Understanding ATP-mediated Vasodilatation in Humans

Negative results, cynicism, the piecing together of puzzles and integrating physiology with experimental approaches are all part of the inspiring and meandering journey on the path to understand ATP-mediated vasodilatation. While it is satisfying to be right as scientists, we know sometimes the most important progress occurs when data are surprising or contrary to our original hypothesis. The pursuit of explanations for unexpected findings often leads to the best advancements. The story of how our laboratory came to investigate the underlying vasodilatator pathways of adenosine triphosphate (ATP) is an example of such a path to our current understanding

Vascular Regulation via KIR Channels and Na+ /K+-ATPase

Despite the longstanding knowledge that blood flow increases in proportion to metabolic activity of skeletal muscle, the underlying mechanisms that govern this response have only recently been identified.1 Given the role of endothelial cells in mediating exercise hyperemia,2 interest has been focused on endothelium-derived vasodilation occurring via the synthesis of nitric oxide (NO) and vasodilating prostaglandins (PGs; i.e. prostacyclin) or endothelium- derived hyperpolarization. A number of studies performed in humans have established a minimal-to-modest role for NO and PGs during mild- and moderate- intensity exercise. In animal preparations, prevention of hyperpolarization attenuates contraction-induced hyperemia; however, performing similar studies in humans has been difficult. Specific candidate contributors to hyperpolarization such as P450 metabolites, calcium-activated potassium (KCa) channels, and ATP-sensitive potassium (KATP) channels have been inhibited with minimal to nonexistent effects. Recently, we inhibited KIR channels and Na+/K+-ATPase [via intraarterial barium chloride (BaCl2) and ouabain] in the human forearm during rhythmic muscle contractions.3 Importantly, we have established that K+-mediated vasodilation (intra-arterial infusion of KCl) is essentially abolished following BaCl2 and ouabain administration, evidence of successful inhibition of KIR channels and Na+/K+-ATPase. Based on the observed reduction in forearm blood flow during contractions with BaCl2, we concluded that activation of KIR channels significantly contributes (~30%) to exercise hyperemia in healthy humans. A reduction of this magnitude is profound, particularly in a small muscle mass such as the forearm

Intravascular ATP and the regulation of blood flow and oxygen delivery in humans

Regulation of vascular tone is a complex response that integrates multiple signals which allow for blood flow and oxygen supply to appropriately match oxygen demand. Here, we discuss the potential role of intravascular ATP as a primary factor in these responses and propose that deficient ATP release may contribute to impairments in vascular control exhibited in aged and diseased populations

Impaired Skeletal Muscle Blood Flow Control With Advancing Age in Humans: Attenuated ATP Release and Local Vasodilation During Erythrocyte Deoxygenation

Rationale: Skeletal muscle blood flow is coupled with the oxygenation state of hemoglobin in young adults, whereby the erythrocyte functions as an oxygen sensor and releases ATP during deoxygenation to evoke vasodilation. Whether this function is impaired in humans of advanced age is unknown. Objective: To test the hypothesis that older adults demonstrate impaired muscle blood flow and lower intravascular ATP during conditions of erythrocyte deoxygenation. Methods and Results: We showed impaired forearm blood flow responses during 2 conditions of erythrocyte deoxygenation (systemic hypoxia and graded handgrip exercise) with age, which was caused by reduced local vasodilation. In young adults, both hypoxia and exercise significantly increased venous [ATP] and ATP effluent (forearm blood flow×[ATP]) draining the skeletal muscle. In contrast, hypoxia and exercise did not increase venous [ATP] in older adults, and both venous [ATP] and ATP effluent were substantially reduced compared with young people despite similar levels of deoxygenation. Next, we demonstrated that this could not be explained by augmented extracellular ATP hydrolysis in whole blood with age. Finally, we found that deoxygenation-mediated ATP release from isolated erythrocytes was essentially nonexistent in older adults. Conclusions: Skeletal muscle blood flow during conditions of erythrocyte deoxygenation was markedly reduced in aging humans, and reductions in plasma ATP and erythrocyte-mediated ATP release may be a novel mechanism underlying impaired vasodilation and oxygen delivery during hypoxemia with advancing age. Because aging is associated with elevated risk for ischemic cardiovascular disease and exercise intolerance, interventions that target erythrocyte-mediated ATP release may offer therapeutic potential

KIR channel activation contributes to onset and steady-state exercise hyperemia in humans

