Cerebral parenchymal arterioles (PAs) play a critical role in assuring appropriate blood flow and perfusion pressure within the brain. PAs are unique in contrast to upstream pial arteries, as defined by their critical roles in neurovascular coupling, distinct sensitivities to vasoconstrictors, and enhanced myogenic responsiveness. Dysfunction of these blood vessels is implicated in numerous cardiovascular diseases. However, treatments are limited due to incomplete understanding of the fundamental control mechanisms at this level of the circulation. One of the key elements within most vascular networks, including the cerebral circulation, is the presence of myogenic tone, an intrinsic process whereby resistance arteries constrict and reduce their diameter in response to elevated arterial pressure. This process is centrally involved in the ability of the brain to maintain nearly constant blood flow over a broad range of systemic blood pressures. The overall goal of this dissertation was to investigate the unique mechanisms of myogenic tone regulation in the cerebral microcirculation. To reveal the contributions of various signaling factors in this process, measurements of diameter, intracellular Ca2+ concentration ([Ca2+]i), membrane potential and ion channel activity were performed. Initial work determined that two purinergic G protein-coupled receptors, P2Y4 and P2Y6 receptors, play a unique role in mediating pressure-induced vasoconstriction of PAs in a ligand-independent manner. Moreover, a particular transient receptor potential (TRP) channel in the melastatin subfamily, i.e. TRPM4, was also identified as a mediator of PA myogenic responses. Notably, the observations that inhibiting TRPM4 channels substantially reduces P2Y receptor-mediated depolarization and vasoconstriction, and that P2Y receptor ligands markedly activate TRPM4 currents provide definitive evidence that this ion channel functions as an important link between mechano-sensitive P2Y receptor activation and the myogenic response in PAs. Next, the signaling cascades that mediate stretch-induced TRPM4 activation in PA myocytes were explored. Interestingly, these experiments determined that the RhoA/Rho kinase signaling pathway is involved in this mechanism by facilitating pressure-induced, P2Y receptor-mediated stimulation of TRPM4 channels, leading to subsequent smooth muscle depolarization, [Ca2+]i increase and contraction. Since Rho kinase is generally accepted as a u27Ca2+-sensitizationu27 mediator, the present, contrasting observations point to an underappreciated role of RhoA/Rho kinase signaling in the excitation-contraction mechanisms within the cerebral microcirculation. Overall, this dissertation provides evidence that myogenic regulation of cerebral PAs is mediated by mechano-sensitive P2Y receptors, which initiate the RhoA/Rho kinase signaling pathway, subsequent TRPM4 channel opening, and concomitant depolarization and contraction of arteriolar smooth muscle cells. Revealing the unique mechanochemical coupling mechanisms in the cerebral microcirculation may lead to development of innovative therapeutic strategies for prevention and treatment of microvascular pathologies in the brain.
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