Zhen Du Zhang

Novel mechanism of endothelin-1-induced vasospasm after subarachnoid hemorrhage

Cerebral vasospasm is a major cause of morbidity and mortality after aneurysmal subarachnoid hemorrhage (SAH). It is a sustained constriction of the cerebral arteries that can be reduced by endothelin (ET) receptor antagonists. Voltage-gated Ca2+ channel antagonists such as nimodipine are relatively less effective. Endothelin-1 is not increased enough after SAH to directly cause the constriction, so we sought alternate mechanisms by which ET-1 might mediate vasospasm. Vasospasm was created in dogs, and the smooth muscle cells were studied molecularly, electro-physiologically, and by isometric tension. During vasospasm, ET-1, 10 nmol/L, induced a nonselective cation current carried by Ca2+ in 64% of cells compared with in only 7% of control cells. Nimodipine and 2-aminoethoxydiphenylborate (a specific antagonist of store-operated channels) had no effect, whereas SKF96365 (a nonspecific antagonist of nonselective cation channels) decreased this current in vasospastic smooth muscle cells. Transient receptor potential (TRP) proteins may mediate entry of Ca2+ through nonselective cationic pathways. We tested their role by incubating smooth muscle cells with anti-TRPC1 or TRPC4, both of which blocked ET-1-induced currents in SAH cells. Anti-TRPC5 had no effect. Anti-TRPC1 also inhibited ET-1 contraction of SAH arteries in vitro. Quantitative polymerase chain reaction and Western blotting of seven TRPC isoforms found increased expression of TRPC4 and a novel splice variant of TRPC1 and increased protein expression of TRPC4 and TRPC1. Taken together, the results support a novel mechanism whereby ET-1 significantly increases Ca2+ influx mediated by TRPC1 and TRPC4 or their heteromers in smooth muscle cells, which promotes development of vasospasm after SAH.

Voltage-dependent calcium channels of dog basilar artery

Electrophysiological and molecular characteristics of voltage‐dependent calcium (Ca2+) channels were studied using whole‐cell patch clamp, polymerase chain reaction and Western blotting in smooth muscle cells freshly isolated from dog basilar artery. Inward currents evoked by depolarizing steps from a holding potential of –50 or –90 mV in 10 mm barium consisted of low‐ (LVA) and high‐voltage activated (HVA) components. LVA current comprised more than half of total current in 24 (12%) of 203 cells and less than 10% of total current in 52 (26%) cells. The remaining cells (127 cells, 62%) had LVA currents between one tenth and one half of total current. LVA current was rapidly inactivating, slowly deactivating, inhibited by high doses of nimodipine and mibefradil (> 0.3 μm), not affected by ω‐agatoxin GVIA (γ100 nm), ω‐conotoxin IVA (1 μm) or SNX‐482 (200 nm) and probably carried by T‐type Ca2+ channels based on the presence of messenger ribonucleic acid (mRNA) and protein for Cav3.1 and Cav3.3α1 subunits of these channels. LVA currents exhibited window current with a maximum of 13% of the LVA current at –37.4 mV. HVA current was slowly inactivating and rapidly deactivating. It was inhibited by nimodipine (IC50= 0.018 μm), mibefradil (IC50= 0.39 μm) and ω‐conotoxin IV (1 μm). Smooth muscle cells also contained mRNA and protein for L‐ (Cav1.2 and Cav1.3), N‐ (Cav2.2) and T‐type (Cav3.1 and Cav3.3) α1 Ca2+ channel subunits. Confocal microscopy showed Cav1.2 and Cav1.3 (L‐type), Cav2.2 (N‐type) and Cav3.1 and Cav3.3 (T‐type) protein in smooth muscle cells. Relaxation of intact arteries under isometric tension in vitro to nimodipine (1 μm) and mibefradil (1 μm) but not to ω‐agatoxin GVIA (100 nm), ω‐conotoxin IVA (1 μm) or SNX‐482 (1 μm) confirmed the functional significance of L‐ and T‐type voltage‐dependent Ca2+ channel subtypes but not N‐type. These results show that dog basilar artery smooth muscle cells express functional voltage‐dependent Ca2+ channels of multiple types.

