Capillary Response to Skeletal Muscle Contraction: Implications for Redundancy in Microvascular Blood Flow Control

Redundancy, during active hyperaemia (AH), describes a regulatory control paradigm in which the vasoactive effect of a dilator can be amplified to compensate for the loss of another vasodilator's effect. This paradigm necessitates that some dilators exert an inhibitory influence on the dilatory ability of others. Thus, the compromised release of one dilator would remove its inhibition on another dilator thereby amplifying its effect to compensate for its loss. We have previously shown redundant signalling between dilators in arterioles of skeletal muscle microcirculation (Lamb & Murrant, 2015). However, given the importance of capillaries in the coordination of blood flow to active skeletal muscle we sought to determine whether redundancy is important at this vascular level. Using intravital microscopy of the hamster cremaster we stimulated capillaries via micropipette application of metabolic dilators and assessed changes in diameter of the upstream terminal arteriole, associated with the stimulated capillary. Alone, potassium (K + , 10mM), nitric oxide (NO) donor S‐Nitroso‐N‐acetylpenicillamine (10 −6 M) and adenosine (ADO, 10 −6 M) caused a conducted response (CR) up the vascular tree. However, the magnitude of NO and ADO's CR was significantly blunted when added in the presence of KCl, demonstrating that dilators relevant to contraction can exert inhibitory influences on other dilators at the capillary level. We then aimed to determine the physiological relevance of redundant interactions during muscle contraction. CRs were elicited by contracting skeletal muscle fibres underlying capillary modules (via a microelectrode) for 2 minutes at 6,15, 24 and 60 contractions per minute (cpm). We found that inhibition of NO, via NO synthase inhibitor L‐N G ‐Nitroarginine methyl ester (L‐NAME, 10 −6 M), significantly blunted the CR elicited by 6, 15 and 24cpm by ~102%, ~92%, and ~46%, respectively, but the CR elicited by 60cpm was unaffected. Presumably NO would be present during all contraction frequencies but we have shown it was not effective in mediating the CR at 60cpm. Based on the data from the micropipette protocol we hypothesized that K + may be inhibiting NO's effectiveness. To determine whether K + was exerting an inhibitory influence on NO at high contraction frequencies we antagonized K + release, via inhibition of voltage‐gated K + channels using 3,4‐Diaminopyridine (DAP, 3×10 −4 M), in addition to L‐NAME during contraction. In the presence of L‐NAME and DAP the CR elicited by 60cpm was significantly blunted by ~79%, showing that in the absence of K + NO plays a significant role in mediating the CR elicited by 60cpm. Given that interstitial K + concentration ([K + ] i ) is correlated with muscle contraction we postulated that at low contraction frequencies [K + ] i may not be sufficient in attenuating NO's dilatory ability, but as contraction frequency increases the associated increase in [K + ] i may be sufficient in exerting an inhibitory effect on NO. Together our data demonstrate that inhibitory effects between vasodilators exist at the level of the capillary. Further, we have shown that these interactions are important during muscle contraction, demonstrating the physiological relevance of redundancy. This fundamentally shifts our understanding of blood flow regulation during skeletal muscle activity, as these data provide evidence that AH is governed by a complex redundant interplay between dilators. Support or Funding Information NSERC Canada.

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