Expression of calcitonin gene-related peptide by cultured rat alveolar type II cells

Calcitonin gene-related peptide (CGRP) immunoreactivity is found in the airways in terminals of primary sensory afferents, in neuroendocrine cells, and in tracheal serous cells. This study shows that rat alveolar epithelial cells express immunoreactive CGRP also. Freshly isolated cells contained 34 +/- 23 fmol CGRP/10(7) cells (n = 4). Cultured type II cells secreted CGRP at a stable rate for 3 days after cell isolation, averaging 206 +/- 14 fmol CGRP/well/day (750,000 cells plated/well with approximately 30% efficiency). The extracellular CGRP immunoreactivity eluted in the same fraction as rat CGRP-beta on high performance liquid chromatography. Secretion of CGRP from type II cells was reversibly blocked by monensin, an inhibitor of secretory protein transport. CGRP secretion was stimulated in a concentration-dependent fashion by phorbol myristate acetate, but it was not affected by forskolin, capsaicin, bradykinin, or nicotine. CGRP was not detected in culture media from alveolar macrophages or fibroblasts, potential contaminants of primary type II cell cultures. Calcitonin is expressed by neuroendocrine cells, but it was not detected in conditioned media from type II cell cultures. Thus, type II alveolar epithelial cells express and secrete CGRP. Secretion occurs constitutively and is regulated by a protein kinase C-dependent pathway. Secretion is unaffected by increases in cyclic adenosine monophosphate or by treatments that induce release of CGRP from sensory afferent nerve terminals in the airways.