Vasoconstrictive responses elicited by endothelin in bovine cerebral arteries

Increased plasma endothelin-1 levels in patients with intracerebral hemorrhage

Endothelins (ETs) are discovered peptides that are widely distributed in neurons and nonneuronal cells of the human nervous system. Previous studies showed that ischemic stroke may be associated with increased plasma ET-1 levels. There are no studies to show plasma ET-1 levels in intracerebral hemorrhage. Plasma ET-1 levels in 30 patients with cerebral hemorrhage within 72 hours after the onset of focal neurological deficit were measured by a microplate enzyme immunoassay. Thirty sex- and age-matched healthy subjects were accepted as a control group. The clinical neurological status in the patients was evaluated according to the modified Matthew Scale. The mean plasma ET-1 level in hemorrhagic stroke patients was significantly higher than in control subjects (2.39±2.08 v 0.65±0.32 fmol/mL, (P < .05). There was a significant difference in ET levels between patients who died in the hospital and patients who survived (P < .05). The mean ET-1 concentration in patients with severe neurologic deficit was significantly higher than in patients with mild neurologic deficit (P < .05). There was a correlation between hematoma volumes and plasma ET-1 levels in the patients (r = 0.66, Pt < .001). The mean plasma ET-1 concentration was found to be significantly higher in patients with intraventricular hemorrhage than in patients without intraventricular hemorrhage (P < 0.05). There were no significant differences in ET-1 levels between supratentorial and infratentorial subgroups or among supratentorial subgroups (P > .05). It was concluded that plasma ET-1 levels were increased in the acute period of hemorrhagic stroke. Plasma ET-1 levels may be associated with hematoma volume, which is related to a poor prognosis of the cerebral hematoma. We suggest that increased plasma ET-1 levels may be a consequence of local cerebral hemorrhage or the acute stress condition of the disease.

Endothelin-1 in plasma, cisternal CSF and microdialysate following aneurysmal SAH

OBJECTIVE

Endothelin-1 (ET-1) is postulated to play an important role in the development of cerebral vasospasm (CVS) following SAH. This study was conducted to investigate the time course of ET-release in three different sources: CSF, plasma and microdialysate.

METHODS

In a prospective study ET-1-concentrations were measured in plasma, cisternal CSF and microdialysate in 20 patients with aneurysmal SAH for at least 8 days after hemorrhage.

RESULTS

ET-1 concentration in microdialysate was almost four times higher compared to CSF and plasma. (p<0.001) Only in CSF ET-1-release showed a significant increase over time with highest values on day 5 post ictus (p = 0.03). This was parallel to the increase of transcranial Doppler velocities. ET-1 in plasma and microdialysate did not change over time.

CONCLUSION

ET-1 may have a different biological function in different biological tissues. Only ET-1 in CSF seemed to be associated with CVS.

New therapeutics that antagonize endothelin: promises and frustrations

The discovery of endothelin--a highly potent endogenous vasoconstrictor - in 1988 has led to considerable efforts to develop antagonists of endothelin receptors that could have therapeutic potential in disorders including hypertension, heart failure and renal diseases. However, in general, the results of trials in humans have not mirrored the highly promising effects in animal disease models. Here, we discuss preclinical and clinical results with endothelin antagonists, and consider possible approaches to fully realizing the potential of endothelin antagonism.

Reduced verapamil inhibition of endothelin-1-constricted rabbit basilar artery due to enhanced non L-type Ca(2+)-channel-dependent constriction

