Nonspecific microvascular vasodilation during iontophoresis is attenuated by application of hyperosmolar saline

Concentration-dependent microvascular responses to repeated iontophoresis of acetylcholine

BACKGROUND

Iontophoresis studies face challenges due to the unknown absolute drug dose delivered and the possible effect of the current used in drug delivery on the microvessels, known as current-induced vasodilation. This study aimed to investigate how various concentrations of acetylcholine (ACh), delivered through transdermal iontophoresis using repeated current pulses, impact the recovery profile of the microvascular response.

METHODS

The study included fifteen healthy volunteers, and microvascular responses to five concentrations of iontophorised ACh (ranging from 0.0055 mM to 55 mM) and sterile water were assessed at six forearm skin sites using polarized reflectance spectroscopy. Iontophoresis at each concentration involved three consecutive pulses separated 8 recovery periods.

RESULTS

Current-induced responses were more pronounced for lower concentrations of ACh and for sterile water. With repeated pulses, lower concentrations of ACh exhibited a recovery profile more akin to higher concentrations.

PERSPECTIVE

Through repeated iontophoresis of ACh, microvascular responses exhibit variation based on the drug concentration and the number of pulses administered. These variations are likely attributed to changes in skin conductivity and permeability.

Neurovascular-modulation: A review of primary vascular responses to transcranial electrical stimulation as a mechanism of action

BACKGROUND

The ubiquitous vascular response to transcranial electrical stimulation (tES) has been attributed to the secondary effect of neuronal activity forming the classic neurovascular coupling. However, the current density delivered transcranially concentrates in: A) the cerebrospinal fluid of subarachnoid space where cerebral vasculature resides after reaching the dural and pial surfaces and B) across the blood-brain-barrier after reaching the brain parenchyma. Therefore, it is anticipated that tES has a primary vascular influence.

OBJECTIVES

Focused review of studies that demonstrated the direct vascular response to electrical stimulation and studies demonstrating evidence for tES-induced vascular effect in coupled neurovascular systems.

RESULTS

tES induces both primary and secondary vascular phenomena originating from four cellular elements; the first two mediating a primary vascular phenomenon mainly in the form of an immediate vasodilatory response and the latter two leading to secondary vascular effects and as parts of classic neurovascular coupling: 1) The perivascular nerves of more superficially located dural and pial arteries and medium-sized arterioles with multilayered smooth muscle cells; and 2) The endothelial lining of all vessels including microvasculature of blood-brain barrier; 3) Astrocytes; and 4) Neurons of neurovascular units.

CONCLUSION

A primary vascular effect of tES is highly suggested based on various preclinical and clinical studies. We explain how the nature of vascular response can depend on vessel anatomy (size) and physiology and be controlled by stimulation waveform. Further studies are warranted to investigate the mechanisms underlying the vascular response and its contribution to neural activity in both healthy brain and pathological conditions - recognizing many brain diseases are associated with alteration of cerebral hemodynamics and decoupling of neurovascular units.

Repeatability of the evaluation of systemic microvascular endothelial function using laser doppler perfusion monitoring: clinical and statistical implications

OBJECTIVE

An awareness of the repeatability of biological measures is required to properly design and calculate sample sizes for longitudinal interventional studies. We investigated the day-to-day repeatability of measures of systemic microvascular reactivity using laser Doppler perfusion monitoring.

METHODS

We performed laser Doppler perfusion monitoring in combination with skin iontophoresis using acetylcholine and sodium nitroprusside as well as post-occlusive reactive and thermal hyperemia twice within two weeks. The repeatability was assessed by calculating the within-subject standard deviations, limits of agreement, typical errors and intra-class correlation coefficients between days 1 and 2. The ratio of the within-subject standard deviation to the mean values obtained on days 1 and 2 (within-subject standard deviation/GM) was used to determine the condition with the best repeatability.

RESULTS

Twenty-four healthy subjects, aged 24.6 ± 3.8 years, were recruited. The area under the curve of the vasodilatory response to post-occlusive reactivity showed marked variability (within-subject standard deviation/GM = 0.83), while the area under the curve for acetylcholine exhibited less variability (within-subject standard deviation/ GM = 0.52) and was comparable to the responses to sodium nitroprusside and thermal treatment (within-subject standard deviations/GM of 0.67 and 0.56, respectively). The area under the blood flow/time curve for vasodilation during acetylcholine administration required the smallest sample sizes, the area under the blood flow/time curve during post-occlusive reactivity required the largest sample sizes, and the area under the blood flow/time curves of vasodilation induced by sodium nitroprusside and thermal treatment required intermediate sizes.

CONCLUSIONS

In view of the importance of random error related to the day-to-day repeatability of laser Doppler perfusion monitoring, we propose an original and robust statistical methodology for use in designing prospective clinical studies.

Nonlinear dynamics of cardiovascular ageing

The application of methods drawn from nonlinear and stochastic dynamics to the analysis of cardiovascular time series is reviewed, with particular reference to the identification of changes associated with ageing. The natural variability of the heart rate (HRV) is considered in detail, including the respiratory sinus arrhythmia (RSA) corresponding to modulation of the instantaneous cardiac frequency by the rhythm of respiration. HRV has been intensively studied using traditional spectral analyses, e.g. by Fourier transform or autoregressive methods, and, because of its complexity, has been used as a paradigm for testing several proposed new methods of complexity analysis. These methods are reviewed. The application of time-frequency methods to HRV is considered, including in particular the wavelet transform which can resolve the time-dependent spectral content of HRV. Attention is focused on the cardio-respiratory interaction by introduction of the respiratory frequency variability signal (RFV), which can be acquired simultaneously with HRV by use of a respiratory effort transducer. Current methods for the analysis of interacting oscillators are reviewed and applied to cardio-respiratory data, including those for the quantification of synchronization and direction of coupling. These reveal the effect of ageing on the cardio-respiratory interaction through changes in the mutual modulation of the instantaneous cardiac and respiratory frequencies. Analyses of blood flow signals recorded with laser Doppler flowmetry are reviewed and related to the current understanding of how endothelial-dependent oscillations evolve with age: the inner lining of the vessels (the endothelium) is shown to be of crucial importance to the emerging picture. It is concluded that analyses of the complex and nonlinear dynamics of the cardiovascular system can illuminate the mechanisms of blood circulation, and that the heart, the lungs and the vascular system function as a single entity in dynamical terms. Clear evidence is found for dynamical ageing.

