Type 2 diabetes impairs vascular responsiveness to nitric oxide, but not the venoarteriolar reflex or post-occlusive reactive hyperaemia in forearm skin

The venoarteriolar reflex (VAR) is a local mechanism by which vasoconstriction is mediated in response to venous congestion. This response may minimize tissue overperfusion, preventing capillary damage and oedema. Post-occlusive reactive hyperaemia (PORH) is used to assess microvascular function by performing a brief local arterial occlusion resulting in a subsequent rapid transient vasodilation. In the current study, we hypothesized that type 2 diabetes (T2D) attenuates VAR and PORH responses in forearm skin in vivo. In 11 healthy older adults (Control, 58 ± 8 years) and 13 older adults with controlled T2D (62 ± 10 years), cutaneous blood flow measured by laser-Doppler flowmetry was monitored following a 3-min venous occlusion of 45 mm Hg that elicited the VAR, followed by a 3-min recovery period and then a 5-min arterial occlusion of 240 mm Hg that induced PORH. Finally, sodium nitroprusside, a nitric oxide donor, was administered to induce maximum vasodilation. VAR and PORH variables were similar between groups. By contrast, maximal cutaneous blood flow induced by sodium nitroprusside was lower in the T2D group. Taken together, our observations indicate that T2D impairs vascular smooth muscle responsiveness to nitric oxide, but not VAR and PORH in forearm skin.

The effect of seasonal acclimatization on whole body heat loss response during exercise in a hot humid environment with different air velocity

Seasonal acclimatization from winter to summer is known to enhance thermoeffector responses in hot-dry environments during exercise whereas its impact on sweat evaporation and core temperature (T_core) responses in hot-humid environments remains unknown. We, therefore, sought to determine whether seasonal acclimatization is able to modulate whole body sweat rate (WBSR), evaporated sweat rate, sweating efficiency, and thermoregulatory function during cycling exercise in a hot-humid environment (32°C, 75% RH). We also determined whether the increase in air velocity could enhance evaporated sweat rate and sweating efficiency before and after seasonal acclimatization. Twelve males cycled for 1 h at 40% V̇o_2max in winter (preacclimatization) and repeated the trial again in summer (after acclimatization). For the last 20 min of cycling at a steady-state of T_core, air velocity increased from 0.2 (0.04) m/s to 1.1 (0.02) m/s by using an electric fan located in front of the participant. Seasonal acclimatization enhanced WBSR, unevaporated sweat rate, local sweat rate and mean skin temperature compared with preacclimatization state (all P < 0.05) whereas sweating efficiency was lower (P < 0.01) until 55 min of exercise. T_core and evaporated sweat rate were unaltered by acclimatization status (all P > 0.70). In conclusion, seasonal acclimatization enhances thermoeffector responses but does not attenuate T_core during exercise in a hot-humid environment. Furthermore, increasing air velocity enhances evaporated sweat rate and sweating efficiency irrespective of acclimated state. NEW & NOTEWORTHY Seasonal acclimatization to humid heat enhances eccrine sweat gland function and thus results in a higher local and whole body sweat rate but does not attenuate T_core during exercise in a hot-humid environment. Sweating efficiency is lower after seasonal acclimatization to humid heat compared with preacclimatization with and without the increase of air velocity. However, having a lower sweating efficiency does not mitigate the T_core response during exercise in a hot-humid environment.

Effects of Isomaltulose Ingestion on Thermoregulatory Responses during Exercise in a Hot Environment

Isomaltulose is a low glycemic and insulinemic carbohydrate available as a constituent of sports drinks. However, it remains unclear whether thermoregulatory responses (sweating and cutaneous vasodilation) after isomaltulose drink ingestion differ from those of sucrose and water during exercise in a hot environment. Ten young healthy males consumed 10% sucrose, 10% isomaltulose, or water drinks. Thirty-five minutes after ingestion, they cycled for fifteen minutes at 75% peak oxygen uptake in a hot environment (30 °C, 40% relative humidity). Sucrose ingestion induced greater blood glucose concentration and insulin secretion at the pre-exercise state, compared with isomaltulose and/or water trials, with no differences during exercise in blood glucose. Change in plasma volume did not differ between the three trials throughout the experiment, but both sucrose and isomaltulose ingestions similarly increased plasma osmolality, as compared with water (main beverage effect, p = 0.040)-a key response that potentially delays the onset of heat loss responses. However, core temperature thresholds and slopes for heat loss responses were not different between the trials during exercise. These results suggest that ingestion of isomaltulose beverages induces low glycemic and insulinemic states before exercise but does not alter thermoregulatory responses during exercise in a hot environment, compared with sucrose or water.

