Effects of endogenous nitric oxide in activation of group IV muscle afferents

Task Failure during Exercise to Exhaustion in Normoxia and Hypoxia Is Due to Reduced Muscle Activation Caused by Central Mechanisms While Muscle Metaboreflex Does Not Limit Performance

To determine whether task failure during incremental exercise to exhaustion (IE) is principally due to reduced neural drive and increased metaboreflex activation eleven men (22 ± 2 years) performed a 10 s control isokinetic sprint (IS; 80 rpm) after a short warm-up. This was immediately followed by an IE in normoxia (Nx, P_IO₂:143 mmHg) and hypoxia (Hyp, P_IO₂:73 mmHg) in random order, separated by a 120 min resting period. At exhaustion, the circulation of both legs was occluded instantaneously (300 mmHg) during 10 or 60 s to impede recovery and increase metaboreflex activation. This was immediately followed by an IS with open circulation. Electromyographic recordings were obtained from the vastus medialis and lateralis. Muscle biopsies and blood gases were obtained in separate experiments. During the last 10 s of the IE, pulmonary ventilation, VO₂, power output and muscle activation were lower in hypoxia than in normoxia, while pedaling rate was similar. Compared to the control sprint, performance (IS-Wpeak) was reduced to a greater extent after the IE-Nx (11% lower P < 0.05) than IE-Hyp. The root mean square (EMG_RMS) was reduced by 38 and 27% during IS performed after IE-Nx and IE-Hyp, respectively (Nx vs. Hyp: P < 0.05). Post-ischemia IS-EMG_RMS values were higher than during the last 10 s of IE. Sprint exercise mean (IS-MPF) and median (IS-MdPF) power frequencies, and burst duration, were more reduced after IE-Nx than IE-Hyp (P < 0.05). Despite increased muscle lactate accumulation, acidification, and metaboreflex activation from 10 to 60 s of ischemia, IS-Wmean (+23%) and burst duration (+10%) increased, while IS-EMG_RMS decreased (−24%, P < 0.05), with IS-MPF and IS-MdPF remaining unchanged. In conclusion, close to task failure, muscle activation is lower in hypoxia than in normoxia. Task failure is predominantly caused by central mechanisms, which recover to great extent within 1 min even when the legs remain ischemic. There is dissociation between the recovery of EMG_RMS and performance. The reduction of surface electromyogram MPF, MdPF and burst duration due to fatigue is associated but not caused by muscle acidification and lactate accumulation. Despite metaboreflex stimulation, muscle activation and power output recovers partly in ischemia indicating that metaboreflex activation has a minor impact on sprint performance.

Afferent nerve sensitivity is decreased by an iNOS-dependent mechanism during indomethacin-induced inflammation in the murine jejunum in vitro

Evidence exists that visceral afferent sensitivity is subject to regulatory mechanisms. We hypothesized that afferent sensitivity is decreased in the small intestine during intestinal inflammation by an inducible nitric oxide synthase (iNOS)-dependent mechanism. C57BL/6 mice were injected twice with vehicle or 60 mg kg(-1) indomethacin subcutaneously to induce intestinal inflammation. Afferent sensitivity was recorded on day 3 from a 2-cm segment of jejunum in vitro by extracellular multi-unit afferent recordings from the mesenteric nerve bundle. In subgroups (n = 6), iNOS was inhibited selectively by L-N6-(1-iminoethyl)-lysine (L-NIL) given either chronically from day 1-3 (3 mg kg(-1) twice daily i.p.) or acutely into the organ bath (30 micromol L(-1)). The indomethacin-induced increase of macroscopic and microscopic scores of intestinal inflammation (both P < 0.05) were unchanged after pretreatment with L-NIL. Peak afferent firing following bradykinin (0.5 micromol L(-1)) was 55 +/- 8 impulse s(-1) during inflammation vs 97 +/- 7 impulse s(-1) in controls (P < 0.05). Normal firing rate was preserved following L-NIL pretreatment (112 +/- 16 impulse s(-1)) or acute administration of L-NIL (108 +/- 14 impulse s(-1)). A similar L-NIL dependent reduction was observed for 5-HT (250 micromol L(-1)) and mechanical ramp distension from 20 to 60 cmH(2)O (both P < 0.05). Intraluminal pressure peaks were decreased to 0.66 +/- 0.1 cmH(2)O during inflammation compared to 2.51 +/- 0.3 in controls (P < 0.01). Afferent sensitivity is decreased by an iNOS-dependent mechanism during intestinal inflammation which appears to be independent of the inflammatory response. This suggests that iNOS-dependent nitric oxide production alters afferent sensitivity during inflammation by interfering with signal transduction to afferent nerves rather than by attenuating intestinal inflammation.