Calcineurin inhibitor effects on glucose metabolism and endothelial function following renal transplantation

BACKGROUND

Calcineurin inhibitors (CNI) are involved in the development of post-transplant diabetes mellitus (PTDM). Changes in insulin secretion and sensitivity contribute to the development of PTDM and are associated with endothelial function.

METHODS

In a pre-defined substudy of a previously published randomized trial in renal transplant recipients we compared the effect of CNI treatment (n = 23) with complete CNI-avoidance (n = 21) on insulin secretion and sensitivity (oral glucose tolerance test) as well as endothelial function (laser Doppler flowmetry), 10 wk and 12 months following transplantation.

RESULTS

Insulin sensitivity differed 10 wk post-transplant and was significantly better after 12 months in patients never treated with CNI drugs [0.091 (0.050) vs. 0.083 (0.036) micromol/kg/min/pmol/L, p = 0.043]. Insulin secretion tended to be higher in CNI treated patients at both time points (p = 0.068). Endothelial function was not significantly different at week 10 [540 (205) vs. 227 (565) arbitary units x minutes, p = 0.35] or month 12 [510 (620) vs. 243 (242), p = 0.33].

CONCLUSIONS

Findings in the present study indicate that long-term CNI treatment negatively affects glucose metabolism and this may contribute to the increased risk for premature cardiovascular disease in CNI treated renal transplant recipients. Further studies to elucidate this hypothesis are, however, needed.

An assessment of traditional and novel therapies for cellulite

There are numerous treatments for cellulite including topical, surgical, laser and other therapies. Many of these treatments are expensive. Part of the difficulty in treating cellulite arises from our incomplete understanding of this phenomenon. As noted previously in this journal, there is no consensus as to the etiology of cellulite. This article will focus on both traditional and novel treatments for cellulite and assess their efficacy based on the scientific literature.

Current concepts in assessment of microvascular endothelial function using laser Doppler imaging and iontophoresis

Effective evaluation of endothelial function is a powerful tool for determining patients at risk of development and progression of cardiovascular disease. As an alternative to invasive tests of endothelial function, several noninvasive methods have been developed, including the use of laser Doppler flowmetry/imaging to measure cutaneous perfusion accompanied by iontophoresis of acetylcholine and sodium nitroprusside. It is clear from previous studies that this technique provides an easy, validated, and reproducible method for investigators to assess and monitor endothelial function in patients with a variety of pathologic conditions, but it may also be used to examine disease progression over time and responsiveness to treatment, thereby facilitating clinical trials. However, a standardization of protocols would help reduce the apparent controversy seen in the literature. With its increasing use by other groups, it is anticipated that further published studies will help to provide a better understanding of the development and progression of cardiovascular disease.

Association between insulin resistance and endothelial dysfunction in renal transplant recipients

BACKGROUND

Endothelial dysfunction is a common finding in renal transplant recipients (RTR) and is related to impaired local regulation of vasodilative and vasoconstrictive substances, such as nitric monoxide (NO) and endothelin-1 (ET-1). In non-transplanted patients, an association between impaired endothelial function and insulin resistance has been shown. Whether such an association also exists in RTR is unknown.

OBJECTIVE

The aim of the present study was to examine whether insulin resistance is associated with endothelial dysfunction in RTR.

MATERIAL AND METHODS

A total of 47 RTR in a stable phase six yr post-transplant were included in the statistical analysis. The immunosuppressive therapy was based on cyclosporine and prednisolone. Non-invasive assessment of endothelial function was performed with laser Doppler flowmetry of the forearm skin vasculature after local acetylcholine stimulation. Oral glucose tolerance tests comprising both glucose and insulin measurements were used to calculate insulin sensitivity (IS) indices. NO, ET-1 and von Willebrand factor were measured in fasting plasma samples.

RESULTS

Normal glucose tolerance was found in 31 RTR. In these subjects, both IS (r(2) = 0.164, p = 0.044) and plasma NO (r(2) = 0.326, p = 0.002) were significantly correlated with endothelial function. Patients with glucose intolerance (n = 16) had higher plasma ET-1 and lower NO levels, but the association between IS and endothelial function was not significant in these subjects. In the total patient cohort, IS and endothelial function tended to be correlated (p = 0.127).

CONCLUSIONS

Endothelial dysfunction is significantly associated with insulin resistance in normoglycemic RTR but explains a rather small part of the variation. In glucose-intolerant recipients, IS appears to be more critically dependent on other factors not revealed in the present study.

Cathodal current-induced vasodilation to single application and the amplified response to repeated application in humans rely on aspirin-sensitive mechanisms

Assumed to rely on an axon reflex, the current-induced vasodilation (CIV) interferes with the microvascular response to iontophoretic drug delivery. Mechanisms resulting in CIV are likely different at the anode and at the cathode. While studies have been conducted to understand anodal CIV, little information is available on cathodal CIV. The present study investigates CIV observed following 0.1-mA cathodal applications on forearms of healthy volunteers and the possible mechanisms involved. Results are expressed in percentage of the cutaneous heat-induced maximal vascular conductance [%MVC (means ± SE)]. 1) The amplitude of CIV was proportional to the duration of cathodal currents for periods of <1 min: r = 0.99. 2) Two current applications of 10 s, with 10-min interstimulation interval, induced a higher peak value of CIV (79.1 ± 8.6% MVC) than the one obtained with all-at-once 20-s current application (39.5 ± 4.3% MVC, P < 0.05). This amplified vascular response due to segmental application was observed for all tested interstimulation intervals (up to 40 min). 3) Two hours and 3 days following pretreatment with 1-g oral aspirin, the CIV observed following cathodal application, as well as the difference of cathodal CIV amplitude between all-at-once and segmented applications, were reduced. These findings suggest a role of prostaglandins, not only released from endothelial or smooth muscle cells, as direct vasodilator and/or as a sensitizer. Thus aspirin pretreatment could be used to decrease CIV resulting from all-at-once and repeated cathodal application and facilitate the study of the specific vascular effect induced by the drug delivered.

