CaV3.2 Channels and the Induction of Negative Feedback In Cerebral Arteries
Rationale: T-type (CaV3.1/CaV3.2) Ca2+ channels are expressed in rat cerebral arterial smooth muscle. While present, their functional significance remains uncertain with findings pointing to a variety of roles.
Objective: This study tested whether CaV3.2 channels mediate a negative feedback response by triggering Ca2+ sparks, discrete events that initiate arterial hyperpolarization by activating BKCa channels.
Methods and Results: Micromolar Ni2+, an agent that selectively blocks CaV3.2 but not CaV1.2/CaV3.1, was first shown to depolarize/constrict pressurized rat cerebral arteries; no effect was observed in CaV3.2-/- arteries. Structural analysis using 3D-tomography, immunolabeling, and a proximity ligation assay next revealed the existence of microdomains in cerebral arterial smooth muscle comprised of sarcoplasmic reticulum and caveolae. Within these discrete structures, CaV3.2 and RyR resided in close apposition to one another. Computational modeling subsequently revealed that Ca2+ influx through CaV3.2 could repetitively activate RyR, inducing discrete Ca2+-induced Ca2+ release events in a voltage dependent manner. In keeping with theoretical observations, rapid Ca2+ imaging and perforated patch clamp electrophysiology demonstrated that Ni2+ suppressed Ca2+ sparks and consequently spontaneous transient outward current, BKCa mediated events. Additional functional work on pressurized arteries noted that paxilline, a BKCa inhibitor, elicited arterial constriction equivalent, and not additive, to Ni2+. Key experiments repeated on human cerebral arteries indicate that CaV3.2 is present and drives a comparable response to moderate constriction.
Conclusions:These findings indicate for the first time that CaV3.2 channels localize to discrete microdomains and drive RyR-mediated Ca2+ sparks enabling BKCa activation, hyperpolarization and an attenuation of cerebral arterial constriction.
- Received March 28, 2014.
- Revision received July 29, 2014.
- Accepted July 31, 2014.