Cyclic Nucleotides and Blood Vessel Contraction

Neointima formation impairs endothelial muscarinic receptors while enhancing prostacyclin-mediated responses in the rabbit carotid artery

The purpose of this study was to determine whether the generation of a neointima, an early step in the development of atherosclerosis, affects endothelium-dependent or -independent vasodilation. The neointima was induced, within 7 days, by positioning a nonocclusive silicone collar around one carotid artery in rabbits. After 1, 2, 7, or 14 days segments were cut from the collar-surrounded region of this artery as well as from the sham-operated contralateral artery and were used for isometric tension recording or for bioassay of nitric oxide (NO). The acetylcholine-induced release of NO was significantly reduced at 7 days. The tension recordings suggested that this already occurred at the earliest stages of neointima formation. Neither the capacity of the endothelial cells to form NO in response to the calcium ionophore A23187 nor the capacity of the underlying smooth muscle cells to relax in response to sources of exogenous NO (3-morpholinosydnonimine and nitroglycerin) was affected by the neointima. Therefore, the impaired endothelium-dependent relaxations to acetylcholine are presumably due to a defect at the level of the endothelial muscarinic receptors. The presence of a fully developed neointima did not alter the responsiveness to isoproterenol and forskolin but enhanced prostacyclin-mediated responses (assessed by iloprost and 13-hydroxyoctadecadienoic acid). These results illustrate selective alterations of endothelial and smooth muscle cell function in intima generation before fatty streak formation.

Effects of a selective inhibitor of cyclic AMP phosphodiesterase on the pial microcirculation in feline cerebral ischemia

We evaluated the effects of cilostazol, a selective inhibitor of cyclic adenosine monophosphate phosphodiesterase, on the pial vessels of adult cats subjected to endothelial damage followed by middle cerebral artery occlusion. Six cats were treated with cilostazol and four with 30% N,N-dimethylformamide in 70% saline (solvent). The brain surface was irradiated with ultraviolet rays through a cranial window for 3 minutes to selectively damage the endothelium of the pial vessels in both groups. Beginning 32 minutes after termination of the irradiation, the middle cerebral artery was occluded for 30 minutes. Thirty minutes before occlusion, intravenous infusion of 30 micrograms/kg/min cilostazol or 0.1 ml/kg/min solvent was begun and continued until the end of the study. Before occlusion, the infusion of cilostazol induced a significant (p less than 0.05) dilatation while the infusion of solvent produced no significant changes in the diameter of the pial arteries. The pial veins of solvent-treated cats showed significant (p less than 0.05) constriction during occlusion, whereas cilostazol-treated cats exhibited only mild constriction of the pial veins. The formation of platelet thrombi after occlusion was significantly (p less than 0.05) inhibited in the pial veins of cilostazol-treated compared with solvent-treated cats. Similarly, the microcirculation of the pial veins was effectively restored after reopening of the middle cerebral artery in cilostazol-treated compared with solvent-treated cats. Our data suggest that cilostazol is an effective antithrombotic agent as well as a potent vasodilator acting on vascular smooth muscle.

Effect of histamine on the energy metabolism of K+-depolarized hog carotid artery

In a previous study (Peterson and Glück, 1982), activation of hog carotid artery with high-K+ medium caused no increase in aerobic glycolysis (lactic acid production). In contrast, histamine alone is found to stimulate aerobic glycolysis in a tension-dependent fashion (GLück and Paul, 1977). In this study, the addition of histamine to the already K+-depolarized hog carotid artery induced additional contraction, elevated tissue cyclic adenosine 3',5'-monophosphate [cAMP] levels, coupled the rate of aerobic glycolysis to external [Ca++] and isometric tension, and caused suprabasal energy utilization to exceed that required to support the contraction alone by approximately 25%. Increased cytoplasmic free-[Ca++] per se does not stimulate aerobic glycolysis. In the presence of histamine, however, aerobic glycolysis is Ca++-sensitive. Whereas a cycle of Ca++ depletion and restoration has only a small effect on basal metabolic rates in arteries activated with high-K+ alone, the same procedure with arteries stimulated by high-K+ with added histamine causes a dramatic shift in basal metabolic rates toward higher levels of glycolysis.

Aortic responses to vanadate: independence from (Na,K)-ATPase and comparison of Dahl salt-sensitive and salt-resistant rats

Vanadate at doses from 10(-4.5) to 10(-3) M caused a dose-dependent contraction of the rat aorta in vitro. Aortas of Dahl salt-hypertension sensitive (S) rats responded to vanadate with a greater contraction than Dahl salt-hypertension resistant (R) rats. In contrast, S and R aortic responses to depolarization with potassium were equal, and responses to norepinephrine were less in S than R. The mechanism by which vanadate causes the aortic response was studied in S rats. In aortic smooth muscle sodium-loaded by exposure to low potassium media followed by a norepinephrine-induced contraction, a relaxation induced with 5 mM potassium was not influenced by 10(-3) M vanadate. Since this potassium-induced relaxation is known to be a reflection of (NaK)-ATPase activity, these data show that vanadate (up to 10(-3) M does not inhibit (Na,K)-ATPase in intact smooth muscle cells although it is a known potent inhibitor of (Na,K)-ATPase in isolated cell membrane preparations. Response to vanadate was not changed by alpha-blockade with phentolamine or by blocking (Na,K)-ATPase with ouabain. Vanadate contraction was blocked by 4,4'-diisothiocyano-2,2'-disulfonic acid stilbene, a known inhibitor of anion transport, suggesting that vanadate anions must enter smooth muscle cells to induce contraction.