Mechanisms of the cutaneous vasodilator response to local external pressure application in rats: involvement of CGRP, neurokinins, prostaglandins and NO

Low skin temperature impairs the cutaneous vasodilator response to local progressive pressure strain

A pressure-induced vasodilation (PIV) was recently reported as a putative protective response in human skin. Therefore, we examined the influence of skin temperatures on cutaneous blood flow responses to local progressive pressure strain. Ten healthy volunteers were studied at different ambient temperatures leading to low (29.0 +/- 0.3 degrees C), intermediate (32.6 +/- 0.1 degrees C), high (33.9 +/- 0.1 degrees C) and very high (36.0 +/- 0.1 degrees C) skin temperatures. We measured cutaneous blood flow using laser Doppler flowmetry on the foot in response to a local progressive pressure increase of 5.0 mm Hg min(-1). Progressive pressure strain led to an almost linear cutaneous laser Doppler flow decrease at both low and intermediate skin temperatures (-40.1 +/- 6.6% and -31.2 +/- 6.5% from baseline at 30 +/- 1.25 mm Hg), whereas at both high and very high skin temperatures, subjects responded with a transient cutaneous vasodilation (+33.6 +/- 10.6% and +50.6 +/- 15.4% from baseline at 30 +/- 1.25 mm Hg). These findings suggest that high skin temperatures are required for the PIV to develop.

Impaired pressure-induced vasodilation at the foot in young adults with type 1 diabetes

Vascular and neurological mechanisms are both likely to be involved in foot ulcer. We recently reported a pressure-induced vasodilation (PIV), relying on unmyelinated afferent excitation. We previously found that cutaneous blood flow in response to locally applied pressure might be impaired in diabetic patients because of the combined effects of low cutaneous temperature and alterations in microcirculatory function. Therefore, we aimed to analyze whether, at a relatively high cutaneous temperature, PIV is present in type 1 diabetes and to assess endothelial-dependent vasodilation and endothelium-independent vasodilation. We measured cutaneous blood flow using laser Doppler flowmetry on the head of the first metatarsus in response to applied pressure at 5.0 mmHg/min in warm conditions (29.5 +/- 0.2 degrees C). Responses to iontophoresis of acetylcholine (endothelium dependent) and sodium nitroprusside (endothelium independent) were measured using laser Doppler flowmetry in the forearm. The data indicate that PIV exists at the foot level in normal subjects, whereas it was not found in diabetic patients. In diabetic patients, the nonendothelial-mediated response to sodium nitroprusside was preserved, whereas the endothelial-mediated response to acetylcholine was impaired. These findings might be relevant to the high prevalence of foot ulcer that occurs in diabetic patients.

Post pressure hyperemia in the rat

In prior studies in man, we have demonstrated that pressure-induced hyperemia lasts for prolonged periods as compared to the short-term hyperemia created by proximal arterial occlusion. We have analyzed this phenomenon in our well-studied rat model of skin blood flow. Skin blood flow was measured using laser Doppler techniques in Wistar Kyoto rats at the back, a nutritively perfused site, and at the plantar surface of the paw, where arteriovenous anastomotic perfusion dominates. A customized pressure feedback control device was used to vary applied pressures. At the back, pressures in excess of 80 mmHg resulted in occlusion, whereas at the paw 150 mmHg was required. The peak hyperemic flow after release of pressure was comparable to that elicited by proximal arterial occlusion with a blood pressure cuff. However, the post pressure hyperemia peak descended to a plateau value, which was 50-100% greater than baseline and continued for up to 20 min while the peak following proximal arterial occlusion returned to baseline within 4 min. At the back, post pressure hyperemia reached a maximum after application of 100 mmHg pressure. The application of higher pressures than required for occlusion produced no greater hyperemic response. At the paw, maximum post pressure hyperemia occurred at 100 mmHg, although this pressure level was not totally occlusive. Higher pressures resulted in no greater hyperemia. At the back, 10 min of occlusion produced a maximal peak value whereas 1 min was sufficient at the paw. The application of pressure to a heated probe with subsequent release, produced a hyperemic response. Normalized to baseline blood flow, there was no difference between the hyperemic responses at basal skin temperature and at 44 degrees C. There is a prolonged hyperemic response following local pressure occlusion compared to a much shorter period following proximal ischemic occlusion. One can presume two different mechanisms, one related to ischemia and the other a separate pressure related phenomenon. The thermal vasodilatory response is additive, not synergistic with the post pressure hyperemia we have demonstrated. This finding suggests that different mechanisms are involved in thermal vasodilation and post pressure hyperemia.