The effect of glucose and mental stress on cutaneous microvascular endothelial function

Glucose and mental stress, independently, have been found to impair arterial endothelial function (an indicator of vascular health). The present study sought to determine whether the combination of glucose and stress would have a greater effect on microvascular endothelial function than each on its own. To assess endothelial function, surges in skin blood flow (reactive hyperemia), following the release of cuff pressure to the upper arm at 200 mmHg for 5 min, were measured with laser Doppler flowmetry in 40 young, healthy females. Endothelial function did not change significantly following a 5-min mathematics stressor or the consumption of 75 g of glucose. However, the combination of glucose and stress impaired endothelium-dependent dilatation 30 min after glucose consumption. These findings suggest that combinations of vascular risk factors may be more threatening to cardiovascular health than singularly occurring factors.

Mechanisms of acetylcholine-mediated vasodilatation in young and aged human skin

Thermoregulatory cutaneous vasodilatation (VD) is attenuated in aged skin. While acetylcholine (ACh) plays a role in thermally mediated VD, the precise mechanisms through which ACh-mediated VD acts and whether those downstream mechanisms change with ageing are unclear. We tested the hypotheses that both nitric oxide (NO)- and prostanoid-mediated pathways contribute to exogenous ACh-mediated VD, and that both are attenuated with advanced age. Twelve young (Y: 23 +/- 1 years) and 10 older (O: 69 +/- 1 years) subjects underwent infusions of 137.5 mum ACh at four intradermal microdialysis sites: control (C, Ringer solution), NO synthase inhibited (NOS-I, 10 mm l-NAME), cyclooxygenase inhibited (COX-I, 10 mm ketorolac) and NOS-I + COX-I. Red blood cell flux was monitored using laser-Doppler flowmetry, and cutaneous vascular conductance (CVC) was calculated (laser-Doppler flux/mean arterial pressure) and normalized to maximal CVC (%CVC(max)) (28 mm sodium nitroprusside + local heating to 43 degrees C). Baseline %CVC(max) was increased in the O at COX-I sites (COX-I 16 +/- 1, NOS-I + COX-I 16 +/- 2 versus C 10 +/- 1%CVC(max); P < 0.001) but not in the young, suggesting an age-related shift toward COX vasoconstrictors contributing to basal cutaneous vasomotor tone. There was no difference in peak %CVC(max) during ACh infusion between age groups, and the response was unchanged by NOS-I (O: NOS-I 35 +/- 5 versus C 38 +/- 5%CVC(max); P = 0.84) (Y: NOS-I 41 +/- 4 versus C 39 +/- 4%CVC(max); P = 0.67). COX-I and NOS-I + COX-I attenuated the peak CVC response to ACh in both groups (COX-I O: 29 +/- 3, Y: 22 +/- 2%CVC(max) versus C; P < 0.001 both groups; NOS-I + COX-I O: 32 +/- 3 versus Y: 29 +/- 2%CVC(max); versus C; P < 0.001 both groups). ACh mediates cutaneous VD through prostanoid and non-NO-, non-prostanoid-dependent pathways. Further, older subjects have a diminished prostanoid contribution to ACh-mediated VD.

Short-term treatment with rosiglitazone improves glucose tolerance, insulin sensitivity and endothelial function in renal transplant recipients

BACKGROUND

Insulin resistance (IR) contributes to the development of glucose intolerance (post-transplant diabetes mellitus or impaired glucose tolerance) following renal transplantation. Furthermore, endothelial dysfunction (ED) is associated with IR. Glucose intolerance, IR and ED are all independent risk factors for cardiovascular disease. Therefore, treatment with insulin sensitizers may benefit glucose-intolerant renal transplant recipients. The main objectives of the present study were to investigate the effect of 4 weeks' treatment with the PPAR-gamma agonist rosiglitazone on insulin sensitivity, plasma glucose and endothelial function in renal transplant recipients with glucose intolerance. Safety parameters were also addressed.

METHODS

A total of 10 glucose-intolerant renal transplant recipients were treated with rosiglitazone (initially 4 mg/day increasing to 8 mg/day after 1 week). A hyperinsulinaemic euglycaemic glucose clamp, an oral glucose tolerance test and endothelial function assessment with laser Doppler flowmetry were performed both at baseline and at follow-up.

RESULTS

Treatment with rosiglitazone was followed by a significantly improved mean glucose disposal rate (from 6.5 to 9.1 g/kg/min; P = 0.02) and a significant decline in fasting and 2 h plasma glucose (from 6.4 to 5.8 mmol/l, P = 0.01 and from 14.2 to 10.6 mmol/l, P = 0.03, respectively). Furthermore, a significant improvement in endothelial function was demonstrated (AUC(ACh); from 389 to 832 AU x min, P = 0.04). No serious adverse events or hypoglycaemic episodes were observed.

CONCLUSIONS

Four weeks' treatment with rosiglitazone was associated with increased insulin sensitivity, lowered fasting and 2 h plasma glucose and improved endothelial function in renal transplant recipients with glucose intolerance. The drug was well tolerated and may be a good alternative for treating these patients.

A protocol for iontophoresis of acetylcholine and sodium nitroprusside that minimises nonspecific vasodilatory effects

Iontophoresis of vasoactive substances is a promising tool for studying pharmacological aspects of the (patho)physiology of the microvasculature. However, nonspecific microvascular responses are a common problem in most protocols used. We studied the effect of current density (mA/cm2), charge density (mC/cm2), drug concentration (mass %) and vehicle concentration (M) on the nonspecific vasodilatation during iontophoresis of sodium chloride, acetylcholine (ACh) and sodium nitroprusside (SNP). We found that nonspecific vasodilatation depended on current density and charge density in both anodal and cathodal iontophoresis. The responses to ACh and SNP were dependent on current density, charge density and drug concentration. We found that by limiting current density (<0.01 mA/cm2) and charge density (<7.8 mC/cm2) and with adjusted concentrations for drugs and vehicles, it is possible to prevent nonspecific effects during iontophoresis of ACh and SNP, while maximum drug effects (plateaus in the dose-response curves) are still obtained. These new findings are important for future iontophoresis studies in which vasoactive drugs are used to assess microvascular function because the presented approach has advantages compared to older techniques, which mainly have attempted to suppress or compensate for the nonspecific responses during iontophoresis by the use of local anaesthetics or the measurement of drug-minus-vehicle responses, both of which present well-known experimental shortcomings.

