Ontogeny of local sarcoplasmic reticulum Ca2+ signals in cerebral arteries: Ca2+ sparks as elementary physiological events

Vascular mechanotransduction

This review aims to survey the current state of mechanotransduction in vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), including their sensing of mechanical stimuli and transduction of mechanical signals that result in the acute functional modulation and longer-term transcriptomic and epigenetic regulation of blood vessels. The mechanosensors discussed include ion channels, plasma membrane-associated structures and receptors, and junction proteins. The mechanosignaling pathways presented include the cytoskeleton, integrins, extracellular matrix, and intracellular signaling molecules. These are followed by discussions on mechanical regulation of transcriptome and epigenetics, relevance of mechanotransduction to health and disease, and interactions between VSMCs and ECs. Throughout this review, we offer suggestions for specific topics that require further understanding. In the closing section on conclusions and perspectives, we summarize what is known and point out the need to treat the vasculature as a system, including not only VSMCs and ECs but also the extracellular matrix and other types of cells such as resident macrophages and pericytes, so that we can fully understand the physiology and pathophysiology of the blood vessel as a whole, thus enhancing the comprehension, diagnosis, treatment, and prevention of vascular diseases.

Aldosterone-Induced Sarco/Endoplasmic Reticulum Ca2+ Pump Upregulation Counterbalances Cav1.2-Mediated Ca2+ Influx in Mesenteric Arteries

In mesenteric arteries (MAs), aldosterone (ALDO) binds to the endogenous mineralocorticoid receptor (MR) and increases the expression of the voltage-gated L-type Ca_v1.2 channel, an essential ion channel for vascular contraction, sarcoplasmic reticulum (SR) Ca²⁺ store refilling, and Ca²⁺ spark generation. In mesenteric artery smooth muscle cells (MASMCs), Ca²⁺ influx through Ca_v1.2 is the indirect mechanism for triggering Ca²⁺ sparks. This process is facilitated by plasma membrane-sarcoplasmic reticulum (PM-SR) nanojunctions that drive Ca²⁺ from the extracellular space into the SR via Sarco/Endoplasmic Reticulum Ca²⁺ (SERCA) pump. Ca²⁺ sparks produced by clusters of Ryanodine receptors (RyRs) at PM-SR nanodomains, decrease contractility by activating large-conductance Ca²⁺-activated K⁺ channels (BK_Ca channels), which generate spontaneous transient outward currents (STOCs). Altogether, Ca_v1.2, SERCA pump, RyRs, and BK_Ca channels work as a functional unit at the PM-SR nanodomain, regulating intracellular Ca²⁺ and vascular function. However, the effect of the ALDO/MR signaling pathway on this functional unit has not been completely explored. Our results show that short-term exposure to ALDO (10 nM, 24 h) increased the expression of Ca_v1.2 in rat MAs. The depolarization-induced Ca²⁺ entry increased SR Ca²⁺ load, and the frequencies of both Ca²⁺ sparks and STOCs, while [Ca²⁺]_cyt and vasoconstriction remained unaltered in Aldo-treated MAs. ALDO treatment significantly increased the mRNA and protein expression levels of the SERCA pump, which counterbalanced the augmented Ca_v1.2-mediated Ca²⁺ influx at the PM-SR nanodomain, increasing SR Ca²⁺ content, Ca²⁺ spark and STOC frequencies, and opposing to hyperpolarization-induced vasoconstriction while enhancing Acetylcholine-mediated vasorelaxation. This work provides novel evidence for short-term ALDO-induced upregulation of the functional unit comprising Ca_v1.2, SERCA2 pump, RyRs, and BK_Ca channels; in which the SERCA pump buffers ALDO-induced upregulation of Ca²⁺ entry at the superficial SR-PM nanodomain of MASMCs, preventing ALDO-triggered depolarization-induced vasoconstriction and enhancing vasodilation. Pathological conditions that lead to SERCA pump downregulation, for instance, chronic exposure to ALDO, might favor the development of ALDO/MR-mediated augmented vasoconstriction of mesenteric arteries.

Regulation of the Cerebral Circulation During Development

The cerebral microcirculation undergoes dynamic changes in parallel with the development of neurons, glia, and their energy metabolism throughout gestation and postnatally. Cerebral blood flow (CBF), oxygen consumption, and glucose consumption are as low as 20% of adult levels in humans born prematurely but eventually exceed adult levels at ages 3 to 11 years, which coincide with the period of continued brain growth, synapse formation, synapse pruning, and myelination. Neurovascular coupling to sensory activation is present but attenuated at birth. By 2 postnatal months, the increase in CBF often is disproportionately smaller than the increase in oxygen consumption, in contrast to the relative hyperemia seen in adults. Vascular smooth muscle myogenic tone increases in parallel with developmental increases in arterial pressure. CBF autoregulatory response to increased arterial pressure is intact at birth but has a more limited range with arterial hypotension. Hypoxia-induced vasodilation in preterm fetal sheep with low oxygen consumption does not sustain cerebral oxygen transport, but the response becomes better developed for sustaining oxygen transport by term. Nitric oxide tonically inhibits vasomotor tone, and glutamate receptor activation can evoke its release in lambs and piglets. In piglets, astrocyte-derived carbon monoxide plays a central role in vasodilation evoked by glutamate, ADP, and seizures, and prostanoids play a large role in endothelial-dependent and hypercapnic vasodilation. Overall, homeostatic mechanisms of CBF regulation in response to arterial pressure, neuronal activity, carbon dioxide, and oxygenation are present at birth but continue to develop postnatally as neurovascular signaling pathways are dynamically altered and integrated. © 2021 American Physiological Society. Compr Physiol 11:1-62, 2021.

Remodeling of Arterial Tone Regulation in Postnatal Development: Focus on Smooth Muscle Cell Potassium Channels

Maturation of the cardiovascular system is associated with crucial structural and functional remodeling. Thickening of the arterial wall, maturation of the sympathetic innervation, and switching of the mechanisms of arterial contraction from calcium-independent to calcium-dependent occur during postnatal development. All these processes promote an almost doubling of blood pressure from the moment of birth to reaching adulthood. This review focuses on the developmental alterations of potassium channels functioning as key smooth muscle membrane potential determinants and, consequently, vascular tone regulators. We present evidence that the pattern of potassium channel contribution to vascular control changes from K_ir2, K_v1, K_v7 and TASK-1 channels to BK_Ca channels with maturation. The differences in the contribution of potassium channels to vasomotor tone at different stages of postnatal life should be considered in treatment strategies of cardiovascular diseases associated with potassium channel malfunction.

Trophic sympathetic influence weakens pro-contractile role of Cl- channels in rat arteries during postnatal maturation

Membrane transporters and their functional contribution in vasculature change during early postnatal development. Here we tested the hypothesis that the contribution of Cl⁻ channels to arterial contraction declines during early postnatal development and this decline is associated with the trophic sympathetic influence. Endothelium‐denuded saphenous arteries from 1- to 2-week-old and 2- to 3-month-old male rats were used. Arterial contraction was assessed in the isometric myograph, in some experiments combined with measurements of membrane potential. mRNA and protein levels were determined by qPCR and Western blot. Sympathectomy was performed by treatment with guanethidine from the first postnatal day until 8–9-week age. Cl⁻ substitution in the solution as well as Cl⁻-channel blockers (MONNA, DIDS) had larger suppressive effect on the methoxamine-induced arterial contraction and methoxamine-induced depolarization of smooth muscle cells in 1- to 2-week-old compared to 2- to 3-month-old rats. Vasculature of younger group demonstrated elevated expression levels of TMEM16A and bestrophin 3. Chronic sympathectomy increased Cl⁻ contribution to arterial contraction in 2-month-old rats that was associated with an increased TMEM16A expression level. Our study demonstrates that contribution of Cl⁻ channels to agonist-induced arterial contraction and depolarization decreases during postnatal development. This postnatal decline is associated with sympathetic nerves development.

TASK-1 channel blockade by AVE1231 increases vasocontractile responses and BP in 1- to 2-week-old but not adult rats

Background and Purpose

The vasomotor role of K2P potassium channels during early postnatal development has never been investigated. We tested the hypothesis that TASK‐1 channel (K2P family member) contribution to arterial vascular tone and BP is higher in the early postnatal period than in adulthood.

Experimental Approach

We studied 10‐ to 15‐day‐old (“young”) and 2‐ to 3‐month‐old (“adult”) male rats performing digital PCR (dPCR) (using endothelium‐intact saphenous arteries), isometric myography, sharp microelectrode technique, quantitative PCR (qPCR) and Western blotting (using endothelium‐denuded saphenous arteries), and arterial pressure measurements under urethane anaesthesia.

Key Results

We found mRNA of Kcnk1–Kcnk7, Kcnk12, and Kcnk13 genes to be expressed in rat saphenous artery, and Kcnk3 (TASK‐1) and Kcnk6 (TWIK‐2) were most abundant in both age groups. The TASK‐1 channel blocker AVE1231 (1 μmol·L⁻¹) prominently depolarized arterial smooth muscle and increased basal tone level and contractile responses to methoxamine of arteries from young rats but had almost no effect in adult rats. The level of TASK‐1 mRNA and protein expression was higher in arteries from young compared with adult rats. Importantly, intravenous administration of AVE1231 (4 mg·kg⁻¹) had no effect on mean arterial pressure in adult rats but prominently raised it in young rats.

Conclusion and Implications

We showed that TASK‐1 channels are important for negative feedback regulation of vasocontraction in young but not adult rats. The influence of TASK‐1 channels most likely contributes to low BP level at perinatal age.

