Alpha-vascular responses after short-term and long-term inhibition of nitric oxide synthesis

Apart from the direct actions of nitric oxide (NO) on vascular smooth muscle, this factor may regulate cardiovascular functions through specific actions on alpha-adrenergic constrictor mechanisms. In this study we aim to establish whether the inhibition of the synthesis of this mediator could alter the vasoconstrictor responses mediated by alpha-adrenoceptor stimulation. We have been able to demonstrate that the blockage of the NO synthase really does exist, when both short- and long-term treatments with Nomega-nitro-Larginine methyl ester (L-NAME) are carried out. We have evaluated the concentration-dependent contractions induced by the selective alpha1-adrenoceptor agonists methoxamine and phenylephrine in isolated rat aorta rings in the following groups of animals: control, short-term L-NAME-treated (100 mg/kg i.p. 20 min before subjecting the animals to the experiments) and long-term L-NAME-treated (100 mg/kg per day in the drinking water for 7, 21, or 45 days). We have also evaluated the pressor responses to methoxamine and to the selective alpha-adrenoceptor agonist B-HT 920 using the pithed rat preparation in the same groups of animals. The contractile responses to methoxamine and phenylephrine were similar in the rat aorta preparations from control and short-term L-NAME-treated animals. On the contrary, in the rat aorta preparations from long-term L-NAME-treated animals these responses were clearly reduced when compared with the corresponding responses in those from control animals, the reduction being more marked when the treatment lasted longer. The pressor responses to methoxamine were also similar in control and short-term L-NAME-treated pithed rats. Nevertheless, the responses to B-HT 920 were greater in the latter. On the other hand, the dose-response curves to both alpha-adrenoceptor agonists were shifted to the right in a non-parallel manner in rats treated long term with L-NAME, the shift being, in the case of B-HT 920, more accentuated when the treatment lasted 21 or 45 days than when it lasted only 7 days. These results indicate that the short-term decrease in NO synthesis does not modify the vascular smooth muscle responses mediated by alpha1-adrenoceptor stimulation, but it does induce a potentiation of sympathetic vasoconstriction mediated by alpha2-adrenoceptors. Nevertheless, the long-term inhibition of NO synthesis causes a compensating decrease in the alpha1- and alpha2-vascular smooth muscle contractile responses.