Caffeine Exacerbates Hyperventilation and Reductions in Cerebral Blood Flow in Physically Fit Men Exercising in the Heat

INTRODUCTION

Caffeine is an exercise performance enhancer widely used by individuals engaged in training or competition under heat-stressed conditions. Caffeine ingestion during exercise in the heat is believed to be safe because it does not greatly affect body temperature responses, heart rate, or body fluid status. However, it remains unknown whether caffeine affects hyperthermia-induced hyperventilation or reductions in the cerebral blood flow index. We tested the hypothesis that under conditions inducing severe hyperthermia, caffeine exacerbates hyperthermia-induced hyperventilation and reduces the cerebral blood flow index during exercise.

METHODS

Using a randomized, single-blind, crossover design, 12 physically active healthy young men (23 ± 2 yr) consumed a moderate dose of caffeine (5 mg·kg-1) or placebo in the heat (37°C). Approximately 60 min after the ingestion, they cycled for ~45 min at a workload equal to ~55% of their predetermined peak oxygen uptake (moderate intensity) until their core temperature increased to 2.0°C above its preexercise baseline level.

RESULTS

In both trials, ventilation increased and the cerebral blood flow index assessed by middle cerebral artery mean blood velocity decreased as core temperature rose during exercise (P < 0.05), indicating that hyperthermia-induced hyperventilation and lowering of the cerebral blood flow occurred. When core temperature was elevated by 1.5°C or more (P < 0.05), ventilation was higher and the cerebral blood flow was lower throughout the caffeine trial than the placebo trial (P < 0.05).

CONCLUSIONS

A moderate dose of caffeine exacerbates hyperthermia-induced hyperventilation and reductions in the cerebral blood flow index during exercise in the heat with severe hyperthermia.

Effects of short-term heat acclimation on whole-body heat exchange and local nitric oxide synthase- and cyclooxygenase-dependent heat loss responses in exercising older men

NEW FINDINGS

What is the central question of this study? Does short-term heat acclimation enhance whole-body evaporative heat loss and augment nitric oxide synthase (NOS)-dependent cutaneous vasodilatation and NOS- and cyclooxygenase (COX)-dependent sweating, in exercising older men? What is the main finding and its importance? Our preliminary data (n = 8) demonstrated that short-term heat acclimation improved whole-body evaporative heat loss, but it did not influence the effects of NOS and/or COX inhibition on cutaneous vasodilatation or sweating in older men during an exercise-heat stress. These outcomes might imply that although short-term heat acclimation enhances heat dissipation in older men, it does not modulate NOS- and COX-dependent control of cutaneous vasodilatation or sweating on the forearm.

ABSTRACT

Ageing is associated with decrements in whole-body heat loss (evaporative + dry heat exchange), which might stem from alterations in nitric oxide synthase (NOS)- and cyclooxygenase (COX)-dependent cutaneous vasodilatation and sweating. We evaluated whether short-term heat acclimation would (i) enhance whole-body heat loss primarily by increasing evaporative heat loss, and (ii) augment NOS-dependent cutaneous vasodilatation and NOS- and COX-dependent sweating, in exercising older men. Eight older men [mean (SD) age, 59 (8) years] completed a calorimetry and microdialysis trial before and after 7 days of exercise-heat acclimation. For the calorimetry trials, whole-body evaporative and dry heat exchange were assessed using direct calorimetry during 30 min bouts of cycling at light, moderate and vigorous metabolic heat productions (150, 200 and 250 W/m² , respectively) in dry heat (40°C, 20% relative humidity). For the microdialysis trials, local cutaneous vascular conductance and sweat rate were assessed during 60 min exercise in the heat (35°C, 20% relative humidity) at four dorsal forearm skin sites treated with lactated Ringer solution (control), NOS inhibitor, COX inhibitor or combined NOS and COX inhibitors, via microdialysis. Evaporative heat loss during moderate (P = 0.036) and vigorous (P = 0.021) exercise increased after acclimation. Inhibition of NOS alone reduced cutaneous vascular conductance to a similar extent before and after acclimation (P < 0.040), whereas separate and combined NOS and COX inhibition had no significant effects on sweating relative to the control site (P = 0.745). Our preliminary results might suggest that short-term heat acclimation improves evaporative heat loss, but does not significantly modulate the contributions of NOS or COX to cutaneous vasodilatation or sweating on the forearm in older men during an exercise-heat stress.