Role of transdermal potential difference during iontophoretic drug delivery

Potential differences have been measured during transdermal iontophoresis in order to establish the effect of voltage, as opposed to current, on cutaneous blood flow. It is known that, even in the absence of drugs, the iontophoresis current can sometimes produce increased blood flow. The role of voltage in this process is studied through single-ended measurements (between electrode and body) of the potential difference during iontophoresis with 100-microA, 20-s current pulses through deionized water, saturated 20.4% NaCl solution, 1% acetylcholine, and 1% sodium nitroprusside. It is found that the voltage needed to deliver the current varied by orders of magnitudes less than the differences in the conductance of these different electrolytes, and it is concluded that, at least for the present current protocol, the voltage as such is not an important factor in increasing the blood flow.

Influence of vehicle resistance on transdermal iontophoretic delivery of acetylcholine and sodium nitroprusside in humans

Iontophoresis is a valuable method of noninvasive drug delivery for assessment of skin microvascular function, but it is important to consider and minimize its potential nonspecific electrical effects on blood flow. The use of sodium chloride (NaCl) instead of water as the iontophoresis vehicle has been reported to reduce these effects because it has a lower electrical resistance. However, this argument may not be valid when an agonist is added to the vehicle because its resistance will be changed. The aim of our study was to determine whether there is a difference in resistance between water and NaCl when used as vehicles for iontophoresis of acetylcholine (ACh) and sodium nitroprusside (SNP). Four cumulative doses of each drug, dissolved in either water or NaCl, were delivered via iontophoresis to the forearm skin of 14 healthy volunteers. We measured the resulting blood flow responses by using laser-Doppler imaging and the voltage across the electrodes for each delivery as an index of resistance. For ACh and SNP, there were no significant differences between the voltages measured when either water or NaCl was used as the vehicle. However, the blood flow responses to both agonists were significantly lower with NaCl (ACh: 25% lower, P < 0.001; SNP: 15% lower, P = 0.019). The use of NaCl is therefore unlikely to decrease any nonspecific electrical effects, and it may in fact reduce the effective dose of drug delivered. Deionized water is a better iontophoresis vehicle for the assessment of microvascular function in skin when using ACh and SNP.

Digital iontophoresis of vasoactive substances as measured by laser Doppler imaging—a non-invasive technique by which to measure microvascular dysfunction in Raynaud's phenomenon

OBJECTIVE

To test the hypothesis that microvascular vasodilation is impaired in patients with systemic sclerosis (SSc) compared with patients with primary Raynaud's phenomenon (PRP) and healthy controls, using the technique of laser Doppler imaging to quantify blood flow responses to iontophoresis of vasoactive agents.

METHODS

Microvascular blood flow was measured by laser Doppler imaging before, during and after 120 s iontophoresis (30 microA) of 1% acetylcholine chloride (ACh, endothelium-dependent) and 1% sodium nitroprusside (NaNP, endothelium-independent). Two adjacent fingers of the left hand were studied, and the procedure then repeated on the right. Ten patients with limited cutaneous SSc (LCSSc), 10 patients with PRP and 11 healthy control subjects were studied.

RESULTS

Vasodilation in response to both ACh and NaNP iontophoresis, as measured by 'area under the blood flow.time curve' (AUC), normalized for baseline flux, was similar in the control and PRP groups, but was diminished in the LCSSc group compared with both control and PRP groups (ACh results: control vs LCSSc P = 0.028, PRP vs LCSSc P = 0.005; NaNP results: control vs LCSSc P = 0.004, PRP vs LCSSc P = 0.005). There were no differences between groups in baseline flux values nor in voltages required to drive the 30 microA current.

CONCLUSIONS

Both endothelium-dependent and endothelium-independent vasodilation are impaired in patients with LCSSc. Vasodilatory responses in patients with PRP are similar to those in controls. If reproducibility is confirmed to be satisfactory, then these techniques could be used to examine disease progression over time and responsiveness to vasoactive treatment, thus facilitating clinical trials.

Evaluation of forearm microvascular blood flow regulation by laser Doppler flowmetry, iontophoresis, and curve analysis: contribution of axon reflex

The relative contribution of vasodilating factors to the control of blood flow in the forearm cutaneous microcirculation is not well defined. Therefore, a mathematical transformation is introduced to decompose the superimposed signal and to investigate the involved mechanisms separately. Transdermal iontophoresis was used for the delivery of acetylcholine (ACh) or sodium nitroprusside (SNP) into the forearm, and cutaneous perfusion was measured using a laser Doppler flowmeter (LDF). The curve fitting procedure used in this study indicates that the LDF signal in response to ACh iontophoresis can be described by the superposition of two independent hyperbolic response curves. Obviously, each component of LDF signal indicates the existence of a separate mechanism, with corresponding rate constant k, latency T, and the saturation level Fmax. Blockade of C-fiber function (axon reflex) with topical anesthesia removes one of the two components of this response and allows the precise quantification of its contribution. SNP-evoked response also has two components, but their parameters were different from those of ACh. Therefore, ACh and SNP cause vasodilation in the skin microcirculation through different pathways. These findings have implications for clinical studies that use the iontophoresis technique for assessing vascular function and comparing responses to ACh and SNP to evaluate endothelial dysfunction.

Wavelet analysis of blood flow dynamics: effect on the individual oscillatory components of iontophoresis with pharmacologically neutral electrolytes

Iontophoresis currents are used in the transcutaneous delivery of vasoactive substances for noninvasive assessment of skin vascular properties. The blood flow rate can be recorded by laser Doppler flowmetry (LDF), its average value and the amplitudes of its oscillatory components being used to evaluate the effect of the drugs. Because non-drug-specific, current-induced, vasodilation could confound the interpretation of the response, we have investigated the effect of currents of both polarities on the spectral components of the LDF signal in the absence of vasoactive substances. It was recorded for healthy volunteers with both high conductance (5 mol/l NaCl) and low conductance (deionized water) electrolytes. The oscillatory components were analysed by wavelet transform within 0.0095-1.6 Hz, divided into five sub-intervals. Only cathodal iontophoresis with deionized water increased the oscillatory energy and amplitude. It did so at all frequencies, but none of the sub-intervals associated with vasodilation (0.0095-0.145 Hz) was selectively affected compared to the others.

