Inhibition of cold-induced vasoconstriction with ketanserin

Role of cyclooxygenase in the vascular responses to extremity cooling in Caucasian and African males

NEW FINDINGS

What is the central question of this study? Compared with Caucasians, African individuals are more susceptible to non-freezing cold injury and experience greater cutaneous vasoconstriction and cooler finger skin temperatures upon hand cooling. We investigated whether the enzyme cyclooxygenase is, in part, responsible for the exaggerated response to local cooling. What is the main finding and its importance? During local hand cooling, individuals of African descent experienced significantly lower finger skin blood flow and skin temperature compared with Caucasians irrespective of cyclooxygenase inhibition. These data suggest that in young African males the cyclooxygenase pathway appears not to be the primary reason for the increased susceptibility to non-freezing cold injury. Individuals of African descent (AFD) are more susceptible to non-freezing cold injury (NFCI) and experience an exaggerated cutaneous vasoconstrictor response to hand cooling compared with Caucasians (CAU). Using a placebo-controlled, cross-over design, this study tested the hypothesis that cyclooxygenase (COX) may, in part, be responsible for the exaggerated vasoconstrictor response to local cooling in AFD. Twelve AFD and 12 CAU young healthy men completed foot cooling and hand cooling (separately, in 8°C water for 30 min) with spontaneous rewarming in 30°C air after placebo or aspirin (COX inhibition) treatment. Skin blood flow, expressed as cutaneous vascular conductance (as flux per millimetre of mercury), and skin temperature were measured throughout. Irrespective of COX inhibition, the responses to foot cooling, but not hand cooling, were similar between ethnicities. Specifically, during hand cooling after placebo, AFD experienced a lower minimal skin blood flow [mean (SD): 0.5 (0.1) versus 0.8 (0.2) flux mmHg^-1 , P < 0.001] and a lower minimal finger skin temperature [9.5 (1.4) versus 10.7 (1.3)°C, P = 0.039] compared with CAU. During spontaneous rewarming, average skin blood flow was also lower in AFD than in CAU [2.8 (1.6) versus 4.3 (1.0) flux mmHg^-1 , P < 0.001]. These data provide further support that AFD experience an exaggerated response to hand cooling on reflection this appears to overstate findings; however, the results demonstrate that the COX pathway is not the primary reason for the exaggerated responses in AFD and increased susceptibility to NFCI.

Different populations of prostaglandin EP3 receptor-expressing preoptic neurons project to two fever-mediating sympathoexcitatory brain regions

The central mechanism of fever induction is triggered by an action of prostaglandin E(2) (PGE(2)) on neurons in the preoptic area (POA) through the EP3 subtype of prostaglandin E receptor. EP3 receptor (EP3R)-expressing POA neurons project directly to the dorsomedial hypothalamus (DMH) and to the rostral raphe pallidus nucleus (rRPa), key sites for the control of thermoregulatory effectors. Based on physiological findings, we hypothesize that the febrile responses in brown adipose tissue (BAT) and those in cutaneous vasoconstrictors are controlled independently by separate neuronal pathways: PGE(2) pyrogenic signaling is transmitted from EP3R-expressing POA neurons via a projection to the DMH to activate BAT thermogenesis and via another projection to the rRPa to increase cutaneous vasoconstriction. In this case, DMH-projecting and rRPa-projecting neurons would constitute segregated populations within the EP3R-expressing neuronal group in the POA. Here, we sought direct anatomical evidence to test this hypothesis with a double-tracing experiment in which two types of the retrograde tracer, cholera toxin b-subunit (CTb), conjugated with different fluorophores were injected into the DMH and the rRPa of rats and the resulting retrogradely labeled populations of EP3R-immunoreactive neurons in the POA were identified with confocal microscopy. We found substantial numbers of EP3R-immunoreactive neurons in both the DMH-projecting and the rRPa-projecting populations. However, very few EP3R-immunoreactive POA neurons were labeled with both the CTb from the DMH and that from the rRPa, although a substantial number of neurons that were not immunoreactive for EP3R were double-labeled with both CTbs. The paucity of the EP3R-expressing neurons that send collaterals to both the DMH and the rRPa suggests that pyrogenic signals are sent independently to these caudal brain regions from the POA and that such pyrogenic outputs from the POA reflect different control mechanisms for BAT thermogenesis and for cutaneous vasoconstriction by distinct sets of POA neurons.