Elementary calcium signaling in arterial smooth muscle

Vascular smooth muscle cells (VSMCs) of small peripheral arteries contribute to blood pressure control by adapting their contractile state. These adaptations depend on the VSMC cytosolic Ca²⁺ concentration, regulated by complex local elementary Ca²⁺ signaling pathways. Ca²⁺ sparks represent local, transient, rapid calcium release events from a cluster of ryanodine receptors (RyRs) in the sarcoplasmic reticulum. In arterial SMCs, Ca²⁺ sparks activate nearby calcium-dependent potassium channels, cause membrane hyperpolarization and thus decrease the global intracellular [Ca²⁺] to oppose vasoconstriction. Arterial SMC Ca_v1.2 L-type channels regulate intracellular calcium stores content, which in turn modulates calcium efflux through RyRs. Ca_v3.2 T-type channels contribute to a minor extend to Ca²⁺ spark generation in certain types of arteries. Their localization within cell membrane caveolae is essential. We summarize present data on local elementary calcium signaling (Ca²⁺ sparks) in arterial SMCs with focus on RyR isoforms, large-conductance calcium-dependent potassium (BK_Ca) channels, and cell membrane-bound calcium channels (Ca_v1.2 and Ca_v3.2), particularly in caveolar microdomains.

Age attenuates the T-type CaV 3.2-RyR axis in vascular smooth muscle

Caveolae position Ca_V3.2 (T‐type Ca²⁺ channel encoded by the α‐3.2 subunit) sufficiently close to RyR (ryanodine receptors) for extracellular Ca²⁺ influx to trigger Ca²⁺ sparks and large‐conductance Ca²⁺‐activated K⁺ channel feedback in vascular smooth muscle. We hypothesize that this mechanism of Ca²⁺ spark generation is affected by age. Using smooth muscle cells (VSMCs) from mouse mesenteric arteries, we found that both Ca_v3.2 channel inhibition by Ni²⁺ (50 µM) and caveolae disruption by methyl‐ß‐cyclodextrin or genetic abolition of Eps15 homology domain‐containing protein (EHD2) inhibited Ca²⁺ sparks in cells from young (4 months) but not old (12 months) mice. In accordance, expression of Ca_v3.2 channel was higher in mesenteric arteries from young than old mice. Similar effects were observed for caveolae density. Using SMAKO Ca_v1.2^−/− mice, caffeine (RyR activator) and thapsigargin (Ca²⁺ transport ATPase inhibitor), we found that sufficient SR Ca²⁺ load is a prerequisite for the Ca_V3.2‐RyR axis to generate Ca²⁺ sparks. We identified a fraction of Ca²⁺ sparks in aged VSMCs, which is sensitive to the TRP channel blocker Gd³⁺ (100 µM), but insensitive to Ca_V1.2 and Ca_V3.2 channel blockade. Our data demonstrate that the VSMC Ca_V3.2‐RyR axis is down‐regulated by aging. This defective Ca_V3.2‐RyR coupling is counterbalanced by a Gd³⁺ sensitive Ca²⁺ pathway providing compensatory Ca²⁺ influx for triggering Ca²⁺ sparks in aged VSMCs.

Role of Ryanodine Type 2 Receptors in Elementary Ca2+ Signaling in Arteries and Vascular Adaptive Responses

Background Hypertension is the major risk factor for cardiovascular disease, the most common cause of death worldwide. Resistance arteries are capable of adapting their diameter independently in response to pressure and flow-associated shear stress. Ryanodine receptors (RyRs) are major Ca2+-release channels in the sarcoplasmic reticulum membrane of myocytes that contribute to the regulation of contractility. Vascular smooth muscle cells exhibit 3 different RyR isoforms (RyR1, RyR2, and RyR3), but the impact of individual RyR isoforms on adaptive vascular responses is largely unknown. Herein, we generated tamoxifen-inducible smooth muscle cell-specific RyR2-deficient mice and tested the hypothesis that vascular smooth muscle cell RyR2s play a specific role in elementary Ca2+ signaling and adaptive vascular responses to vascular pressure and/or flow. Methods and Results Targeted deletion of the Ryr2 gene resulted in a complete loss of sarcoplasmic reticulum-mediated Ca2+-release events and associated Ca2+-activated, large-conductance K+ channel currents in peripheral arteries, leading to increased myogenic tone and systemic blood pressure. In the absence of RyR2, the pulmonary artery pressure response to sustained hypoxia was enhanced, but flow-dependent effects, including blood flow recovery in ischemic hind limbs, were unaffected. Conclusions Our results establish that RyR2-mediated Ca2+-release events in VSCM s specifically regulate myogenic tone (systemic circulation) and arterial adaptation in response to changes in pressure (hypoxic lung model), but not flow. They further suggest that vascular smooth muscle cell-expressed RyR2 deserves scrutiny as a therapeutic target for the treatment of vascular responses in hypertension and chronic vascular diseases.

Negative feedback regulation of vasocontraction by potassium channels in 10- to 15-day-old rats: Dominating role of Kv 7 channels

AIM

Potassium channels are key regulators of smooth muscle membrane potential and arterial tone. However, the roles of potassium channels in vascular tone regulation in the systemic circulation during early postnatal development are poorly understood. Therefore, this study tested the hypothesis that the negative feedback regulation of vasocontraction by potassium channels changes during maturation.

METHODS

Experiments were performed on endothelium-denuded saphenous arteries from 10- to 15-day-old and 2- to 3-month-old male rats. Isometric force and membrane potential were recorded using wire myography and the sharp microelectrode technique respectively; mRNA and protein contents were determined by qPCR and Western blotting.

RESULTS

The effects of K_v 1, K_ir and K_v 7 channel blockers (DPO-1, BaCl₂ , XE991) on methoxamine-induced contraction were larger in arteries of 10- to 15-day-old compared to 2- to 3-month-old animals. In contrast, the BK_{C a} channel blocker iberiotoxin had a stronger influence in 2- to 3- month-old rats. The effects of K_ATP and K_v 2 channel blockers (glibenclamide, stromatoxin) were not pronounced at both ages. The larger influence of K_v 7 and K_ir channel blockade on arterial contraction in 10- to 15-day-old rats was associated with more prominent smooth muscle depolarization. The developmental alterations in potassium channel functioning were generally consistent with their mRNA and protein expression levels in arterial smooth muscle.

CONCLUSION

The negative feedback regulation of vasocontraction by potassium channels varies during maturation depending on the channel type. A dominating contribution of K_v 7 channels to the regulation of basal tone and agonist-induced contraction was observed in arteries of 10- to 15-day-old animals.

Perivascular adipose tissue and the dynamic regulation of Kv 7 and Kir channels: Implications for resistant hypertension

Resistant hypertension is defined as high blood pressure that remains uncontrolled despite treatment with at least three antihypertensive drugs at adequate doses. Resistant hypertension is an increasingly common clinical problem in older age, obesity, diabetes, sleep apnea, and chronic kidney disease. Although the direct vasodilator minoxidil was introduced in the early 1970s, only recently has this drug been shown to be particularly effective in a subgroup of patients with treatment-resistant or uncontrolled hypertension. This pharmacological approach is interesting from a mechanistic perspective as minoxidil is the only clinically used K⁺ channel opener today, which targets a subclass of K⁺ channels, namely K_ATP channels in VSMCs. Beside K_ATP channels, two other classes of VSMC K⁺ channels could represent novel effective targets for treatment of resistant hypertension, namely K_v 7 (KCNQ) and inward rectifier potassium (K_ir 2.1) channels. Interestingly, these channels are unique among VSMC potassium channels. First, both have been implicated in the control of microvascular tone by perivascular adipose tissue. Second, they exhibit biophysical properties strongly controlled and regulated by membrane voltage, but not intracellular calcium. This review focuses on K_v 7 (K_v 7.1-5) and K_ir (K_ir 2.1) channels in VSMCs as potential novel drug targets for treatment of resistant hypertension, particularly in comorbid conditions such as obesity and metabolic syndrome.

Differential targeting and signalling of voltage-gated T-type Cav 3.2 and L-type Cav 1.2 channels to ryanodine receptors in mesenteric arteries

KEY POINTS

In arterial smooth muscle, Ca2+ sparks are elementary Ca2+ -release events generated by ryanodine receptors (RyRs) to cause vasodilatation by opening maxi Ca2+ -sensitive K+ (BKCa ) channels. This study elucidated the contribution of T-type Cav 3.2 channels in caveolae and their functional interaction with L-type Cav 1.2 channels to trigger Ca2+ sparks in vascular smooth muscle cells (VSMCs). Our data demonstrate that L-type Cav 1.2 channels provide the predominant Ca2+ pathway for the generation of Ca2+ sparks in murine arterial VSMCs. T-type Cav 3.2 channels represent an additional source for generation of VSMC Ca2+ sparks. They are located in pit structures of caveolae to provide locally restricted, tight coupling between T-type Cav 3.2 channels and RyRs to ignite Ca2+ sparks.

ABSTRACT

Recent data suggest that T-type Cav 3.2 channels in arterial vascular smooth muscle cells (VSMCs) and pits structure of caveolae could contribute to elementary Ca2+ signalling (Ca2+ sparks) via ryanodine receptors (RyRs) to cause vasodilatation. While plausible, their precise involvement in igniting Ca2+ sparks remains largely unexplored. The goal of this study was to elucidate the contribution of caveolar Cav 3.2 channels and their functional interaction with Cav 1.2 channels to trigger Ca2+ sparks in VSMCs from mesenteric, tibial and cerebral arteries. We used tamoxifen-inducible smooth muscle-specific Cav 1.2-/- (SMAKO) mice and laser scanning confocal microscopy to assess Ca2+ spark generation in VSMCs. Ni2+ , Cd2+ and methyl-β-cyclodextrin were used to inhibit Cav 3.2 channels, Cav 1.2 channels and caveolae, respectively. Ni2+ (50 μmol L-1 ) and methyl-β-cyclodextrin (10 mmol L-1 ) decreased Ca2+ spark frequency by ∼20-30% in mesenteric VSMCs in a non-additive manner, but failed to inhibit Ca2+ sparks in tibial and cerebral artery VSMCs. Cd2+ (200 μmol L-1 ) suppressed Ca2+ sparks in mesenteric arteries by ∼70-80%. A similar suppression of Ca2+ sparks was seen in mesenteric artery VSMCs of SMAKO mice. The remaining Ca2+ sparks were fully abolished by Ni2+ or methyl-β-cyclodextrin. Our data demonstrate that Ca2+ influx through CaV 1.2 channels is the primary means of triggering Ca2+ sparks in murine arterial VSMCs. CaV 3.2 channels, localized to caveolae and tightly coupled to RyR, provide an additional Ca2+ source for Ca2+ spark generation in mesenteric, but not tibial and cerebral, arteries.