The relative contribution of α- and β-adrenergic sweating during heat exposure and the influence of sex and training status

While human eccrine sweat glands respond to adrenergic agonists, there remains a paucity of information on the factors modulating this response. Thus, we assessed the relative contribution of α- and β-adrenergic sweating during a heat exposure and as a function of individual factors of sex and training status. α- and β-adrenergic sweating was assessed in forty-eight healthy young men (n = 35) and women (n = 13) including endurance-trained (n = 12) and untrained men (n = 12) under non-heat exposure (temperate, 25°C; n = 17) and heat exposure (hot, 35°C; n = 48) conditions using transdermal iontophoresis of phenylephrine (α-adrenergic agonist) and salbutamol (β-adrenergic agonist) on the ventral forearm, respectively. Adrenergic sweating was also measured after iontophoretic administration of atropine (muscarinic receptor antagonist) or saline (control) to evaluate how changes in muscarinic receptor activity modulate the adrenergic response to a heat exposure (n = 12). α- and β-adrenergic sweating was augmented in hot compared with temperate conditions (both P ≤ .014), albeit the relative increase was greater in β (~5.4-fold)- as compared to α (~1.5-fold)-adrenergic-mediated sweating response. However, both α- and β-adrenergic sweating was abolished by atropinization (P = .001). Endurance-trained men showed an augmentation in α- (P = .043) but not β (P = .960)-adrenergic sweating as compared to untrained men. Finally, a greater α- and β-adrenergic sweating response (both P ≤ .001) was measured in habitually active men than in women. We show that heat exposure augments α-and β-adrenergic sweating differently via mechanisms associated with altered muscarinic receptor activity. Sex and training status modulate this response.

TRPV4 channel blockade does not modulate skin vasodilation and sweating during hyperthermia or cutaneous postocclusive reactive and thermal hyperemia

Transient receptor potential vanilloid 4 (TRPV4) channels exist on vascular endothelial cells and eccrine sweat gland secretory cells in human skin. Here, we assessed whether TRPV4 channels contribute to cutaneous vasodilation and sweating during whole body passive heat stress (protocol 1) and to cutaneous vasodilation during postocclusive reactive hyperemia and local thermal hyperemia (protocol 2). Intradermal microdialysis was employed to locally deliver pharmacological agents to forearm skin sites, where cutaneous vascular conductance (CVC) and sweat rate were assessed. In protocol 1 (12 young adults), CVC and sweat rate were increased by passive whole body heating, resulting in a body core temperature elevation of 1.2 ± 0.1°C. The elevated CVC and sweat rate assessed at sites treated with TRPV4 channel antagonist (either 200 µM HC-067047 or 125 µM GSK2193874) were not different from the vehicle control site (5% dimethyl sulfoxide). After whole body heating, the TRPV4 channel agonist (100 µM GSK1016790A) was administered to each skin site, eliciting elevations in CVC. Relative to control, this response was partly attenuated by both TRPV4 channel antagonists, confirming drug efficacy. In protocol 2 (10 young adults), CVC was increased following a 5-min arterial occlusion and during local heating from 33 to 42°C. These responses did not differ between the control and the TRPV4 channel antagonist sites (200 µM HC-067047). We show that TRPV4 channels are not required for regulating cutaneous vasodilation or sweating during a whole body passive heat stress. Furthermore, they are not required for regulating cutaneous vasodilation during postocclusive reactive hyperemia and local thermal hyperemia.

The nitric oxide dependence of cutaneous microvascular function to independent and combined hypoxic cold exposure

When separated from local cooling, whole body cooling elicited cutaneous reflex vasoconstriction via mechanisms independent of nitric oxide removal. Hypoxia elicited cutaneous vasodilatation via mechanisms mediated primarily by nitric oxide synthase, rather than xanthine oxidase-mediated nitrite reduction. Cold-induced vasoconstriction was blunted by the opposing effect of hypoxic vasodilatation, whereas the underpinning mechanisms did not interrelate in the absence of local cooling. Full vasoconstriction was restored with nitric oxide synthase inhibition.

Ageing augments β-adrenergic cutaneous vasodilatation differently in men and women, with no effect on β-adrenergic sweating

NEW FINDINGS

What is the central question of this study? β-Adrenergic receptor activation modulates cutaneous vasodilatation and sweating in young adults. In this study, we assessed whether age-related differences in β-adrenergic regulation of these responses exist and whether they differ between men and women. What is the main finding and its importance? We showed that ageing augmented β-adrenergic cutaneous vasodilatation, although the pattern of response differed between men and women. Ageing had no effect on β-adrenergic sweating in men or women. Our findings advance our understanding of age-related changes in the regulation of cutaneous vasodilatation and sweating and provide new directions for research on the significance of enhanced β-adrenergic cutaneous vasodilatation in older adults.

