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

Abstract

1. Unanaesthetized, unrestrained rabbits were exposed for 3 hr in a chamber to either air, hypoxic gas mixtures (10% or 14% O(2) in N(2)) or a hyperoxic gas mixture (50% O(2) in N(2)). The carotid bodies were then removed and incubated for 3 hr in modified Tyrode media equilibrated with 100% O(2) and containing either [(3)H]tyrosine or [(3)H]DOPA. The contents of [(3)H]DA and [(3)H]NA in the tissue were determined as described in the preceding paper.2. When [(3)H]DOPA was used as precursor, neither labelled dopamine (DA) or noradrenaline (NA) synthesis was increased in carotid bodies from rabbits exposed to 10% O(2) in N(2). Following exposure to 10% O(2) in N(2) and incubation with [(3)H]tyrosine, however, [(3)H]DA synthesis was increased by 72% above control (air) values while [(3)H]NA synthesis was unchanged. Less severe hypoxia, 14% O(2) in N(2), resulted in a smaller increase in [(3)H]DA synthesis, i.e. 53% above control value. Again, [(3)H]NA synthesis was unchanged. Similar experiments with the superior cervical ganglion involving exposure of the animals to either 10% or 14% O(2) in N(2) did not produce any change in the amounts of [(3)H]DA or [(3)H]NA synthesized from [(3)H]tyrosine when compared to control animals breathing air.3. Sympathectomy of the carotid body or transection of the carotid sinus nerve 12-15 days prior to hypoxic exposure (10% O(2) in N(2)) did not alter the increase in [(3)H]DA synthesis compared to normally innervated carotid bodies.4. Carotid bodies incubated with [(3)H]tyrosine for 2 hr in an alternating O(2)/N(2) sequence (5 min in media equilibrated with 100% O(2) followed by 3 min in media equilibrated with 100% N(2)) synthesized 37% more [(3)H]DA than control carotid bodies similarly exposed to an alternating O(2)/O(2) sequence. [(3)H]NA synthesis was unchanged. However, tissue levels of non-metabolized [(3)H]tyrosine were reduced by 19% in the carotid bodies exposed to the O(2)/N(2) sequence.5. Exposure of rabbits for 3 hr to 50% O(2) in N(2), followed by incubation of their carotid bodies in [(3)H]tyrosine, resulted in a 19% decrease in the absolute value for [(3)H]DA synthesis compared to control carotid bodies, but this difference was not significant (P > 0.05). However, [(3)H]NA synthesis was significantly reduced (51%; P < 0.05) in the hyperoxic carotid bodies. Similar experiments with the superior cervical ganglion showed that [(3)H]DA and [(3)H]NA synthesis were unchanged under control vs. hyperoxic conditions.6. Carotid bodies incubated with [(3)H]tyrosine for 3 hr, then transferred for 1 hr to unlabelled media equilibrated with 10% O(2) in N(2), released 81% more [(3)H]DA, and contained 38% less [(3)H]DA, than similarly treated carotid bodies exposed to 100% O(2). [(3)H]NA was not detectable in the media, and tissue levels of [(3)H]NA were the same in both hypoxic and control carotid bodies.