CARBONIC ANHYDRASE-II EXPRESSION IN RAT TYPE-II PNEUMOCYTES

Pulmonary carbonic anhydrase (CA) activity plays important roles in ca rbon dioxide exchange, fluid secretion, and pH regulation. This study reports the use of molecular and immunologic techniques to characteriz e expression of the high-activity cytosolic isoenzyme CA II in rat lun g tissue. Northern blot analysis of RNA isolated from various rat tiss ues revealed that the lung is a site of abundant tissue-specific CA II gene expression. The cell type primarily responsible for CA II expres sion in the lung was identified by immunohistochemistry as the alveola r type II pneumocyte. RNA blot and immunoblot analyses of isolated rat type II cells in culture confirmed CA II expression by this cell type . Little immunoreactive CA I and no CA IV was detected in these cells. Inhibition studies confirmed that the majority of CA activity in isol ated type II cells is attributable to CA II. CA II expression was foun d to continue in these cells beyond 72 h in culture, a timeframe durin g which these cells had dedifferentiated. The ontologic pattern of CA II expression in the lung was found by RNA blot analysis to be dispara te from that of the surfactant-associated proteins. These observations suggest roles for CA II in alveolar pneumocytes independent of (or in addition to) participation in surfactant biology. Such roles may incl ude the regulation of fluid secretion or facilitation of carbon dioxid e elimination.