Local anesthesia reduces the maximal skin vasodilation during iontophoresis of sodium nitroprusside and heating

AIM

To evaluate the effect of local anesthesia on the skin vasodilation induced by the iontophoresis of sodium nitroprusside and heating.

METHODS

Skin vascular reactivity, in response to iontophoresis of sodium nitroprusside (SNP), was evaluated at the forearm and foot in 13 neuropathic diabetic (DN) and 11 nonneuropathic diabetic (D) patients and 9 healthy, nondiabetic subjects who served as controls (C). The direct (DI) and nerve axon reflex-related (N-V) vasodilation were measured by using two single-point laser Doppler probes. The vasodilation in response to local warming was also assessed. A topical anesthetic was applied on the contralateral forearm and foot and all the measurements were repeated.

RESULTS

Dermal anesthesia resulted in a reduction of the direct vasodilation to SNP at the forearm [C: 58.1 +/- 16, D: 60.6 +/- 11%, and DN: 48.3 +/- 37% (postanesthesia percentage of reduction; mean +/- SEM), P<0.01] and at the foot in all three groups (D: 38.5 +/- 12%, P<0.01; C: 27.2 +/- 14% and DN: 11.3 +/- 17.5%, P=NS). The N-V related vasodilation was very low before and did not change after local anesthesia. The postanesthesia hyperemic response to warming was significantly reduced at low temperatures but did not change at 44 degrees C.

CONCLUSION

The sodium nitroprusside-related vasodilation is reduced after local anesthesia in a similar way in healthy subjects and diabetic patients with and without neuropathy. The response to heating is also reduced at low temperatures. This indicates a stabilizing effect of local anesthesia on the smooth muscle cell.

Fluvastatin reduces atherogenic lipids without any effect on native endothelial function early after kidney transplantation

BACKGROUND

Cardiovascular risk is greatly increased in renal transplant recipients. 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor (statin) therapy may reduce cardiovascular risk by improving both dyslipidemia and endothelial function. We therefore performed this study to assess the effect of fluvastatin on endothelial function in renal transplant recipients.

METHODS

This randomized, placebo-controlled, double-blind designed study investigated the effect of fluvastatin on endothelial function. Thirty-seven recipients received fluvastatin 40 mg/d and 35 received placebo during the first 12 wk following transplantation. All patients initially received cyclosporin A, prednisolone and azathioprine. At the end of treatment, endothelial function was assessed in the forearm skin microvasculature by laser Doppler flowmetry following acetylcholine stimulation. Samples were taken for measurements of serum lipids and vasoactive markers.

RESULTS

There were no differences in endothelial function between fluvastatin recipients and controls, AUCACh was 656 +/- 479 and 627 +/- 518 AU min, respectively (fluv vs. control, p > 0.65). In the placebo limb, total cholesterol and LDL cholesterol increased 22 +/- 12% and 22 +/- 18%, respectively in the first 12 wk following transplantation. The respective values were 18 +/- 13% (p = 0.010) and 34 +/- 19% (p = 0.0013) lower at 12 wk in the fluvastatin treated patients. Plasma ET-1, BigET-1 and urinary excretion of cGMP were not significantly different between treatment groups (p > 0.55).

CONCLUSION

Although fluvastatin 40 mg/d significantly lowers cholesterol it does not affect endothelial function the first 3 months after renal transplantation. The lack of effect on endothelial function is consistent with a lack of effect on vasoactive substances.

Prolonged aspirin inhibition of anodal vasodilation is not due to the trafficking delay of neural mediators

We previously reported that forearm vasodilation to a delivered all-at-once over 5 min or a 1-min repeated monopolar anodal 0.10-mA current application is aspirin sensitive and that a single high-dose aspirin exerts a long-lived effect in the former case. We hypothesized that 1) in the latter case, the effect of aspirin would also be long lived and 2) the time required to resupply nerve endings with unblocked cyclooxygenase through axonal transport could explain this phenomenon. We studied the time course for the recovery of vasodilation to repeated current application after placebo or 1-g aspirin treatment. We then searched for a difference at a proximal vs. distal site in the recovery of the response. Aspirin abolished current-induced vasodilation at 2 h, 10 h, and 3 days, with a progressive recovery thereafter, but no difference between distal and proximal site was observed for the recovery of the response. This suggests that, although neural cyclooxygenase could participate in the response, the time course of aspirin inhibition of current-induced cutaneous vasodilation is not due to the time required through neural transport to resupply nerve endings with unblocked proteins.

Nonspecific vasodilatation during transdermal iontophoresis-the effect of voltage over the skin

We used laser Doppler perfusion imaging (LDPI) to study nonspecific vasodilatation during iontophoresis. In iontophoresis studies, nonspecific vasodilatation occurs as a result either of galvanic currents or of the applied voltage over the skin. We made dose-response measurements to study the effect of ionic strength of the vehicle on the nonspecific vasodilatation during iontophoresis of sodium chloride and deionized water, while we monitored the voltage over the skin. We found that anodal and cathodal ionotophoresis induced a voltage over the skin that was dependent on the ionic strength of the test solution. The nonspecific vasodilatation during anodal iontophoresis was less pronounced than during cathodal iontophoresis, and was independent of the voltage over the skin. The nonspecific vasodilatation in cathodal iontophoresis was related to the voltage over the skin, and was possibly mediated by depolarization of local sensory nerves. In experiments using cathodal iontophoresis, therefore, the ionic strengths of the vehicle and the drug are important when vasoactive drugs are examined, as the nonspecific vasodilatation needs to be controlled for. As the vasodilatation that we observed was heterogeneously distributed within the area of iontophoresis, LDPI may provide more accurate measurements than conventional laser Doppler perfusion monitoring.

