Different contractile responses of carotid artery and thoracic aorta in rabbits

Effects of inhibition of nitric oxide synthase by N?-nitro L-arginine (L-NA) on acethylcholine (ACh)-induced relaxations, potassium chloride (KCl)- and phenylephrine (PH)-induced contractions on rabbit thoracic aorta and carotid artery were investigated. Rabbits (n=15) weighing 2-2.5 kg from both sex were used. A pair of thoracic aorta rings was isolated, as was a pair of carotid artery rings. One ring from each pair was treated with Krebs solution containing L-NA (0.1 mM) throughout the experiment and the other ring served as the control. Inhibition of nitric oxide synthesis by L-NA did not completely block ACh-induced relaxations in either artery. Inhibition of nitric oxide synthesis by L-NA significantly reduced 60 mM KCl-induced and (1nM- 0.1mM) PH-induced contractile responses in rabbit thoracic aorta whereas it did not affect these responses in carotid artery. Inhibition of nitric oxide synthesis by L-NA did not affect the sensitivity (pD2) to PH in any of the arteries. In conclusion, the results of the present study showed that chronicle inhibition of nitric oxide synthesis by L-NA did not completely block nitric oxide-dependent relaxations in thoracic aorta and common carotid artery and decreased the contractile responses to PH and KCl only in thoracic aorta