Biphasic response of cutaneous blood flow induced by passive cutaneous anaphylaxis in rats

In the immediate phase of passive cutaneous anaphylaxis, sensitized skin mast cells release various mediators when activated by antigen. The present study investigated the effects of the mediators on cutaneous blood flow at the antigen-antibody reaction site. Induction of passive cutaneous anaphylaxis produced a biphasic response consisting of an initial decrease, followed by a sustained increase, in the cutaneous blood flow. The initial phase was almost eliminated by the 5-hydroxytryptamine receptor antagonist methysergide, whereas the second phase was sensitive to the histamine H(2) receptor antagonist ranitidine. The histamine H(1) receptor antagonist chlorpheniramine, the denervation of sensory nerves with capsaicin, the cyclooxygenase inhibitor indomethacin, or the bradykinin B(2) receptor antagonist D-arginyl-L-arginyl-L-prolyl-trans-4-hydroxy-L-prolylglycyl-3-(2-thienyl)-L-alanyl-L-seryl-D-1,2,3,4-tetrahydro-3-isoquinolinecarbonyl-L-(2alpha,3beta,7abeta)-octahydro-1H-indole-2-carbonyl-L-arginine (HOE140) did not affect the blood-flow changes caused by the anaphylaxis. These results suggest that 5-hydroxytryptamine and histamine H(2) receptors mediate the initial decrease and the subsequent increase in cutaneous blood flow, respectively, induced by passive cutaneous anaphylaxis in rats.

Effects of intra-arterial ketanserin in Raynaud's phenomenon assessed by 99MTc-pertechnetate scintigraphy

The severity of the peripheral vasopastic condition in 10 patients with Raynaud's phenomenon was investigated using scintigraphy of both hands. 99MTc-pertechnetate was used as a diffusable tracer and scintigrams were recorded with normal hand skin temperature and following immersion of the hands in ice-cold water. Cold provocation resulted in a marked reduction of the rate of blood inflow and tissue perfusion in affected hands. The selective peripheral S2-antagonist ketanserin and placebo were administered intra-arterially according to a double-blind cross-over design. As compared to placebo, ketanserin significantly improved the rate of inflow following cold provocation, and it increased tissue perfusion in the hands of the patients to values normally observed in healthy individuals. The results support the use of ketanserin in the treatment of Raynaud's phenomenon.

Hemodynamic effects of prostacyclin infusions in healthy volunteers

The selective hemodynamic effects of an intravenous infusion of prostacyclin on skin and on forearm blood flow during increasing doses up to 6 ng.kg.-1 min.-1 were evaluated in 12 healthy volunteers. The highest dose raised total skin blood flow (p = 0.0001) as measured by laser Doppler with a concomitant rise in finger skin temperature (p = 0.02). Capillary perfusion as measured by transcutaneous oxygen monitoring remained relatively unaffected (p = 0.25). Cold-induced vasoconstriction and the recovery thereafter were not influenced. Diastolic blood pressure tended to decrease (p = 0.06), but there was no change in systolic blood pressure, forearm blood flow, and calculated forearm vascular resistance. The authors conclude that prostacyclin may have dissimilar vasoactive potencies in different vascular beds as shown by a pronounced increase in total skin blood flow, owing to vasodilation of shunt vessels, simultaneously with an insignificant change in nutritional skin blood flow and forearm muscle blood flow.

Serotoninergic mechanisms in hypertension. Focus on the effects of ketanserin

Aggregating platelets release serotonin, which induces contraction of most vascular smooth muscle by activation of S2-serotoninergic receptors. Serotonin released in the circulation may contribute to the increase in peripheral resistance of hypertension as the responsiveness of blood vessels from hypertensive animals and humans to the vasoconstrictor action of the monoamine is augmented. The data obtained with the new antihypertensive agent ketanserin may favor that interpretation. Ketanserin is a selective S2-serotoninergic antagonist with additional alpha 1-adrenergic blocking properties. In humans, it has a terminal half-life of 12 to 25 hours and is eliminated predominantly by the liver. The hemodynamic profile of ketanserin is that of a vasodilator drug with actions on both resistance and capacitance vessels. On short-term intravenous administration, it lowers blood pressure in hypertensive patients with minimal reflex changes in cardiovascular function. When given orally long term to hypertensive patients, ketanserin causes a sustained reduction in arterial blood pressure, comparable to that obtained with either beta-adrenergic blockers or diuretics. Several studies have shown a greater efficacy in older (greater than 60 years of age) than in younger patients independent of starting pressure. Side effects mainly consist of dizziness, somnolence, and dry mouth, but they are usually not severe. The mechanism underlying the antihypertensive effect of ketanserin is unclear. It cannot be attributed to either S2-serotoninergic or alpha 1-adrenergic blockade alone, but an interaction between the two effects appears to be required.