Hypoxia and N6,O2'-dibutyryladenosine 3',5'-cyclic monophosphate, but not nerve growth factor, induce Na+ channels and hypertrophy in chromaffin-like arterial chemoreceptors.

Abstract

Chronic hypoxia sensitizes the ventilatory reflex in mammals and causes enlargement of the carotid body, a peripheral arterial chemosensory organ. To investigate possible underlying mechanisms, in the absence of circulatory changes, we exposed cultures of dissociated rat carotid body containing the oxygen sensors (i.e., chromaffin-like glomus cells) to chronic hypoxia (6% 02) over a period of 2 weeks. After a delay of a few days, the Na+ current density in hypoxia-treated glomus cells increased gn ntly, reaching values up to 6 times that seen in normoxic (20% 02) controls.In addition the whole-cell capacitance, an indicator of cell size, was also significantly larger (3-4 times control) in glomus cells exposed to chronic hypoxia. Both effects were mimicked qualitatively by chronic treatment of normoxic cultures with N6,02-dibutyryladenosine 3',S'-cycic monophosphate, but not nerve growth factor, which is known to induce similar changes in the chromaffm cell line PC12. Thus, the physiological and morphological effects of chronic hypoxia on the carotid body in vivo may be due in part to a cAMP-mediated stimulation of Na+ channel expression and hypertrophy in the chemosensory glomus cells.Conditions of chronic hypoxia, as occur in humans and animals living at high altitude (1) or in patients with hypoxic lung disease (2), cause enlargement or hypertrophy of the carotid body, an organ that senses blood Po2 and controls ventilation. In addition, there is a time-dependent sensitization of this chemosensory pathway (3, 4), resulting in an increased respiratory drive that is thought to be important during acclimatization to hypoxia. The above structural and functional changes may be mediated via a direct action of low arterial Po2 on the carotid-body oxygen sensors (i.e., the chromaffin-like glomus cells), which contain a unique class of 02-sensitive K+ channels (5-7), or indirectly, by way of blood-borne factors arising elsewhere in the circulation. Though there is evidence that the hypertrophic response of the carotid body during chronic hypoxia is accompanied in part by an increased size of the glomus cells (8, 9), the underlying mechanisms associated with the morphological and physiological adaptations of the carotid body to chronic hypoxia are largely unknown. Moreover, these mechanisms are especially difficult to investigate in vivo, where circulatory factors cannot be readily controlled or eliminated.This laboratory has been investigating (7, 10, 11) chemotransduction mechanisms by using dissociated cell cultures of the rat carotid body in which the chemosensory glomus cells survive for several weeks and are accessible for electrophysiological patch-clamp/whole-cell recording. The ability to control the cellular, fluid, and gaseous environments in these cultures allowed us to investigate whether direct exposure ofcarotid body cells alone to chronic hypoxia could alter the physiological and morphological properties of the glomus cells, in the absence of extraneous circulatory factors. In this study, we com...

Adrenomedullin expression and function in the rat carotid body

“…Unlike its effects in other organs, hypoxia causes the carotid body to grow by increasing the number, size, and excitability of glomus cells (Stea et al 1992). These changes are most likely mediated by the transcription factor, HIF-1 (López- Barneo et al 2001).…”

Co-expression of NGF and Its High-affinity Receptor TakA in the Rat Carotid Body Chief Cells

“…Several immature members of this lineage are known to respond to peptide growth factors, particularly nerve growth factor (NGF) and basic fibroblast growth factor (bFGF), by undergoing mitosis and/or neuronal differentiation [3,10,29,31,35,43]. Though NGF has been implicated in the fetal and early postnatal development of carotid body chief cells in vivo [1], several studies in vitro have failed to demonstrate NGF-responsiveness of these cells in the perinatal period [12,17,27,34]. These studies raised questions about the relationship of chief cells to more established members of the sympathoadrenal lineage.…”

The influence of chronic hypoxia upon chemoreception

“…10% of the level found in the squid giant axon), they are important for glomus cell membrane and action potentials (Gonzalez et al, 1994). Na + channel density increases in glomus cells cultured in chronic hypoxia, which could contribute to increased excitability of glomus cells after stimulation by hypoxia (Stea et al, 1992). (Hempleman, 1995) found increased Na+ currents in glomus cells from chronically hypoxic neonatal rat pups, which could increase excitability given the decrease he found in K + channel density in the same preparation (see above).…”