Figure 2. Arteriolar regulation of cerebral blood flow Neurons. Nitric oxide (NO) is a major moderator of functional hyperemia. NO produced by neurons acts directly on VSMCs, leading to VSMC hyperpolarization and relaxation. Adenosine triphosphate (ATP) and adenosine released by neuronal activity can also act directly on VSMCs through purinergic P2XR and P2YR receptors resulting in constriction, or adenosine 2A receptors (A2R) resulting in relaxation, respectively. Neuronal-mediated large increases in extracellular [K+] activate VSMC voltage-gated calcium channels (VGCCs), resulting in VSMCs intracellular [Ca2+] increases, leading to depolarization and contraction. Astrocytes. The role of astrocytes in neurovascular coupling to arterioles is controversial. Glutamate or ATP released from neurons are postulated to act on metabotropic glutamate receptors (mGluR) or P2YR on astrocytes, respectively, to initiate 1,4,5-trisphosphate (IP3)-dependent intracellular [Ca2+] increase, which has been shown by some studies to contribute to neurovascular coupling, but disputed by others. According to some studies, intracellular [Ca2+] rise launches signaling cascades in the astrocytes and release of vasoactive ions and molecules from astrocyte endfeet to VSMCs, mainly K+ ions from large conductance calcium- activated potassium channels (BKCa), arachidonic acid (AA), through phospholipase A2 (PLA2) pathway, and AA metabolic products epoxyeicosatetraenoic acids (EETs), and prostaglandin E2 (PGE2), via cytochrome P450 (p450) and cyclooxygenase 1 (Cox1), respectively. Extracellular Ca2+ intake from transient receptor potential vanilloid channel 4 (TRPV4) provides another means of increasing intracellular [Ca2+]. Dashed lines and question marks indicate pathways for which there is limited or conflicting data in the literature. VSMCs. EETs and moderate increases in extracellular [K+] both act on VSMC potassium channels including BKCa and inward rectifier potassium channels (KIR), resulting in hyperpolarization and relaxation of the VSMCs (left). However, large increases in extracellular [K+] activate VGCCs, resulting in intracellular [Ca2+] increases leading to VSMCs depolarization and contraction (right). PGE2, which acts through prostaglandin EP4 receptors (EP4R) on VSMCs, generates cyclic adenosine monophosphate (cAMP) from intracellular ATP, also producing hyperpolarization and relaxation. PGE2 levels can be modulated by extracellular lactate levels, which can block reuptake of PGE2 by prostaglandin transporters (PGT). Lactate levels depend on the oxygen content of the tissue. Conversely, AA taken in by VSMCs can be metabolized to 20-hydroxyeicosatetraenoic acid (20-HETE), a potent VSMC depolarizer, resulting in VSMC contraction. NO released by neurons or endothelial cells can block VSMC 20-HETE production, modulating VSMC contraction and favoring relaxation through facilitation of conversion of guanosine triphosphate (GTP) to cyclic guanosine monophosphate (cGMP) via soluble guanylate cyclase (sGC). Adenosine released by neurons acts on A2R and blocks VSMC VGCC activation leading to VSMCs hyperpolarization and relaxation. In contrast, neuronal release of ATP to VSMCs P2XR and P2YR can increase intracellular [Ca2+], resulting in depolarization and contraction. Endothelium. Vasoactive compounds, including ATP, adenosine diphosphate (ADP), uridine triphosphate (UTP), acetylcholine (ACh), and bradykinin (BK), in the blood stream can bind their respective receptors (P2XR, P2YR, muscarinic M3R, and B2R) to initiate signaling pathways in endothelial cells similar to the pathways in astrocytes, with the addition of the diacyl-glycerol (DAG) AA pathway mediated by phospholipase C (PLC), generating vasoactive molecules that are released to VSMCs. Intracellular [Ca2+] increases initiated by receptor-mediated signaling of endothelium can produce endothelial AA, EETs, and prostacyclin (PGI2), which act on VSMCs similarly to PGE2, to generate VSMC hyperpolarization and relaxation. The intracellular [Ca2+] rise can also activate endothelial nitric oxide synthase (eNOS) leading to NO production, and endothelial KCa channels, releasing K+ that can act on VSMCs as well as hyperpolarizing the endothelium. Endothelial-derived hyperpolarizing factor (EDHF) can also trigger VSMC hyperpolarization. Additionally, shear stress on the endothelial vessel walls and red blood cells (RBCs) due to blood flow triggers ATP release from RBCs and signaling pathways, including activation of endothelial eNOS, and direct production of AA and its metabolites. Endothelial and endothelial-VSMCs gap junctions (GJs) facilitate retrograde endothelial signal propagation and signaling to VSMCs.