Date of Award

2010

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Pharmacology

First Advisor

Wellman, George

Abstract

Subarachnoid hemorrhage (SAH) following cerebral aneurysm rupture is associated with substantial morbidity and mortality. The ability of SAH to induce vasospasm in large diameter pial arteries has been extensively studied, although the contribution of this phenomenon to patient outcome is unclear. Conversely, little is known regarding the impact of SAH on intracerebral (parenchymal) arterioles, which are critical for regulation of cerebral blood flow. To assess the function of parenchymal arterioles following SAH, measurements of diameter, intracellular Ca2+ ([Ca2+]i) and membrane potential were performed in intact arterioles from unoperated (control), sham-operated and SAH model rats. At physiological intravascular pressure, parenchymal arterioles from SAH animals exhibited significantly elevated [Ca2+]i and enhanced constriction compared with arterioles from control and sham-operated animals. Elevated [Ca2+]i and enhanced tone following SAH were observed in the absence of vascular endothelium and were abolished by the L-type voltage-dependent Ca2+ channel (VDCC) inhibitor nimodipine. Molecular assessment of the L-type VDCC CaV1.2 indicated unchanged mRNA and protein expression in arterioles from SAH animals. Increased CaV1.2 activity following SAH may also reflect enhanced pressure-induced membrane potential depolarization of arteriolar smooth muscle. Membrane potential measurements in arteriolar myocytes using intracellular microelectrodes revealed approximately 7 mV depolarization at 40 mmHg in myocytes from SAH animals. Further, when membrane potential was adjusted to similar values, arteriolar [Ca2+]i and tone were similar between groups. These results demonstrate that greater pressure-dependent membrane potential depolarization results in increased activity of CaV1.2 channels, elevated [Ca2+]i and enhanced constriction of parenchymal arterioles from SAH animals. Thus, impaired regulation of parenchymal arteriolar [Ca2+]i and diameter may restrict cerebral blood flow in SAH patients. Although nimodipine is used clinically to prevent delayed neurological deficits in SAH patients, the use of this drug has been limited by hypotension and treatment options remain inadequate. Therefore, our next objective was to explore strategies to selectively suppress CaV1.2 channels in the cerebral vasculature. To do so, we examined the physiological role of smooth muscle CaV1.2 splice variants containing the alternatively-spliced exon 9* in cerebral artery constriction. Using antisense oligonucleotides, we demonstrate that suppression of exon 9*-containing CaV1.2 splice variants results in substantially reduced cerebral artery constriction to elevated extracellular [K+]. In addition, no further reduction in constriction was observed following suppression of all Cav1.2 splice variants, suggesting that exon 9* splice variants are functionally dominant in cerebral artery constriction. In summary, results shown in this dissertation demonstrate that increased CaV1.2 activity following SAH results in enhanced constriction of parenchymal arterioles. Furthermore, evidence is provided supporting the concept that CaV1.2 splice variants with exon 9* are critical for cerebral artery constriction and may provide a novel target for the prevention of delayed ischemic deficits in SAH patients.

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