Long-term high-altitude hypoxia influences pulmonary arterial L-type calcium channel-mediated Ca2+ signals and contraction in fetal and adult sheep

Long-term hypoxia (LTH) has a profound effect on pulmonary arterial vasoconstriction in the fetus and adult. Dysregulation in Ca²⁺ signaling is important during the development of LTH-induced pulmonary hypertension. In the present study, we tested the hypothesis that L-type Ca²⁺ channels (Ca_L), which are voltage dependent and found in smooth, skeletal, and cardiac muscle, are important in the adaptation of pulmonary arterial contractions in postnatal maturation and in response to LTH. Pulmonary arteries were isolated from fetal or adult sheep maintained at low or high altitude (3,801 m) for >100 days. The effects were measured using an L-type Ca²⁺ channel opener FPL 64176 (FPL) in the presence or absence of an inhibitor, Nifedipine (NIF) on arterial contractions, intracellular Ca²⁺ oscillations, and ryanodine receptor-driven Ca²⁺ sparks. FPL induced pulmonary arterial contractions in all groups were sensitive to NIF. However, when compared with 125 mM K⁺, FPL contractions were greater in fetuses than in adults. FPL reduced Ca²⁺ oscillations in myocytes of adult but not fetal arteries, independently of altitude. The FPL effects on Ca²⁺ oscillations were reversed by NIF in myocytes of hypoxic but not normoxic adults. FPL failed to enhance Ca²⁺ spark frequency and had little impact on spatiotemporal firing characteristics. These data suggest that Ca_L-dependent contractions are largely uncoupled from intracellular Ca²⁺ oscillations and the development of Ca²⁺ sparks. This raises questions regarding the coupling of pulmonary arterial contractility to membrane depolarization, attendant Ca_L facilitation, and the related associations with the activation of Ca²⁺ oscillations and Ca²⁺ sparks.

Distinct Effects of Ca2+ Sparks on Cerebral Artery and Airway Smooth Muscle Cell Tone in Mice and Humans

The effects of Ca²⁺ sparks on cerebral artery smooth muscle cells (CASMCs) and airway smooth muscle cells (ASMCs) tone, as well as the underlying mechanisms, are not clear. In this investigation, we elucidated the underlying mechanisms of the distinct effects of Ca²⁺ sparks on cerebral artery smooth muscle cells (CASMCs) and airway smooth muscle cells (ASMCs) tone. In CASMCs, owing to the functional loss of Ca²⁺-activated Cl^- (Clca) channels, Ca²⁺ sparks activated large-conductance Ca²⁺-activated K⁺ channels (BKs), resulting in a decreases in tone against a spontaneous depolarization-caused high tone in the resting state. In ASMCs, Ca²⁺ sparks induced relaxation through BKs and contraction via Clca channels. However, the integrated result was contraction because Ca²⁺ sparks activated BKs prior to Clca channels and Clca channels-induced depolarization was larger than BKs-caused hyperpolarization. However, the effects of Ca²⁺ sparks on both cell types were determined by L-type voltage-dependent Ca²⁺ channels (LVDCCs). In addition, compared with ASMCs, CASMCs had great and higher amplitude Ca²⁺ sparks, a higher density of BKs, and higher Ca²⁺ and voltage sensitivity of BKs. These differences enhanced the ability of Ca²⁺ sparks to decrease CASMC and to increase ASMC tone. The higher Ca²⁺ and voltage sensitivity of BKs in CASMCs than ASMCs were determined by the β1 subunits. Moreover, Ca²⁺ sparks showed the similar effects on human CASMC and ASMC tone. In conclusions, Ca²⁺ sparks decrease CASMC tone and increase ASMC tone, mediated by BKs and Clca channels, respectively, and finally determined by LVDCCs.

Differential distribution and functional impact of BK channel beta1 subunits across mesenteric, coronary, and different cerebral arteries of the rat

Large conductance, Ca²⁺_i- and voltage-gated K⁺ (BK) channels regulate myogenic tone and, thus, arterial diameter. In smooth muscle (SM), BK channels include channel-forming α and auxiliary β1 subunits. BK β1 increases the channel's Ca²⁺ sensitivity, allowing BK channels to negatively feedback on depolarization-induced Ca²⁺ entry, oppose SM contraction and favor vasodilation. Thus, endothelial-independent vasodilation can be evoked though targeting of SM BK β1 by endogenous ligands, including lithocholate (LCA). Here, we investigated the expression of BK β1 across arteries of the cerebral and peripheral circulations, and the contribution of such expression to channel function and BK β1-mediated vasodilation. Data demonstrate that endothelium-independent, BK β1-mediated vasodilation by LCA is larger in coronary (CA) and basilar (BA) arteries than in anterior cerebral (ACA), middle cerebral (MCA), posterior cerebral (PCA), and mesenteric (MA) arteries, all arterial segments having a similar diameter. Thus, differential dilation occurs in extracranial arteries which are subjected to similar vascular pressure (CA vs. MA) and in arteries that irrigate different brain regions (BA vs. ACA, MCA, and PCA). SM BK channels from BA and CA displayed increased basal activity and LCA responses, indicating increased BK β1 functional presence. Indeed, in the absence of detectable changes in BK α, BA and CA myocytes showed an increased location of BK β1 in the plasmalemma/subplasmalemma. Moreover, these myocytes distinctly showed increased BK β1 messenger RNA (mRNA) levels. Supporting a major role of enhanced BK β1 transcripts in artery dilation, LCA-induced dilation of MCA transfected with BK β1 complementary DNA (cDNA) was as high as LCA-induced dilation of untransfected BA or CA.

Do KV 7.1 channels contribute to control of arterial vascular tone?

BACKGROUND AND PURPOSE

K_V 7.1 voltage-gated potassium channels are expressed in vascular smooth muscle cells (VSMC) of diverse arteries, including mesenteric arteries. Based on pharmacological evidence using R-L3 (K_V 7.1 channel opener), HMR1556, chromanol 293B (K_V 7.1 channel blockers), stimulation of these channels has been suggested to evoke profound relaxation in various vascular beds of rats. However, the specificity of these drugs in vivo is uncertain.

EXPERIMENTAL APPROACH

We used Kcnq1^-/- mice and pharmacological tools to determine whether K_V 7.1 channels play a role in the regulation of arterial tone.

KEY RESULTS

R-L3 produced similar concentration-dependent relaxations (EC₅₀  ~ 1.4 μM) of arteries from wild-type (Kcnq1^+/+ ) and Kcnq1^-/- mice, pre-contracted with either phenylephrine or 60 mM KCl. This relaxation was not affected by 10 μM chromanol 293B, 10 μM HMR1556 or 30 μM XE991 (pan-K_V 7 channel blocker). The anti-contractile effects of the perivascular adipose tissue (PVAT) were normal in Kcnq1^-/- arteries. Chromanol 293B and HMR1556 did not affect the anti-contractile effects of (PVAT). Isolated VSMCs from Kcnq1^-/- mice exhibited normal peak K_V currents. The K_V 7.2-5 channel opener retigabine caused similar relaxations in Kcnq1^-/- and wild-type vessels.

CONCLUSION AND IMPLICATIONS

We conclude that K_V 7.1 channels were apparently not involved in the control of arterial tone by α₁ -adrenoceptor agonists and PVAT. In addition, R-L3 is an inappropriate pharmacological tool for studying the function of native vascular K_V 7.1 channels in mice.

The Role of DPO-1 and XE991-Sensitive Potassium Channels in Perivascular Adipose Tissue-Mediated Regulation of Vascular Tone

The anti-contractile effect of perivascular adipose tissue (PVAT) is an important mechanism in the modulation of vascular tone in peripheral arteries. Recent evidence has implicated the XE991-sensitive voltage-gated KV (KCNQ) channels in the regulation of arterial tone by PVAT. However, until now the in vivo pharmacology of the involved vascular KV channels with regard to XE991 remains undetermined, since XE991 effects may involve Ca(2+) activated BKCa channels and/or voltage-dependent KV1.5 channels sensitive to diphenyl phosphine oxide-1 (DPO-1). In this study, we tested whether KV1.5 channels are involved in the control of mesenteric arterial tone and its regulation by PVAT. Our study was also aimed at extending our current knowledge on the in situ vascular pharmacology of DPO-1 and XE991 regarding KV1.5 and BKCa channels, in helping to identify the nature of K(+) channels that could contribute to PVAT-mediated relaxation. XE991 at 30 μM reduced the anti-contractile response of PVAT, but had no effects on vasocontraction induced by phenylephrine (PE) in the absence of PVAT. Similar effects were observed for XE991 at 0.3 μM, which is known to almost completely inhibit mesenteric artery VSMC KV currents. 30 μM XE991 did not affect BKCa currents in VSMCs. Kcna5 (-/-) arteries and wild-type arteries incubated with 1 μM DPO-1 showed normal vasocontractions in response to PE in the presence and absence of PVAT. KV current density and inhibition by 30 μM XE991 were normal in mesenteric artery VSMCs isolated from Kcna5 (-/-) mice. We conclude that KV channels are involved in the control of arterial vascular tone by PVAT. These channels are present in VSMCs and very potently inhibited by the KCNQ channel blocker XE991. BKCa channels and/or DPO-1 sensitive KV1.5 channels in VSMCs are not the downstream mediators of the XE991 effects on PVAT-dependent arterial vasorelaxation. Further studies will need to be undertaken to examine the role of other KV channels in the phenomenon.

