No effect of systemic isocapnic hypoxia on α-adrenergic vasoconstrictor responsiveness in human skin

Mechanistic involvement of noradrenergic neuronal neurotransmitter release in cutaneous vasoconstriction during autonomic dysreflexia in persons with spinal cord injury

INTRODUCTION

Autonomic dysreflexia (AD) is a potentially life-threatening consequence in high (above T6) spinal cord injury that involves multiple incompletely understood mechanisms. While peripheral arteriolar vasoconstriction, which controls systemic vascular resistance, is documented to be pronounced during AD, the pathophysiological neurovascular junction mechanisms of this vasoconstriction are undefined. One hypothesized mechanism is increased neuronal release of norepinephrine and co-transmitters. We tested this by examining the effects of blockade of pre-synaptic neural release of norepinephrine and co-transmitters on cutaneous vasoconstriction during AD, using a novel non-invasive technique; bretylium (BT) iontophoresis followed by skin blood flow measurements via laser doppler flowmetry (LDF).

METHODS

Bretylium, a sympathetic neuronal blocking agent (blocks release of norepinephrine and co-transmitters) was applied iontophoretically to the skin of a sensate (arm) and insensate (leg) area in 8 males with motor complete tetraplegia. An nearby untreated site served as control (CON). Cutaneous vascular conductance (CVC) was measured (CVC = LDF/mean arterial pressure) at normotension before AD was elicited by bladder stimulation. The percent drop in CVC values from pre-AD vs. AD was compared among BT and CON sites in sensate and insensate areas.

RESULTS

There was a significant effect of treatment but no significant effect of limb/sensation or interaction of limb x treatment on CVC. The percent drop in CVC between BT and CON treated sites was 25.7±1.75 vs. 39.4±0.87, respectively (P = 0.004).

CONCLUSION

Bretylium attenuates, but does not fully abolish vasoconstriction during AD. This suggests release of norepinephrine and cotransmitters from cutaneous sympathetic nerves is involved in cutaneous vasoconstriction during AD.

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.

Evidence for a functional vasoconstrictor role for ATP in the human cutaneous microvasculature

NEW FINDINGS

What is the central question of this study? In young adults, about half of the cold-related reduction in skin blood flow during cold exposure is mediated by noradrenaline, while the remainder is attributable to other substances co-released with noradrenaline that have yet to be identified. What is the main finding and its importance? Purinergic receptor blockade blunted the vasoconstriction response to whole-body cooling and to intradermal administration of tyramine. These results indicate that ATP is necessary to vasoconstrict blood vessels in the skin adequately and prevent heat loss in a cold environment. Noradrenaline is responsible for eliciting ∼60% of the reflex cutaneous vasoconstriction (VC) response in young adults, while the remainder is attributable to one or more unidentified co-released sympathetic adrenergic neurotransmitter(s). Inconsistent evidence has placed neuropeptide Y in this role; however, other putative cotransmitters have yet to be tested. We hypothesize that ATP contributes to the reflex cutaneous VC response. Two protocols were conducted in young adults (n = 10); both involved the placement of three microdialysis probes in forearm skin and whole-body cooling (skin temperature = 30.5°C). In protocol 1, the following solutions were infused: (i) lactated Ringer solution (control); (ii) 10 mm l-NAME; and (iii) purinergic receptor blockade with 1 mm suramin plus l-NAME. In protocol 2, the following solutions were infused: (i) lactated Ringer solution; (ii) suramin plus l-NAME; and (iii) suramin plus l-NAME plus adrenoreceptor blockade with 5 mm yohimbine plus 1 mm propranolol. Laser Doppler flux (LDF) was measured over each microdialysis site, and cutaneous vascular conductance (CVC) was calculated (CVC = LDF/MAP) and expressed as percentage changes from baseline (%ΔCVC_BASELINE ). l-NAME was used to block the vasodilatory influence of ATP and unmask the P₂ X-mediated VC response to exogenous ATP infusion (-21 ± 6%ΔCVC_BASELINE ). During cooling, the VC response (control, -39 ± 8%ΔCVC_BASELINE ) was attenuated at the suramin site (-21 ± 4%ΔCVC_BASELINE ) and further blunted with combined adrenoreceptor blockade (-9 ± 3%ΔCVC_BASELINE ; P < 0.05). Compared with the control site (-22 ± 5%ΔCVC_BASELINE ), suramin inhibited pharmacologically induced VC to tyramine (-12 ± 6%ΔCVC_BASELINE ; P < 0.05), which displaces adrenergic neurotransmitters from axon terminals. These data indicate that ATP contributes to the cutaneous VC response in humans.

Responses of the hands and feet to cold exposure

An initial response to whole-body or local exposure of the extremities to cold is a strong vasoconstriction, leading to a rapid decrease in hand and foot temperature. This impairs tactile sensitivity, manual dexterity, and muscle contractile characteristics while increasing pain and sympathetic drive, decreasing gross motor function, occupational performance, and survival. A paradoxical and cyclical vasodilatation often occurs in the fingers, toes, and face, and this has been termed the hunting response or cold-induced vasodilatation (CIVD). Despite being described almost a century ago, the mechanisms of CIVD are still disputed; research in this area has remained largely descriptive in nature. Recent research into CIVD has brought increased standardization of methodology along with new knowledge about the impact of mediating factors such as hypoxia and physical fitness. Increasing mechanistic analysis of CIVD has also emerged along with improved modeling and prediction of CIVD responses. The present review will survey work conducted during this century on CIVD, its potential mechanisms and modeling, and also the broader context of manual function in cold conditions.

Dynamic adaptation of the peripheral circulation to cold exposure

Humans residing or working in cold environments exhibit a stronger cold-induced vasodilation (CIVD) reaction in the peripheral microvasculature than those living in warm regions of the world, leading to a general assumption that thermal responses to local cold exposure can be systematically improved by natural acclimatization or specific acclimation. However, it remains unclear whether this improved tolerance is actually due to systematic acclimatization, or alternately due to the genetic pre-disposition or self-selection for such occupations. Longitudinal studies of repeated extremity exposure to cold demonstrate only ambiguous adaptive responses. In field studies, general cold acclimation may lead to increased sympathetic activity that results in reduced finger blood flow. Laboratory studies offer more control over confounding parameters, but in most studies, no consistent changes in peripheral blood flow occur even after repeated exposure for several weeks. Most studies are performed on a limited amount of subjects only, and the variability of the CIVD response demands more subjects to obtain significant results. This review systematically surveys the trainability of CIVD, concluding that repeated local cold exposure does not alter circulatory dynamics in the peripheries, and that humans remain at risk of cold injuries even after extended stays in cold environments.