Cerebral Capillary TRPA1 Channels Mediate Functional Hyperemia via Retrograde Conducted Vasodilation

Abstract

Cerebral capillaries are the sites of nutrient exchange and removal of metabolic by‐products within the brain parenchyma. Recent studies showed that cerebral capillary endothelial cells (ECs) are central players in matching augmented neuronal metabolism to an increase in local blood perfusion, a process known as functional hyperemia. However, little is known about the molecular sensors present in capillary EC that orchestrate dilation of upstream parenchymal arterioles (PAs). We previously showed that the transient receptor potential ankyrin 1 (TRPA1) channel, a Ca2+‐permeable non‐selective cation channel, is present in ECs from cerebral arteries, but not in other peripheral vascular beds. We tested the hypothesis that TRPA1 is also present in cerebral capillary EC and its activation induces dilation of upstream PAs through a Ca2+‐dependent mechanism. Immunofluorescence staining of freshly isolated cerebral capillaries showed the presence of TRPA1, which was absent in EC‐specific TRPA1 knockout mice (eTRPA1−/−). Further, currents evoked by the TRPA1 agonist allyl isothyocyanate (AITC, 30 μM) in whole‐cell patch clamp electrophysiology experiments were blocked by the selective TRPA1 antagonist HC‐030031 (10 μM) and were absent from eTRPA1−/− mice. Using a recently‐described ex vivo capillary‐parenchymal arteriole preparation in which the capillary bed remains attached to pressurized PAs, we observed that picospritzing capillaries with AITC induced dilation of the upstream PA, an effect inhibited by HC‐030031 and absent in eTRPA1−/− mice. We then investigated the mechanisms underlying propagation of the vasodilatory signal from capillary to arteriolar EC. Using mice expressing the genetically‐encoded Ca2+ biosensor GCaMP6f in EC, we observed that picospritzing AITC onto capillary beds generated an intercellular Ca2+ wave that propagated from capillaries to the upstream PA. Pre‐incubation of preparations with the TRPA1 inhibitor HC‐030031 abolished the generation of Ca2+ waves. Moreover, the frequency of Ca2+ transients in arteriolar EC was increased immediately after dissipation of the Ca2+ wave, a process blocked by HC‐030031. Increases in arteriolar EC intracellular Ca2+ can activate intermediate and small conductance Ca2+‐activated K+ channels (IK and SK, respectively), leading to vasodilation. To test if this mechanism was responsible for PA dilation, preparations were incubated with the IK and SK channels inhibitors TRAM‐34 (1 μM) and apamin (1 μM), which blunted arteriolar dilation induced by picospritzing AITC onto capillaries. Somatosensory stimulation of the whisker barrel cortex, a model of functional hyperemia, caused a smaller increase in perfusion in eTRPA1−/− mice than in wildtype littermates. Taken together, these data suggest that functional TRPA1 channels are present in cerebral capillary EC and that activation of the channel elicits intercellular Ca2+ waves that propagate to upstream PAs, where it activates IK and SK channels to cause hyperpolarization and dilation, thus mediating functional hyperemia responses.Support or Funding InformationSupported by the National Institutes of Health (R01HL091095 to SE) and the American Heart Association (15POST2472002 to PWP)This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.