Pannexin's Role in Mediating Skeletal Muscle Active Hyperaemia

Abstract

The dynamic metabolic rate of skeletal muscle coupled with the fact that discrete motor units can be recruited within a larger muscle bed necessitates a level of vascular control that allows blood flow to be directed to single muscle fibers. While the locus of blood flow regulation is undetermined, mounting evidence suggests capillaries (CAPS) may play a central role in blood flow coordination. This stems from CAPS close proximity to muscle fibres and, that CAPS can be stimulated by products of contraction to increase their own perfusion, via initiating a vasodilatory signal conducted upstream to its controlling arteriole. This cell‐cell communication is necessary to achieve the coordination needed to distribute blood flow to active skeletal muscle fibres. Gap junction dependent spread of hyperpolarization is prevalent in microvascular cell‐cell signaling, however the archetypal characteristics consistent with hyperpolarization are not always observed during conducted responses elicited by muscle contraction. This may suggest a 2nd pathway involved in conducted signaling. Pannexin (PANX) channels have been implicated in cell‐cell signaling in other tissues. PANX allow the cellular release of ATP into the interstitial space to activate purinergic receptors on neighboring cells. Activated receptors increase intracellular Ca2+ leading to PANX‐mediated ATP release and propagation of the signal to adjacent cells. We sought to determine if there was a role of PANX in CAP‐initiated condcuted responses. Using intravital microscopy of the hamster cremaster we stimulated CAPS with 1) vasoactive agents relevant to muscle contraction and 2) muscle contraction itself, while observing changes in diameter of the upstream arteriole associated with the stimulated CAP. 1) Micropipetting vasoactive agents; potassium chloride (KCl; 10 mM; n=7), adenosine (ADO, 10−4 M; n=9), pinacidil (10−5 M; n=8), SNAP (nitric oxide donor; 10−6 M; n=13), and acetylcholine (10−4 M; n=7) produced an upstream arteriolar dilation. However, when PANX blocker, mefloquine (MEF, 10−5 M) was applied between the CAP stimulation site and observation site, the conducted responses elicited by KCl and ADO were inhibited by 111% and 67% respectively, while having no effect on the conducted response of other dilators. 2) Conducted responses were elicited by contracting muscle fibres underlying CAPS for 2 min while altering contraction frequency (6, 15 & 60 contractions per minute (CPM)) or stimulus frequency (4, 20 & 40 Hz). While each contraction parameter produced an upstream arteriolar dilation, we found that, in the presence of MEF, conducted responses at low (6CPM, n=15) and intermediate (15CPM; n=15) contraction frequency were unaffected but at high (60CPM; n=15) vasodilation was blunted by 35%. Further, MEF blunted the conducted response at low (4 Hz; n=7) and intermediate (20 Hz; n=7) stimulus frequency by 88% and 73%, respectively but not at high (40 Hz; n=7). These data show CAPS elicit a conducted response involving PANX channel signaling, and that this signal is physiologically relevant during skeletal muscle contraction. Evidence of a 2nd pathway involved in mediating the conducted response begs the question of why 2 pathways exist when they seem to be mediating the same function.Support or Funding InformationNSERC Canada.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.