AuthorHoesch, Robert Eugene
AdvisorKao, Joseph P. Y.
MetadataShow full item record
AbstractMicrofluorimetric, electrophysiological, pharmacological, and intracellular photorelease techniques were used to study Ca2+ physiology in rabbit vagal sensory neurons (nodose ganglion neurons, NGNs). NGNs exhibit robust Ca2+-induced Ca2+ release (CICR) that can be triggered by caffeine, the classic CICR agonist. A caffeine-induced increase in cytosolic free Ca2+ concentration ([Ca2+ ]i) was traditionally taken as diagnostic of the existence of CICR. However, when CICR was disabled through depletion of intracellular Ca2+ stores or pharmacological blockade of intracellular Ca 2+ release channels (ryanodine receptors, RyRs), caffeine still elicited a significant rise in [Ca2+]i in ∼50% of NGNs. We demonstrated that this rise in [Ca2+]i results from Ca2+ influx, and that Ca2+ influx is one component of a non-selective cation current that is activated by caffeine. Therefore, in approximately half of all NGNs, caffeine elicits both CICR and Ca2+ influx. We determined that d-myo-inositol 1,4,5-trisphosphate receptors (IP3Rs), another type of intracellular Ca2+ release channel, coexist with RyRs in NGNs. ATP, an extracellular physiological signaling molecule, consistently evoked robust transient increases in [Ca 2+]i (Ca2+ transients) that have a dual origin: (1) Ca2+ influx via P2X receptors and voltage-gated Ca2+ channels; and (2) intracellular Ca2+ release, via IP3Rs and RyRs, that requires activation of P2Y receptors and phospholipase C. We determined that Ca2+ transients evoked by IP3 photorelease can gate a Ca2+-activated K+ current (I IP3) in NGNs. IIP3 is unaffected by three common antagonists of Ca2+-activated K+ currents: iberiotoxin, apamin, and 8-Br-CAMP. Using caffeine to selectively deplete intracellular Ca2+ stores, we show that while CICR does not contribute significantly to global IP3-evoked Ca2+ transients, surprisingly, ∼20% of IIP3 is activated by CICR. We propose a model of Ca2+ signaling microdomains that rationalize these observations. We showed that ATP can evoke a Ca2+-activated K + current in 57% of NGNs. ATP-evoked Ca2+ influx activates ∼75% of this current, while ATP-evoked intracellular Ca2+ release activates the other ∼25%. We also determined that NGNs express P2X receptors that mediate robust influx, therefore our data suggest that ATP can exert both excitatory and inhibitory effects in NGNs.
DescriptionUniversity of Maryland, Baltimore. Molecular Medicine. Ph.D. 2002
Biology, Animal Physiology