Tacrolimus inhibits vasoconstriction by increasing Ca(2+) sparks in rat aorta

The present study attempted to test a novel hypothesis that Ca(2+) sparks play an important role in arterial relaxation induced by tacrolimus. Recorded with confocal laser scanning microscopy, tacrolimus (10 µmol/L) increased the frequency of Ca(2+) sparks, which could be reversed by ryanodine (10 µmol/L). Electrophysiological experiments revealed that tacrolimus (10 µmol/L) increased the large-conductance Ca(2+)-activated K(+) currents (BKCa) in rat aortic vascular smooth muscle cells (AVSMCs), which could be blocked by ryanodine (10 µmol/L). Furthermore, tacrolimus (10 and 50 µmol/L) reduced the contractile force induced by norepinephrine (NE) or KCl in aortic vascular smooth muscle in a concentration-dependent manner, which could be also significantly attenuated by iberiotoxin (100 nmol/L) and ryanodine (10 µmol/L) respectively. In conclusion, tacrolimus could indirectly activate BKCa currents by increasing Ca(2+) sparks released from ryanodine receptors, which inhibited the NE- or KCl-induced contraction in rat aorta.

The role of Ca(2+) influx in spontaneous Ca(2+) wave propagation in interstitial cells of Cajal from the rabbit urethra

KEY POINTS

Tonic contractions of rabbit urethra are associated with spontaneous electrical slow waves that are thought to originate in pacemaker cells termed interstitial cells of Cajal (ICC). ICC pacemaker activity results from their ability to generate propagating Ca(2+) waves, although the exact mechanisms of propagation are not understood. In this study, we have identified spontaneous localised Ca(2+) events for the first time in urethral ICC; these were due to Ca(2+) release from the endoplasmic reticulum (ER) via ryanodine receptors (RyRs) and, while they often remained localised, they sometimes initiated propagating Ca(2+) waves. We show that propagation of Ca(2+) waves in urethral ICC is critically dependent upon Ca(2+) influx via reverse mode NCX. Our data provide a clearer understanding of the intracellular mechanisms involved in the generation of ICC pacemaker activity. Interstitial cells of Cajal (ICC) are putative pacemaker cells in the rabbit urethra. Pacemaker activity in ICC results from spontaneous propagating Ca(2+) waves that are modulated by [Ca(2+)]o and whose propagation is inhibited by inositol tri-phosphate receptor (IP3 R) blockers. The purpose of this study was to further examine the role of Ca(2+) influx and Ca(2+) release in the propagation of Ca(2+) waves. Intracellular Ca(2+) was measured in Fluo-4-loaded ICC using a Nipkow spinning disc confocal microscope at fast acquisition rates (50 fps). We identified previously undetected localised Ca(2+) events originating from ryanodine receptors (RyRs). Inhibiting Ca(2+) influx by removing [Ca(2+)]o or blocking reverse mode sodium-calcium exchange (NCX) with KB-R 7943 or SEA-0400 abolished Ca(2+) waves, while localised Ca(2+) events persisted. Stimulating RyRs with 1 mm caffeine restored propagation. Propagation was also inhibited when Ca(2+) release sites were uncoupled by buffering intracellular Ca(2+) with EGTA-AM. This was reversed when Ca(2+) influx via NCX was increased by reducing [Na(+)]o to 13 mm. Low [Na(+)]o also increased the frequency of Ca(2+) waves and this effect was blocked by tetracaine and ryanodine but not 2-aminoethoxydiphenyl borate (2-APB). RT-PCR revealed that isolated ICC expressed both RyR2 and RyR3 subtypes. We conclude: (i) RyRs are required for the initiation of Ca(2+) waves, but wave propagation normally depends on activation of IP3 Rs; (ii) under resting conditions, propagation by IP3 Rs requires sensitisation by influx of Ca(2+) via reverse mode NCX; (iii) propagation can be maintained by RyRs if they have been sensitised to Ca(2+).

Calcium-activated potassium channels in ischemia reperfusion: a brief update

Ischemia and reperfusion (IR) injury constitutes one of the major causes of cardiovascular morbidity and mortality. The discovery of new therapies to block/mediate the effects of IR is therefore an important goal in the biomedical sciences. Dysfunction associated with IR involves modification of calcium-activated potassium channels (KCa) through different mechanisms, which are still under study. Respectively, the KCa family, major contributors to plasma membrane calcium influx in cells and essential players in the regulation of the vascular tone are interesting candidates. This family is divided into two groups including the large conductance (BKCa) and the small/intermediate conductance (SKCa/IKCa) K(+) channels. In the heart and brain, these channels have been described to offer protection against IR injury. BKCa and SKCa channels deserve special attention since new data demonstrate that these channels are also expressed in mitochondria. More studies are however needed to fully determine their potential use as therapeutic targets.

Stretch-activation of angiotensin II type 1a receptors contributes to the myogenic response of mouse mesenteric and renal arteries

RATIONALE

Vascular wall stretch is the major stimulus for the myogenic response of small arteries to pressure. The molecular mechanisms are elusive, but recent findings suggest that G protein-coupled receptors can elicit a stretch response.

OBJECTIVE

To determine whether angiotensin II type 1 receptors (AT1R) in vascular smooth muscle cells exert mechanosensitivity and identify the downstream ion channel mediators of myogenic vasoconstriction.

METHODS AND RESULTS

We used mice deficient in AT1R signaling molecules and putative ion channel targets, namely AT1R, angiotensinogen, transient receptor potential channel 6 (TRPC6) channels, or several subtypes of the voltage-gated K+ (Kv7) gene family (KCNQ3, 4, or 5). We identified a mechanosensing mechanism in isolated mesenteric arteries and in the renal circulation that relies on coupling of the AT1R subtype a to a Gq/11 protein as a critical event to accomplish the myogenic response. Arterial mechanoactivation occurs after pharmacological block of AT1R and in the absence of angiotensinogen or TRPC6 channels. Activation of AT1R subtype a by osmotically induced membrane stretch suppresses an XE991-sensitive Kv channel current in patch-clamped vascular smooth muscle cells, and similar concentrations of XE991 enhance mesenteric and renal myogenic tone. Although XE991-sensitive KCNQ3, 4, and 5 channels are expressed in vascular smooth muscle cells, XE991-sensitive K+ current and myogenic contractions persist in arteries deficient in these channels.

CONCLUSIONS

Our results provide definitive evidence that myogenic responses of mouse mesenteric and renal arteries rely on ligand-independent, mechanoactivation of AT1R subtype a. The AT1R subtype a signal relies on an ion channel distinct from TRPC6 or KCNQ3, 4, or 5 to enact vascular smooth muscle cell activation and elevated vascular resistance.

Antenatal hypoxia and pulmonary vascular function and remodeling

This review provides evidence that antenatal hypoxia, which represents a significant and worldwide problem, causes prenatal programming of the lung. A general overview of lung development is provided along with some background regarding transcriptional and signaling systems of the lung. The review illustrates that antenatal hypoxic stress can induce a continuum of responses depending on the species examined. Fetuses and newborns of certain species and specific human populations are well acclimated to antenatal hypoxia. However, antenatal hypoxia causes pulmonary vascular disease in fetuses and newborns of most mammalian species and humans. Disease can range from mild pulmonary hypertension, to severe vascular remodeling and dangerous elevations in pressure. The timing, length, and magnitude of the intrauterine hypoxic stress are important to disease development, however there is also a genetic-environmental relationship that is not yet completely understood. Determining the origins of pulmonary vascular remodeling and pulmonary hypertension and their associated effects is a challenging task, but is necessary in order to develop targeted therapies for pulmonary hypertension in the newborn due to antenatal hypoxia that can both treat the symptoms and curtail or reverse disease progression.

Abnormal Ca2+ spark/STOC coupling in cerebral artery smooth muscle cells of obese type 2 diabetic mice

Diabetes is a major risk factor for stroke. However, the molecular mechanisms involved in cerebral artery dysfunction found in the diabetic patients are not completely elucidated. In cerebral artery smooth muscle cells (CASMCs), spontaneous and local increases of intracellular Ca2+ due to the opening of ryanodine receptors (Ca2+ sparks) activate large conductance Ca2+-activated K+ (BK) channels that generate spontaneous transient outward currents (STOCs). STOCs have a key participation in the control of vascular myogenic tone and blood pressure. Our goal was to investigate whether alterations in Ca(2+) spark and STOC activities, measured by confocal microscopy and patch-clamp technique, respectively, occur in isolated CASMCs of an experimental model of type-2 diabetes (db/db mouse). We found that mean Ca(2+) spark amplitude, duration, size and rate-of-rise were significantly smaller in Fluo-3 loaded db/db compared to control CASMCs, with a subsequent decrease in the total amount of Ca(2+) released through Ca(2+) sparks in db/db CASMCs, though Ca(2+) spark frequency remained. Interestingly, the frequency of large-amplitude Ca(2+) sparks was also significantly reduced in db/db cells. In addition, the frequency and amplitude of STOCs were markedly reduced at all voltages tested (from -50 to 0 mV) in db/db CASMCs. The latter correlates with decreased BK channel β1/α subunit ratio found in db/db vascular tissues. Taken together, Ca(2+) spark alterations lead to inappropriate BK channels activation in CASMCs of db/db mice and this condition is aggravated by the decrease in the BK β1 subunit/α subunit ratio which underlies the significant reduction of Ca(2+) spark/STOC coupling in CASMCs of diabetic animals.

