ACETYLCHOLINE INDUCES RELAXATION VIA THE RELEASE OF NITRIC-OXIDE FROMENDOTHELIAL-CELLS OF THE GARTER SNAKE (THAMNOPHIS-SIRTALIS-PARIETALIS) AORTA

1. The role of nitric oxide (NO) in the regulation of vascular tone of the dorsal aorta of the garter snake (Thamnophis sirtalis parietalis) was investigated. 2. In noradrenaline (NA) preconstricted vessels, bo th acetylcholine (ACh) and sodium nitroprusside (SNP) caused concentra tion-dependent relaxations. 3. L-N(G)-Nitroarginine methyl ester (L-NA ME; 10-100 muM) concentration-dependently inhibited vasodilatation in response to ACh, but had no effect on the responses to SNP. 4. The inh ibitory effect Of L-NAME was reversed by the presence Of L-arginine (L -Arg; 100 times the concentration Of L-NAME) at all concentrations Of L-NAME used. 5. This study demonstrates the presence of an endothelium -dependent vasodilator response to ACh in the snake aorta, acting thro ugh the generation of NO derived froM L-Arg. 6. This is discussed in r elation to other lower vertebrate and mammalian groups and it is sugge sted that dual control of vascular tone by perivascular nerves and end othelium first appears in the more advanced groups of amphibians and r eptiles.