Hypertension has been linked to opening of the bloodbrain barrier and may be related to the expression of the smooth muscle a-actin gene in contractile cells at the brain microvasculature. However, the cellular origin (i.e., endothelial cells, pericytes, smooth muscle cells) of the a-actin mRNA in the brain microvasculature is not clearly identified. Therefore, we investigated the abundance of actin mRNA by Northern blot analysis in isolated brain microvessels and in brain microvascular endothelia or pericytes in tissue culture. All samples showed the characteristic 2.1 kb transcript corresponding to cytoplasmic p and y isoform mRNA. The 1.7 kb transcript corresponding to smooth muscle a-actin was detected in freshly isolated bovine brain microvessels, in primary cultures of brain microvascular pericytes, or endothelial cells; the latter cultures contain both endothelial cells and pericytes. The a-actin mRNA was absent in a cloned bovine brain endothelial cell line. The relative abundance of the a/@ + y ) actin transcript ratio was: cultured pericytes > freshly isolated microvessels > endothelial primary. The cellular distribution of the smooth muscle a-actin immunoreactive protein was studied by immunocytochemistry in cytospun/methanol-fixed isolated bovine brain microvessels with a monoclonal antibody directed to the amino-terminal decapeptide of the smooth muscle a-actin isoform. This antibody reacted strongly with precapillary arterioles of isolated microvessels, whereas no immunostaining was observed in either capillary endothelial cells or in pericytes. In conclusion, the a-actin mRNA is expressed in brain microvascular pericytes in tissue culture, but the immunoreactive a-actin protein is not expressed in brain microvascular pericytes in vivo. These data suggest that either 1) a-actin gene expression is induced in capillary pericytes in tissue culture or 2) a-actin mRNA in brain capillary pericytes in vivo is subject to translational repression re-sulting in no detectable a-actin protein under normal conditions.