Maternal high-altitude hypoxia and suppression of ryanodine receptor-mediated Ca2+ sparks in fetal sheep pulmonary arterial myocytes

Ca(2+) sparks are fundamental Ca(2+) signaling events arising from ryanodine receptor (RyR) activation, events that relate to contractile and dilatory events in the pulmonary vasculature. Recent studies demonstrate that long-term hypoxia (LTH) can affect pulmonary arterial reactivity in fetal, newborn, and adult animals. Because RyRs are important to pulmonary vascular reactivity and reactivity changes with ontogeny and LTH we tested the hypothesis that RyR-generated Ca(2+) signals are more active before birth and that LTH suppresses these responses. We examined these hypotheses by performing confocal imaging of myocytes in living arteries and by performing wire myography studies. Pulmonary arteries (PA) were isolated from fetal, newborn, or adult sheep that lived at low altitude or from those that were acclimatized to 3,801 m for > 100 days. Confocal imaging demonstrated preservation of the distance between the sarcoplasmic reticulum, nucleus, and plasma membrane in PA myocytes. Maturation increased global Ca(2+) waves and Ca(2+) spark activity, with sparks becoming larger, wider, and slower. LTH preferentially depressed Ca(2+) spark activity in immature pulmonary arterial myocytes, and these sparks were smaller, wider, and slower. LTH also suppressed caffeine-elicited contraction in fetal PA but augmented contraction in the newborn and adult. The influence of both ontogeny and LTH on RyR-dependent cell excitability shed new light on the therapeutic potential of these channels for the treatment of pulmonary vascular disease in newborns as well as adults.

An investigation of the mechanisms of hydrogen sulfide-induced vasorelaxation in rat middle cerebral arteries

Hydrogen sulfide (H(2)S) is an endogenous mediator with peripheral vasorelaxant effects; however, the mechanism of H(2)S-induced vasorelaxation in cerebral blood vessels has not been extensively studied. Vasorelaxation studies were performed on middle cerebral arteries from male Sprague Dawley rats using wire myography. Immunofluorescence staining was used to detect the presence of the H(2)S-producing enzyme cystathionine-γ-lyase (CSE). CSE was present in the endothelium and smooth muscle of middle cerebral arteries. The CSE substrate, L-cysteine, induced vasorelaxation that was sensitive to the CSE inhibitor DL-propargylglycine. This relaxation was independent of endothelium, suggesting that H(2)S was produced in the vascular smooth muscle. The H(2)S donor, sodium hydrogen sulfide (NaHS; 0.1-3.0 mM) produced concentration-dependent relaxation, which was unaffected by endothelium removal. Nifedipine (3 μM) significantly reduced the maximum relaxation elicited by NaHS. Inhibiting potassium (K(+)) conductance with 50 mM K(+) significantly attenuated NaHS-induced relaxation, however, selective blockers of ATP sensitive (K(ATP)), calcium sensitive (K(Ca)), voltage dependent (K(V)), or inward rectifier (K(ir)) channels alone or in combination did not affect the response to NaHS. 4,4-diisothiocyanatostilbene-2,2-disulfonic acid (DIDS; 300 μM) caused a significant rightward shift of the NaHS concentration-response curve, but this effect could not be explained by inhibition of Cl(-) channels or Cl(-)/HCO (3)(-) exchange, as selective blockade of these mechanisms had no effect. These findings suggest endogenous H(2)S can regulate cerebral vascular function. The H(2)S-mediated relaxation of middle cerebral arteries is DIDS sensitive and partly mediated by inhibition of L-type calcium channels, with an additional contribution by K channels but not K(ATP), K(Ca), K(V), or K(ir) subtypes.

Relationship between Ca2+ sparklets and sarcoplasmic reticulum Ca2+ load and release in rat cerebral arterial smooth muscle

Ca(+) sparklets are subcellular Ca(2+) signals produced by the opening of sarcolemmal L-type Ca(2+) channels. Ca(2+) sparklet activity varies within the sarcolemma of arterial myocytes. In this study, we examined the relationship between Ca(2+) sparklet activity and sarcoplasmic reticulum (SR) Ca(2+) accumulation and release in cerebral arterial myocytes. Our data indicate that the SR is a vast organelle with multiple regions near the sarcolemma of these cells. Ca(2+) sparklet sites were located at or <0.2 μm from SR-sarcolemmal junctions. We found that while Ca(2+) sparklets increase the rate of SR Ca(2+) refilling in arterial myocytes, their activity did not induce regional variations in SR Ca(2+) content or Ca(2+) spark activity. In arterial myocytes, L-type Ca(2+) channel activity was independent of SR Ca(2+) load. This ruled out a potential feedback mechanism whereby SR Ca(2+) load regulates the activity of these channels. Together, our data suggest a model in which Ca(2+) sparklets contribute Ca(2+) influx into a cytosolic Ca(2+) pool from which sarco(endo)plasmic reticulum Ca(2+)-ATPase pumps Ca(2+) into the SR, indirectly regulating SR function.

Cytochrome P450 pathway contributes to methanandamide-induced vasorelaxation in rat aorta

PURPOSE

The generation of hyperpolarising vasorelaxant endothelial cytochrome P450 epoxygenase (CYP)-derived metabolites of arachidonic may provide beneficial effects for the treatment of cardiovascular diseases in which the bioavailability of NO is impaired. The cannabinoid methanandamide has vasodilatory properties linked to hyperpolarisation. The aim of the present work was to investigate the vasorelaxant effects of methanandamide in rat aorta, focusing on the role of cytochrome P450 pathway.

METHODS

Changes in isometric tension in response to a cumulative concentration-response curve of methanandamide (1 nM-100 μM) were recorded in aortic rings from male Wistar rats. The involvement of cannabinoid receptors, endothelial nitric oxide (NO)-, prostacyclin- and some hyperpolarising-mediated pathways were investigated. The activation of large-conductance Ca(2+)-activated K(+) (BKCa) channels have also been evaluated.

RESULTS

Methanandamide provoked an endothelium-dependent vasorelaxation in rat aorta, reaching a maximal effect (Rmax) of 67% ± 2.6%. This vasorelaxation was clearly inhibited by the combination of CB(1) and CB(2) cannabinoid antagonists (Rmax: 21.6% ± 1.3%) and by the combination of guanylate cyclase and CYP inhibitors (Rmax: 16.7% ± 1.1%). The blockade induced separately by guanylate cyclase (31.3% ± 2.8%) or CYP (36.3% ± 6.6%) inhibitors on methanandamide vasorelaxation was not significantly modified by either CB(1) or CB(2) inhibition. BKCa channels inhibition caused a partial and significant inhibition of the methanandamide vasorelaxation (Rmax: 39.9% ± 3.3%).

CONCLUSIONS

Methanandamide endothelium-dependent vasorelaxation is mediated by CB(1) and CB(2) cannabinoid receptors. The NO- and CYP-mediated pathways contribute in a concurrent manner in this vascular effect. Stimulation of both cannabinoid receptor subtypes is indistinctly linked to NO or CYP routes to cause vasorelaxation.

Amplification of EDHF-type vasodilatations in TRPC1-deficient mice

BACKGROUND AND PURPOSE

TRPC1 channels are expressed in the vasculature and are putative candidates for intracellular Ca(2+) handling. However, little is known about their role in endothelium-dependent vasodilatations including endothelium-derived hyperpolarizing factor (EDHF) vasodilatations, which require activation of Ca(2+) -activated K(+) channels (K(Ca)). To provide molecular information on the role of TRPC1 for K(Ca) function and the EDHF signalling complex, we examined endothelium-dependent and independent vasodilatations, K(Ca) currents and smooth muscle contractility in TRPC1-deficient mice (TRPC1-/-).

EXPERIMENTAL APPROACH

Vascular responses were studied using pressure/wire myography and intravital microscopy. We performed electrophysiological measurements, and confocal Ca(2+) imaging for studying K(Ca) channel functions and Ca(2+) sparks.

KEY RESULTS

TRPC1 deficiency in carotid arteries produced a twofold augmentation of TRAM-34- and UCL1684-sensitive EDHF-type vasodilatations and of endothelial hyperpolarization to acetylcholine. NO-mediated vasodilatations were unchanged. TRPC1-/- exhibited enhanced EDHF-type vasodilatations in resistance-sized arterioles in vivo associated with reduced spontaneous tone. Endothelial IK(Ca) /SK(Ca)-type K(Ca) currents, smooth muscle cell Ca(2+) sparks and associated BK(Ca)-mediated spontaneous transient outward currents were unchanged in TRPC1-/-. Smooth muscle contractility induced by receptor-operated Ca(2+) influx or Ca(2+) release and endothelium-independent vasodilatations were unaltered in TRPC1-/-. TRPC1-/- exhibited lower systolic blood pressure as determined by tail-cuff blood pressure measurements.

CONCLUSIONS AND IMPLICATIONS

Our data demonstrate that TRPC1 acts as a negative regulator of endothelial K(Ca) channel-dependent EDHF-type vasodilatations and thereby contributes to blood pressure regulation. Thus, we propose a specific role of TRPC1 in the EDHF-K(Ca) signalling complex and suggest that pharmacological inhibition of TRPC1, by enhancing EDHF vasodilatations, may be a novel strategy for lowering blood pressure.

