Astrocyte-Derived CO Is a Diffusible Messenger That Mediates Glutamate-Induced Cerebral Arteriolar Dilation by Activating Smooth Muscle Cell K _Ca Channels

Abstract

Astrocyte signals can modulate arteriolar tone, contributing to regulation of cerebral blood flow, but specific intercellular communication mechanisms are unclear. Here we used isolated cerebral arteriole myocytes, astrocytes, and brain slices to investigate whether carbon monoxide (CO) generated by the enzyme heme oxygenase (HO) acts as an astrocyte-to-myocyte gasotransmitter in the brain. Glutamate stimulated CO production by astrocytes with intact HO-2, but not those genetically deficient in HO-2. Glutamate activated transient K(Ca) currents and single K(Ca) channels in myocytes that were in contact with astrocytes, but did not affect K(Ca) channel activity in myocytes that were alone. Pretreatment of astrocytes with chromium mesoporphyrin (CrMP), a HO inhibitor, or genetic ablation of HO-2 prevented glutamate-induced activation of myocyte transient K(Ca) currents and K(Ca) channels. Glutamate decreased arteriole myocyte intracellular Ca2+ concentration and dilated brain slice arterioles and this decrease and dilation were blocked by CrMP. Brain slice arteriole dilation to glutamate was also blocked by L-2-alpha aminoadipic acid, a selective astrocyte toxin, and paxilline, a K(Ca) channel blocker. These data indicate that an astrocytic signal, notably HO-2-derived CO, is used by glutamate to stimulate arteriole myocyte K(Ca) channels and dilate cerebral arterioles. Our study explains the astrocyte and HO dependence of glutamatergic functional hyperemia observed in the newborn cerebrovascular circulation in vivo.

Glutamate-induced calcium signals stimulate CO production in piglet astrocytes

“…Our previous studies in piglets demonstrated that glutamate stimulates astrocytic CO production in primary-cultured astrocytes and in vivo (24,26,31). We also demonstrated that astrocytes release CO and astrocyte-derived CO causes glutamate-induced pial arteriolar dilation in vivo (24), supporting a regulatory role for astrocytic CO in neurovascular coupling in the newborn brain.…”

“…We also demonstrated that astrocytes release CO and astrocyte-derived CO causes glutamate-induced pial arteriolar dilation in vivo (24), supporting a regulatory role for astrocytic CO in neurovascular coupling in the newborn brain. Indeed, in brain slices from newborn piglets, we demonstrated that glutamate regulates Ca 2ϩ signals in smooth muscle cells of arterioles through astrocyte-and HO-dependent mechanisms (26,39).Before this study, the mechanism by which glutamate stimulates astrocytic CO production was not known. Data acquired from other cell types, such as neurons, cerebral microvessels, and endothelial cells, suggest that calmodulin (CaM) could be involved in CO production by astrocytes.…”

“…Functional hyperemia has been investigated extensively, and many astrocyte-derived vasoactive mediators, including ADP (40, 41), ATP (34, 36), epoxyeicosatrienoic acids (1-3), K ϩ (11, 13, 33), and prostacyclin (42), appear to be involved, depending on experimental models. We have shown that carbon monoxide (CO) is another critical regulator of cerebrovascular tone in newborn piglets (19,24,26,31,39).CO dilates newborn pig pial arterioles (22,25). CO causes vasodilation by stimulating Ca 2ϩ sparks and large-conductance Ca 2ϩ -activated K ϩ (BK Ca ) channels and by increasing effective coupling between Ca 2ϩ sparks and BK Ca channels in arterial smooth muscle cells (17,18,25,38).…”

“…Functional hyperemia has been investigated extensively, and many astrocyte-derived vasoactive mediators, including ADP (40,41), ATP (34,36), epoxyeicosatrienoic acids (1-3), K ϩ (11,13,33), and prostacyclin (42), appear to be involved, depending on experimental models. We have shown that carbon monoxide (CO) is another critical regulator of cerebrovascular tone in newborn piglets (19,24,26,31,39).…”

Lack of direct effect of adiponectin on vascular smooth muscle cell BKCa channels or Ca2+ signaling in the regulation of small artery pressure-induced constriction

“…2012) or Glutamate (Li et al. 2008). However, even adopting this approach, there was no evidence for an effect by Adiponectin on STOC amplitude.…”