Effects of hypoxia on catecholamine synthesis in rabbit carotid body in vitro

Abstract

Unanesthetized, unrestrained rabbits were exposed for 3 h in a chamber to either air, hypoxic gas mixtures (10 or 14% O2 in N2) or a hyperoxic gas mixture (50% O2 in N2). The carotid bodies were then removed and incubated for 3 h in modified Tyrode media equilibrated with 100% O2 and containing either [3H]tyrosine or [3H]dopa. The contents of [3H]DA [dopamine] and [3H]NA [noradrenaline [norepinephrine]] in the tissue were determined as described previously. When [3H]dopa was used as precursor, neither labeled DA or NA synthesis was increased in carotid bodies from rabbits exposed to 10% O2 in N2. Following exposure to 10% O2 in N2 and incubation with [3H]tyrosine, [3H]DA synthesis was increased by 72% above control (air) values while [3H]NA synthesis was unchanged. Less severe hypoxia, 14% O2 in N2, resulted in a smaller increase in [3H]DA synthesis, i.e., 53% above control value and [3H]NA synthesis was unchanged. Similar experiments with the superior cervical ganglion involving exposure of the animals to either 10% or 14% O2 in N2 did not produce any change in the amounts of [3H]DA or [3H]NA synthesized from [3H]tyrosine when compared to control animals breathing air. Sympathectomy of the carotid body or transection of the carotid sinus nerve 12-15 days prior to hypoxic exposure (10% O2 in N2) did not alter the increase in [3H]DA synthesis compared to normally innervated carotid bodies. Carotid bodies incubated with [3H]tyrosine for 2 h in an alternating O2/N2 sequence (5 min in media equilibrated with 100% O2 followed by 3 min in media equilibrated with 100% N2) synthesized 37% more [3H]DA than control carotid bodies similarly exposed to an alternating O2/O2 sequence. [3H]NA synthesis was unchanged. Tissue levels of nonmetabolized [3H]tyrosine was reduced by 19% in the carotid bodies exposed to the O2/N2 sequence. Exposure of rabbits for 3 h to 50% O2 in N2, followed by incubation of their carotid bodies in [3H]tyrosine, resulted in a 19% decrease in the absolute value for [3H]DA synthesis compared to control carotid bodies, but this difference was not significant (P > 0.05). [3H]NA synthesis was significantly reduced (51%; P < 0.05) in the hyperoxic carotid bodies. Similar experiments with the superior cervical ganglion showed that [3H]DA and [3H]NA synthesis were unchanged under control vs. hyperoxic conditions. Carotid bodies incubated with [3H]tyrosine for 3 h, then transferred for 1 h to unlabeled media equilibrated with 10% O2 in N2, released 81% more [3H]DA and contained 38% less [3H]DA than similarly treated carotid bodies exposed to 100% O2. [3H]NA was not detectable in the media, and tissue levels of [3H]NA were the same in both hypoxic and control carotid bodies.