Regulation of human cutaneous circulation evaluated by laser Doppler flowmetry, iontophoresis, and spectral analysis: importance of nitric oxide and prostaglandines

Nitric oxide (NO) and prostaglandines (PGs) are important in regulation of vascular tone and blood flow. Their contribution in human cutaneous circulation is still uncertain. We inhibited NO synthesis by infusing N(G)-monomethyl-L-arginine (L-NMMA) in the brachial artery (16 micromol/min for 5 min) and reversed it by intraarterial infusion of L-arginine (40 micromol/min for 7.5 min). PG synthesis was inhibited by the cyclooxygenase inhibitor aspirin (600 mg over 5 min intravenously). Basal cutaneous perfusion and perfusion responses during iontophoresis with the endothelium-dependent vasodilator acetylcholine (ACh) and the endothelium-independent vasodilator sodium nitroprusside (SNP) were recorded by laser Doppler flowmetry (LDF). We performed wavelet transforms of the measured signals. Mean spectral amplitude within the frequency interval from 0.0095 to 1.6 Hz and mean and normalized amplitudes of five intervals around 1, 0.3, 0.1, 0.04, and 0.01 Hz were analysed. The oscillations with frequencies around 1, 0.3, 0.1, and 0.04 Hz are influenced by the heartbeat, the respiration, the intrinsic myogenic activity of vascular smooth muscle, and the neurogenic activity of the vessel wall, respectively. We have previously shown that the oscillation with a frequency around 0.01 Hz is modulated by the vascular endothelium. L-NMMA reduced mean value of the LDF signal by approximately 20% (P = 0.0067). This reduction was reversed by L-arginine. Mean value of the LDF signals during ACh and SNP iontophoresis did not change after infusion of L-NMMA. Aspirin did not affect mean value of the LDF signal or the LDF signal during ACh or SNP iontophoresis. Before interventions the only significant difference between the effects of ACh and SNP was observed in the frequency around 0.01 Hz, where ACh increased normalized amplitude to a greater extent than SNP. L-NMMA abolished this difference, whereas it reappeared after infusion of L-arginine (P = 0.0084). Aspirin did not affect this difference (P = 0.006). We conclude that basal cutaneous blood flow and the endothelial dependency of the oscillation around 0.01 Hz are partly mediated by NO, but not by endogenous PGs. Other aspects of human cutaneous circulation studied are not regulated by NO or PGs.

Transient hyperaemic response to assess vascular reactivity of skin: effect of locally iontophoresed acetylcholine, bradykinin, epinephrine and phenylephrine

BACKGROUND

Recently, the transient hyperaemic response (THR) to brief compression (20 s) of the brachial artery has been described as a way to assess vascular reactivity of the forearm skin. We studied the effects of locally iontophoresed vasoactive agents on this response in 20 male volunteers.

METHODS

An iontophoresis chamber attached to the anterior forearm permitted simultaneous administration of drugs by iontophoresis and measurement of skin blood flow-flux by laser Doppler probe. Three THR tests were performed before and after iontophoresis by compressing the brachial artery with digital pressure for 20 s and then releasing. The following were iontophoresed: saline 0.9% (iontophoresis vehicle control), acetylcholine, bradykinin, epinephrine and phenylephrine. The THR ratio (THRR) was calculated as F2/F1 where F1 was baseline blood flow-flux immediately before compression and F2 was peak blood flow-flux after release.

RESULTS

When compared with saline 0.9%, acetylcholine and bradykinin increased median F1 from 9.2 (range 5.2-23.8) to 22.1 (8.7-61.5) and from 4.8 (3.0-23.2) to 15.0 (2.5-31.8), respectively, and reduced THRR from 1.26 (1.07-2.2) to 0.99 (0.93-1.04) and from 1.63 (1.06-2.58) to 1.09 (0.93-1.19), respectively. Epinephrine, but not phenylephrine, caused a significant reduction in F1 from 9.2 (5.2-23.8) to 4.0 (1.5-22.3). Neither epinephrine nor phenylephrine had significant effect on THRR.

CONCLUSIONS

Iontophoresed acetylcholine and bradykinin significantly increase the flow-flux and impair THR in forearm skin, further validating the concept that THR represents true vasodilatation during arterial occlusion. In addition, iontophoresis of vasoconstrictors does not appear to have any consistent effect.

Break excitation alone does not explain the delay and amplitude of anodal current-induced vasodilatation in human skin

In iontophoresis experiments, a 'non-specific' current-induced vasodilatation interferes with the effects of the diffused drugs. This current-induced vasodilatation is assumed to rely on an axon reflex due to excitation of cutaneous nociceptors and is weaker and delayed at the anode as compared to the cathode. We analysed whether these anodal specificities could result from a break excitation of nociceptors. Break excitation is the generation of action potentials at the end of a square anodal DC current application, which are generally weaker than those observed at the onset of a same application at the cathode. In eight healthy volunteers, we studied forearm cutaneous laser Doppler flow (LDF) responses to: (1) anodal and cathodal 100 microA current applications of 1, 2, 3, 4 or 5 min; (2) 100 microA anodal applications of 3 min with a progressive ending over 100 s (total charge 23 mC); these were compared to square-ended 100 microA anodal applications of the same total charge (23 mC) or duration (3 min); (3) a 4 min 100 microA anodal application with a 333 msec break at half time. Results (mean +/- S.D.) are expressed as percentage of heat-induced maximal vasodilatation (%MVD). Onset (T(vd)) and amplitude (LDF(peak)) of vasodilatation were determined. We observed that: T(vd) was linearly related to the duration of current application at the anode (slope = 1.01, r(2) = 0.99, P < 0.0001) but not at the cathode (slope = 0.03, r(2) = 0.02, n.s.). Progressive ending of anodal current did not decrease LDF(peak) (63.3 +/- 24.6 %MVD) as compared to square-ending of current application of the same duration (36.9 +/- 22.2 %MVD) or the same total charge (57.1 +/- 23.5 %MVD). A transient break of anodal current did not allow for the vasodilatation to develop until current was permanently stopped. We conclude that, during iontophoresis, anodal break excitation alone cannot account for the delay and amplitude of the vascular response.

