Involvement of nitric oxide production in the impairment of skin blood flow response to local cooling in diabetic db/db mice

Differential contribution of calcium channels to α1-adrenoceptor-mediated contraction is responsible for diverse responses to cooling between rat tail and iliac arteries

Our previous studies have shown that α₁-adrenoceptors, in addition to α₂-adrenoceptors, are involved in enhanced contraction of cutaneous blood vessels during cooling. The present study aimed to elucidate the mechanism underlying it. In tail and iliac arteries isolated from rats, isometric contraction was measured using a myograph and the phosphorylation level of myosin phosphatase target subunit 1 (MYPT1) was quantified by western blotting. The phenylephrine-induced contraction was enhanced by cooling to 24 °C in tail arteries, but was suppressed in iliac arteries. Endothelium denudation or treatment with iberiotoxin enhanced the phenylephrine-induced contraction in tail arteries at 37 °C; however, neither affected the contraction at 24 °C. The phenylephrine-induced contraction at 37 °C was largely suppressed by nifedipine in iliac arteries, but only slightly in tail arteries. The Rho kinase inhibitor H-1152 largely suppressed the phenylephrine-induced contraction at 24 °C, but only slightly at 37 °C, in both arteries. The phosphorylation level of MYPT1 at Thr855 in tail arteries was increased by the cooling. Taken together, these results suggest the following mechanism in regard to cooling-induced enhancement of α₁-adrenoceptor-mediated contraction in tail arteries: Cooling enhances the contraction of tail arteries via α₁-adrenoceptor stimulation by reducing endothelium-dependent, large-conductance Ca²⁺-activated K⁺ channel-mediated relaxation and by inducing Rho kinase-mediated Ca²⁺ sensitization, although the latter occurs even in iliac arteries. A smaller contribution of voltage-dependent Ca²⁺ channels, which are largely suppressed by cooling, to α₁-adrenoceptor-mediated contraction in tail arteries seems to be more crucially involved in the appearance of the enhanced contractile response to cooling.

Skin vasoconstriction as a heat conservation thermoeffector

Cold exposure stimulates heat production and conservation to protect internal temperature. Heat conservation is brought about via reductions in skin blood flow. The focus, here, is an exploration of the mechanisms, particularly in humans, leading to that cutaneous vasoconstriction. Local skin cooling has several effects: (1) reduction of tonic nitric oxide formation by inhibiting nitric oxide synthase and element(s) downstream of the enzyme, which removes tonic vasodilator effects, yielding a relative vasoconstriction; (2) translocation of intracellular alpha-2c adrenoceptors to the vascular smooth-muscle cell membrane, enhancing adrenergic vasoconstriction; (3) increased norepinephrine release from vasoconstrictor nerves; and (4) cold-induced vasodilation, seen more clearly in anastomoses-rich glabrous skin. Cold-induced vasodilation occurs in nonglabrous skin when nitric oxide synthase or sympathetic function is blocked. Reflex responses to general body cooling complement these local effects. Sympathetic excitation leads to the increased release of norepinephrine and its cotransmitter neuropeptide Y, each of which contributes significantly to the vasoconstriction. The contributions of these two transmitters vary with aging, disease and, in women, reproductive hormone status. Interaction between local and reflex mechanisms is in part through effects on baseline and in part through removal of the inhibitory effects of nitric oxide on adrenergic vasoconstriction.

G-protein coupled estrogen receptor-mediated non-genomic facilitatory effect of estrogen on cooling-induced reduction of skin blood flow in mice

An enhanced vasoconstrictor activity of cutaneous arteries participates in the reduction of skin blood flow induced by cooling stimulation. Raynaud's phenomenon, which is characterized by intense cooling-induced constriction of cutaneous arteries, is more common in women during the period from menarche to menopause. We thus investigated the effect of 17β-estradiol (E2) on cooling-induced reduction of plantar skin blood flow (PSBF) in mouse in vivo. Ovariectomized female ddY mice, anaesthetized with pentobarbital, were treated with tetrodotoxin for eliminating the sympathetic nerve tone and artificially ventilated. The PSBF was measured by laser Doppler flowmetry. Cooling air temperature around the foot from 25 to 20, 15, or 10°C decreased the PSBF in a temperature-dependent manner, which was suppressed by the specific α_2C-adrenoceptor antagonist MK-912. When E2 was intravenously administered as a bolus followed by a constant infusion for 10min just before the cooling stimulation, the cooling-induced reduction of PSBF was facilitated by E2 in a dose-dependent manner. The facilitatory effect of E2 was not induced after the treatment with MK-912. Similar facilitatory effect was induced by an intravenous application of G-1, an agonist of G protein-coupled estrogen receptor (GPER, also termed GPR30). Moreover, the facilitatory effect of E2 was abolished by the GPER antagonist G15. These results suggest that acute administration of E2 leads to the facilitation of cooling-induced, α_2C-adrenoceptor-mediated reduction of skin blood flow via the activation of the non-genomic estrogen receptor GPER.