Alteration in the release of endothelium-derived relaxing factors by alpha-adrenoceptor stimulation in the aorta of stroke-prone spontaneously hypertensive rats

[Endothelium-derived factors in hypertensive blood vessels, especially nitric oxide and hypertension]

Endothelium-dependent relaxation (EDR) in the blood vessels of spontaneously hypertensive rats (SHR) and the role of nitric oxide (NO) in the initiation of hypertension are reviewed. EDR was impaired in blood vessels of SHR depending on age and degree of hypertension when compared with those of normotensive rats. The cause of the impairment varied among the type of blood vessels: a decrease in the production of NO and endothelium-derived relaxing factor (EDRF) and an increase in the production of endothelium-derived contracting factor (EDCF) are the main causes of the impairment in large arteries, while a decrease in endothelium-dependent hyperpolarization and increased release of EDCF are the main causes of the impairment in small arteries. Interactions among these endothelium-derived factors and changes in the interactions are also causes of impairment. Superoxide may be involved in the impairment of EDR by destroying NO. The endothelium depresses smooth muscle contraction, including spontaneous tone developed in vascular smooth muscle, and the depressing effect of the endothelium is impaired in the preparations from SHR. The endothelium of blood vessels of SHR are structurally injured as demonstrated by scanning electron microscopy. Antihypertensive treatment prevented these functional and structural changes. Chronic treatment with inhibitors of NO production in normotensive rats impaired EDR and elevated blood pressure. The impairment of EDR is a secondary change due to continued hypertension, and early initiation of antihypertensive therapy is recommended.

Alpha-2 adrenoceptors are present in rat aorta smooth muscle cells, and their action is mediated by ATP-sensitive K(+) channels

The role of alpha(2)-adrenoceptors in the response of aorta smooth muscle rings to the alpha(2)-adrenoceptors agonists UK 14,304 and clonidine was studied. Stimulation by 1 - 10 nM UK 14,304 caused dose-dependent relaxant responses in BaCl(2)-contracted endothelium-denuded aorta rings, and hyperpolarization in rings with or without endothelium, which were inhibited by yohimbine and glibenclamide, but not affected by prazosin, propranolol, apamin or iberiotoxin. At higher concentrations (10 nM - 10 microM) UK 14,304 also induced a depolarizing effect which was potentiated by yohimbine and inhibited by prazosin. These results indicate that UK 14,304 acts on alpha(2)-adrenoceptors at lower concentrations and on both alpha(1)- and alpha(2)-adrenoceptors above 10 nM. In rings, with or without endothelium, noradrenaline had a depolarizing effect which was inhibited by prazosin. Adrenaline did not affect the membrane potential but in the presence of prazosin caused hyperpolarization, which was inhibited by yohimbine and glibenclamide. These results indicate that noradrenaline is more selective for alpha(1)-, whereas adrenaline has similar affinities for alpha(1)- and alpha(2)-adrenoceptors. In aortae with endothelium, L-NNA caused a small depolarization but did not affect the hyperpolarization induced by UK 14,304, indicating that NO is not involved in that response. Glibenclamide induced a small depolarization in aortae, with or without endothelium, indicating that ATP-sensitive K(+) channels may play a role in maintaining the smooth muscle's membrane potential. Our results indicate that, in rat aorta, alpha(2)-adrenoceptors are also present in the smooth muscle, and that these receptors act through small-conductance ATP-sensitive K(+) channels.

Actions of L-NAME and methylene blue on the hypotensive effects of clonidine and rilmenidine in the anesthetized rat

The antihypertensive mechanism of alpha2-adrenoceptor agonists, such as clonidine and rilmenidine, is not completely elucidated, although it is probably due to reduction of sympathetic tone mediated by stimulation of central alpha2-adrenoceptors. Because activation of alpha2-adrenoceptors on endothelial cells induces release of endothelium-derived relaxing factor (EDRF), we determined whether nitric oxide (NO) release is involved in the antihypertensive action of clonidine and rilmenidine. In chloralose-anesthetised Wistar rats, systolic and diastolic arterial blood pressures were recorded on a polygraph. Intravenous injection of clonidine or rilmenidine (control group) caused a rapid increase of arterial blood pressure. followed by a long-lasting hypotensive effect. The hypotensive effects, estimated as the area enclosed by the decrease in diastolic pressure during the 20 min after clonidine and rilmenidine injections, were 574+/-60 and 410+/-59 mm Hg/min, respectively. The delta decrease in diastolic arterial blood pressure observed 20 min after intravenous injections of clonidine and rilmenidine was 48+/-5 and 34+/-3 mm Hg, respectively. Clonidine and rilmenidine injected 5-10 min after intravenous pretreatment with L-NAME (2 and 1 mg/kg) or methylene blue (10 mg/kg) induced hypotensive effects that were significantly smaller than that observed for the control group. These results suggest that the antihypertensive effects of clonidine and rilmenidine also may be modulated by the NO-cyclic guanosine monophosphate (cGMP) pathway at the level of the central nervous system and/or at the vascular peripheral circulation.