ABSTRACT

β-Adrenergic receptor agonists, such as isoprenaline, can induce cutaneous vasodilatation and sweating in young adults. Given that cutaneous vasodilatation and sweating responses to whole-body heating and to pharmacological agonists, such as acetylcholine, ATP and nicotine, can differ in older adults, we assessed whether ageing also modulates β-adrenergic cutaneous vasodilatation and sweating and whether responses differ between men and women. In the context of the latter, prior reports showed that the effects of ageing on cutaneous vasodilatation (evoked with ATP and nicotine) and sweating (stimulated by acetylcholine) were sex dependent. Thus, in the present study, we assessed the role of β-adrenergic receptor activation on forearm cutaneous vasodilatation and sweating in 11 young men (24 ± 4 years of age), 11 young women (23 ± 5 years of age), 11 older men (61 ± 8 years of age) and 11 older women (60 ± 8 years of age). Initially, a high dose (100 µm) of isoprenaline was administered via intradermal microdialysis for 5 min to induce maximal β-adrenergic sweating. Approximately 60 min after the washout period, three incremental doses of isoprenaline were administered (1, 10 and 100 µm, each for 25 min) to assess dose-dependent cutaneous vasodilatation. Isoprenaline-mediated cutaneous vasodilatation was greater in both older men and older women relative to their young counterparts. Augmented cutaneous vasodilatory responses were observed at 1 and 10 µm in women and at 100 µm in men. Isoprenaline-mediated sweating was unaffected by ageing, regardless of sex. We show that ageing augments β-adrenergic cutaneous vasodilatation differently in men and women, without influencing β-adrenergic sweating.

Voluntary hypocapnic hyperventilation lasting 5 min and 20 min similarly reduce aerobic metabolism without affecting power outputs during Wingate anaerobic test

AbstractTwenty minutes of voluntary hypocapnic hyperventilation prior to exercise reduces the aerobic metabolic rate with a compensatory increase in the anaerobic metabolic rate without affecting exercise performance during the Wingate anaerobic test (WAnT). Thus, pre-exercise hypocapnic hyperventilation may be a useful means of stressing the anaerobic energy system during training, ultimately improving anaerobic exercise performance. However, it remains unclear whether a shorter (e.g., 5 min) pre-exercise hypocapnic hyperventilation is sufficient to reduce the aerobic metabolic rate during high-intensity exercise. We therefore compared the effects of 5-min and 20-min pre-exercise hypocapnic hyperventilation on aerobic metabolism during the 30-s WAnT. Ten healthy young males and one female performed the WAnT following 20 min of spontaneous breathing (control trial) or 5 or 20 min of voluntary hypocapnic hyperventilation. Both the 5-min and 20-min hyperventilation reduced end-tidal CO₂ partial pressure (an index of arterial CO₂ partial pressure) to ∼23 mmHg, whereas it remained unchanged during the spontaneous breathing. The peak, mean and minimum power outputs during the WAnT did not differ among the three trials. Oxygen uptake during the WAnT was lower in both the 5-min (1493 ± 257 mL min^-1) and 20-min (1397 ± 447 mL min^-1) hyperventilation trials than during the control trial (1847 ± 286 mL min^-1), and was similar in the two hyperventilation trials. These results suggest that 5 min of pre-exercise hypocapnic hyperventilation reduces aerobic metabolism during the 30-s WAnT to a level similar to that seen with the 20-min hyperventilation. Moreover, exercise performance was unaffected, which implies anaerobic metabolism was enhanced.

Sex-differences in cholinergic, nicotinic, and β-adrenergic cutaneous vasodilation: Roles of nitric oxide synthase, cyclooxygenase, and K+ channels

Previous studies indicate that sex-related differences exist in the regulation of cutaneous vasodilation, however, the mechanisms remain unresolved. We assessed if sex-differences in young adults exist for cholinergic, nicotinic, and β-adrenergic cutaneous vasodilation with a focus on nitric oxide synthase (NOS), cyclooxygenase (COX), and K⁺ channel mechanisms. In twelve young men and thirteen young women, four intradermal forearm skin sites were perfused with the following: 1) lactated Ringer's solution (control), 2) 10 mM N^ω-nitro-l-arginine, a non-selective NOS inhibitor, 3) 10 mM ketorolac, a non-selective COX inhibitor, or 4) 50 mM BaCl₂, a nonspecific K⁺ channel blocker. At all four sites, cutaneous vasodilation was induced by 1) 10 mM nicotine, a nicotinic receptor agonist, 2) 100 μM isoproterenol, a nonselective β-adrenergic receptor agonist, and 3) 2 mM and 2000 mM acetylcholine, an acetylcholine receptor agonist. Nicotine and isoproterenol were administered for 3 min, whereas each acetylcholine dose was administered for 25 min. Regardless of treatment site, cutaneous vasodilation in response to nicotine and a high dose of acetylcholine (2000 mM) were lower in women than men. By contrast, isoproterenol induced cutaneous vasodilation was greater in women vs. men. Irrespective of sex, NOS inhibition or K⁺ channel blockade attenuated isoproterenol-mediated cutaneous vasodilation, whereas K⁺ channel blockade decreased nicotine-induced cutaneous vasodilation. Taken together, our findings indicate that while the mechanisms underlying cutaneous vasodilation are comparable between young men and women, sex-related differences in the magnitude of cutaneous vasodilation do exist and this response differs as a function of the receptor agonist.

