Abstract
Somatostatin released from capsaicin-sensitive sensory nerves of the lung during endotoxin-induced murine pneumonitis inhibits inflammation and hyperresponsiveness, presumably via somatostatin receptor subtype 4 (sst(4)). The goal of the present study was to identify sst(4) receptors in mouse and human lungs and to reveal its inflammation-induced alterations with real-time quantitative PCR, Western blot, and immunohistochemistry. In non-inflamed mouse and human lungs, mRNA expression and immunolocalization of sst(4) are very similar. They are present on bronchial epithelial, vascular endothelial, and smooth-muscle cells. The sst(4) receptor protein in the mouse lung significantly increases 24 hr after intranasal endotoxin administration as well as in response to 3 months of whole-body cigarette smoke exposure, owing to the infiltrating sst(4)-positive mononuclear cells and neutrophils. In the chronically inflamed human lung, the large number of activated macrophages markedly elevate sst(4) mRNA levels, although there is no change in acute purulent pneumonia, in which granulocytes accumulate. Despite mouse granulocytes, human neutrophils do not show sst(4) immunopositivity. We provide the first evidence for the expression, localization, and inflammation-induced alterations of sst(4) receptors in murine and human lungs. Inasmuch as tissue distribution of this receptor is highly similar, extrapolation of murine experimental results to human conditions might be possible.