Roles of nitric oxide synthase isoforms in cutaneous vasodilation induced by local warming of the skin and whole body heat stress in humans

Antagonism of soluble guanylyl cyclase attenuates cutaneous vasodilation during whole body heat stress and local warming in humans

We hypothesized that nitric oxide activation of soluble guanylyl cyclase (sGC) participates in cutaneous vasodilation during whole body heat stress and local skin warming. We examined the effects of the sGC inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), on reflex skin blood flow responses to whole body heat stress and on nonreflex responses to increased local skin temperature. Blood flow was monitored by laser-Doppler flowmetry, and blood pressure by Finapres to calculate cutaneous vascular conductance (CVC). Intradermal microdialysis was used to treat one site with 1 mM ODQ in 2% DMSO and Ringer, a second site with 2% DMSO in Ringer, and a third site received Ringer. In protocol 1, after a period of normothermia, whole body heat stress was induced. In protocol 2, local heating units warmed local skin temperature from 34 to 41°C to cause local vasodilation. In protocol 1, in normothermia, CVC did not differ among sites [ODQ, 15 ± 3% maximum CVC (CVC(max)); DMSO, 14 ± 3% CVC(max); Ringer, 17 ± 6% CVC(max); P > 0.05]. During heat stress, ODQ attenuated CVC increases (ODQ, 54 ± 4% CVC(max); DMSO, 64 ± 4% CVC(max); Ringer, 63 ± 4% CVC(max); P < 0.05, ODQ vs. DMSO or Ringer). In protocol 2, at 34°C local temperature, CVC did not differ among sites (ODQ, 17 ± 2% CVC(max); DMSO, 18 ± 4% CVC(max); Ringer, 18 ± 3% CVC(max); P > 0.05). ODQ attenuated CVC increases at 41°C local temperature (ODQ, 54 ± 5% CVC(max); DMSO, 86 ± 4% CVC(max); Ringer, 90 ± 2% CVC(max); P < 0.05 ODQ vs. DMSO or Ringer). sGC participates in neurogenic active vasodilation during heat stress and in the local response to direct skin warming.

Nitric oxide synthase inhibition attenuates cutaneous vasodilation during postmenopausal hot flash episodes

OBJECTIVE

The purpose of this study was to test the hypothesis that local inhibition of nitric oxide and prostaglandin synthesis attenuates cutaneous vasodilator responses during postmenopausal hot flash episodes.

METHODS

Four microdialysis membranes were inserted into the forearm skin (dorsal surface) of eight postmenopausal women (mean +/- SD age, 51 +/- 7 y). Ringer solution (control), 10 mM ketorolac (Keto) to inhibit prostaglandin synthesis, 10 mM N-l-arginine methyl ester (l-NAME) to inhibit nitric oxide synthase, and a combination of 10 mM Keto + 10 mM l-NAME were each infused at the separate sites. Skin blood flow at each site was indexed using laser-Doppler flowmetry. Cutaneous vascular conductance (CVC) was calculated as laser-Doppler flux/mean arterial blood pressure and was expressed as a percentage of the maximal calculated CVC (CVCmax) obtained after infusion of 50 mM sodium nitroprusside at all sites at the end of the study. Data from 13 hot flash episodes were analyzed.

RESULTS

At the control site, the mean +/- SD peak increase in CVC was 15.5% +/- 6% CVCmax units. This value was not different relative to the peak increase in CVC at the Keto site (13.0% +/- 5% CVCmax units; P = 0.09). However, the peak increase in CVC during hot flash episodes were attenuated at the l-NAME and l-NAME + Keto sites (7.4% +/- 4% and 8.7% +/- 7% CVCmax units, respectively) relative to both the control and the Keto sites (P < 0.05 for both comparisons). There were no significant differences in the peak increases in sweat rate between any of the sites (P = 0.24).

CONCLUSIONS

These data demonstrate that the mechanism for cutaneous vasodilation during hot flash episodes has a nitric oxide component. Increases in CVC despite the inhibition of prostaglandin synthesis suggest that prostaglandins do not contribute to cutaneous vasodilation during hot flash episodes.

