The effect of a nitric oxide donor, sodium nitroprusside, on the release of acetylcholine from the in vitro cat carotid body

The purpose of the present study was to determine the impact of a nitric oxide (NO) donor, sodium nitroprusside (SNP), on the release of acetylcholine (ACh), an essential excitatory neurotransmitter, from the in vitro cat carotid body (CB). Bilateral CBs were harvested from five deeply anesthetized cats according to the regulations contained in the policies of the Johns Hopkins Animal Care and Use Committee. After recovering from the surgical procedures for extraction and cleaning, the CBs were taken through a 15-step protocol in which they were exposed to a hyperoxic gas mixture (40% O2/5% CO2; 20 min), then a hypoxic gas mixture (6% O2/5% CO2; 20 min), and a final 10 min hyperoxic mixture. This sequence was applied twice, followed by the same sequence in the presence, first, of 5 μM SNP, and secondly in the presence of 10 μM SNP. After washing and a recovery period the CBs were again exposed to the gases as in the first two non-SNP trials. The SNP exposures significantly reduced the overall release of ACh by about 20% (P = 0.039). Further, SNP significantly reduced the hypoxia-induced increase in ACh release (without SNP: 82.4 ± 19.1 fmol/20 μL versus with SNP: 49.7 ± 15.0 fmol/20 μL; mean ± S.E.M.; P = 0.032). Trials #1 and #2 which preceded the application of SNP and Trial #3 which followed SNP were statistically indistinguishable. The CBs had recovered their original status. The data support the hypothesis that the frequently reported NO-induced reduction in CB neural output during hypoxia is at least in part due to the reduction in ACh release. The results are consistent with a previous report in which l-arginine, an NO precursor, had the same reducing effect. Possible mechanisms are discussed.