaging of dorsal root ganglion (DRG) neurons. SGIP1 knockout (KO) mice of both sexes showed a selective reduction in heat sensitivity, with no change in mechanical withdrawal threshold. Following carrageenan‐induced inflammation, both wild‐type (WT) and KO mice developed mechanical allodynia, whereas thermal hyperalgesia in KO mice reached only the control values of WT mice. Intrathecal application of cannabinoid agonist WIN55,212‐2 produced analgesia in both WT and KO mice, with enhanced effect of the lower dose (5 µg vs . 10 µg) in KO animals. WIN55,212‐2 (1 µM) in slices from inflamed SGIP1 KO mice induced significantly greater reduction in miniature excitatory postsynaptic current (mEPSC) frequency, without amplitude change, indicating enhanced presynaptic inhibition in KO mice. These enhanced inhibitory effects of WIN55,212‐2 in KO mice after inflammation were further demonstrated in DRG cultured neurons. Our results establish SGIP1 as a state‐dependent negative modulator of CB 1 function and identify SGIP1–CB 1 interaction as a candidate target for selective enhancement of cannabinoid analgesia under inflammatory pain conditions. image Key points SGIP1 deletion reduced basal heat sensitivity in mice of both sexes without altering mechanical threshold, suggesting a sex‐independent role in thermal nociception. Inflammation increased mechanical and thermal sensitivity in both SGIP1 KO and WT mice, whereas the increased thermal sensitivity of KO reached only the baseline levels of WT mice. Intrathecal cannabinoid agonist WIN55,212‐2 produced analgesia in both genotypes, with a bigger effect of the lower dose (5 vs . 10 µg) in SGIP1 KO mice. WIN55,212‐2 induced strong inhibition of miniature excitatory postsynaptic current (mEPSC) frequency in dorsal horn neurons that was selectively potentiated by SGIP1 deletion under inflammatory, but not control, conditions, consistent with a presynaptic locus of action. These effects were further confirmed by calcium imaging in DRG cultured neurons. SGIP1 acts as an inflammation‐dependent presynaptic CB 1 modulator of spinal nociceptive transmission, positioning the SGIP1–CB 1 interaction as a candidate target for selective enhancement of cannabinoid analgesia.
Abstract Localized application of heat is used therapeutically to reduce skeletal muscle pain and improve mobility and recovery in various populations. The efficacy of this thermal therapy is considered to be partly a result of increased blood flow in the skeletal muscle directly exposed to heat (i.e. thermal hyperaemia). However, the mechanisms mediating thermal hyperaemia in skeletal muscle of humans remain poorly understood. Voltage‐dependent potassium (K V ) channels play a pivotal role linking upstream vasodilatory signals to electro‐mechanical mechanisms that mediate vascular smooth muscle relaxation. Therefore, the present study aimed to test the hypothesis that K V channel antagonism would attenuate thermal hyperaemia in skeletal muscle of humans. We used microdialysis to directly measure local blood flow via the ethanol washout technique at two sites in the vastus lateralis muscle of 10 healthy adults (seven women). One probe was perfused with 0.9% saline, whereas the other was perfused with the K V channel antagonist 4‐aminopyridine (26.6 m m ). Pulsed short‐wave diathermy was used to induce deep heating of the vastus lateralis for 90 min. Muscle temperature was measured midway between the microdialysis probes via a thermocouple. Baseline blood flow did not differ between probes ( P = 0.684). Pulsed short‐wave diathermy increased skeletal muscle temperature through 90 min of heating ( P < 0.001). However, the accompanying increase in blood flow at 90 min was attenuated when perfused with 4‐aminopyridine ( P = 0.0222). These data suggest that K V channels substantially contribute to thermal hyperaemia in skeletal muscle of humans. image Key points The efficacy of thermal therapy is considered to be partly a result of increased blood flow in the skeletal muscle directly exposed to heat (i.e. thermal hyperaemia). However, the mechanisms mediating thermal hyperaemia in skeletal muscle remain poorly understood. Voltage‐dependent potassium (K V ) channels play a pivotal role as downstream effectors that mediate vascular smooth muscle relaxation. Therefore, we tested the hypothesis that K V channel antagonism would attenuate thermal hyperaemia in skeletal muscle of humans. Using microdialysis, we found that K V channel antagonism with 4‐aminopyridine attenuated thermal hyperaemia at 90 min of localized heating, when this vasodilatory response was the greatest. We identified a critical role for K V channels in human skeletal muscle thermal hyperaemia, establishing these channels as important downstream effectors of this vasodilatory response.
