Pyridoxine, P2 Receptor Antagonism, and ATP-mediated Vasodilation in Young Adults

Study Overview
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Brief Summary
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Previous research has identified adenosine triphosphate (ATP) as an important vasodilator that is released from red blood cells during exercise and exposure to hypoxic environments in adult humans. Further, older adults appear to have lower blood flow during both of these stressors and also have lower amounts of ATP released from their red blood cells. However, the contribution of ATP to vasodilation in response to exercise and hypoxia is currently unknown due to the lack of an effective ATP receptor antagonist. We aim to determine whether Vitamin B6 or its metabolite, Pyridoxal-5-Phosphate (PLP) is an effective ATP receptor antagonist.

Detailed Description
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Adenosine triphosphate (ATP) is an established vasodilator that is released from red blood cells during a variety of stimuli including exercise and exposure to hypoxic environments. Many studies have shown that infusion of ATP can lead to vasodilation similar to that which is achieved during exercise, and that plasma ATP concentrations increase in a graded fashion during graded exercise. Further, older adults have lower levels of blood flow during exercise and hypoxia compared to their younger counterparts, and the reduced blood flow is coupled with impaired release of ATP from red blood cells during these stimuli. Thus, ATP is believed to be an important vasodilator. However, the role of ATP in the regulation of blood flow is not fully understood due to the lack of an effective ATP receptor (P2Y2) antagonist. Development of an effective P2Y2 antagonist will allow researchers to determine the role of ATP in vasodilation to stimuli such as exercise by comparing blood flow during exercise with and without the blocker. Preliminary data from our laboratory suggests that Vitamin B6 (pyridoxine hydrochloride) or its metabolite Pyridoxal-5-Phosphate (PLP) may be an effective blocker of ATP-mediated vasodilation. As a result, the purpose of this study is to determine whether Vitamin B6 or PLP can inhibit vasodilation in response to intra-arterial infusions of ATP. This study also aims to determine the specificity of Vitamin B6 or PLP by measuring its effect on vasodilation in response to infusion of several other vasodilators. Participants will be asked to complete one screening visit and one study visit. Once study eligibility has been determined, participants will report to the Human Performance Clinical Research Laboratory at Colorado State University following an overnight fast. A physician will then place a catheter in the brachial artery of the non-dominant arm, and participants will be randomized into one of three study arms to determine which drugs will be infused into the artery. Each arm includes ATP and two other vasodilators. The study will begin by measuring vasodilation in response to four standard doses of each vasodilator. Vasodilation in response to the vasodilators will then be assessed again following infusion of Vitamin B6 or PLP. Reduced vasodilation to any of the drugs during the second trial will suggest that Vitamin B6 or PLP is an antagonist to the channel through which the drug signals. Each study visit will last approximately 4-5 hours.

Official Title
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Pyridoxine, P2 Receptor Antagonism, and ATP-mediated Vasodilation in Young Adults

Conditions
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Receptor Blockade

Intervention / Treatment
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* Drug: Adenosine Triphosphate
* Drug: Acetylcholine
* Drug: Sodium Nitroprusside
* Drug: Adenosine Diphosphate
* Drug: Adenosine Monophosphate
* Drug: Uridine Triphosphate
* Drug: Adenosine
* Drug: Vitamin B 6
* Drug: Pyridoxal 5'-Phosphate


Participation Criteria
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Eligibility Criteria
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Inclusion Criteria: Sedentary-moderately active Free of chronic disease Exclusion Criteria: Current smoker BMI > 29.9 kg/m2 Blood pressure equal to or greater than 140/90 mmHg Use of any medications including vitamin B6 supplements or antioxidants

Ages Eligible for Study
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Minimum Age: 18 Years
Maximum Age: 30 Years

Sexes Eligible for Study
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All

Accepts Healthy Volunteers
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Accepts Healthy Volunteers

Study Plan
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How is the study designed?
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Design Details

Primary Purpose: Basic Science
Allocation: Randomized
Intervention Model: Parallel Assignment
Interventional Model Description: Participants will be assigned to one of three arms of the study. The arms of the study are based on the vasodilators being tested in order to determine the effect of pyridoxine or PLP on vasodilation in response to intra-arterial administration of the vasodilators.
Masking: None (Open Label)


