Signal transduction pathways in constriction of the basilar artery in vivo

Prostaglandins in migraine: update

PURPOSE OF REVIEW

This review presents recent findings on the role of prostaglandins in migraine pathophysiology.

RECENT FINDINGS

Experimental studies have shown that prostaglandins are distributed in the trigeminal-vascular system and its receptors are localized in the trigeminal ganglion and the trigeminal nucleus caudalis. Prostaglandins were found in smooth muscles of cranial arteries, and functional studies in vivo showed that prostaglandins induced dilatation of cranial vessels. Human studies showed that intravenous infusion of vasodilating prostaglandins such as prostaglandin E₂ (PGE₂), prostaglandin I₂ (PGI₂) and prostaglandin D₂ (PGD₂) induced headache and dilatation of intra-cranial and extra-cranial arteries in healthy volunteers. In contrast, infusion of non-dilating prostaglandin F₂α (PGF₂α) caused no headache or any vascular responses in cranial arteries. PGE₂ and PGI₂ triggered migraine-like attacks in migraine patients without aura, accompanied by dilatation of the intra-cerebral and extra-cerebral arteries. A novel EP4 receptor antagonist could not prevent PGE₂-induced headache in healthy volunteers.

SUMMARY

Recent in-vitro/in-vivo data demonstrated presence and action of prostaglandins within the trigeminal pain pathways. Migraine induction after intravenous administration of PGE₂ and PGI₂ suggests a specific blockade of their receptors, EP and IP respectively, as a new potential drug target for the acute treatment of migraine.

Pro-inflammatory and vasoconstricting prostanoid PGF2α causes no headache in man

Background: During two decades of migraine provocation studies with naturally occurring signalling molecules, vasodilators such as prostaglandin E2, prostaglandin I2 (prostacyclin) and prostaglandin D2 were shown to be able to induce headache in man. To elucidate the role of inflammation and vasodilatation in the generation of headache, we investigated whether the pro-inflammatory and vasoconstricting prostanoid prostaglandin F2α (PGF2α) would cause headache in a human model of headache.

Methods: Twelve healthy volunteers were randomly allocated to receive 3.5 µg/kg/min PGF2α or placebo over 20 min in a two-way crossover study. We recorded headache intensity on a verbal rating scale, middle cerebral artery blood flow velocity (VMCA) and the diameters of the superficial temporal artery (STA) and radial artery (RA).

Results: We found no difference in the area under the curve (AUC) for immediate headache (0–90 min) between PGF2α and placebo ( p = 0.144). The McNemar's test showed no difference in the incidence of immediate and delayed headache between verum and placebo ( p = 0.500 and p = 1.000, respectively). There was no difference in VMCA ( p = 0.776) and in the diameter of the STA ( p = 0.460) or RA ( p = 0.780) between PGF2α and placebo.

Conclusion: The present study shows that PGF2α, unlike vasodilating prostaglandins, does not provoke headache. We suggest that the vasodilating abilities of prostaglandins are important for the induction of experimental headache in healthy volunteers.

Importance of PKC and PI3Ks in ethanol-induced contraction of cerebral arterial smooth muscle

We investigated the relationships of two potential intracellular signaling pathways, protein kinase C (PKC) and phosphatidylinositol 3-kinases (PI3Ks), to ethanol-induced contractions in cerebral arteries. Ethanol (20-200 mM) induces concentration-dependent constriction in isolated canine basilar arteries that is inhibited in a concentration-dependent manner by pretreatment of these vessels with 10(-9)-10(-3) M Gö-6976 (an antagonist selective for PKC-alpha and PKC-betaI), 10(-10)-10(-4) M bisindolylmaleimide I (a specific antagonist of PKC), and 10(-10)-10(-4) M wortmannin or 10(-8)-10(-2) M LY-294002 (selective antagonists of PI3Ks). Ethanol-induced increases in intracellular Ca(2+) concentration (from approximately 100 to approximately 500 nM) in canine basilar smooth muscle cells are also suppressed markedly (approximately 20-70%) in the presence of a similar concentration range of Gö-6976, bisindolymaleimide I, wortmannin, or LY-294002. This study suggests that activation of PKC isoforms and PI3Ks appears to be an important signaling pathway in ethanol-induced vasoconstriction of cerebral blood vessels.

