removing the microdialysis probes after performing the dose-response experiments

Investigating Novel Mechanisms of Microvascular Dysfunction in Humans

Cardiovascular disease (CVD) is the leading cause of death in the United States1. Hypertension (HTN) is an independent risk factor for stroke, coronary heart disease, and heart failure and is estimated to affect upward of ~50% of the United States population2. HTN can develop as an independent CVD (primary HTN) or as a result of another condition, such as polycystic kidney disease and/or endocrine disorders (secondary HTN). The breadth of etiologies of HTN complicates investigations into the underlying mechanisms and end-organ damage observed with HTN. Diverse and novel research approaches into the pathophysiology of the end-organ damage associated with HTN are needed.
One of the earliest pathological signs of CVD is endothelial dysfunction, as characterized by impaired nitric oxide (NO)-mediated vasodilation3,4,5. Flow-mediated dilation is a common approach used to quantify the endothelial dysfunction associated with CVD, but endothelial dysfunction in microvascular beds can be both independent of and precursory to that of large conduit arteries6,7,8. Furthermore, resistance arterioles are more directly acted on by local tissue than conduit arteries and have more immediate control over the delivery of oxygen-rich blood. Microvascular function is predictive of adverse cardiovascular event-free survival9,10,11. The cutaneous microvasculature is an accessible vascular bed that can be used to examine responses to physiological and pharmacological vasoconstrictive or vasodilatory stimuli. Intradermal microdialysis is a minimally invasive technique, the goal of which is to investigate the mechanisms of both vascular smooth muscle and endothelial function in the cutaneous microvasculature with targeted pharmacological dissection. This method contrasts with other techniques, such as post-occlusive reactive hyperemia, which does not allow for pharmacological dissection, and iontophoresis, which allows for pharmacological delivery but is less precise in its mechanism of action (reviewed thoroughly elsewhere12).
The rationale behind the development and use of this technique is extensively reviewed elsewhere13. This approach was originally developed for use in neurological research in rodents and then was first applied to humans to investigate the mechanisms underlying active vasodilation from a thermoregulatory standpoint. In the late 1990s, this method was used to examine both neural and endothelial mechanisms with regard to local heating of the skin. Since that time, the technique has been utilized to investigate a number of neurovascular signaling mechanisms in the skin.
Using this technique, our group and others have interrogated the mechanisms of endothelial dysfunction in the microvasculature of several clinical populations, including, but not limited to, dyslipidemia, primary aging, diabetes, chronic kidney disease, polycystic ovary syndrome, preeclampsia, major depressive disorder14,15,16,17,18,19, and hypertension20,21,22,23,24. For example, a previous study found that normotensive women with a history of preeclampsia, who are at an increased risk for CVD, had reduced NO-mediated vasodilation in the cutaneous circulation compared with women with a history of normotensive pregnancy20. In another study, adults diagnosed with primary HTN demonstrated increased angiotensin II sensitivity in the microvasculature compared with healthy controls21, and chronic sulfhydryl-donating antihypertensive pharmacotherapy in primary HTN patients has been shown to decrease blood pressure and improve both hydrogen sulfide- and NO-mediated vasodilation22. Wong et al.23 found impaired sensory-mediated and NO-mediated vasodilation in prehypertensive adults, coinciding with our finding of a progression of endothelial dysfunction with increasing HTN stages, as categorized by the 2017 American Heart Association and American College of Cardiology guidelines24.
The intradermal microdialysis technique allows for tightly controlled mechanistic investigations into microvascular function in health and disease states. Therefore, this paper aims to describe the intradermal microdialysis technique as applied by our group and others. We detail the procedures for both pharmacological stimulation of the endothelium with acetylcholine (ACh) to examine the dose-response relationship and physiological stimulation of endogenous NO production with either a 39 &#176;C or 42 &#176;C local heating stimulus protocol. We present representative results for each approach and discuss the clinical implications of the findings that have arisen from this technique.