Heterogeneous function of ryanodine receptors, but not IP3 receptors, in hamster cremaster muscle feed arteries and arterioles

The roles played by ryanodine receptors (RyRs) and inositol 1,4,5-trisphosphate receptors (IP₃Rs) in vascular smooth muscle in the microcirculation remain unclear. Therefore, the function of both RyRs and IP₃Rs in Ca(²+) signals and myogenic tone in hamster cremaster muscle feed arteries and downstream arterioles were assessed using confocal imaging and pressure myography. Feed artery vascular smooth muscle displayed Ca(²+) sparks and Ca(²+) waves, which were inhibited by the RyR antagonists ryanodine (10 μM) or tetracaine (100 μM). Despite the inhibition of sparks and waves, ryanodine or tetracaine increased global intracellular Ca(²+) and constricted the arteries. The blockade of IP₃Rs with xestospongin D (5 μM) or 2-aminoethoxydiphenyl borate (100 μM) or the inhibition of phospholipase C using U-73122 (10 μM) also attenuated Ca(2+) waves without affecting Ca(²+) sparks. Importantly, the IP₃Rs and phospholipase C antagonists decreased global intracellular Ca(2+) and dilated the arteries. In contrast, cremaster arterioles displayed only Ca(²+) waves: Ca(²+) sparks were not observed, and neither ryanodine (10-50 μM) nor tetracaine (100 μM) affected either Ca(²+) signals or arteriolar tone despite the presence of functional RyRs as assessed by responses to the RyR agonist caffeine (10 mM). As in feed arteries, arteriolar Ca(²+) waves were attenuated by xestospongin D (5 μM), 2-aminoethoxydiphenyl borate (100 μM), and U-73122 (10 μM), accompanied by decreased global intracellular Ca(²+) and vasodilation. These findings highlight the contrasting roles played by RyRs and IP₃Rs in Ca(²+) signals and myogenic tone in feed arteries and demonstrate important differences in the function of RyRs between feed arteries and downstream arterioles.

Roles of cytosolic Ca2+ concentration and myofilament Ca2+ sensitization in age-dependent cerebrovascular myogenic tone

In light of evidence that immature arteries contain a higher proportion of noncontractile smooth muscle cells than found in fully differentiated mature arteries, the present study explored the hypothesis that age-related differences in the smooth muscle phenotype contribute to age-related differences in contractility. Because Ca(2+) handling differs markedly between contractile and noncontractile smooth muscle, the present study specifically tested the hypothesis that the relative contributions of Ca(2+) influx and myofilament sensitization to myogenic tone are upregulated, whereas Ca(2+) release is downregulated, in immature [14 days postnatal (P14)] compared with mature (6 mo old) rat middle cerebral arteries (MCAs). Myofilament Ca(2+) sensitivity measured in β-escin-permeabilized arteries increased with pressure in P14 but not adult MCAs. Cyclopiazonic acid (an inhibitor of Ca(2+) release from the sarcoplasmic reticulum) increased diameter and reduced Ca(2+) in adult MCAs but increased diameter with no apparent change in Ca(2+) in P14 MCAs. La(3+) (Ca(2+) influx inhibitor) increased diameter and decreased Ca(2+) in adult MCAs, but in P14 MCAs, La(3+) increased diameter with no apparent change in Ca(2+). After treatment with both La(3+) and CPA, diameters were passive in both adult and P14 MCAs, but Ca(2+) was decreased only in adult MCAs. To quantify the fraction of smooth muscle cells in the fully differentiated contractile phenotype, extents of colocalization between smooth muscle α-actin and SM2 myosin heavy chain were determined and found to be at least twofold greater in adult than pup MCAs. These data suggest that compared with adult MCAs, pup MCAs contain a greater proportion of noncontractile smooth muscle and, as a consequence, rely more on myofilament Ca(2+) sensitization and Ca(2+) influx to maintain myogenic reactivity. The inability of La(3+) to reduce cytosolic Ca(2+) in the pup MCA appears due to La(3+)-insensitive noncontractile smooth muscle cells, which contribute to the spatially averaged measurements of Ca(2+) but not contraction.

Endothelium-dependent control of vascular tone during early postnatal and juvenile growth

Endothelial dysfunction can develop at an early age in children with risk factors for cardiovascular disease. A clear understanding of the nature of this dysfunction and how it can worsen over time requires detailed information on the normal growth-related changes in endothelial function on which the pathological changes are superimposed. This review summarizes our current understanding of these normal changes, as derived from studies in four different mammalian species. Although the endothelium plays an important role in controlling vascular tone from birth onward, the vasoactive molecules that mediate this control often change during postnatal or juvenile growth. The specifics of this transition to an adult endothelial cell phenotype can vary depending on the vascular bed. During growth, the contribution of nitric oxide to endothelium-dependent dilation generally increases in the lung, cerebral cortex, and skeletal muscle, but decreases in the intestine. Endothelial capacity for release of other vasoactive factors (e.g., cyclooxygenase products, hydrogen peroxide, carbon monoxide) can also increase or decrease during growth. Although these changes have been well documented, there is less information on their underlying cellular or molecular events. Further research is required to clarify these mechanisms, and to evaluate the functional significance of such shifts in endothelial phenotype.

Sarcoplasmic reticulum function in smooth muscle

The sarcoplasmic reticulum (SR) of smooth muscles presents many intriguing facets and questions concerning its roles, especially as these change with development, disease, and modulation of physiological activity. The SR's function was originally perceived to be synthetic and then that of a Ca store for the contractile proteins, acting as a Ca amplification mechanism as it does in striated muscles. Gradually, as investigators have struggled to find a convincing role for Ca-induced Ca release in many smooth muscles, a role in controlling excitability has emerged. This is the Ca spark/spontaneous transient outward current coupling mechanism which reduces excitability and limits contraction. Release of SR Ca occurs in response to inositol 1,4,5-trisphosphate, Ca, and nicotinic acid adenine dinucleotide phosphate, and depletion of SR Ca can initiate Ca entry, the mechanism of which is being investigated but seems to involve Stim and Orai as found in nonexcitable cells. The contribution of the elemental Ca signals from the SR, sparks and puffs, to global Ca signals, i.e., Ca waves and oscillations, is becoming clearer but is far from established. The dynamics of SR Ca release and uptake mechanisms are reviewed along with the control of luminal Ca. We review the growing list of the SR's functions that still includes Ca storage, contraction, and relaxation but has been expanded to encompass Ca homeostasis, generating local and global Ca signals, and contributing to cellular microdomains and signaling in other organelles, including mitochondria, lysosomes, and the nucleus. For an integrated approach, a review of aspects of the SR in health and disease and during development and aging are also included. While the sheer versatility of smooth muscle makes it foolish to have a "one model fits all" approach to this subject, we have tried to synthesize conclusions wherever possible.

Minding the calcium store: Ryanodine receptor activation as a convergent mechanism of PCB toxicity

Chronic low-level polychlorinated biphenyl (PCB) exposures remain a significant public health concern since results from epidemiological studies indicate that PCB burden is associated with immune system dysfunction, cardiovascular disease, and impairment of the developing nervous system. Of these various adverse health effects, developmental neurotoxicity has emerged as a particularly vulnerable endpoint in PCB toxicity. Arguably the most pervasive biological effects of PCBs could be mediated by their ability to alter the spatial and temporal fidelity of Ca2+ signals through one or more receptor-mediated processes. This review will focus on our current knowledge of the structure and function of ryanodine receptors (RyRs) in muscle and nerve cells and how PCBs and related non-coplanar structures alter these functions. The molecular and cellular mechanisms by which non-coplanar PCBs and related structures alter local and global Ca2+ signaling properties and the possible short and long-term consequences of these perturbations on neurodevelopment and neurodegeneration are reviewed.

Role of ryanodine receptor subtypes in initiation and formation of calcium sparks in arterial smooth muscle: comparison with striated muscle

Calcium sparks represent local, rapid, and transient calcium release events from a cluster of ryanodine receptors (RyRs) in the sarcoplasmic reticulum. In arterial smooth muscle cells (SMCs), calcium sparks activate calcium-dependent potassium channels causing decrease in the global intracellular [Ca2+] and oppose vasoconstriction. This is in contrast to cardiac and skeletal muscle, where spatial and temporal summation of calcium sparks leads to global increases in intracellular [Ca2+] and myocyte contraction. We summarize the present data on local RyR calcium signaling in arterial SMCs in comparison to striated muscle and muscle-specific differences in coupling between L-type calcium channels and RyRs. Accordingly, arterial SMC Ca(v)1.2 L-type channels regulate intracellular calcium stores content, which in turn modulates calcium efflux though RyRs. Downregulation of RyR2 up to a certain degree is compensated by increased SR calcium content to normalize calcium sparks. This indirect coupling between Ca(v)1.2 and RyR in arterial SMCs is opposite to striated muscle, where triggering of calcium sparks is controlled by rapid and direct cross-talk between Ca(v)1.1/Ca(v)1.2 L-type channels and RyRs. We discuss the role of RyR isoforms in initiation and formation of calcium sparks in SMCs and their possible molecular binding partners and regulators, which differ compared to striated muscle.

Glutamate regulates Ca2+ signals in smooth muscle cells of newborn piglet brain slice arterioles through astrocyte- and heme oxygenase-dependent mechanisms

Glutamate is the principal cerebral excitatory neurotransmitter and dilates cerebral arterioles to match blood flow to neural activity. Arterial contractility is regulated by local and global Ca(2+) signals that occur in smooth muscle cells, but modulation of these signals by glutamate is poorly understood. Here, using high-speed confocal imaging, we measured the Ca(2+) signals that occur in arteriole smooth muscle cells of newborn piglet tangential brain slices, studied signal regulation by glutamate, and investigated the physiological function of heme oxygenase (HO) and carbon monoxide (CO) in these responses. Glutamate elevated Ca(2+) spark frequency by approximately 188% and reduced global intracellular Ca(2+) concentration ([Ca(2+)](i)) to approximately 76% of control but did not alter Ca(2+) wave frequency in brain arteriole smooth muscle cells. Isolation of cerebral arterioles from brain slices abolished glutamate-induced Ca(2+) signal modulation. In slices treated with l-2-alpha-aminoadipic acid, a glial toxin, glutamate did not alter Ca(2+) sparks or global [Ca(2+)](i) but did activate Ca(2+) waves. This shift in Ca(2+) signal modulation by glutamate did not occur in slices treated with d-2-alpha-aminoadipic acid, an inactive isomer of l-2-alpha-aminoadipic acid. In the presence of chromium mesoporphyrin, a HO blocker, glutamate inhibited Ca(2+) sparks and Ca(2+) waves and did not alter global [Ca(2+)](i). In isolated arterioles, CORM-3 [tricarbonylchloro(glycinato)ruthenium(II)], a CO donor, activated Ca(2+) sparks and reduced global [Ca(2+)](i). These effects were blocked by 1H-(1,2,4)-oxadiazolo-(4,3-a)-quinoxalin-1-one, a soluble guanylyl cyclase inhibitor. Collectively, these data indicate that glutamate can modulate Ca(2+) sparks, Ca(2+) waves, and global [Ca(2+)](i) in arteriole smooth muscle cells via mechanisms that require astrocytes and HO. These data also indicate that soluble guanylyl cyclase is involved in CO activation of Ca(2+) sparks in arteriole smooth muscle cells.