Evidence that Rho-kinase activity contributes to cerebral vascular tone in vivo and is enhanced during chronic hypertension: comparison with protein kinase C

The small G protein Rho and its target Rho-kinase may participate in the mechanisms underlying vascular contractile tone via inhibition of myosin light chain phosphatase. The present study has tested the hypothesis that Rho-kinase activity normally contributes to cerebral vascular tone in vivo, and that this effect is augmented during chronic hypertension. Comparative studies also examined the role of protein kinase C (PKC) in regulation of cerebral artery tone. Two Rho-kinase inhibitors, Y-27632 (0.1 to 100 micromol/L) and HA1077 (1 to 10 micromol/L), caused marked concentration-dependent increases in basilar artery diameter of anesthetized normotensive rats (Sprague-Dawley and Wistar-Kyoto [WKY] strains), as measured using a cranial window approach. By comparison, the selective PKC inhibitors calphostin C (0.01 to 0.5 micromol/L) and Ro 31-8220 (5 micromol/L) had little or no effect on basilar artery diameter. Vasodilator responses to Y-27632 were unaffected by PKC inhibition or activation. In two models of chronic hypertension (spontaneously hypertensive rats and WKY rats treated with N-nitro-L-arginine methyl ester for 4 weeks), Y-27632 elicited cerebral vasodilator responses that were significantly greater than in control WKY rats (P<0.05), indicating that the chronically hypertensive state and not genetic factors contributed to the increased responses to Rho-kinase inhibition. PKC inhibition had no significant effect on basilar artery diameter in chronically hypertensive rats. These data suggest that Rho-kinase, but not PKC, activity contributes substantially to cerebral artery tone in vivo, and this effect is augmented in the cerebral circulation during chronic hypertension.

Increased Ca2+ sensitivity as a key mechanism of PKC-induced constriction in pressurized cerebral arteries

The effects of activating protein kinase C (PKC) with indolactam V (Indo-V) and 1,2-dioctanoyl-sn-glycerol (DOG) on smooth muscle intracellular Ca2+ concentrations ([Ca2+]i) and arterial diameter were determined using ratiometric Ca2+ imaging and video edge detection of pressurized rat posterior cerebral arteries. Elevation of intraluminal pressure from 10 to 60 mmHg resulted in an increase in [Ca2+]i from 74 +/- 5 to 219 +/- 8 nM and myogenic constriction. Application of Indo-V (0.01-3 microM) or DOG (0.1-30 microM) induced constriction and decreased [Ca2+]i to 140 +/- 11 and 127 +/- 12 nM, respectively, at the highest concentrations used. In the presence of Indo-V, the dihydropyridine Ca2+-channel-blocker nisoldipine produced nearly maximum dilation and decreased [Ca2+]i to 97 +/- 7 nM. In alpha-toxin-permeabilized arteries, the constrictor effects of Indo-V and DOG were not observed in the absence of Ca2+. Both PKC activators significantly increased the degree of constriction of permeabilized arteries at different [Ca2+]i. We conclude that 1) Indo-V- or DOG-induced constriction of pressurized arteries requires Ca2+ influx through voltage-dependent Ca2+ channels, and 2) PKC-induced constriction of pressurized rat cerebral arteries is associated with a decrease in [Ca2+]i, suggesting an increase in the Ca2+ sensitivity of the contractile process.

Temperature and protein kinase C modulate myofilament Ca2+ sensitivity in pressurized rat cerebral arteries

The effects of pharmacological activation and inhibition of protein kinase C (PKC) and temperature on the relationship between cytoplasmic Ca2+ and lumen diameter were studied in pressurized (50 mmHg) rat posterior cerebral arteries permeabilized with alpha-toxin. Increasing Ca2+ concentrations (30 nM-10 microM, 22 degrees C) induced stable, concentration-dependent constrictions with a half-maximal effective concentration (EC50) of 112 nM. The maximal constriction was 80% of baseline diameter and 157% of that during depolarization of nonpermeabilized vessels with 124 mM KCl. Elevation of temperature to 37 degrees C increased the EC50 to 246 nM and enhanced the steepness of concentration-response curves. Exposure of permeabilized arteries to indolactam V, an activator of PKC, resulted in a significant myofilament Ca2+ sensitization (e.g., EC50 at 5 microM = 126 nM) without changing efficacy. The effects of calphostin C, a PKC inhibitor, on Ca2+ sensitivity were minimal; however, the amplitude of Ca2+-induced constrictions in both control and indolactam-treated arteries was suppressed in a concentration-dependent manner. Thus 1) temperature is an important variable in studies of arterial Ca2+ sensitivity, and 2) changes in PKC activity can significantly alter both myofilament sensitivity to and constrictor efficacy of cytosolic Ca2+.

Role of tyrosine kinase in serotonin-induced constriction of the basilar artery in vivo

BACKGROUND AND PURPOSE

Serotonin is one of the most potent constrictors of cerebral blood vessels and is implicated in several pathological conditions, including migraine and cerebral ischemia. Recent evidence has suggested that tyrosine kinase is involved in vasocontractile responses. The objective of this study was to test the hypothesis that activation of tyrosine kinase contributes to serotonin-induced constriction of the basilar artery in vivo.

