Impact of subarachnoid hemorrhage on local and global calcium signaling in cerebral artery myocytes.
BACKGROUND
Ca2+ signaling mechanisms are crucial for proper regulation of vascular smooth muscle contractility and vessel diameter. In cerebral artery myocytes, a rise in global cytosolic Ca2+ concentration ([Ca2+]i) causes contraction while an increase in local Ca2+ release events from the sarcoplasmic reticulum (Ca2+ sparks) leads to increased activity of large-conductance Ca2+-activated (BK) K+ channels, hyperpolarization and relaxation. Here, we examined the impact of SAH on Ca2+ spark activity and [Ca2+]i in cerebral artery myocytes following SAH.


METHODS
A rabbit double injection SAH model was used in this study. Five days after the initial intracisternal injection of whole blood, small diameter cerebral arteries were dissected from the brain for study. For simultaneous measurement of arterial wall [Ca2+]i and diameter, vessels were cannulated and loaded with the ratiometric Ca2+ indicator fura-2. For measurement of Ca2+ sparks, individual myocytes were enzymatically isolated from cerebral arteries and loaded with the Ca2+ indicator fluo-4. Sparks were visualized using laser scanning confocal microscopy.


RESULTS
Arterial wall [Ca2+]i was significantly elevated and greater levels of myogenic tone developed in arteries isolated from SAH animals compared with arteries isolated from healthy animals. The L-type voltage-dependent Ca2+ channel (VDCC) blocker nifedipine attenuated increases in [Ca2+]i and tone in both groups suggesting increased VDCC activity following SAH. Membrane potential measurement using intracellular microelectrodes revealed significant depolarization of vascular smooth muscle following SAH. Further, myocytes from SAH animals exhibited significantly reduced Ca2+ spark frequency (~50%).


CONCLUSIONS
Our findings suggest decreased Ca2+ spark frequency leads to reduced BK channel activity in cerebral artery myocytes following SAH. This results in membrane potential depolarization, increased VDCC activity, elevated [Ca2+]i and decreased vessel diameter. We propose this mechanism of enhanced cerebral artery myocyte contractility may contribute to decreased cerebral blood flow and development of neurological deficits in SAH patients.
Department of Pharmacology, University of Vermont College of Medicine, Burlington, VT 05405-0068, USA.
Masayo
M
Koide
Matthew A
MA
Nystoriak
Joseph E
JE
Brayden
George C
GC
Wellman
D000818:Animals; D001794:Blood Pressure / Q000502:physiology; D002118:Calcium / Q000378:metabolism; D002121:Calcium Channel Blockers / Q000494:pharmacology; D020746:Calcium Channels, L-Type / Q000378:metabolism; D020013:Calcium Signaling / Q000502:physiology*; D002536:Cerebral Arteries / Q000473:pathology*; D004195:Disease Models, Animal; D005456:Fluorescent Dyes; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D018613:Microscopy, Confocal / Q000379:methods; D008954:Models, Biological; D032342:Muscle Cells / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D011817:Rabbits; D013345:Subarachnoid Hemorrhage / Q000473:pathology* / Q000503:physiopathology
D002121:Calcium Channel Blockers; D020746:Calcium Channels, L-Type; D005456:Fluorescent Dyes; C419385:L-type calcium channel alpha(1C); D002118:Calcium