Effect of inspiratory muscle-loaded exercise training on peak oxygen uptake and ventilatory response during incremental exercise under normoxia and hypoxia

Background

Although numerous studies have reported the effect of inspiratory muscle training for improving exercise performance, the outcome of whether exercise performance is improved by inspiratory muscle training is controversial. Therefore, this study investigated the influence of inspiratory muscle-loaded exercise training (IMLET) on peak oxygen uptake (VO_2peak), respiratory responses, and exercise performance under normoxic (N) and hypoxic (H) exercise conditions. We hypothesised that IMLET enhances respiratory muscle strength and improves respiratory response, thereby improving VO_2peak and work capacity under H condition.

Methods

Sixteen university track runners (13 men and 3 women) were randomly assigned to the IMLET (n = 8) or exercise training (ET) group (n = 8). All subjects underwent 4 weeks of 20-min 60% VO_2peak cycling exercise training, thrice per week. IMLET loaded 50% of maximal inspiratory pressure during exercise. At pre- and post-training periods, subjects performed exhaustive incremental cycling under normoxic (N; 20.9 ± 0%) and hypoxic (H; 15.0 ± 0.1%) conditions.

Results

Although maximal inspiratory pressure (PImax) significantly increased after training in both groups, the extent of PImax increase was significantly higher in the IMLET group (from 102 ± 20 to 145 ± 26 cmH₂O in IMLET; from 111 ± 23 to 127 ± 23 cmH₂O in ET; P < 0.05). In both groups, VO_2peak and maximal work load (W_max) similarly increased both under N and H conditions after training (P < 0.05). Further, the extent of W_max decrease under H condition was lower in the IMLET group at post-training test than at pre-training (from − 14.7 ± 2.2% to − 12.5 ± 1.7%; P < 0.05). Maximal minute ventilation in both N and H conditions increased after training than in the pre-training period.

Conclusions

Our IMLET enhanced the respiratory muscle strength, and the decrease in work capacity under hypoxia was reduced regardless of the increase in VO_2peak.

Intradermal Administration of Atrial Natriuretic Peptide Attenuates Cutaneous Vasodilation but Not Sweating in Young Men during Exercise in the Heat

INTRODUCTION

Prolonged exercise in the heat stimulates plasma release of atrial natriuretic peptide (ANP) in association with dehydration-induced reductions in blood volume. Elevated plasma ANP levels under these conditions may indirectly attenuate cutaneous blood flow and sweating responses due to the effects of this hormone on central blood volume and plasma osmolality and the resulting stimulation of nonthermal reflexes. However, it remains unclear whether cutaneous blood flow and sweating are directly modulated by ANP at the level of the cutaneous end organs (cutaneous microvessels and eccrine sweat glands) during prolonged exercise in the heat.

OBJECTIVE

Therefore, we evaluated the effects of local ANP administration on forearm cutaneous vascular conductance (CVC) and local sweat rate (LSR) during rest and exercise in the heat.

METHODS

In 9 habitually active young men (26 ± 6 years) CVC and LSR were evaluated at 3 intradermal microdialysis sites continuously perfused with lactated Ringer solution (control) or ANP (0.1 or 1.0 μM). Participants rested in a non-heat-stress condition (25°C) for approximately 60 min followed by 70 min in the heat (35°C). They then performed 50 min of moderate-intensity cycling (approx. 55% VO2 peak), with a 30-min recovery. Thereafter, 50 mM sodium nitroprusside was administered at all sites to elicit maximum CVC, which was subsequently used to normalize all values (CVC%max).

RESULTS

No effects of ANP on CVC%max were observed in the non-heat-stress resting condition compared to the untreated control site (both p > 0.05). Conversely during rest in the heat there was an 11% (5-17%) reduction in CVC%max at the 1.0 μM ANP site relative to the untreated control site (p < 0.05). At the end of exercise CVC%max was attenuated by 12% (1-23%) at the 0.1 μM ANP site and by 21% (7-35%) at the 1.0 μM ANP site relative to the untreated control site (all p < 0.05). Conversely, neither concentration of ANP influenced sweating at any time point (all p > 0.05).

CONCLUSION

Intradermal ANP administration directly attenuated cutaneous blood flow, but not sweating, in habitually active young men during rest and exercise in the heat.