The contribution of skin blood flow in warming the skin after the application of local heat; the duality of the Pennes heat equation

As predicted by the Pennes equation, skin blood flow is a major contributor to the removal of heat from an external heat source. This protects the skin from erythema and burns. But, for a person in a thermally neutral room, the skin is normally much cooler than arterial blood. Therefore, if skin blood flow (BF) increases, it should initially warm the skin paradoxically. To examine this phenomenon, 10 young male and female subjects participated in a series of experiments to examine the contribution of skin blood flow in the initial warming the skin after the application of local heat. Heat flow was measured by the use of a thermode above the brachioradialis muscle. The thermode was warmed by constant temperature water at 44°C entering the thermode at a water flow rate of 100 cm(3)/min. Skin temperature was measured by a thermistor and blood flow in the underlying skin was measured by a laser Doppler imager in single point mode. The results of the experiments showed that, when skin temperature is cool (31-32°C), the number of calories being transferred to the skin from the thermode cannot account for the rise in skin temperature alone. A significant portion of the rise in skin temperature is due to the warm arterialized blood traversing the skin from the core areas of the body. However, as skin temperature approaches central core temperature, it becomes less of a heat source and more of a heat sync such that when skin temperature is at or above core temperature, the blood flow to the skin, as predicted by Pennes, becomes a heat sync pulling heat from the thermode.

Cutaneous vascular and sudomotor responses in human skin grafts

Each year millions of individuals sustain burns. Within the US 40,000-70,000 individuals are hospitalized for burn-related injuries, some of which are quite severe, requiring skin grafting. The grafting procedure disrupts neural and vascular connections between the host site and the graft, both of which are necessary for that region of skin to contribute to temperature regulation. With the use of relatively modern techniques such as laser-Doppler flowmetry and intradermal microdialysis, a wealth of information has become available regarding the consequences of skin grafting on heat dissipation and heat conservation mechanisms. The prevailing data suggest that cutaneous vasodilator capacity to an indirect heat stress (i.e., heating the individual but not the evaluated graft area) and a local heating stimulus (i.e., directly heating the graft area) is impaired in grafted skin. These impairments persist for ≥4 yr following the grafting procedures and are perhaps permanent. The capacity for grafted skin to vasodilate to an endothelial-dependent vasodilator is likewise impaired, whereas its capacity to vasodilate to an endothelial-independent vasodilator is generally preserved. Sweating responsiveness is minimal to nonexistent in grafted skin to both a whole body heat stress and local administration of the primary neurotransmitter responsible for stimulating sweat glands (i.e., acetylcholine). Likewise, there is no evidence that this absence of sweat gland responsiveness improves as the graft matures. In contrast to the heating stimuli, cutaneous vasoconstrictor responses to both indirect whole body cooling (i.e., exposing the individual to a cold stress but not at the evaluated graft area) and direct local cooling (i.e., directly cooling the graft area) are preserved in grafted skin as early as 5-9 mo postgrafting. If uninjured skin does not compensate for impaired heat dissipation of grafted skin, individuals having skin grafts encompassing significant fractions of their body surface area will be at a greater risk for a hyperthermic-related injury. Conversely, the prevailing data suggest that such individuals will not be at a greater risk of hypothermia upon exposure to cold environmental conditions.

Local thermal control of the human cutaneous circulation

The level of skin blood flow is subject to both reflex thermoregulatory control and influences from the direct effects of warming and cooling the skin. The effects of local changes in temperature are capable of maximally vasoconstricting or vasodilating the skin. They are brought about by a combination of mechanisms involving endothelial, adrenergic, and sensory systems. Local warming initiates a transient vasodilation through an axon reflex, succeeded by a plateau phase due largely to nitric oxide. Both phases are supported by sympathetic transmitters. The plateau phase is followed by the die-away phenomenon, a slow reversal of the vasodilation that is dependent on intact sympathetic vasoconstrictor nerves. The vasoconstriction with local skin cooling is brought about, in part, by a postsynaptic upregulation of α(2c)-adrenoceptors and, in part, by inhibition of the nitric oxide system at at least two points. There is also an early vasodilator response to local cooling, dependent on the rate of cooling. The mechanism for that transient vasodilation is not known, but it is inhibited by intact sympathetic vasoconstrictor nerve function and by intact sensory nerve function.