Author Spotlight: A Pharmacodissection Approach to Uncover Mechanisms in Cardiovascular Disease Risk Populations 

Intradermal Microdialysis: An Approach to Investigating Novel Mechanisms of Microvascular Dysfunction in Humans

My research examines mechanisms of vascular dysfunction in humans with traditional and non-traditional cardiovascular disease risk. I have developed a model to examine microvascular dysfunction mechanisms in humans, which allows pharmaco dissection that was previously impossible. We have explored several molecular targets, mediating vascular dysfunction, translating preclinical research to humans using this novel bioassay.One of the current challenges in experiments is that vasodilator and vasoconstrictor responses are caused by multiple mechanisms. To isolate the mechanisms of interest, pharmacological agents must be used. While this approach is limited, it does allow for a more comprehensive understanding of cutaneous vascular responses.In clinical cohorts such as hypertension and major depressive disorder, or MDD, we have discovered microvascular dysfunction. The possible reasons for this dysfunction in these groups are increased arginase and reactive oxygen species activity, respectively. Short-term salicylate treatment has been found to be effective in improving microvascular function and MDD.This technique is minimally invasive and allows for well controlled pharmacological dissection of the mechanisms of function of a local vascular bed. This is opposed to techniques like postocclusive reactive hyperemia and iontophoresis, which are useful, but allow for a little control of a pharmacologic tool. Our laboratory is currently investigating the contribution of inflammation and mediating microvascular dysfunction in women with endometriosis.In addition, we plan to investigate similar mechanisms in non-Hispanic black women.

Watch this Scientific Journal Video about Removing the Microdialysis Probes After Performing the Dose-Response Experiments at JoVE.com

Removing the Microdialysis Probes After Performing the Dose-Response Experiments  

After performing the dose-response experiment, use surgical scissors to cut the microdialysis probes. Carefully remove the laser doppler flowmetry probes from the heaters and detach the heaters from the skin. Gently remove the tape securing the probes to the skin.Inspect the puncture sites on either side of the probe to identify the smallest blood clot. Cut the probe near the site with the smaller clot, leaving approximately one inch of the probe outside the skin uncut. Then clean the skin area around the probe's entry and exit sites using an alcohol swab, including the one-inch length on the less-clotted side.Grasp the probe extending from the puncture site with the larger clot and gently pull it towards the clot. Lastly, cover any bleeding from the probe's removal with sterile gauze and apply pressure.

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90 Views.  The Pennsylvania State University. After performing the dose-response experiment, use surgical scissors to cut the microdialysis probes. Carefully remove the laser doppler flowmetry probes from the heaters and detach the heaters from the skin. Gently remove the tape securing the probes to the skin.Inspect the puncture sites on either side of the probe to identify the smallest blood clot. Cut the probe near the site with the smaller clot, leaving approximately one inch of the probe outside the skin uncut. Then clean the ...

Video: Removing the Microdialysis Probes After Performing the Dose-Response Experiments  

The cutaneous vasculature is an accessible tissue that can be used to assess microvascular function in humans. Intradermal microdialysis is a minimally invasive technique used to investigate mechanisms of vascular smooth muscle and endothelial function in the cutaneous circulation. This technique allows for the pharmacological dissection of the pathophysiology of microvascular endothelial dysfunction as indexed by decreased nitric oxide-mediated vasodilation, an indicator of cardiovascular disease development risk. In this technique, a microdialysis probe is placed in the dermal layer of the skin, and a local heating unit with a laser Doppler flowmetry probe is placed over the probe to measure the red blood cell flux. The local skin temperature is clamped or stimulated with direct heat application, and pharmacological agents are perfused through the probe to stimulate or inhibit intracellular signaling pathways in order to induce vasodilation or vasoconstriction or to interrogate mechanisms of interest (co-factors, antioxidants, etc.). The cutaneous vascular conductance is quantified, and mechanisms of endothelial dysfunction in disease states can be delineated.

Intradermal microdialysis is a minimally invasive technique used to investigate microvascular function in health and disease. Both dose-response and local heating protocols can be utilized for this technique to explore mechanisms of vasodilation and vasoconstriction in the cutaneous circulation.

The cutaneous vasculature is an accessible tissue that can be used to assess microvascular function in humans. Intradermal microdialysis is a minimally ...

