The endocrine lung and its response to hypoxia

Functional facets of the pulmonary neuroendocrine system

Pulmonary neuroendocrine cells (PNECs) have been around for 60 years in the scientific literature, although phylogenetically they are ancient. Their traditionally ascribed functions include chemoreception and regulation of lung maturation and growth. There is recent evidence that neuroendocrine (NE) differentiation in the lung is regulated by genes and pathways that are conserved in the development of the nervous system from Drosophila to humans (such as achaete-scute homolog-1), or implicated in the carcinogenesis of the nervous or NE system (such as the retinoblastoma tumor suppressor gene). In addition, complex neural networks are in place to regulate chemosensory and other functions. Even solitary PNECs appear to be innervated. For the first time ever, we have mouse models for lung NE carcinomas, including the most common and virulent small cell lung carcinoma. Moreover, PNECs may be important for inflammatory responses, and pivotal for lung stem cell niches. These discoveries signify an exciting new era for PNECs and are likely to have therapeutic and diagnostic applications.

Pulmonary neuroendocrine cell system in pediatric and adult lung disease

In humans lungs affected by naturally occurring pulmonary disease, the pulmonary neuroendocrine cell system, which is normally arranged in a sparse but even distribution throughout the respiratory tract, increases in size. It is likely that the stimulus for this is pulmonary injury and that its purpose is the paracrine regulation of the restoration of pulmonary tissues to their normal state, an hypothesis supported by studies of animal lungs subjected to experimental injury as well as of the development of human and animal lungs in utero. Initially, this increase involves the development of interrupted rows of neuroendocrine cells. In the later stages, however, development of more disorderly intraepithelial aggregates can occur and the small, locally invasive neuroendocrine cell lesions known as tumourlets may occasionally result. Both of these latter structures often contain secretory products not found in the neuroendocrine cells of normal human lungs, probably indicating a derangement of what appears to be a fundamentally physiological response. It is likely that, in some circumstances, this disorderly change may contribute to pulmonary disease as well as being the result of it.

Structural and pathologic changes in the lung vasculature in chronic liver disease

The hepatopulmonary syndrome results from erythrocytes bypassing the lung without oxygenation. In addition to ventilation-perfusion mismatching, the hypoxemia may result from portapulmonary shunting, passage around alveoli through pleural and hilar blood vessels, and intrapulmonary vascular dilatations. Dilated vascular channels between arteries and veins on the pleural surface are seen more often than dilated intrapulmonary capillaries in chronic liver disease. These anastomoses appear grossly as vascular "spider nevi" on the pleura. Portal vein-to-pulmonary vein anastomoses could produce arterial hypoxemia because the deoxygenated portal venous blood can mix with oxygenated pulmonary venous blood. There is an association of esophageal varices with the hepatopulmonary syndrome and anastomoses between the portal veins and pulmonary veins have been found in both animals and humans. As portal pressures increase, the mediastinal veins enlarge, enhancing the chance that they may penetrate the pleura and drain into pulmonary veins. Direct splenic injections in patients, however, suggest that this shunt pathway is uncommon and small. Pulmonary artery injection studies have demonstrated dilated intrapulmonary vascular segments in humans and animals. Dilation of capillaries may allow a more rapid flow through the lung and the greater distance between the erythrocyte and alveolar wall may make it more difficult to oxygenate rapidly passing erythrocytes. Pulmonary capillary dilation can explain the abnormalities of the perfusion lung scan and contrast echocardiogram that portapulmonary shunting cannot. Pulmonary hypertension may occur in chronic liver disease even without arterial hypoxemia, but it is rare. The prevalence of hypertensive pulmonary vascular disease in patients with cirrhosis of the liver is less than 1%, although a higher percentage (2%) has been found when patients with portal hypertension were studied by cardiac catheterization. The hypertensive pulmonary vascular disease (pulmonary arteriopathy with plexiform lesions) that occurs in patients with liver disease appears identical to that encountered in patients with congenital cardiac shunts and unexplained (primary) pulmonary hypertension.