Vasodilatation in response to repeated anodal current application in the human skin relies on aspirin-sensitive mechanisms

The vasodilatation resulting from prolonged square-wave monopolar current application as used in iontophoresis is assumed to rely on an axon reflex. Involvement of prostaglandins in the anodal current-induced vasodilatation remains unclear. We tested the hypothesis that prostaglandins participate in a sensitisation mechanism to current application rather than as direct vasodilators. In healthy volunteers, laser Doppler flowmetry (LDF) was recorded in the forearm during and following isolated or repeated 0.1 mA transcutaneous anodal current applications, using deionised water as a vehicle. Segmented current applications of 6 or 12 mC resulted in an LDF increase twice that observed following current applications of comparable total charge delivered all at once (P < 0.05). Following a 1 min anodal application, a slow and prolonged LDF drift occurred (slope: 0.3 +/- 0.5 arbitrary units min(-1)). When the same current application was repeated after intervals of 5 and 20 min, an abrupt vasodilatation occurred, with maximal LDF amplitude of 53.5 +/- 34.0 and 48.2 +/- 19.1 arbitrary units, respectively. Pretreatment with 1 g oral aspirin abolished the abrupt vasodilatation to repeated current application but not the initial slow drift. We suggest that vasodilatation occurs through two parallel pathways: (1) a slow progressive drift of LDF of limited amplitude insensitive to aspirin pretreatment, and (2) an abrupt vasodilatation probably resulting from afferent fibre activation, appearing if a preliminary sensitisation by current application is performed. Sensitisation lasts for at least 20 min, and is blocked by aspirin, suggesting participation of prostanoids.

Vascular and neural mechanisms of ACh-mediated vasodilation in the forearm cutaneous microcirculation

The relative contribution of endothelial vasodilating factors to acetylcholine (ACh)-mediated vasodilation in the forearm cutaneous microcirculation is unclear. The aims of this study were to investigate the contributions of prostanoids and cutaneous C fibers to basal cutaneous blood flow (CuBF) and ACh-mediated vasodilation. ACh was iontophoresed into the forearm, and cutaneous perfusion was measured by laser-Doppler flowmetry. To inhibit the production of prostanoids, four doses of acetylsalicylic acid (ASA; 81, 648, 972, and 1,944 mg) were administered orally. Cutaneous nerve fibers were blocked with topical anesthesia. Cyclooxygenase inhibition did not change basal CuBF or endothelium-mediated vasodilation to ACh. In contrast, ASA (972 and 1,944 mg) significantly reduced the C-fiber-mediated axon reflex in a dose-dependent fashion. Blockade of C-fiber function significantly reduced axon reflex-mediated vasodilation but did not affect basal CuBF or endothelium-dependent vasodilation. The findings suggest that prostanoids do not contribute significantly to basal CuBF or endothelium-dependent vasodilation in the forearm microcirculation. In contrast, prostanoids are mediators of the ACh-provoked axon reflex.

The effect of aspirin and various iontophoresis solution vehicles on skin microvascular reactivity

The two main objectives of this study were: (1) to examine the effect of aspirin on the endothelial function in healthy subjects and (2) to examine the effect of deionized water and 5% NaCl as iontophoresis solution vehicles. The skin microcirculation was evaluated at the forearm level of healthy subjects. A laser Doppler scanner was employed to measure vasodilation in response to the iontophoresis of 1% acetylcholine (endothelium-dependent) and 1% sodium nitroprusside (endothelium-independent). In the first experiment, nine healthy subjects were given 500 mg aspirin daily for 3 days. The microvascular reactivity was measured at the beginning and the end of the study. In the second experiment, the response to iontophoresis of acetylcholine and sodium nitroprusside as 1% solutions of deionized water was compared to the responses that were achieved after the iontophoresis of deionized water or 5% NaCl solution. After 3 days of aspirin intake, there were no changes in the vasodilatory response to acetylcholine (endothelium-dependent vasodilation) [81 +/- 11 vs 77 +/- 10 (% of increase over baseline at the beginning vs the end of the study, mean +/- SE), P = NS] or sodium nitroprusside (endothelium-independent vasodilation) (69 +/- 8 vs 64 +/- 12, P = NS). There was also a negligible response after the iontophoresis of 5% NaCl (3 +/- 4) and deionized water (6 +/- 4) in anodal mode (the mode employed for the iontophoresis of acetylcholine). In cathodal mode, employed for the iontophoresis of sodium nitroprusside, the response to 5% NaCl was still negligible but a considerable response was found after the iontophoresis of deionized water. In normal healthy subjects, aspirin administration has no effect on forearm skin microvascular reactivity, including both endothelium-dependent and endothelium-independent vasodilation. In addition, a NaCl solution would be preferable to deionized water as the iontophoresis solution vehicle.

Current-induced vasodilation during water iontophoresis (5 min, 0.10 mA) is delayed from current onset and involves aspirin sensitive mechanisms

Study of the microcirculation by iontophoresis is potentially confounded by any non-specific effects of current application. Laser Doppler flow (LDF, mean +/- SD; arbitrary units; AU) was recorded on the forearms of healthy volunteers during and 20 min following application of 0.10-mA current for 1, 3 and 5 min, using deionised water as a vehicle. Local heating to 44 degrees C was then applied for 24 min to assess maximal vasodilation. Cathodal current applications resulted in delayed and prolonged vasodilation (peak values: 78 +/- 29, 75 +/- 19, 80 +/- 37 AU) whereas anodal peak LDF was 13 +/- 6, 27 +/- 34 and 72 +/- 40 AU for 1-, 3- and 5-min periods of current applications, respectively. From current onset, inflexion points in the responses to 3- and 5-min anodal current applications occurred at 4.5 and 6.5 min, respectively, and at approximately 1.5 min for all cathodal current applications. For 5-min current applications: a preliminary tourniquet ischaemia neither changed the time course nor the amplitude of the response to current application. In this situation, local anaesthesia abolished the current-induced vasodilation. Chronic capsaicin pretreatment decreased the amplitude of the vasodilation. Pretreatment with 500 mg oral aspirin decreased the cathodal vasodilation and abolished the anodal vasodilation, even in the absence of preliminary ischaemia. We conclude that vasodilation to prolonged application of 0.10-mA continuous monopolar current after transient tourniquet ischaemia cannot be exclusively the result of an axon reflex initiated by current onset. This current-induced vasodilation is at least partly dependent on capsaicin-sensitive afferent fibres and relies on aspirin-sensitive mechanisms at both polarities.

Atorvastatin improves endothelial function in renal-transplant recipients

BACKGROUND

Hyperlipidaemia and endothelial dysfunction are common features in cyclosporin A (CsA)-treated renal transplant recipients. Endothelial dysfunction may contribute to the risk of premature atherosclerosis and cardiovascular death in these patients. A beneficial effect of statin therapy beyond cholesterol lowering may be an improvement of endothelial function. The present study was designed to assess the effect of atorvastatin on serum lipids and endothelial function in CsA treated renal transplant recipients.

