The carotid bodies, which are the primary organs for sensing O_2 tension in arterial blood, are enlarged in rat exposed to chronic hypoxia (380 torr, 12 weeks). We noted some ultrastructural features with chronic hypoxia which are characteristic of the amphibian carotid labyrinth glomus cells : 1) incomplete covering of glomus cells with the supporting cell missing over a wide area, 2) long thin cytoplasmic projections in the intervascular stroma, and 3) intimate apposition of the glomus cells and endothelial cells, pericytes, and others. Threedimensional analysis using a combination of an ultra high voltage electron microscope (400kV) and semi-thin sections clearly demonstrated that the incomplete covering of glomus cells is based on a difference in an extent of hypertrophy between glomus cells and supporting cells. Because arterial PO_2 is generally low in amphibia, these ultrastructural similarities may be general features of hypoxic adaptation and facilitate both uptake of oxygen from blood and release of catecholamine into the blood.
When the distribution and abundance of the peptidergic fibers were compared between normoxic and chronically hypoxic carotid bodies and tracheas, there was a difference in number of some peptide-immunoreactive fibers (SP,CGRP,VIP,and others). The increase of these immunoreactive fibers suggests that altered airway reflexes may be a feature of hypoxic adaptation. To clarify a physiological mechanism in the hypoxic adaptation, we are now studying an effect of intracellular second messengers, such as cyclic AMP and others, in axoplasmic transport of nerve fibers in unfixed sections of chronically hypoxic carotid body.