Ca _V 3.2 Channels and the Induction of Negative Feedback in Cerebral Arteries

Abstract

Rationale T-type (CaV3.1/CaV3.2) Ca2+ channels are expressed in rat cerebral arterial smooth muscle. Although 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 large-conductance Ca2+-activated K+ 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 3-dimensional tomography, immunolabeling, and a proximity ligation assay next revealed the existence of microdomains in cerebral arterial smooth muscle which comprised sarcoplasmic reticulum and caveolae. Within these discrete structures, CaV3.2 and ryanodine receptor resided in close apposition to one another. Computational modeling revealed that Ca2+ influx through CaV3.2 could repetitively activate ryanodine receptor, 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 K+ currents, large-conductance Ca2+-activated K+ channel mediated events. Additional functional work on pressurized arteries noted that paxilline, a large-conductance Ca2+-activated K+ channel inhibitor, elicited arterial constriction equivalent, and not additive, to Ni2+. Key experiments 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 ryanodine receptor–mediated Ca2+ sparks, enabling large-conductance Ca2+-activated K+ channel activation, hyperpolarization, and attenuation of cerebral arterial constriction.

T-type Ca2+channels and autoregulation of local blood flow

“…46 However, also at higher pressure (100 mmHg) did low concentration of mibefradil (60 nM) significantly reduce cerebral myogenic responses. 83 Conversely, myogenic tone in rat middle cerebral arteries was increased by the Ca V 3.2 T-type channel blocker Ni 2C (50 mM) 105 which is in line with the results found in the renal vasculature in Ca V 3.2 Ttype KO mice 94 where efferent arteriolar resistance increased. This increase in myogenic tone following addition of Ni 2C was not seen in mesenteric arteries from Ca V 3.2 ¡/¡ mice.…”

“…50 These results are generally in agreement with the hypothesis that pressure-dependent activation of Ca V 3.2 channels in the VSMC plasma membrane causes activation of Ca 2C sparks via Ryanodine receptors (RyRs) closely apposed to Sarcoplasmatic Reticulum (SR). This leads to activation of spontaneous transient outward currents (STOC) via BK Ca channels, and negative feedback on the cerebral myogenic tone, 63,105 as previously suggested for the involvement of Ca V 3.2 channels in relaxation of coronary arteries. 103 These effects of Ca V 3.2 channel deletion on myogenic tone were only observed in young mice (2-4 months) but not in mature adult mice (7-12 months) leading to the conclusion that Ca V 3.2 channel expression may protect against excessive tone and high blood pressure in young individuals.…”

“…This increase in myogenic tone following addition of Ni 2C was not seen in mesenteric arteries from Ca V 3.2 ¡/¡ mice. 50,63,105 The increased tone was mimicked by addition of the specific BK Ca channel blocker paxilline (1 mM) in rat cerebral arteries, 105 and this effect of paxilline was not seen in small mesenteric arteries from young Ca V 3.2-deficient mice. 50 These results are generally in agreement with the hypothesis that pressure-dependent activation of Ca V 3.2 channels in the VSMC plasma membrane causes activation of Ca 2C sparks via Ryanodine receptors (RyRs) closely apposed to Sarcoplasmatic Reticulum (SR).…”

Functional importance of T-type voltage-gated calcium channels in the cardiovascular and renal system: news from the world of knockout mice

“…In agreement, Ca v 3 channels colocalize with BK Ca , and T-type currents are able to activate the potassium channels in rat medial vestibular neurons (85). A recent article confirms the surprising finding that activation of T-type channels (Ca v 3.2) is involved in the relaxation of blood vessels (36). The article demonstrates that in rat smooth muscle cells, calcium influx through Ca v 3.2 channels leads to activation of ryanodine receptors, calcium release from internal stores, activation of BK Ca channels, hyperpolarization, and vascular relaxation.…”

“…Unfortunately, both models have neurological defects that have to be kept in mind when studying the cardiovascular system. T-type channels are now known to be involved in vascular function being involved in ACh-induced relaxation of coronary and cerebral arteries (Ca v 3.2) (14,36) and contractility in mesenteric and pulmonary vessels (Ca v 3.1) (111). Furthermore, these channels regulate heart rate (Ca v 3.1) (67).…”