Time–frequency analysis of laser Doppler flowmetry signals recorded in response to a progressive pressure applied locally on anaesthetized healthy rats

The laser Doppler flowmetry technique has recently been used to report a significant transient increase of the cutaneous blood flow signal, in response to a local non-noxious pressure applied progressively on the skin of both healthy humans and rats. This phenomenon is not entirely understood yet. In the present work, a time-frequency analysis is applied to signals recorded on anaesthetized healthy rats, at rest and during a cutaneous pressure-induced vasodilation (PIV). The comparison, at rest and during PIV, of the scalogram relative energies and scalogram relative amplitudes in five bands, corresponding to five characteristic frequencies, shows an increased contribution for the endothelial related metabolic activity in PIV signals, till 400 s after the beginning of the progressive pressure application. The other subsystems (heart, respiration, myogenic and neurogenic activities) contribute relatively less during PIV than at rest. The differences are statistically significant for all the relative activities in the interval 0-200 s following the beginning of the pressure. These results and others obtained on patients, such as diabetics, could increase the understanding of some cutaneous pathologies involved in various neurological diseases and in the pathophysiology of decubitus ulcers.

Signal Processing Methodology to Study the Cutaneous Vasodilator Response to a Local External Pressure Application Detected by Laser Doppler Flowmetry

The existence of a cutaneous pressure-induced vasodilation (PIV) has recently been reported. This paper proposes a signal processing methodology to improve PIV knowledge. Temporal variations of laser Doppler signals rhythmic activities are first analyzed on anesthetized rats. The results lead to a method that provides a better PIV understanding.

Evidence for the involvement of VPAC1 and VPAC2 receptors in pressure-induced vasodilatation in rodents

A transient increase in skin blood flow in response to an innocuous local pressure application, defined as pressure-induced vasodilatation (PIV), delays the occurrence of ischaemia, suggesting a protective feature against applied pressure. The PIV response depends on capsaicin-sensitive nerve fibres and calcitonin gene-related peptide (CGRP) has been shown to be involved. In these fibres, CGRP coexists with pituitary adenylate cyclase-activating polypeptide (PACAP). Three distinct receptors mediate the biological effects of PACAP: VPAC1 and VPAC2 receptors binding with the same affinity for PACAP and vasoactive intestinal peptide and PAC1 receptors showing high selectivity for PACAP. Because the receptors are widely expressed in the nervous system and in the skin, we hypothesized that at least one of them is involved in PIV development. To verify this hypothesis, we used [D-p-Cl-Phe(6),Leu(17)]-VIP (nonspecific antagonist of VPAC1/VPAC2 receptors), PG 97-269 (antagonist of VPAC1 receptors), PACAP(6-38) (antagonist of VPAC2/PAC1 receptors) and Max.d.4 (antagonist of PAC1 receptors) in anaesthetized rodents. The blockade of VPAC1/VPAC2, VPAC1 or VPAC2/PAC1 receptors eliminated the PIV response, whereas PAC1 blockade had no effect, demonstrating an involvement of VPAC1/VPAC2 receptors in PIV development. Moreover, endothelium-independent and -dependent vasodilator responses were unchanged by the VPAC1/VPAC2 antagonist. Thus, the absence of a PIV response following VPAC1/VPAC2 blockade cannot be explained by any dysfunction of the vascular smooth muscle or endothelial vasodilator capacity. The involvement of VPAC1/VPAC2 receptors in the development of PIV seems to imply a series relationship in which each receptor type (CGRP, VPAC1, VPAC2) is necessary for the full transmission of the response.

Early decrease of skin blood flow in response to locally applied pressure in diabetic subjects

Pressure ulcers are common debilitating complications of diabetes that are caused by tissue ischemia. Skin blood flow in response to locally applied pressure might be impaired in diabetic patients because of the combined effects of a typically low skin temperature and alterations in microcirculatory function, and could be worsened by neuropathy. We measured skin blood flow by laser Doppler flowmetry over the internal anklebone in response to local pressure applied at 5.0 mmHg/min in three groups of diabetic patients (with clinical and subclinical neuropathy and without neuropathy) and in healthy matched control subjects at usual room temperature. Compared with in matched control subjects with comparable skin temperatures (29.3 +/- 0.4 vs. 28.7 +/- 0.4 degrees C), in diabetic patients the skin blood flow response to locally applied pressure was further impeded, even in those without neuropathy. Indeed, skin blood flow decreased significantly from baseline at much lower applied pressure (7.5 mmHg) in diabetic subjects, again even in those without neuropathy, than in control subjects (48.8 mmHg). The large difference between these pressures could partially explain diabetic patients' high risk of developing decubitus and plantar ulcers.