Arms and Interventions

| Participant Group/Arm | Intervention/Treatment |
| --- | --- |
| Experimental: ATP, Ach, SNP<br>All drugs will be administered via intra-arterial (brachial artery) infusion, and the dosages below will be administered two times: once before administration of Vitamin B6 (pyridoxine) and once following administration of the Vitamin B6 (pyridoxine) loading dose. Adenosine Triphosphate: 1.25, 2.5, 5, and 10 μg/dl forearm volume/min for 3 minutes each. Acetylcholine: 1, 4, 8, and 16 μg/dl forearm volume/min for 3 minutes each. Sodium Nitroprusside: 0.25, 0.5, 1, and 2 μg/dl forearm volume/min for 3 minutes each. Vitamin B6 (pyridoxine): up to 200 mg of pyridoxine will be infused over 20 minutes. A maintenance dose of 2.5 mg/min may be used throughout the remainder of the protocol. Pyridoxal-5-Phosphate (PLP) may be used as an alternative blocker instead of pyridoxine. It will be infused at doses up to 200 µg/dl forearm volume/min. | Drug: Adenosine Triphosphate<br>* See arm/group descriptions<br>* Other names: ATP;Drug: Acetylcholine<br>* See arm/group descriptions<br>* Other names: Ach;Drug: Sodium Nitroprusside<br>* See arm/group descriptions<br>* Other names: SNP;Drug: Vitamin B 6<br>* See arm/group descriptions<br>* Other names: Pyridoxine Hydrochloride;Drug: Pyridoxal 5'-Phosphate<br>* See arm/group descriptions<br>* Other names: MC-1;|
| Experimental: ATP, ADP, AMP<br>All drugs will be administered via intra-arterial (brachial artery) infusion, and the dosages below will be administered two times: once before administration of Vitamin B6 (pyridoxine) and once following administration of the Vitamin B6 (pyridoxine) loading dose. Adenosine Triphosphate: 1.25, 2.5, 5, and 10 μg/dl forearm volume/min for 3 minutes each. Adenosine Diphosphate: 20, 40, 80, and 160 μg/dl forearm volume/min for 3 minutes each. Adenosine Monophosphate: 25, 50, 100, and 200 μg/dl forearm volume/min for 3 minutes each. Vitamin B6 (pyridoxine): up to 200 mg of pyridoxine will be infused over 20 minutes. A maintenance dose of 2.5 mg/min may be used throughout the remainder of the protocol. Pyridoxal-5-Phosphate (PLP) may be used as an alternative blocker instead of pyridoxine. It will be infused at doses up to 200 µg/dl forearm volume/min. | Drug: Adenosine Triphosphate<br>* See arm/group descriptions<br>* Other names: ATP;Drug: Adenosine Diphosphate<br>* See arm/group descriptions<br>* Other names: ADP;Drug: Adenosine Monophosphate<br>* See arm/group descriptions<br>* Other names: AMP;Drug: Vitamin B 6<br>* See arm/group descriptions<br>* Other names: Pyridoxine Hydrochloride;Drug: Pyridoxal 5'-Phosphate<br>* See arm/group descriptions<br>* Other names: MC-1;|
| Experimental: ATP, UTP, Adenosine<br>All drugs will be administered via intra-arterial (brachial artery) infusion, and the dosages below will be administered two times: once before administration of Vitamin B6 (pyridoxine) and once following administration of the Vitamin B6 (pyridoxine) loading dose. Adenosine Triphosphate: 1.25, 2.5, 5, and 10 μg/dl forearm volume/min for 3 minutes each. Uridine Triphosphate: 1.25, 2.5, 5, and 10 μg/dl forearm volume/min for 3 minutes each. Adenosine: 3.125, 6.25, 12.5, and 25 μg/dl forearm volume/min for 3 minutes each. Vitamin B6 (pyridoxine): up to 200 mg of pyridoxine will be infused over 20 minutes. A maintenance dose of 2.5 mg/min may be used throughout the remainder of the protocol. Pyridoxal-5-Phosphate (PLP) may be used as an alternative blocker instead of pyridoxine. It will be infused at doses up to 200 µg/dl forearm volume/min. | Drug: Adenosine Triphosphate<br>* See arm/group descriptions<br>* Other names: ATP;Drug: Uridine Triphosphate<br>* See arm/group descriptions<br>* Other names: UTP;Drug: Adenosine<br>* See arm/group descriptions<br>* Other names: Adenocard;Drug: Vitamin B 6<br>* See arm/group descriptions<br>* Other names: Pyridoxine Hydrochloride;Drug: Pyridoxal 5'-Phosphate<br>* See arm/group descriptions<br>* Other names: MC-1;|

What is the study measuring?
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Primary Outcome Measures

| Outcome Measure | Measure Description | Time Frame |
| --- | --- | --- |
| Vascular Conductance | Vascular conductance is an index of vascular tone through which vasodilation can be determined. Vascular conductance is calculated by measuring blood flow in response to infusion of a vasodilator and accounting for blood pressure. Thus, the change in blood flow is due to a change in vascular conductance. | Continuous measurement of vascular conductance during the 12 minute dose response for each drug. Measures are repeated following administration of Vitamin B6 or PLP. | 

 Terms related to the study
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Keywords Provided by Centre Hospitalier Valida
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Adenosine Triphosphate, Pyridoxine, Cardiovascular, PLP, P5P