This study tested whether (1) L-type Ca(2+) channel blockade and extracellular Ca(2+) removal prior to endothelin-1, as compared to during the endothelin-1 constriction, resulted in lesser inhibition, and (2) the reduced inhibition due to prior L-type Ca(2+) channel blockade resulted from enhanced non L-type Ca(2+)-channel-dependent constriction. Pretreatment of rabbit basilar artery in vitro with 1 microM verapamil, an L-type Ca(2+) channel blocker, inhibited 3, 10, 30, and 100 nM endothelin-1 constrictions to a lesser extent than verapamil addition during the plateau endothelin-1 constriction. Ni(2+) (0.03 and 0.1 mM), a nonselective cation channel blocker, relaxed the plateau endothelin-1 constrictions in vessels pretreated with verapamil to greater magnitudes than vessels unexposed to verapamil. Extracellular Ca(2+) removal prior to 10, 30, and 100 nM endothelin-1 also inhibited the endothelin-1 constrictions to smaller magnitudes than Ca(2+) removal during the plateau endothelin-1 constrictions. These results suggest that the reduced inhibition of the endothelin-1 constriction following pretreatment with L-type Ca(2+) channel blocker or Ca(2+)-free solution, as compared to addition of these agents during the endothelin-1 constriction, is the result of non L-type Ca(2+) channel opening and enhanced Ca(2+)-independent constriction, respectively.

Modulation by the endothelium of the inhibitory effects of pinacidil and nimodipine on endothelin-induced contraction in cerebral arteries

The effects of pinacidil and nimodipine on endothelin-1-induced contractions in isolated cerebral arteries with and without endothelium were compared. The sensitivity to endothelin-1 was increased (0.5 log units) in the rabbit basilar artery after removal of the endothelium. The nitric oxide synthase inhibitor N omega-nitro-L-arginine (0.1 mM) also increased the sensitivity to endothelin-1 (0.6 log units) in basilar arteries with endothelium, whereas N omega-nitro-D-arginine (0.1 mM) and indomethacin (3 microM) had no effect, indicating that withdrawal of endothelium-derived nitric oxide may account for the enhancement of the endothelin-1-induced contraction after endothelial denudation. Pinacidil (1 microM) shifted the concentration-response curve for endothelin-1 to the right without affecting the maximal response in arteries without endothelium, but had no effect on the endothelin-1-induced contraction in vessels with endothelium. Nimodipine (1 microM) reduced the maximal endothelin-1-induced contraction by approximately 50% in both the presence and absence of endothelium, whereas the sensitivity to endothelin-1 was reduced only in vessels without endothelium. Incubation in "calcium-free" medium reduced the maximal endothelin-1-induced contraction by 69% and 80% in vessels with and without endothelium, respectively. In human pial arteries with endothelium, pinacidil did not affect the endothelin-1-induced contraction, whereas nimodipine and exposure to "calcium-free" solution reduced the maximal response by 31% and 74% respectively. The results show that, in the rabbit, pinacidil and to a lesser extent nimodipine preferentially act on cerebral arteries with disrupted endothelium, indicating that vasoactive factors liberated from the endothelium may modify the effect of a vasodilator.

Potentiation by endothelin-1 of Ca2+ sensitivity of contractile elements depends on Ca2+ influx through L-type Ca2+ channels in the canine cerebral artery

1. Endothelin-1 (ET-1) contracted canine cerebral artery in a concentration-dependent manner with an increase in intracellular Ca2+ concentration ([Ca2+]i), and at higher concentrations it produced a greater contraction with a smaller increase in [Ca2+]i. 2. Ca2+ channel antagonist such as d-cis-diltiazem inhibited the tension more effectively than the [Ca2+]i increased by ET-1. 3. In Ca(2+)-free solution containing 0.2 mM EGTA, ET-1 elicited a transient increase in [Ca2+]i and tension. 4. In the Staphylococcus aureus alpha-toxin-permeabilized artery, ET-1 shifted the pCa-tension relationship leftwards in the presence of GTP. 5. These findings suggest that ET-1 contracts the canine cerebral artery by increasing not only the Ca2+ influx through L-type Ca2+ channels, but also Ca2+ release from the intracellular storage sites, and also Ca2+ sensitivity of contractile elements. The degree of Ca2+ sensitivity is strongly affected by [Ca2+]i which is increased by the Ca2+ influx through L-type Ca2+ channels.

