Role of Astrocytes and CO in ADP induced cerebrovascular dilation in newborn pigs

Abstract

Astrocytes with neurons and microvessels form a neurovascular unit allowing blood flow to match neuronal activity. Adenosine diphosphate (ADP) functions as an important signaling molecule in the brain. Dilations to ADP have been shown to be astrocyte dependent in rats and newborn pigs. CO, produced endogenously by catabolism of heme to CO, free iron and biliverdin by heme oxygenase (HO), is an endogenous gaseous biological messenger in the brain and a major component of regulation of cerebrovascular circulation in the newborn. The present study addresses the hypothesis that ADP stimulates production of CO by perivascular astrocytes and this CO causes pial arteriolar dilation. Experiments used anesthetized newborn pigs with closed cranial windows, and freshly isolated piglet astrocytes and microvessels. Astrocyte injury was caused by topical application of L-2-alpha aminoadipic acid, (L-AAA, 2mM, 5h). CSF was collected from under the cranial windows for measurement of CO. Isolated astrocytes and microvessels were used for measurement of CO production in response to ADP. CO was measured by GC-MS analysis of headspace gas from sealed vials and quantified using heavy CO as an internal standard. In vivo, topical application of ADP produced dilation of pial arterioles that was absent following astrocyte injury as shown before. ADP increased CO production by the brain before but not after astrocyte injury. Chromium mesoporphyrin (CrMP), prevented the ADP-induced increase in CSF CO and blocked dilation of pial arterioles to ADP. Neither L-AAA nor CrMP altered pial arteriolar dilation to sodium nitroprusside. ADP also increased CO production by piglet astrocytes. ADP increased CO production more in astrocytes than in cerebral microvessels. Therefore astrocytes may employ CO as a gasotransmitter to cause cerebrovascular dilation in response to ADP.