NO-mediated activation of KATP channels contributes to cutaneous thermal hyperemia in young adults

Local skin heating to 42°C causes cutaneous thermal hyperemia largely via nitric oxide (NO) synthase (NOS)-related mechanisms. We assessed the hypothesis that ATP-sensitive K⁺ (K_ATP) channels interact with NOS to mediate cutaneous thermal hyperemia. In 13 young adults (6 women, 7 men), cutaneous vascular conductance (CVC) was measured at four intradermal microdialysis sites that were continuously perfused with 1) lactated Ringer solution (control), 2) 5 mM glibenclamide (K_ATP channel blocker), 3) 20 mM N^G-nitro-l-arginine methyl ester (NOS inhibitor), or 4) a combination of K_ATP channel blocker and NOS inhibitor. Local skin heating to 42°C was administered at all four treatment sites to elicit cutaneous thermal hyperemia. Thirty minutes after the local heating, 1.25 mM pinacidil (K_ATP channel opener) and subsequently 25 mM sodium nitroprusside (NO donor) were administered to three of the four sites (each 25-30 min). The local heating-induced prolonged elevation in CVC was attenuated by glibenclamide (19%), but the transient initial peak was not. However, glibenclamide had no effect on the prolonged elevation in CVC in the presence of NOS inhibition. Pinacidil caused an elevation in CVC, but this response was abolished at the glibenclamide-treated skin site, demonstrating its effectiveness as a K_ATP channel blocker. The pinacidil-induced increase in CVC was unaffected by NOS inhibition, whereas the increase in CVC elicited by sodium nitroprusside was partly (15%) inhibited by glibenclamide. In summary, we showed an interactive effect of K_ATP channels and NOS for the plateau of cutaneous thermal hyperemia. This interplay may reflect a vascular smooth muscle cell K_ATP channel activation by NO.

KCa and KV channels modulate the venoarteriolar reflex in non-glabrous human skin with no roles of KATP channels, NOS, and COX

The venoarteriolar reflex is a local mechanism that induces vasoconstriction during venous congestion in various tissues, including skin. This response is thought to play a critical role in minimizing capillary damage or edema resulting from overperfusion, though factors that modulate this response remain largely unknown. Here, we hypothesized that nitric oxide synthase (NOS), cyclooxygenase (COX), and Ca²⁺-activated, ATP-sensitive, and voltage-gated K⁺ channels (K_Ca, K_ATP, and K_V channels, respectively) modulate the venoarteriolar reflex in human skin. Cutaneous blood flow (laser-Doppler flowmetry) was monitored during a 3-min pre-occlusion baseline and following a 3-min venous occlusion of 45 mmHg, the latter maneuver was used to induce the venoarteriolar reflex. The venoarteriolar reflex was assessed at the following forearm skin sites: Experiment 1 (n = 11): 1) lactated Ringer solution (Control), 2) 10 mM N^ω-nitro-_L-arginine (NOS inhibitor), 3) 10 mM ketorolac (COX inhibitor), and 4) combined NOS + COX inhibition; Experiment 2 (n = 15): 1) lactated Ringer solution (Control), 2) 50 mM tetraethylammonium (K_Ca channel blocker), 3) 5 mM glybenclamide (K_ATP channel blocker), and 4) 10 mM 4-aminopyridine (K_V channel blocker). Separate and combined NOS and COX inhibition as well as K_ATP channel blocker had no effect on venoarteriolar reflex. Conversely, venoarteriolar reflex was attenuated by K_Ca channel blockade (36-38%) and augmented by K_V channel blockade (38-55%). We showed that K_Ca and K_V channels modulate the venoarteriolar reflex with minimum roles of NOS, COX, and K_ATP channels in human non-glabrous forearm skin in vivo. Thus, cutaneous venoarteriolar reflex changes could reflect altered K⁺ channel function.

Ageing augments nicotinic and adenosine triphosphate-induced, but not muscarinic, cutaneous vasodilatation in women

NEW FINDINGS

What is the central question of this study? Does ageing augment muscarinic, nicotinic and/or ATP-mediated cutaneous vasodilatation in women? What is the main finding and its importance? Ageing augments nicotinic and ATP-induced, but not muscarinic, cutaneous vasodilatation in women. This will stimulate future studies assessing the pathophysiological significance of the augmented microvascular responsiveness in older women compared to their young counterparts.