Hypertension of Kcnmb1-/- is linked to deficient K secretion and aldosteronism

Mice lacking the beta1-subunit (gene, Kcnmb1; protein, BK-beta1) of the large Ca-activated K channel (BK) are hypertensive. This phenotype is thought to result from diminished BK currents in vascular smooth muscle where BK-beta1 is an ancillary subunit. However, the beta1-subunit is also expressed in the renal connecting tubule (CNT), a segment of the aldosterone-sensitive distal nephron, where it associates with BK and facilitates K secretion. Because of the correlation between certain forms of hypertension and renal defects, particularly in the distal nephron, it was determined whether the hypertension of Kcnmb1(-/-) has a renal origin. We found that Kcnmb1(-/-) are hypertensive, volume expanded, and have reduced urinary K and Na clearances. These conditions are exacerbated when the animals are fed a high K diet (5% K; HK). Supplementing HK-fed Kcnmb1(-/-) with eplerenone (mineralocorticoid receptor antagonist) corrected the fluid imbalance and more than 70% of the hypertension. Finally, plasma [aldo] was elevated in Kcnmb1(-/-) under basal conditions (control diet, 0.6% K) and increased significantly more than wild type when fed the HK diet. We conclude that the majority of the hypertension of Kcnmb1(-/-) is due to aldosteronism, resulting from renal potassium retention and hyperkalemia.

Calcium sparks

The calcium ion (Ca(2+)) is the simplest and most versatile intracellular messenger known. The discovery of Ca(2+) sparks and a related family of elementary Ca(2+) signaling events has revealed fundamental principles of the Ca(2+) signaling system. A newly appreciated "digital" subsystem consisting of brief, high Ca(2+) concentration over short distances (nanometers to microns) comingles with an "analog" global Ca(2+) signaling subsystem. Over the past 15 years, much has been learned about the theoretical and practical aspects of spark formation and detection. The quest for the spark mechanisms [the activation, coordination, and termination of Ca(2+) release units (CRUs)] has met unexpected challenges, however, and raised vexing questions about CRU operation in situ. Ample evidence shows that Ca(2+) sparks catalyze many high-threshold Ca(2+) processes involved in cardiac and skeletal muscle excitation-contraction coupling, vascular tone regulation, membrane excitability, and neuronal secretion. Investigation of Ca(2+) sparks in diseases has also begun to provide novel insights into hypertension, cardiac arrhythmias, heart failure, and muscular dystrophy. An emerging view is that spatially and temporally patterned activation of the digital subsystem confers on intracellular Ca(2+) signaling an exquisite architecture in space, time, and intensity, which underpins signaling efficiency, stability, specificity, and diversity. These recent advances in "sparkology" thus promise to unify the simplicity and complexity of Ca(2+) signaling in biology.

Gq-coupled receptors as mechanosensors mediating myogenic vasoconstriction

Despite the central physiological function of the myogenic response, the underlying signalling pathways and the identity of mechanosensors in vascular smooth muscle (VSM) are still elusive. In contrast to present thinking, we show that membrane stretch does not primarily gate mechanosensitive transient receptor potential (TRP) ion channels, but leads to agonist-independent activation of G(q/11)-coupled receptors, which subsequently signal to TRPC channels in a G protein- and phospholipase C-dependent manner. Mechanically activated receptors adopt an active conformation, allowing for productive G protein coupling and recruitment of beta-arrestin. Agonist-independent receptor activation by mechanical stimuli is blocked by specific antagonists and inverse agonists. Increasing the AT(1) angiotensin II receptor density in mechanically unresponsive rat aortic A7r5 cells resulted in mechanosensitivity. Myogenic tone of cerebral and renal arteries is profoundly diminished by the inverse angiotensin II AT(1) receptor agonist losartan independently of angiotensin II (AII) secretion. This inhibitory effect is enhanced in blood vessels of mice deficient in the regulator of G-protein signalling-2. These findings suggest that G(q/11)-coupled receptors function as sensors of membrane stretch in VSM cells.

Calcium sparks activate calcium-dependent Cl− current in rat corpus cavernosum smooth muscle cells

Spontaneous transient currents, due to activation of Ca2+-dependent K+ and Cl− channels, occur in corpus cavernosum smooth muscle cells (CCSMC) of the penis. The Ca2+ events responsible for triggering Ca2+-dependent Cl− channels have never been identified in vascular muscle. We used high-speed fluorescence imaging combined with patch-clamp electrophysiology to provide the first characterization of Ca2+ events underlying these currents. Freshly isolated rat CCSMC loaded with fluo-4 exhibited localized, spontaneous elevations of intracellular Ca2+ (Ca2+ sparks) in 57% of cells. There was an average of 6.4 ± 0.5 release sites/cell with a frequency of 0.9 ± 1 Hz/cell and peak amplitude ΔF/Fo of 67 ± 10%. We addressed the controversy of whether these events are mediated by ryanodine or inositol 1,4,5 trisphosphate (IP3) receptors. Caffeine caused either a global Ca2+ rise at high concentrations or an increase in spark frequency at lower concentrations, whereas ryanodine dramatically reduced the amplitude and frequency of sparks. 2-Aminoethoxydiphenyl borate, an inhibitor of IP3 receptors, had no effect on spark frequency. Combined imaging and electrophysiological recording revealed strong coupling between Ca2+ sparks and biphasic transient currents, a relationship never before shown in vascular muscle. Moreover, spark frequency increased on depolarization, an effect abolished with the blockade of Ca2+ channels, consistent with Ca2+ influx regulating Ca2+ release from stores. We establish for the first time that Ca2+ sparks occur in CCSMC and arise from Ca2+ release through ryanodine receptors. Moreover, the voltage dependence of spark frequency demonstrated here provides novel functional evidence for voltage-dependent Ca2+ influx in CCSMC.

The vasodilator 17,18-epoxyeicosatetraenoic acid targets the pore-forming BK alpha channel subunit in rodents

17,18-Epoxyeicosatetraenoic acid (17,18-EETeTr) stimulates vascular large-conductance K(+) (BK) channels. BK channels are composed of the pore-forming BK alpha and auxiliary BK beta1 subunits that confer an increased sensitivity for changes in membrane potential and calcium to BK channels. Ryanodine-sensitive calcium-release channels (RyR3) in the sarcoplasmic reticulum (SR) control the process. To elucidate the mechanism of BK channel activation, we performed whole-cell and perforated-patch clamp experiments in freshly isolated cerebral and mesenteric artery vascular smooth muscle cells (VSMC) from Sprague-Dawley rats, BK beta1 gene-deficient (-/-), BK alpha (-/-), RyR3 (-/-) and wild-type mice. The 17,18-EETeTr (100 nm) increased tetraethylammonium (1 mm)-sensitive outward K(+) currents in VSMC from wild-type rats and wild-type mice. The effects were not inhibited by the epoxyeicosatrienoic acid (EET) antagonist 14,15-epoxyeicosa-5(Z)-enoic acid (10 mum). BK channel currents were increased 3.5-fold in VSMC from BK beta1 (-/-) mice, whereas a 2.9-fold stimulation was observed in VSMC from RyR3 (-/-) mice (at membrane voltage 60 mV). The effects were similar compared with those observed in cells from wild-type mice. The BK current increase was neither influenced by strong internal calcium buffering (Ca(2)(+), 100 nm), nor by external calcium influx. The 17,18-EETeTr did not induce outward currents in VSMC BK alpha (-/-) cells. We next tested the vasodilator effects of 17,18-EETeTr on isolated arteries of BK alpha-deficient mice. Vasodilatation was largely inhibited in cerebral and mesenteric arteries isolated from BK alpha (-/-) mice compared with that observed in wild-type and BK beta1 (-/-) arteries. We conclude that 17,18-EETeTr represents an endogenous BK channel agonist and vasodilator. Since 17,18-EETeTr is active in small arteries lacking BK beta1, the data further suggest that BK alpha represents the molecular target for the principal action of 17,18-EETeTr. Finally, the action of 17,18-EETeTr is not mediated by changes of the internal global calcium concentration or local SR calcium release events.

Indirect coupling between Cav1.2 channels and ryanodine receptors to generate Ca2+ sparks in murine arterial smooth muscle cells

In arterial vascular smooth muscle cells (VSMCs), Ca(2+) sparks stimulate nearby Ca(2+)-activated K(+) (BK) channels that hyperpolarize the membrane and close L-type Ca(2+) channels. We tested the contribution of L-type Ca(v)1.2 channels to Ca(2+) spark regulation in tibial and cerebral artery VSMCs using VSMC-specific Ca(v)1.2 channel gene disruption in (SMAKO) mice and an approach based on Poisson statistical analysis of activation frequency and first latency of elementary events. Ca(v)1.2 channel gene inactivation reduced Ca(2+) spark frequency and amplitude by approximately 50% and approximately 80%, respectively. These effects were associated with lower global cytosolic Ca(2+) levels and reduced sarcoplasmic reticulum (SR) Ca(2+) load. Elevating cytosolic Ca(2+) levels reversed the effects completely. The activation frequency and first latency of elementary events in both wild-type and SMAKO VSMCs weakly reflected the voltage dependency of L-type channels. This study provides evidence that local and tight coupling between the Ca(v)1.2 channels and ryanodine receptors (RyRs) is not required to initiate Ca(2+) sparks. Instead, Ca(v)1.2 channels contribute to global cytosolic [Ca(2+)], which in turn influences luminal SR calcium and thus Ca(2+) sparks.