Equipment Setup and Microdialysis Fiber Placement for Intradermal Microdialysis

To begin, turn on the local heating unit and the laser Doppler flowmeter. Open a LabChart software file. Next, identify the large visible blood vessels on the forearm's ventral aspect, and mark them with a permanent marker.Clean the marked area with Betadine followed by alcohol. Place a sterile drape over the sterilized skin and apply ice for approximately five minutes. Then, remove the ice and insert an introducer needle with the bevel facing upwards into the skin's dermal layer at 2-3 millimeters depth.Carefully advance the needle within the dermal layer and exit the skin around 20 millimeters from the insertion point. Connect the probe to a syringe containing lactated Ringer's solution, leaving the needle in place. Carefully insert the opposite end of the probe into the introducer needle until the semipermeable membrane is close, but not inside of the opening of the needle.Then, slowly perfuse a small amount of Ringer's solution through the fiber until the solution visibly flows through the membrane's pores. Insert the Harvard Bioscience microdialysis probe into the introducer needle until the membrane is fully inside the dermal layer of the skin within the needle. Next, secure the probe using a finger proximal to the needle.Carefully withdraw the needle in the opposite insertion direction, and tape the external portion of the fiber in place on the skin. Adjust the probe and tape the external fiber on the skin to prevent displacement of the semipermeable membrane during the experiment.

Exploring Mechanisms of Vasodilation and Vasoconstriction in the Cutaneous Circulation

After placing the microdialysis fiber in the forearm, place the single-use syringe into the syringe holder tray on the microinfusion pumps. During the hyperemia phase, perfuse lactated ringers, saline, or vehicle solution. Attach the local heating unit to the skin, ensuring it covers the semipermeable membrane, using the probe adhesive disc.Insert the laser doppler flowmetry probe into the opening at the center of the local heater, and begin recording red blood cell or RBC flux values by clicking Start on the data acquisition software. Place an automatic blood pressure cuff on the arm of an uninstrumented subject. Set the local heaters to 33 degrees Celsius to maintain a thermo neutral skin temperature.To add a comment to the continuous recording in the data acquisition software, click on the text box in the top right corner of the screen. Type a comment, select which channel should receive the comment, and click on Add. Start baseline data collection once the RBC flux stabilizes at 33 degrees Celsius local heat by entering the comment, start baseline.Measure and record the baseline blood pressure after five to 10 minutes of baseline data collection. Enter the comment, end baseline in the data acquisition software. Turn off the microinfusion pumps and replace the lactated ringer solution syringes with the syringe containing the lowest concentration of acetylcholine.After securing the new syringes and confirming fluid perfusion through the probe, turn the microinfusion pumps on again. Enter start minus 10 as a comment in the data acquisition software recording. During the last minute of perfusion, measure and record the blood pressure.Once the perfusion for a given concentration ends, replace the syringe with the next highest concentration. After the final perfusion, raise the local heater temperature to 43 degrees Celsius. Allow the RBC flux to stabilize, and replace the ringers solution with sodium nitroprusside.Measure and record the blood pressure approximately every three minutes during this maximal vasodilation phase. Once a maximal RBC flux plateau has occurred, select Stop in the bottom right corner of the data acquisition software. Record the baseline data collection as demonstrated earlier.Based on the protocol's requirements, increase the local heaters to 39 or 42 degrees Celsius. After the RBC flux plateaus in response to local heat application, perfuse L-Name through the microdialysis probe. Next, set local heaters to 43 degrees Celsius.Then start microdialysis probe profusion with sodium nitroprusside after the RBC flux plateaus at 43 degrees Celsius, allowing the flux to reach its maximal plateau. Once the profusion for each dose began, a continual increase in the RBC flux to a peak was observed, followed by a heat and SNP-induced plateau. Profusion of sodium nitroprusside produced maximal local vasodilation, depicted as a rise in the RBC flux.A relatively lower RBC flux was observed in the presence of a nitric oxide synthase inhibitor. During maximal local vasodilation, there will be an exponential rise in the RBC flux, due to the previous nitric oxide synthase inhibition. In the local heating protocol, L-Name was perfused after achieving a stable plateau in the RBC flux.A rapid decline was observed in the RBC flux until it reached a new plateau in response to L-Name. The heating produced an additional peak and nadir response in the RBC flux.