Voltage-gated K + channel dysfunction in myocytes from a dog model of subarachnoid hemorrhage

Delayed cerebral vasospasm after subarachnoid hemorrhage is primarily due to sustained contraction of arterial smooth muscle cells. Its pathogenesis remains unclear. The degree of arterial constriction is regulated by membrane potential that in turn is determined predominately by K+ conductance (GK). Here, we identified the main voltage-gated K+ (Kv) channels contributing to outward delayed rectifier currents in dog basilar artery smooth muscle as Kv2 class through a combination of electrophysiological and pharmacological methods. Kv2 current density was nearly halved in vasospastic myocytes after subarachnoid hemorrhage (SAH) in dogs, and Kv2.1 and Kv2.2 were downregulated in vasospastic myocytes when examined by quantitative mRNA, Western blotting, and immunohistochemistry. Vasospastic myocytes were depolarized and had a smaller contribution of GK toward maintenance of their membrane potential. Pharmacological block of Kv current in control myocytes mimicked the depolarization observed in vasospastic arteries. The degree of membrane depolarization was found to be compatible with the amount of vasoconstriction observed after SAH. We conclude that Kv2 dysfunction after SAH contributes to the pathogenesis of delayed cerebral vasospasm. This may confer a novel target for treatment of delayed cerebral vasospasm.

Expression and function of inwardly rectifying potassium channels after experimental subarachnoid hemorrhage

Cerebral vasospasm after subarachnoid hemorrhage (SAH) is because of smooth muscle contraction, although the mechanism of this contraction remains unresolved. Membrane potential controls the contractile state of arterial myocytes by gating voltage-sensitive calcium channels and is in turn primarily controlled by K+ ion conductance through several classes of K+ channels. We characterized the role of inwardly rectifying K+ (KIR) channels in vasospasm. Vasospasm was created in dogs using the double-hemorrhage model of SAH. Electrophysiological, real-time quantitative reverse-transcriptase polymerase chain reaction, Western blotting, immunohistochemistry, and isometric tension techniques were used to characterize the expression and function of KIR channels in normal and vasospastic basilar artery 7 days after SAH. Subarachnoid hemorrhage resulted in severe vasospasm of the basilar artery (mean of 61% ± 5% reduction in diameter). Membrane potential of pressurized vasospastic basilar arteries was significantly depolarized compared with control arteries (+-46 ± 1.4 mV versus −29.8 ± 1.8 mV, respectively, P < 0.01). In whole-cell patch clamp of enzymatically isolated basilar artery myocytes, average KIR conductance was 1.6 ± 0.5 pS/pF in control cells and 9.2 ± 2.2 pS/pF in SAH cells (P = 0.007). Blocking KiR channels with BaCI2 (0.1 mmol/L) resulted in significantly greater membrane depolarization in vasospastic compared with normal myocytes. Expression of KIR 2.1 messenger ribonucleic acid (mRNA) was increased after SAH. Western blotting and immunohistochemistry also showed increased expression of KIR protein in vasospastic smooth muscle. Blockage of KIR channels in arteries under isometric tension produced a greater contraction in SAH than in control arteries. These results document increased expression of KIR 2.1 mRNA and protein during vasospasm after experimental SAH and suggest that this increase is a functionally significant adaptive response acting to reduce vasospasm.

Intracisternal Sodium Nitroprusside Fails to Prevent Vasospasm in Nonhuman Primates