Regulator of G protein signalling 2 ameliorates angiotensin II-induced hypertension in mice

Angiotensin II (Ang II) activates signalling pathways predominantly through the G-protein-coupled Ang II type 1 receptor (AT(1)R). The regulator of G protein signalling 2 (RGS2) is a negative G protein regulator. We hypothesized that RGS2 deletion changes blood pressure regulation by increasing the response to Ang II. To address this issue, we infused Ang II (0.5 mg kg(-1) day(-1)) chronically into conscious RGS2-deleted (RGS2(-/-)) and wild-type (RGS2(+/+)) mice, measured mean arterial blood pressure and heart rate (HR) with telemetry and assessed vasoreactivity and gene expression of AT(1A), AT(1B) and AT(2) receptors. Angiotensin II infusion increased blood pressure more in RGS2(-/-) than in RGS2(+/+) mice, while HR was not different between the groups, indicating a resetting of the baroreceptor reflex. Urinary catecholamine excretion was similar in Ang II-infused RGS2(-/-) and RGS2(+/+) mice, indicating a minor role of sympathetic tone for blood pressure differences. Myogenic tone and vasoreactivity in response to Ang II, endothelin-1 and phenylephrine were increased in isolated renal interlobar arterioles of RGS2(-/-) mice compared with RGS2(+/+) mice. The AT(1A), AT(1B) and AT(2) receptor gene expression was not different between RGS2(-/-) and RGS2(+/+) mice. Our findings suggest that RGS2 deletion promotes Ang II-dependent hypertension primarily through an increase of myogenic tone and vasoreactivity, probably by sensitization of AT(1) receptors.

Myogenic contractility is more dependent on myofilament calcium sensitization in term fetal than adult ovine cerebral arteries

Regulation of cytosolic calcium and myofilament calcium sensitivity varies considerably with postnatal age in cerebral arteries. Because these mechanisms also govern myogenic tone, the present study used graded stretch to examine the hypothesis that myogenic tone is less dependent on calcium influx and more dependent on myofilament calcium sensitization in term fetal compared with adult cerebral arteries. Term fetal and adult posterior communicating cerebral arteries exhibited similar myogenic responses, with peak tensions averaging 24 and 26% of maximum contractile force produced in any given tissue in response to an isotonic Krebs buffer containing 122 mM K+ (Kmax) at optimum stretch ratios (working diameter/unstressed diameter) of 2.19 and 2.23, respectively. Graded stretch increased cytosolic Ca2+ concentration at stretch ratios >2.0 in adult arteries, but increased Ca2+ concentration only at stretch ratios >2.3 in fetal arteries. In permeabilized arteries, myogenic tone peaked at a stretch ratio of 2.1 in both fetal and adult arteries. The fetal %Kmax values at peak myogenic tone were not significantly different at either pCa 7.0 (23%) or pCa 5.5 (25%) but were significantly less at pCa 8.0 (8.4 ± 2.3%). Conversely, adult %Kmax values at peak myogenic tone were significantly less at both pCa 8.0 (10.4 ± 1.8%) and pCa 7.0 (16%) than at pCa 5.5 (27%). The maximal extents of stretch-induced increases in myosin light chain phosphorylation in intact fetal (20%) and adult (17%) arteries were similar. The data demonstrate that the cerebrovascular myogenic response is highly conserved during postnatal maturation but is mediated differently in fetal and adult cerebral arteries.

Sorcin modulation of Ca2+ sparks in rat vascular smooth muscle cells

Spontaneous, local Ca(2+) release events or Ca(2+) sparks by ryanodine receptors (RyRs) are important determinants of vascular tone and arteriolar resistance, but the mechanisms that modulate their properties in smooth muscle are poorly understood. Sorcin, a Ca(2+)-binding protein that associates with cardiac RyRs and quickly stops Ca(2+) release in the heart, provides a potential mechanism to modulate Ca(2+) sparks in vascular smooth muscle, but little is known about the functional role of sorcin in this tissue. In this work, we characterized the expression and intracellular location of sorcin in aorta and cerebral artery and gained mechanistic insights into its functional role as a modulator of Ca(2+) sparks. Sorcin is present in endothelial and smooth muscle cells, as assessed by immunocytochemical and Western blot analyses. Smooth muscle sorcin translocates from cytosolic to membranous compartments in a Ca(2+)-dependent manner and associates with RyRs, as shown by coimmunoprecipitation and immunostaining experiments. Ca(2+) sparks recorded in saponin-permeabilized vascular myocytes have increased frequency, duration and spatial spread but reduced amplitude with respect to Ca(2+) sparks in intact cells, suggesting that permeabilization disrupts the normal organization of RyRs and releases diffusible substances that control Ca(2+) spark properties. Perfusion of 2 mum sorcin onto permeabilized myocytes reduced the amplitude, duration and spatial spread of Ca(2+) sparks, demonstrating that sorcin effectively regulates Ca(2+) signalling in vascular smooth muscle. Together with a dense distribution in the perimeter of the cell along a pool of RyRs, these properties make sorcin a viable candidate to modulate vascular tone in smooth muscle.

Modulation of BK channel calcium affinity by differential phosphorylation in developing ovine basilar artery myocytes

Large-conductance Ca2+-sensitive K+ (BK) channel activity is greater in basilar artery smooth muscle cells (SMCs) of the fetus than the adult, and this increased activity is associated with a lower BK channel Ca2+ set point (Ca0). Associated PKG activity is three times greater in BK channels from fetal than adult myocytes, whereas associated PKA activity is three times greater in channels from adult than fetal myocytes. We hypothesized that the change in Ca0 during development results from different levels of channel phosphorylation. In inside-out membrane patch preparations of basilar artery SMCs from adult and fetal sheep, we measured BK channel activity in four states of phosphorylation: native, dephosphorylated, PKA phosphorylated, and PKG phosphorylated. BK channels from adult and fetus exhibited similar voltage-activation curves, Ca0 values, and Ca2+ dissociation constants (Kd) for the dephosphorylated, PKA phosphorylated, and PKG phosphorylated states. However, voltage-activation curves of native fetal BK channels shifted significantly to the left of those of the adult, with Ca0 and Kd values half those of the adult. For the two age groups at each of the phosphorylation states, Ca0 and Kd produced linear relations when plotted against voltage at half-maximal channel activation. We conclude that the Ca0 and Kd values of the BK channel can be modulated by differential channel phosphorylation. Lower Ca0 and Kd values in BK channels of fetal myocytes can be explained by a greater extent of channel phosphorylation of fetal than adult myocytes.

Increased vascular smooth muscle contractility in TRPC6-/- mice

Among the TRPC subfamily of TRP (classical transient receptor potential) channels, TRPC3, -6, and -7 are gated by signal transduction pathways that activate C-type phospholipases as well as by direct exposure to diacylglycerols. Since TRPC6 is highly expressed in pulmonary and vascular smooth muscle cells, it represents a likely molecular candidate for receptor-operated cation entry. To define the physiological role of TRPC6, we have developed a TRPC6-deficient mouse model. These mice showed an elevated blood pressure and enhanced agonist-induced contractility of isolated aortic rings as well as cerebral arteries. Smooth muscle cells of TRPC6-deficient mice have higher basal cation entry, increased TRPC-carried cation currents, and more depolarized membrane potentials. This higher basal cation entry, however, was completely abolished by the expression of a TRPC3-specific small interference RNA in primary TRPC6(-)(/)(-) smooth muscle cells. Along these lines, the expression of TRPC3 in wild-type cells resulted in increased basal activity, while TRPC6 expression in TRPC6(-/-) smooth muscle cells reduced basal cation influx. These findings imply that constitutively active TRPC3-type channels, which are up-regulated in TRPC6-deficient smooth muscle cells, are not able to functionally replace TRPC6. Thus, TRPC6 has distinct nonredundant roles in the control of vascular smooth muscle tone.

Pharmacological modulation of sarcoplasmic reticulum function in smooth muscle

The sarco/endoplasmic reticulum (SR/ER) is the primary storage and release site of intracellular calcium (Ca2+) in many excitable cells. The SR is a tubular network, which in smooth muscle (SM) cells distributes close to cellular periphery (superficial SR) and in deeper aspects of the cell (deep SR). Recent attention has focused on the regulation of cell function by the superficial SR, which can act as a buffer and also as a regulator of membrane channels and transporters. Ca2+ is released from the SR via two types of ionic channels [ryanodine- and inositol 1,4,5-trisphosphate-gated], whereas accumulation from thecytoplasm occurs exclusively by an energy-dependent sarco-endoplasmic reticulum Ca2+-ATPase pump (SERCA). Within the SR, Ca2+ is bound to various storage proteins. Emerging evidence also suggests that the perinuclear portion of the SR may play an important role in nuclear transcription. In this review, we detail the pharmacology of agents that alter the functions of Ca2+ release channels and of SERCA. We describe their use and selectivity and indicate the concentrations used in investigating various SM preparations. Important aspects of cell regulation and excitation-contractile activity coupling in SM have been uncovered through the use of such activators and inhibitors of processes that determine SR function. Likewise, they were instrumental in the recent finding of an interaction of the SR with other cellular organelles such as mitochondria. Thus, an appreciation of the pharmacology and selectivity of agents that interfere with SR function in SM has greatly assisted in unveiling the multifaceted nature of the SR.