Endothelin-induced contraction and relaxation of rat isolated basilar artery: effect of BQ-123

In ring segments from rat basilar artery (BA) the endothelin (ET) peptides ET-1, ET-2, and ET-3 induced concentration-related contractions. The order of potency was ET-1 = ET-2 > ET-3, while no differences occurred in the maximum contraction. The selective ETA receptor antagonist, BQ-123 (10(-10)-10(-4) M) alone elicited a small contraction only at 10(-4) M. In the presence of BQ-123 (10(-7)-10(-5) M), the concentration-response curve for ET-1 was shifted to the right without any decrease in maximum contraction, indicating competitive inhibition of ET-1 binding to the ETA receptor by BQ-123. The pA2 value calculated for BQ-123 was 6.935; the slope of the regression curve was 0.734. In contrast to ET-1, the contractile action of ET-3 was abolished by 10(-5) M BQ-123. In segments precontracted with 10(-6) M serotonin, ET-3, but not ET-1, induced relaxation at low concentrations (10(-11)-10(-8) M), with maximum relaxation amounting to 17.8 +/- 14.7% of precontraction (mean +/- SD; n = 16). The relaxant action of ET-3 was abolished in vessels incubated with NG-nitro-L-arginine (10(-5) M), an inhibitor of nitric oxide synthase. These results indicate that the ET-induced contraction of the isolated rat BA involves activation of the ETA receptor. The ET-3-induced relaxation of precontracted rat BA is apparently mediated by release of nitric oxide from the endothelium.

Intracellular Ca2+ release in cerebral arteries

Vasoconstricting agonists elevate the intracellular Ca2+ concentration and induce tension development in vascular smooth muscle cells by inducing both Ca2+ influx from the extracellular space and Ca2+ release from cellular stores. The relative importance of Ca2+ release has been found to vary between different sites in the vasculature. This review examines the role of Ca2+ release in the activation of cerebral arteries produced by several vasoconstricting stimuli. Although the activation of cerebral arteries by agonists such as 5-hydroxytryptamine and noradrenaline has typically been found to have little dependence on Ca2+ release, other vasoconstrictors such as thromboxane A2, which may be released from the endothelium by other agonists, appear to induce a substantial intracellular Ca2+ release in cerebral arteries. The limited efficacy of Ca2+ influx blockers in the treatment of delayed cerebrovascular constriction occurring as a result of subarachnoid haemorrhage suggests that intracellular mechanisms such as Ca2+ release and/or the activation of protein kinase C may be important determinants of vasoconstriction under pathological conditions.

Cerebral vasoconstrictor mediators

Endogenous cerebral vasoconstrictor mediators regulate vascular resistance and blood flow in the brain as a whole and in various regions and participate in the pathogenesis of cerebral circulatory disturbances. Vasoconstrictors are effective in the treatment of diseases associated with cerebral vasodilatation. There are variations in the response of cerebral arteries from primate and subprimate mammals; therefore, information as to similarities and differences in their response is quite important in evaluating the physiological role, involvement in pathogenesis and therapeutic usefulness of the mediators in healthy men and patients. In this review we described characteristics of the action of vasoconstrictors (amines, peptides, prostanoids, and others) on isolated cerebral arteries from mammals, including humans and monkeys.

Regulation of the cerebral circulation by endothelium

Endothelium exerts an important influence on cerebral vascular tone through the production and release of a diverse group of vasoactive factors. Relaxing factors produced by endothelium include nitric oxide (or a nitric oxide-containing compound), a hyperpolarizing factor, and prostacyclin. Endothelium-derived contracting factors include cyclooxygenase products of arachidonic acid and endothelins. Several pathophysiological conditions are associated with increased formation of endothelium-derived contracting factors. Such endothelial dysfunction in the cerebral circulation may shift the balance of vascular tone toward constriction and may potentially contribute to the onset or maintainance of cerebral ischemia and stroke.