OBJECTIVE Hemoglobin contributes to vasospasm after subarachnoid hemorrhage. One mechanism may involve binding of nitric oxide, destruction of nitric oxide, or both. Support for this mechanism would be evidence that nitric oxide donors prevent vasospasm. This study attempted to provide such evidence. METHODS A randomized, blinded study was conducted in which 13 monkeys underwent cerebral angiography and creation of a right subarachnoid hemorrhage. Subcutaneous osmotic pumps were implanted to deliver sodium nitroprusside (n = 7) or vehicle (n = 6) via catheters into the right basal cisterns. Seven days later, angiography was repeated, and the animals were humanely killed. Levels of cyclic nucleotides, hemoglobins, and thiocyanate were measured. RESULTS Significant vasospasm of the right middle cerebral artery was present in animals treated with sodium nitroprusside (35 ± 22% reduction in diameter, P < 0.05, paired t test) and placebo (28 ± 20% reduction, P < 0.05, not significantly different from nitroprusside group by unpaired t test). Adequate delivery of sodium nitroprusside was supported by the finding of a significant increase in cyclic guanosine monophosphate levels in the cerebral arteries of treated animals compared with placebo (P < 0.05, unpaired t test). Thiocyanate was not present in significantly increased amounts in animals treated with nitroprusside, although this group did display elevated concentrations of nitrosyl hemoglobin (measured by electron paramagnetic resonance spectroscopy) and cyanomethemoglobin (measured by spectrophotometry) in the cerebrospinal fluid on Day 7. CONCLUSION The lack of effect of sodium nitroprusside was not the result of inadequate drug delivery because cyclic guanosine monophosphate levels were significantly increased in vasospastic arteries. Vasospasm may not have been prevented because of a toxic effect of sodium nitroprusside metabolites, involvement of smooth muscle relaxation or contraction processes downstream of cyclic guanosine monophosphate, or both.

Preserved BK Channel Function in Vasospastic Myocytes from a Dog Model of Subarachnoid Hemorrhage

Cerebral vasospasm after subarachnoid hemorrhage (SAH) is due to contraction of smooth muscle cells in the cerebral arteries. The mechanism of this contraction, however, is not well understood. Smooth muscle contraction is regulated in part by membrane potential, which is determined by K+ conductance in smooth muscle. Voltage-gated (Kv) and large-conductance, Ca2+-activated K+ (BK) channels dominate arterial smooth muscle K+ conductance. Vasospastic smooth muscle cells are depolarized relative to normal cells, but whether this is due to altered Kv or BK channel function has not been determined. This study determined if BK channels are altered during vasospasm after SAH in dogs. We first characterized BK channels in basilar-artery smooth muscle using whole-cell patch clamping and single-channel recordings. Next, we compared BK channel function between normal and vasospastic cells. There were no significant differences between normal and vasospastic cells in BK current density, kinetics, Ca2+ and voltage sensitivity, single-channel conductance or apparent Ca2+ affinity. Basilar-artery myocytes had no, small- or intermediate-conductance, Ca2+-activated K+ channels. The lack of difference in BK channels between vasospastic and control cells suggests alteration(s) in other K+ channels or other ionic conductances may underlie the membrane depolarization and vasoconstriction observed during vasospasm after SAH.

Temporal profile of potassium channel dysfunction in cerebrovascular smooth muscle after experimental subarachnoid haemorrhage

The pathogenesis of cerebral vasospasm after subarachnoid haemorrhage (SAH) involves sustained contraction of arterial smooth muscle cells that is maximal 6-8 days after SAH. We reported that function of voltage-gated K+ (KV) channels was significantly decreased during vasospasm 7 days after SAH in dogs. Since arterial constriction is regulated by membrane potential that in turn is determined predominately by K+ conductance, the compromised K+ channel dysfunction may cause vasospasm. Additional support for this hypothesis would be demonstration that K+ channel dysfunction is temporally coincident with vasospasm. To test this hypothesis, SAH was created using the double haemorrhage model in dogs and smooth muscle cells from the basilar artery, which develops vasospasm, were isolated 4 days (early vasospasm), 7 days (during vasospasm) and 21 days (after vasospasm) after SAH and studied using patch-clamp electrophysiology. We investigated the two main K+ channels (KV and large-conductance voltage/Ca2+-activated (KCa) channels). Electrophysiologic function of KCa channels was preserved at all times after SAH. In contrast, function of KV channels was significantly decreased at all times after SAH. The decrease in cell size and degree of KV channel dysfunction was maximal 7 days after SAH. The results suggest that KV channel dysfunction either only partially contributes to vasospasm after SAH or that compensatory mechanisms develop that lead to resolution of vasospasm before KV channels recover their function.