METHODS

This pilot study was an open trial of 4 weeks atorvastatin (10 mg per day) treatment in renal transplant recipients (n=22). All patients received a CsA- and prednisolone-based immunosuppressive regimen. Endothelial function was assessed in the forearm skin microvasculature by acetylcholine stimulation and laser Doppler flowmetry, before and after atorvastatin treatment. Serum lipids, plasma endothelin-1 (ET-1), nitric oxide (NO), and von Willebrand factor (vWF) were also measured.

RESULTS

Both total and LDL cholesterol were significantly reduced by 26.8 +/- 8.4 and 41.5 +/- 11.0% respectively, after 4 weeks of treatment. Endothelial function was significantly improved during atorvastatin treatment, area under the flux versus time curve (AUC)(ACh) was 538 +/- 362 AU x min before and 682 +/- 276 AU x min after treatment (P=0.042). Plasma NO levels also showed a borderline significant increase from 49 +/- 30 to 57 +/- 37 micromol/l during the treatment period (P=0.051), though plasma ET-1 (0.37+/-0.08 vs 0.37+/-0.12 fmol/ml) and vW (196+/-57 vs 197+/-37%) were unchanged.

CONCLUSION

Atorvastatin lowered serum cholesterol significantly and improved endothelial function in renal transplant recipients after 4 weeks of treatment. Plasma NO levels were increased during atorvastatin treatment, indicating a possible endothelial protective effect through an "endothelial-NO pathway".

Better microvascular function on long-term treatment with lisinopril than with nifedipine in renal transplant recipients

BACKGROUND

The prevalence of hypertension in renal transplant recipients is high but the pathophysiology is poorly defined. Impaired endothelial function may be a factor of major importance. The present study addresses the effects of long-term treatment with either lisinopril or slow-release nifedipine on microvascular function and plasma endothelin in renal transplant recipients on cyclosporin A (CsA).

METHODS

Seventy-five hypertensive renal transplant recipients were double-blind randomized to receive slow-release nifedipine (NIF, n=40) or lisinopril (LIS, n=35). Ten normotensive, age-matched recipients served as controls. All patients received CsA-based immunosuppressive therapy including prednisolone and azathioprine. Microvascular function was assessed in the forearm skin vasculature, using laser Doppler flowmetry in combination with post-occlusive reactive hyperaemia and endothelial-dependent function during local acetylcholine (ACh) stimulation.

RESULTS

The analysis of microvascular function (AUC(rh)) showed that nifedipine-treated patients had significantly lower responses compared with lisinopril-treated patients (20+/-17 and 43+/-20 AU x min respectively, P=0.0016). Endothelial function was borderline significantly lower in the NIF group compared with the LIS group (640+/-345 and 817+/-404 AU x min respectively, P=0.056). The responses in the LIS group were comparable with those in non-hypertensive controls (AUC(rh) was 37+/-16 and AUC(ACh) was 994+/-566 AU x min). Plasma endothelin-1 concentrations were significantly higher in the NIF group compared with the LIS group (0.44+/-0.19 vs. 0.34+/-0.10 fmol/ml respectively, P=0.048), and were 0.29+/-0.09 fmol/ml in the control patients. AUC(ACh) was associated with plasma endothelin-1 (P=0.0053), while AUC(rh) was not (P=0.080).

CONCLUSIONS

The study indicates that long-term treatment with lisinopril, when compared with nifedipine, yields a more beneficial effect on microvascular function in hypertensive renal transplant recipients on CsA. The beneficial microvascular effect may be mediated in part by an endothelin-1-associated effect on the endothelium.

Methods for evaluation of peripheral neurovascular dysfunction

Measurement of skin blood flow is a sensitive marker of C-fiber neurovascular dysfunction. It precedes development of abnormalities in diabetes mellitus, correlates with in vivo indices of the metabolic syndrome, and may be a "benchmark" for future studies on agents to improve microvascular dysfunction in diabetes mellitus. Skin blood flow can be measured under basal and stimulated conditions. There are different methods of evaluation. Iontophoresis and microdialysis are novel methods of drug delivery and the latter may be used as a means of extracting analytes in the skin. Theses methods are not invasive (iontophoresis) or minimally invasive (microdialysis). They can be performed repeatedly and safely in most patients. The use of microdialysis may be limited by sampling only water-soluble molecules. An alternative to microdialysis is iontophoresis, which works better with polar molecules. A combination of microdialysis and iontophoresis techniques can be useful in assessment of the pharmacokinetics of polar and nonpolar agents and the physiology and pathophysiology of the skin neurovascular system.

Each administration of cyclosporin A enhances skin microvascular reactivity in renal transplant recipients

Both impaired and enhanced microvascular function have been described in humans on cyclosporin A (CsA) therapy. In the present study we investigated the acute microvascular effects of a single CsA administration in renal transplant recipients on maintenance CsA therapy. Fourteen renal transplant recipients, median age 48 years (range 24-63 years), transplanted 4-12 weeks earlier, were included in this placebo-controlled, double-blinded, crossover study. All recipients had stable renal function; median serum creatinine was 116 micromol/L (range 80-184 micromol/L). Immunosuppressive therapy consisted of CsA, prednisolone, and either azathioprine or mycophenolate. Microvascular function was assessed by laser Doppler flowmetry in combination with acetylcholine (endothelium dependent) stimulation and the postocclusive reactive hyperemia test. Measurements were performed before (control) and 2.5 h following administration of CsA (Neoral) or matching placebo and repeated with reversed medication after at least 6 days. Vasodilative responses to acetylcholine stimulation were significantly higher following CsA ingestion compared with placebo. The mean change in AUC(1.5) (area under the flux versus time curve) from control to 2.5 h was 100 +/-145 for CsA and -292 +/- 140 AU x min for placebo (P = 0.047, n = 10). The postocclusive hyperemic response AUC(rh) was also significantly higher following CsA intake (39 +/- 4 AU x min) compared to placebo (30 +/- 4 AU x min) (P = 0.006, n = 12). This study shows that each dose of CsA induces a transient increase in skin microvascular reactivity in renal transplant recipients. We speculate that this might be due to the potentiation of one or several endothelial-dependent compensatory vasodilative mechanisms in the microvascular bed.