The carotid body is a major chemoreceptor organ whose excitation causes reflex responses in cardiopulmonary, renal and endocrine systems. Although the mechanisms of carotid body excitation are not yet clear, essential steps include the depolarization of chemosensitive glomus cells, the increase in glomus cell intracellular calcium and the release of neurotransmitters. Many studies point to the involvement of oxygen-sensitive potassium channels, but a causal relationship between the inhibition of these channels and the depolarization of glomus cells during hypoxia has not yet been established. Since cat glomus cells release acetylcholine even under normoxic/normocapnic conditions, we hypothesize that hypoxia augments the activity of neuronal nicotinic acetylcholine receptors and/or enhances the sensitivity of acetylcholine receptors for acetylcholine. This initiates the depolarization of glomus cells and the increase in intracellular calcium. Oxygen-sensitive potassium channels and voltage-gated calcium channels participate in the later phase of the changes. Preliminary data have shown that: 1) cat glomus cells expressed alpha-4 subunit containing nicotinic acetylcholine receptors, 2) acetylcholine-induced inward current and carotid body neural output were enhanced by a mild decrease in oxygen tension from normoxic levels, 3) acetylcholine increased calcium of carotid body cells, 4) oxygen-sensitive potassium current was linearly inhibited by decreasing oxygen, and 5) increased carotid body neural output in hypoxia was inhibited by L-type voltage gated calcium channels.
Specific aims are to investigate: 1) the role of acetylcholine and nicotinic acetylcholine receptors for initiating the depolarization of glomus cells and the increase in calcium, 2) the contribution of oxygen sensitive potassium channels in the late phase of glomus cell depolarization during hypoxia, 3) the contribution of voltage gated calcium channels to the late phase of the calcium increase in glomus cells during hypoxia. Patch clamp, microfluorometric, and immunocytochemical techniques are to be used. This innovative proposal will advance the understanding of the excitation mechanisms of glomus cells. Once the chemotransductive mechanisms are understood, pharmacological or genetic tools can be developed to alter the carotid body function to levels desirable for treating carotid body related pathological conditions such as sudden infant death syndrome and hemodynamic changes in sleep apnea patients.
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