ACETYLCHOLINE INDUCES RELAXATION VIA THE RELEASE OF NITRIC-OXIDE FROM ENDOTHELIAL-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, both acetylcholine (ACh) and sodium nitroprusside (SNP) caused concentration-dependent relaxations. 3. L-N(G)-Nitroarginine methyl ester (L-NAME; 10-100 muM) concentration-dependently inhibited vasodilatation in response to ACh, but had no effect on the responses to SNP. 4. The inhibitory 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 through the generation of NO derived froM L-Arg. 6. This is discussed in relation to other lower vertebrate and mammalian groups and it is suggested that dual control of vascular tone by perivascular nerves and endothelium first appears in the more advanced groups of amphibians and reptiles.