ABSTRACT

We previously reported that ageing attenuates adenosine triphosphate (ATP)-induced, but not muscarinic and nicotinic, cutaneous vasodilatation in men, and that ageing may augment cutaneous vascular responses in women. In the present study, we evaluated the hypothesis that ageing augments muscarinic, nicotinic and/or ATP-mediated cutaneous vasodilatation in healthy women. In 11 young (23 ± 5 years) and 11 older (60 ± 8 years) women, cutaneous vascular conductance was evaluated at three forearm skin sites that were perfused with (1) methacholine (muscarinic receptor agonist, 5 doses: 0.0125, 0.25, 5, 100, 2000 mm), (2) nicotine (nicotinic receptor agonist, 5 doses: 1.2, 3.6, 11, 33, 100 mm), or (3) ATP (purinergic receptor agonist, 5 doses: 0.03, 0.3, 3, 30, 300 mm). Each agonist was administered for 25 min per dose. Methacholine-induced increases in cutaneous vascular conductance were not different between groups at all doses (all P > 0.05). However, a nicotine-induced elevation in cutaneous vascular conductance at the lowest concentration (1.2 mm) was greater in older vs. young women (43 ± 15 vs. 26 ± 10%max, P = 0.04). ATP-induced increases in cutaneous vascular conductance at moderate and high doses (3 and 30 mm) were also greater in older relative to young women (3 mm, 44 ± 11 vs. 28 ± 10%max, P = 0.02; 30 mm, 83 ± 14 vs. 64 ± 17%max, P = 0.05). Therefore, ageing augments nicotinic and ATP-induced, but not muscarinic, cutaneous vasodilatation in women.

Effects of isomaltulose ingestion on postexercise hydration state and heat loss responses in young men

NEW FINDINGS

What is the central question of this study? What are the effects of isomaltulose, an ingredient in carbohydrate-electrolyte beverages to maintain glycaemia and attenuate the risk of dehydration during exercise heat stress, on postexercise rehydration and physiological heat loss responses? What is the main finding and its importance? Consumption of a 6.5% isomaltulose-electrolyte beverage following exercise heat stress restored hydration following a 2 h recovery as compared to a 2% solution or water only. While the 6.5% isomaltulose-electrolytes increased plasma volume and plasma osmolality, which are known to modulate postexercise heat loss, sweating and cutaneous vascular responses did not differ between conditions. Consequently, ingestion beverages containing 6.5% isomaltulose-electrolytes enhanced postexercise rehydration without affecting heat loss responses.

ABSTRACT

Isomaltulose is a disaccharide carbohydrate widely used during exercise to maintain glycaemia and hydration. We investigated the effects of ingesting a beverage containing isomaltulose and electrolytes on postexercise hydration state and physiological heat loss responses. In a randomized, single-blind cross-over design, 10 young healthy men were hypohydrated by performing up to three 30 min successive moderate-intensity (50% heart rate reserve) bouts of cycling, each separated by 10 min, while wearing a water-perfusion suit heated to 45°C. The protocol continued until a 2% reduction in body mass was achieved. Thereafter, participants performed a final 15 min moderate-intensity exercise bout followed by a 2 h recovery. Following cessation of exercise, participants ingested a beverage consisting of (i) water only (Water), (ii) 2% isomaltulose (CHO-2%), or (iii) 6.5% isomaltulose (CHO-6.5%) equal to the volume of 2% body mass loss within the first 30 min of the recovery. Changes in plasma volume (ΔPV) after fluid ingestion were greater for CHO-6.5% compared with CHO-2% (120 min postexercise) and Water (90 and 120 min) (all P ≤ 0.040). Plasma osmolality remained elevated with CHO-6.5% compared with consumption of the other beverages at 30 and 90 min postexercise (all P ≤ 0.050). Urine output tended to be reduced with CHO-6.5% compared to other fluid conditions (main effect, P = 0.069). Rectal and mean skin temperatures, chest sweat rate and cutaneous perfusion did not differ between conditions (all P > 0.05). In conclusion, compared with CHO-2% and Water, consuming a beverage consisting of CHO-6.5% and electrolytes during recovery under heat stress enhances PV recovery without modulating physiological heat loss responses.

Exogenous Activation of Protease-Activated Receptor 2 Attenuates Cutaneous Vasodilatation and Sweating in Older Men Exercising in the Heat

BACKGROUND

Protease-activated receptor 2 (PAR2) exists in the cutaneous vasculature and eccrine sweat glands. We previously showed that in young habitually active men, exogenous PAR2 activation via the agonist SLIGKV-NH2 had no effect on heat loss responses of cutaneous vasodilatation and sweating during rest or exercise in the heat. However, ageing is associated with altered mechanisms governing these responses. Thus, the effect of exogenous PAR2 activation on cutaneous vasodilatation and sweating in older individuals may differ from that in young adults.

