Hypothesis: the bronchial Kulchitsky (K) cell as a source of humoral biologic activity

The bronchial Kulchitsky cells are scattered specific cells which lie close to the basement membrane of the bronchi and bronchioles. Electron microscopy reveals that they contain electron-dense granules similar to that seen in cells with a known endocrine function. In addition, experimentally induced degranulation of the K cell suggestive of a secretory process, and the likelihood that these cells are precursors of small cell lung carcinoma (which often elaborates humoral substances) has led to the postulate that this bronchial cell serves a humoral role of either a paracrine or endocrine nature. We have found that the bronchial K cell of man contains a calcitonin-like polypeptide which, immunologically and chemically, is not dissimilar to the hormone produced by the C cells of human thyroid. This finding may help explain the persistence of serum immunoreactive calcitonin (iCT) after total thyroidectomy, the fact that thyroidectomized man does not manifest any profound alteration of calcium metabolism, and why small cell cancer of the lung is frequently associated with hypercalcitonemia. In addition, the finding of K cell hyperplasia in chronic bronchitis and emphysema may explain the occurrence of hypercalcitonemia in patients with these diseases and some lung cancers of cell types other than the small cell variety. Further studies are needed to elucidate the role of K cell iCT, and to determine what other hormones might also be elaborated by this diffuse system of bronchial cells.