Regulation of brain capillary endothelial cells by P2Y receptors coupled to Ca2+, phospholipase C and mitogen-activated protein kinase

1. The blood-brain barrier is formed by capillary endothelial cells and is regulated by cell-surface receptors, such as the G protein-coupled P2Y receptors for nucleotides. Here we investigated some of the characteristics of control of brain endothelial cells by these receptors, characterizing the phospholipase C and Ca²⁺ response and investigating the possible involvement of mitogen-activated protein kinases (MAPK).
2. Using an unpassaged primary culture of rat brain capillary endothelial cells we showed that ATP, UTP and 2-methylthio ATP (2MeSATP) give similar and substantial increases in cytosolic Ca²⁺, with a rapid rise to peak followed by a slower decline towards basal or to a sustained plateau. Removal of extracellular Ca²⁺ had little effect on the peak Ca²⁺-response, but resulted in a more rapid decline to basal. There was no response to α,β-MethylATP (α,βMeATP) in these unpassaged cells, but a response to this P2X agonist was seen after a single passage.
3. ATP (log EC₅₀ −5.1±0.2) also caused an increase in the total [³H]-inositol (poly)phosphates ([³H]-InsP_x) in the presence of lithium with a rank order of agonist potency of ATP=UTP=UDP>ADP, with 2MeSATP and α,βMeATP giving no detectable response.
4. Stimulating the cells with ATP or UTP gave a rapid rise in the level of inositol 1,4,5-trisphosphate (Ins(1,4,5)P₃), with a peak at 10 s followed by a decline to a sustained plateau phase. 2MeSATP gave no detectable increase in the level of Ins(1,4,5)P₃.
5. None of the nucleotides tested affected basal cyclic AMP, while ATP and ATPγS, but not 2MeSATP, stimulated cyclic AMP levels in the presence of 5 μM forskolin.
6. Both UTP and ATP stimulated tyrosine phosphorylation of p42 and p44 mitogen-activated protein kinase (MAPK), while 2MeSATP gave a smaller increase in this index of MAPK activation. By use of a peptide kinase assay, UTP gave a substantial increase in MAPK activity with a concentration-dependency consistent with activation at P2Y₂ receptors. 2MeSATP gave a much smaller response with a lower potency than UTP.
7. These results are consistent with brain endothelial regulation by P2Y₂ receptors coupled to phospholipase C, Ca²⁺ and MAPK; and by P2Y₁-like (2MeSATP-sensitive) receptors which are linked to Ca²⁺ mobilization by a mechanism apparently independent of agonist stimulated Ins (1,4,5)P₃ levels. A further response to ATP, acting at an undefined receptor, caused an increase in cyclic AMP levels in the presence of forskolin. The differential MAPK coupling of these receptors suggests that they exert fundamentally distinct influences over brain endothelial function.