We tested the hypothesis that activation of inwardly rectifying potassium (KIR) channels and Na+-K+-ATPase, two pathways that lead to hyperpolarization of vascular cells, contributes to both the onset and steady-state hyperemic response to exercise. We also determined whether after inhibiting these pathways nitric oxide (NO) and prostaglandins (PGs) are involved in the hyperemic response. Forearm blood flow (FBF; Doppler ultrasound) was determined during rhythmic handgrip exercise at 10% maximal voluntary contraction for 5 min in the following conditions: control [saline; trial 1 (T1)]; with combined inhibition of KIR channels and Na+-K+-ATPase alone [via barium chloride (BaCl2) and ouabain, respectively; trial 2(T2)]; and with additional combined nitric oxide synthase (NG-monomethyl-l-arginine) and cyclooxygenase inhibition [ketorolac; trial 3 (T3)]. In T2, the total hyperemic responses were attenuated ∼50% from control (P \u3c 0.05) at exercise onset, and there was minimal further effect in T3 (protocol 1; n= 11). In protocol 2 (n = 8), steady-state FBF was significantly reduced during T2 vs. T1 (133 ± 15 vs. 167 ± 17 ml/min; Δ from control: −20 ± 3%; P \u3c 0.05) and further reduced during T3 (120 ± 15 ml/min; −29 ± 3%; P \u3c 0.05 vs. T2). In protocol 3 (n = 8), BaCl2 alone reduced FBF during onset (∼50%) and steady-state exercise (∼30%) as observed in protocols 1 and 2, respectively, and addition of ouabain had no further impact. Our data implicate activation of KIR channels as a novel contributing pathway to exercise hyperemia in humans

Muscle Contraction Duration and Fibre Recruitment Influence Blood Flow and VO2 Independent of Contractile Work during Steady-State Exercise in Humans

We tested the hypothesis that, among conditions of matched contractile work, shorter contraction durations and greater muscle fibre recruitment result in augmented skeletal muscle blood flow and oxygen consumption (O2) during steady-state exercise in humans. To do so, we measured forearm blood flow (FBF; Doppler ultrasound) during 4 minutes of rhythmic handgrip exercise in 24 healthy young adults and calculated forearm O2 via blood samples obtained from a catheter placed in retrograde fashion into a deep vein draining the forearm muscle. In Protocol 1 (n = 11), subjects performed rhythmic isometric handgrip exercise at mild and moderate intensities under conditions in which tension time index (TTI; isometric analog of work) was held constant but contraction duration was manipulated. In this protocol, shorter contraction durations led to greater FBF (184 ± 25 vs. 164 ± 25 ml·min-1) and O2 (23 ± 3 vs. 17 ± 2 ml·min-1; both PPper se during steady-state exercise in humans

Reactive Hyperemia Occurs Via Activation of Inwardly Rectifying Potassium Channels and Na+/K+-ATPase in Humans

Rationale: Reactive hyperemia (RH) in the forearm circulation is an important marker of cardiovascular health, yet the underlying vasodilator signaling pathways are controversial and thus remain unclear. Objective: We hypothesized that RH occurs via activation of inwardly rectifying potassium (KIR) channels and Na+/K+-ATPase and is largely independent of the combined production of the endothelial autocoids nitric oxide (NO) and prostaglandins in young healthy humans. Methods and Results: In 24 (23±1 years) subjects, we performed RH trials by measuring forearm blood flow (FBF; venous occlusion plethysmography) after 5 minutes of arterial occlusion. In protocol 1, we studied 2 groups of 8 subjects and assessed RH in the following conditions. For group 1, we studied control (saline), KIR channel inhibition (BaCl2), combined inhibition of KIR channels and Na+/K+-ATPase (BaCl2 and ouabain, respectively), and combined inhibition of KIR channels, Na+/K+-ATPase, NO, and prostaglandins (BaCl2, ouabain, L-NMMA [NG-monomethyl-L-arginine] and ketorolac, respectively). Group 2 received ouabain rather than BaCl2 in the second trial. In protocol 2 (n=8), the following 3 RH trials were performed: control; L-NMMA plus ketorolac; and L-NMMA plus ketorolac plus BaCl2 plus ouabain. All infusions were intra-arterial (brachial). Compared with control, BaCl2 significantly reduced peak FBF (−50±6%; P2 (−61±3%) and ouabain (−44±12%) alone, and this effect was enhanced when combined (−87±4%), nearly abolishing RH. L-NMMA plus ketorolac did not impact total RH FBF before or after administration of BaCl2 plus ouabain. Conclusions: Activation of KIR channels is the primary determinant of peak RH, whereas activation of both KIR channels and Na+/K+-ATPase explains nearly all of the total (AUC) RH in humans

Characteristics of Physiology and Physiology-Related Pre-Health Degree Programs in the Physiology Majors Interest Group

The Physiology Majors Interest Group (P-MIG), a grassroots organization of educators, has collected data on the history and characteristics of Physiology and highly related undergraduate programs (ex: Human Biology, Pre-Medicine, Biomedical Sciences, etc.) that serve a common population of prehealth students. Data was obtained as part of an online survey sent out to P-MIG conference attendees at the 2017-2019 annual meetings (n=30). Participating institutions indicate that 25.9% have degrees called Physiology aligned with 28% being housed in a department of physiology, 75.9% are a Bachelor of Science program, 34.9% are affiliated with a College of Arts and Sciences, and 80% have a human/integrative physiology emphasis. Further, 47.6% of programs are greater than 10 years old and 100% have seen either no change or an increase in enrollment over the past 5 years. Most programs have a dedicated advising staff (68.2%) and formalized learning objectives for the major (61.9%). 34.1% have a curriculum committee who oversees the major. Program sizes vary widely from less than 50 to over 2000 students. While there is diversity in departmental organization and management structure in the programs, a commonality is that all programs are preparing students with aspirations in careers in healthcare. We report the similarities and differences between these programs to allow for advisors to better understand the broad landscape of pre-health programs at the undergraduate level