METHODS

Local cutaneous vascular conductance (CVC) and sweat rate were measured in 9 older males (62 ± 4 years) at four forearm skin sites treated with the following: (1) lactated Ringer solution (control), (2) 0.05 mM, (3) 0.5 mM, or (4) 5 mM SLIGKV-NH2. Measurements were performed while participants rested in a non-heat-stress environment (25°C) for ∼60 min and an additional 50 min thereafter in the heat (40°C). Participants then performed 50 min of cycling at a fixed metabolic heat load of 200 W/m2 (to maintain the same thermal drive for heat loss between participants) followed by a 30-min recovery.

RESULTS

CVC during non-heat-stress resting was elevated from the control site with 5 mM SLIGKV-NH2 (p ≤ 0.05), but this response was not observed during ambient heat exposure. By contrast, 5 mM SLIGKV-NH2 lowered CVC during the early stage (10 and 20 min) of exercise compared to the control site (all p ≤ 0.05). Although sweating during non-heat-stressed and heat-stressed resting was not affected by any dose of SLIGKV-NH2, it was reduced with all SLIGKV-NH2 doses relative to the control site during and following exercise (all p ≤ 0.05).

CONCLUSION

We show that while exogenous PAR2 activation induces cutaneous vasodilatation at rest under non-heat-stressed conditions, it attenuates cutaneous vasodilatation and sweating during and following an exercise-induced heat stress in older men.

Separate and combined effects of KCa and KATP channel blockade with NOS inhibition on cutaneous vasodilation and sweating in older men during heat stress

Our objective in this study was to examine the separate and combined effects of potassium (K⁺) channels and nitric oxide synthase (NOS) on cutaneous vasodilation and sweating in older men during rest and exercise in the heat. In 13 habitually active men (61 ± 4 yr), cutaneous vascular conductance and local sweat rate were assessed at six dorsal forearm skin sites continuously perfused with either 1) lactated Ringer (control), 2) 10 mM N^G-nitro-l-arginine methyl ester (l-NAME, NOS inhibitor), 3) 50 mM tetraethylammonium (TEA; Ca²⁺-activated K⁺ channel blocker), 4) 5 mM glybenclamide (GLY; ATP-sensitive K⁺ channel blocker), 5) 50 mM TEA + 10 mM l-NAME, and 6) 5 mM GLY + 10 mM l-NAME via microdialysis. Participants rested in non-heat stress (25°C) and heat stress (35°C) conditions for ∼60 min each, followed by 50 min of moderate-intensity cycling (∼55% V̇o_2peak) and 30 min of recovery in the heat. During rest and exercise in the heat, l-NAME, TEA + l-NAME, and GLY + l-NAME attenuated CVC relative to control (all P ≤ 0.05), although l-NAME was not different from TEA + l-NAME or GLY + l-NAME (all P > 0.05). TEA attenuated CVC during rest, whereas GLY attenuated CVC during exercise (both P ≤ 0.05). Additionally, whereas neither l-NAME nor TEA altered sweating throughout the protocol (all P > 0.05), combined TEA + l-NAME attenuated sweating during exercise in the heat (P ≤ 0.05). We conclude that in habitually active older men blockade of K_Ca and K_ATP channels attenuates cutaneous vasodilation during rest and exercise in the heat, respectively, and these effects are NOS dependent. Furthermore, combined NOS inhibition and K_Ca channel blockade attenuates sweating during exercise in the heat.

Low-intensity exercise delays the shivering response to core cooling

Hypothermia can occur during aquatic exercise despite production of significant amounts of heat by the active muscles. Because the characteristics of human thermoregulatory responses to cold during exercise have not been fully elucidated, we investigated the effect of low-intensity exercise on the shivering response to core cooling in cool water. Eight healthy young men (24 ± 3 yr) were cooled through cool water immersion while resting (rest trial) and during loadless pedaling on a water cycle ergometer (exercise trial). Before the cooling, body temperature was elevated by hot water immersion to clearly detect a core temperature at which shivering initiates. Throughout the cooling period, mean skin temperature remained around the water temperature (25°C) in both trials, whereas esophageal temperature (Tes) did not differ between the trials ( P > 0.05). The Tes at which oxygen uptake (V̇o2) rapidly increased, an index of the core temperature threshold for shivering, was lower during exercise than rest (36.2 ± 0.4°C vs. 36.5 ± 0.4°C, P < 0.05). The sensitivity of the shivering response, as indicated by the slope of the Tes-V̇o2 relation, did not differ between the trials (−441.3 ±177.4 ml·min−1·°C−1 vs. −411.8 ± 268.1 ml·min−1·°C−1, P > 0.05). The thermal sensation response to core cooling, assessed from the slope and intercept of the regression line relating Tes and thermal sensation, did not differ between the trials ( P > 0.05). These results suggest that the core temperature threshold for shivering is delayed during low-intensity exercise in cool water compared with rest although shivering sensitivity is unaffected.