Hyperoxia enhances metaboreflex sensitivity during static exercise in humans

Peripheral chemoreflex inhibition with hyperoxia decreases sympathetic nerve traffic to muscle circulation [muscle sympathetic nerve activity (MSNA)]. Hyperoxia also decreases lactate production during exercise. However, hyperoxia markedly increases the activation of sensory endings in skeletal muscle in animal studies. We tested the hypothesis that hyperoxia increases the MSNA and mean blood pressure (MBP) responses to isometric exercise. The effects of breathing 21% and 100% oxygen at rest and during isometric handgrip at 30% of maximal voluntary contraction on MSNA, heart rate (HR), MBP, blood lactate (BL), and arterial O2 saturation (SaO2) were determined in 12 healthy men. The isometric handgrips were followed by 3 min of postexercise circulatory arrest (PE-CA) to allow metaboreflex activation in the absence of other reflex mechanisms. Hyperoxia lowered resting MSNA, HR, MBP, and BL but increased Sa(O2) compared with normoxia (all P < 0.05). MSNA and MBP increased more when exercise was performed in hyperoxia than in normoxia (MSNA: hyperoxic exercise, 255 +/- 100% vs. normoxic exercise, 211 +/- 80%, P = 0.04; and MBP: hyperoxic exercise, 33 +/- 9 mmHg vs. normoxic exercise, 26 +/- 10 mmHg, P = 0.03). During PE-CA, MSNA and MBP remained elevated (both P < 0.05) and to a larger extent during hyperoxia than normoxia (P < 0.05). Hyperoxia enhances the sympathetic and blood pressure (BP) reactivity to metaboreflex activation. This is due to an increase in metaboreflex sensitivity by hyperoxia that overrules the sympathoinhibitory and BP lowering effects of chemoreflex inhibition. This occurs despite a reduced lactic acid production.

Capsaicin-induced effects on c-fos expression and NADPH-diaphorase activity in the feline spinal cord

The distribution of c-fos expression and NADPH-diaphorase reactivity in the cervical and lumbar segments after stimulation of the vanilloid receptors in the dorsal neck muscles with capsaicin was studied in cats anaesthetized with alpha-chloralose. After the unilateral intramuscular injection of capsaicin, the mean number of Fos-immunoreactive neurons detected with an avidin-biotin-peroxidase technique was significantly increased in the superficial laminae (I), neck of the dorsal horn (V), and area around the central canal (VII) within both the cervical and lumbar spinal cord. Most Fos-immunoreactive neurons in the cervical spinal cord were giant and small cells. The widespread distribution of Fos-immunoreactive cells throughout the cervical cord within the intermediate zone (VII) coincided with the sites of localization of last-order premotor interneurons and cells of origin of inter-segmental crossed and uncrossed descending propriospinal pathways to the lumbar spinal cord. Fos-immunoreactive neurons were co-distributed with nitric oxide-generating cells at both levels of the spinal cord, although the double-labeled cells were not observed. In conclusion, the analysis of c-fos expression and NADPH-diaphorase reactivity shows that stimulation of vanilloid receptors in the neck muscles can initiate distinctive neuronal plasticity in the cervical (C1-C8) and lumbar (L1-L7) segments, and confirms the anatomical and functional coupling of both regions during processing of nociceptive signals from the dorsal neck muscles.

Interactions between endogenous nitric oxide and hypoxemia in activation of group IV muscle afferents

It has previously been shown that both hypoxemia and nitric oxide (NO) synthase blockade depress the activation of group IV muscle afferents after muscle stimulation (MS). In the present study, we questioned whether hypoxemia exerts a specific inhibitory influence, independently from its effects on endogenous NO formation. This hypothesis was tested in two groups of anesthetized rabbits in which we examined the effects of hypoxemia, and then of subsequent NO synthase blockade by N(G)-nitro-L-arginine methyl ester (L-NAME), and vice versa. In each protocol, group IV afferent activity was recorded from the resting tibialis anterior muscle and after 3-min periods of MS that elicited a significant decrease in muscle force. NO synthase blockade in normoxemia suppressed the group IV afferent response to MS, and hypoxemia alone significantly reduced the post-MS activation of these nerve afferents (+18% vs. +28% in normoxemia). In hypoxemic rabbits, further NO synthase blockade abolished the post-MS activation of group IV afferents. Moreover, when hypoxemia followed the NO synthase blockade, MS significantly reduced the discharge of group IV afferents (-28%). Thus, while these muscle afferents are activated after fatiguing muscle contractions when the endogenous NO production is present, they are deactivated by hypoxemia when NO production is blocked. We conclude that endogenous NO production and hypoxemia exert opposite effects on the activation of the group IV afferents. Our data anticipate the neuromuscular side effects of treatments using exogenous NO or drugs acting on endogenous NO production.

Effects of chronic hypoxemia on the afferent nerve activities from skeletal muscle

An acute reduction of the oxygen supply to contracting muscles not only affects their metabolism but also modifies their sensorimotor control through changes in afferent discharge of the group I and group III-IV nerve fibers, the latter playing a pivotal role in the protective mechanisms against muscle fatigue. The effects of chronic hypoxemia on the muscle sensitivity are totally unknown. In the present study, group I fibers (mechanosensory afferents) and group III-IV fibers (mechanosensory and chemosensory afferents) from the anterior tibial muscle were recorded in normoxemic and chronic hypoxemic rats. Hypoxemic rats breathed for 45 d a gas mixture containing 9.5 to 10% O(2) in N(2). The data were compared with those obtained in normoxemic animals of the same age. To activate the different muscle afferents, we used different test agents, including electrically induced fatigue (EIF), KCl, lactic acid injections, as well as tendon vibrations. The conduction velocity of all nerve fibers was significantly (p < 0.01) higher in hypoxemic rats than in the normoxemic group. Chronic hypoxemia significantly depressed the response of the group III-IV muscle afferents to KCl injections and even abolished their response to lactic acid and EIF. However, the response to tendon vibrations of the group I afferents was similar in hypoxemic and normoxemic rats. These results suggest that chronic hypoxemia markedly alters the chemosensitivity of the group III-IV muscle afferents, which may explain the higher fatigability of hypoxemic subjects.