METHODS

Using a cranial window in anesthetized Sprague-Dawley rats, we examined effects of inhibitors of tyrosine kinase and tyrosine phosphatase on constrictor responses of the basilar artery to serotonin in vivo.

RESULTS

Serotonin (10(-8), 10(-7), and 10(-6) mol/L) produced constriction of the basilar artery by 12+/-2%, 27+/-2%, and 37+/-3%, respectively. Genistein (3 x 10(-6) mol/L), an inhibitor of tyrosine kinase, did not affect baseline diameter of the basilar artery but attenuated serotonin-induced vasoconstriction (P<.05 versus control responses). Daidzein, an inactive analogue of genistein, did not affect serotonin-induced constriction of the basilar artery. Tyrphostin 47 (10(-5) mol/L), another inhibitor of tyrosine kinase, also attenuated serotonin-induced vasoconstriction, and tyrphostin 63, an inactive analogue of tyrphostin 47, did not affect the vasoconstriction. Sodium orthovanadate (10(-5) mol/L), an inhibitor of tyrosine phosphatase, enhanced serotonin-induced vasoconstriction. Phorbol 12,13-dibutyrate, a direct activator of protein kinase C, also caused constriction of the basilar artery, which was not affected by genistein or sodium orthovanadate.

CONCLUSIONS

These results suggest that serotonin-induced constriction of the basilar artery is mediated, at least in part, by activation of tyrosine kinase in vivo.

20-Hydroxyeicosatetraenoic acid-induced vasoconstriction and inhibition of potassium current in cerebral vascular smooth muscle is dependent on activation of protein kinase C

20-Hydroxyeicosatetraenoic acid (20-HETE), a cytochrome P450 metabolite of arachidonic acid, is a potent vasoconstrictor, and has been implicated in the myogenic activation of renal and cerebral arteries. We examined the role of protein kinase C (PKC) in the signal transduction pathway by which 20-HETE induces vasoconstriction and inhibition of whole-cell K+ current in cat cerebral vascular smooth muscle. 20-HETE induced a concentration-dependent constriction in isolated pressurized cat middle cerebral arteries (-29 +/- 8% at 1 microM). However, in the presence of an N-myristoylated PKC pseudosubstrate inhibitor peptide (MyrPsiPKC-I(19-27)), 20-HETE induced a concentration-dependent vasodilation (26 +/- 4% at 1 microM). In whole-cell voltage clamp studies, application of 20-HETE inhibited whole-cell K+ current recorded in cat cerebral vascular smooth muscle cells, an effect that was attenuated by MyrPsiPKC-I(19-27). Further evidence for the role of PKC activation in response to 20-HETE is the finding that 20-HETE increased the phosphorylation of myristoylated, alanine-rich PKC substrate in cultured cat cerebral vascular smooth muscle cells in a concentration- and PKC-dependent manner. These data provide evidence that PKC is an integral part of the signal transduction pathway by which 20-HETE elicits vasoconstriction of cerebral arteries and inhibition of whole-cell K+ current in cat cerebral vascular smooth muscle.

Mechanisms of vasodilation of cerebral vessels induced by the potassium channel opener nicorandil in canine in vivo experiments

BACKGROUND AND PURPOSE

Nicorandil, a potent antianginal agent characterized as a potassium channel opener, could produce cerebrovascular dilation in in vitro studies. Our aim was to investigate the pharmacologic response to the topical application of nicorandil on the vasomotor tone of pial vessels in vivo. To elucidate its mechanism, we also studied the inhibitory action of methylene blue and glibenclamide against nicorandil-induced vasodilation.

METHODS

In 14 dogs prepared with a parietal cranial window, we administered five different concentrations of nicorandil solution (10(-7), 10(-6), 10(-5), 10(-4), and 10(-3) mol/L) under the window and measured pial arterial and venular diameters. After pretreating pial vessels with either 10(-5) mol/L methylene blue or 10(-5) mol/L glibenclamide, we examined inhibitory action after the application of 10(-5) mol/L nicorandil. In additional experiments with 9 dogs, we evaluated the effects of nitroglycerin and cromakalim on pial vessels in the absence or presence of 10(-5) mol/L methylene blue and 10(-5) mol/L glibenclamide, respectively.

RESULTS

Nicorandil produced significant, concentration-dependent dilation of pial vessels (P < .05). Methylene blue blocked nicorandil-induced dilation, whereas glibenclamide only attenuated such action of nicorandil. Nitroglycerin and cromakalim also produced a concentration-dependent increase in pial arteriolar and venular diameters (P < .05), and those effects were blocked in the presence of methylene blue or glibenclamide, respectively.

CONCLUSIONS

Our in vivo study demonstrates that topical application of nicorandil dilates both pial arterioles and venules in a concentration-dependent manner and suggests that the mechanisms of such actions are most likely due to both cyclic GMP-mediated vascular smooth muscle dilation and the regulation of K+ flux.