The model of SAH and vasospasm used most frequently is the canine "two-hemorrhage" model, in which two injections of blood into the dog's basal cistern performed 48 hours apart result in greater arterial vasoconstriction than that effected by a single injection of blood. On the basis of its ability to accurately predict what occurs in human SAH, the best model of vasospasm seems to be the primate model in which a blood clot is surgically placed around the large cerebral vessels at the base of the monkey's brain.

Abstract-Although there is evidence that the Rho/Rho kinase pathway and protein kinase C (PKC) are involved in the development of cerebral vasospasm, the mechanism by which subarachnoid hemorrhage (SAH) activates these pathways is unclear. A large body of evidence points to oxyhemoglobin (OxyHb) as a major causative component of blood clot responsible for vasospasm. Therefore, the present studies were conducted to explore whether the Rho/Rho kinase and PKC may be involved in a sustained vasoconstriction induced by OxyHb in cerebral arteries.

1 Although statins have been reported to inhibit the prepro-endothelin-1 (ET-1) gene transcription in endothelial cells, their effects on the vascular function of ET-1 have not been explored. We, therefore, examined the effects of statins on contraction and DNA synthesis mediated by ET-1 in vascular smooth muscle. The effects of statins on contraction induced by ET-1 were compared to those mediated by noradrenaline (NA) and KCl. 2 Simvastatin (SV) induced a concentration-dependent relaxation of tonic contraction mediated by ET-1 (10 nM) (IC 50 value of 1.3 mM). The relaxation was also observed in rings precontracted with NA (0.1 mM) and KCl (60 mM). In contrast, pravastatin did not have any effect on the contractions. 3 Endothelial denudation or pretreatment with L-NAME did not prevent the relaxation, but did reduce the relaxant activity of SV. 4 SV prevented Rho activation caused by ET-1 and KCl in aortic homogenates, as assessed by a Rho pulldown assay. 5 The Rho kinase inhibitor HA-1077 mimicked the effects of SV on tonic contractions induced by ET-1, NA and KCl. 6 Pretreatment with the Kv channels inhibitor, 4-aminopyridine, attenuated the ability of SV to relax contractions mediated by ET-1 and NA.

An investigation was undertaken of the mechanism by which oxyhemoglobin and its analog methemoglobin might cause cerebrovascular spasm. The effect of these compounds on the levels of intracellular inositol triphosphate and calcium in cultured primate cerebrovascular smooth-muscle cells and the contractile action of oxyhemoglobin on isolated rings of primate cerebral arteries were also examined. Oxyhemoglobin, but not methemoglobin, produces a transient but highly significant increase in the intracellular levels of inositol triphosphate. Intracellular calcium levels in these cells are increased by thrombin, aluminum tetrafluoride, and oxyhemoglobin, and the sustained elevation in intracellular calcium is prevented by ethyleneglycol tetra-acetic acid and the phospholipase C inhibitor neomycin. Removal of the oxyhemoglobin after as long as 48 hours' incubation with this compound allowed cells to rapidly reduce their intracellular calcium levels to near normal. Oxyhemoglobin produced contractions of isolated rings of both normal and spastic cerebral arteries, although the response of spastic vessels was significantly smaller. This effect was inhibited by neomycin. The addition of neomycin relaxed arteries that were contracted with oxyhemoglobin, 5-hydroxytryptamine, or potassium chloride. It is thus likely that activation of phospholipase C is a critical step in the development of vasospasm, but the transient nature of the response to inositol triphosphate suggests that the sustained contraction may arise from other phospholipase C-dependent mechanisms.

Endothelin-1 (ET-1) and oxyhemoglobin (OxyHb) have been implicated in the pathogenesis of cerebral vasospasm after subarachnoid hemorrhage. However, the contribution of ET-1 to this condition has not been definitely established. In this study, we investigated whether threshold concentration of ET-1 enhances cerebrovascular smooth muscle (CVSM) contraction to OxyHb by activating the RhoA/Rho kinase and protein kinase C (PKC) pathways. CVSM contraction was measured in endothelium-denuded rabbit basilar arteries. Cytosolic and particulate fractions of CVSM cells were examined for RhoA and PKC reactivity with specific antibodies using immunoblotting procedures. ET-1 (0.1 nM) alone did not produce any significant contraction, but it markedly potentiated the magnitude (223% of control) and rate (149% of control) of contraction in response to OxyHb, which was attenuated by the inhibitors of Rho kinase Y-27632 and HA-1077. ET-1-mediated potentiation of the contraction was also inhibited by inhibitors of PKC, Ro-32-0432, and GF-109203X. BQ-123 prevented potentiation of vasoconstriction mediated by ET-1, indicating that the action of ET-1 was mediated by the endothelin type A receptor. Pretreatment with ET-1 significantly enhanced OxyHb-mediated RhoA translocation in CVSM cells and intact basilar arteries. ET-1 also caused potentiation of PKC-epsilon expression in membranes of CVSM cells exposed to OxyHb for 10 and 60 min but did not markedly change the distribution of PKC-alpha. Thus, in CVSM, threshold concentration of ET-1 potentiates contraction induced by OxyHb via RhoA/Rho kinase- and PKC-epsilon-dependent mechanisms. This process may contribute to the pathological contraction of cerebral arteries observed after subarachnoid hemorrhage.