Ca2+ regulation of vascular smooth muscle

Regulation of intracellular free Ca2+ concentrations in vascular smooth muscle is accomplished mainly by Ca2+ channels and ATP-dependent Ca2+ pumps in the plasmalemma and sarcoplasmic reticulum (SR). Ca2+ entry through the plasmalemma is apparently mediated by four different pathways: leak; receptor-operated Ca2+ channels; potential sensitive Ca2+ channels; and stretch-activated channels. The agonist releasable intracellular Ca2+ store appears to be identical with the SR. Evidence for the involvement of Ca2+-induced Ca2+ release and inositol-1,4,5-trisphosphate in the release of SR Ca2+ is discussed. Smooth muscle contractions induced by certain agonists may be further enhanced by inhibition of Ca2+ uptake by the SR and of active Ca2+ extrusion across the plasmalemma. At the moment it is not clear from a consideration of the Ca2+ regulatory mechanisms present in vascular smooth muscle how dietary Ca2+ affects vascular tone. The increased Ca2+ permeation through smooth muscle cell membranes of resistance arteries taken from spontaneously hypertensive rats may be relevant to this problem.     

Resting calcium influx in airway smooth muscle

Plasma membrane Ca2+ leak remains the most uncertain of the cellular Ca2+ regulation pathways. During passive Ca2+ influx in non-stimulated smooth muscle cells, basal activity of constitutive Ca2+ channels seems to be involved. In vascular smooth muscle, the 3 following Ca2+ entry pathways contribute to this phenomenon: (i) via voltage-dependent Ca2+ channels, (ii) receptor gated Ca2+ channels, and (iii) store operated Ca2+ channels, although, in airway smooth muscle it seems only 2 passive Ca2+ influx pathways are implicated, one sensitive to SKF 96365 (receptor gated Ca2+ channels) and the other to Ni2+ (store operated Ca2+ channels). Resting Ca2+ entry could provide a sufficient amount of Ca2+ and contribute to resting intracellular Ca2+ concentration ([Ca2+]i), maintenance of the resting membrane potential, myogenic tone, and sarcoplasmic reticulum-Ca2+ refilling. However, further research, especially in airway smooth muscle, is required to better explore the physiological role of this passive Ca2+ influx pathway as it could be involved in airway hyperresponsiveness.     

Thyrotropin-releasing hormone activates KCa channels in gastric smooth muscle cells via intracellular Ca2+ release

Thyrotropin-releasing hormone (TRH) is released in high concentrations into gastric juice, but its direct effect on gastric smooth muscles has not been studied yet. We undertook studies on TRH effect on gastric smooth muscle using contraction and patch clamp methods. TRH was found to inhibit both acetylcholine- and BaCl2-induced contractions of gastric strips. TRH, applied to single cells, inhibited the voltage-dependent Ca2+ currents and activated the whole-cell K+ currents. The TRH-induced changes in K+ currents and membrane potential were effectively abolished by inhibitors of either intracellular Ca2+ release channels or phospholipase C. Neither activators, nor blockers of protein kinase C could affect the action of TRH on K+ currents. In conclusion, TRH activates K+ channels via inositol-1,4,5-trisphosphate-induced release of Ca2+ in the direction to the plasma membrane, which in turn leads to stimulation of the Ca2+-sensitive K+ conductance, membrane hyperpolarization and relaxation. The data imply that TRH may act physiologically as a local modulator of gastric smooth muscle tone.     

Caffeine activates a Ca(2+)-permeable, nonselective cation channel in smooth muscle cells

The effects of caffeine on cytoplasmic [Ca2+] ([Ca2+]i) and plasma membrane currents were studied in single gastric smooth muscle cells dissociated from the toad, Bufo marinus. Experiments were carried out using Fura-2 for measuring [Ca2+]i and tight-seal voltage-clamp techniques for recording membrane currents. When the membrane potential was held at -80 mV, in 15% of the cells studied caffeine increased [Ca2+]i without having any effect on membrane currents. In these cells ryanodine completely abolished any caffeine induced increase in [Ca2+]i. In the other cells caffeine caused both an increase in [Ca2+]i and activation of an 80-pS nonselective cation channel. In this group of cells ryanodine only partially blocked the increase in [Ca2+]i induced by caffeine; moreover, the change in [Ca2+]i that did occur was tightly coupled to the time course and magnitude of the cation current through these channels. In the presence of ryanodine, blockade of the 80-pS channel by GdCl3 or decreasing the driving force for Ca2+ influx through the plasma membrane by holding the membrane potential at +60 mV almost completely blocked the increase in [Ca2+]i induced by caffeine. Thus, the channel activated by caffeine appears to be permeable to Ca2+. Caffeine activated the cation channel even when [Ca2+]i was clamped to below 10 nM when the patch pipette contained 10 mM BAPTA suggesting that caffeine directly activates the channel and that it is not being activated by the increase in Ca2+ that occurs when caffeine is applied to the cell. Corroborating this suggestion were additional results showing that when the membrane was depolarized to activate voltage-gated Ca2+ channels or when Ca2+ was released from carbachol- sensitive internal Ca2+ stores, the 80-pS channel was not activated. Moreover, caffeine was able to activate the channel in the presence of ryanodine at both positive and negative potentials, both conditions preventing release of Ca2+ from stores and the former preventing its influx. In summary, in gastric smooth muscle cells caffeine transiently releases Ca2+ from a ryanodine-sensitive internal store and also increases Ca2+ influx through the plasma membrane by activating an 80- pS cation channel by a mechanism which does not seem to involve an elevation of [Ca2+]i.     

Cyclopiazonic acid activates a Ca2+-permeable, nonselective cation conductance in porcine and bovine tracheal smooth muscle.

Capacitative Ca2+ entry has been examined in several tissues and, in some, appears to be mediated by nonselective cation channels collectively referred to as "store-operated" cation channels; however, relatively little is known about the electrophysiological properties of these channels in airway smooth muscle. Consequently we examined the electrophysiological characteristics and changes in intracellular Ca2+ concentration associated with a cyclopiazonic acid (CPA)-evoked current in porcine and bovine airway smooth muscle using patch-clamp and Ca2+-fluorescence techniques. In bovine tracheal myocytes, CPA induced an elevation of intracellular Ca2+ that was dependent on extracellular Ca2+ and was insensitive to nifedipine (an L-type voltage-gated Ca2+ channel inhibitor). Using patch-clamp techniques and conditions that block both K+ and Cl- currents, we found that CPA rapidly activated a membrane conductance (I(CPA)) in porcine and bovine tracheal myocytes that exhibits a linear current-voltage relationship with a reversal potential around 0 mV. Replacement of extracellular Na+ resulted in a marked reduction of I(CPA) at physiological membrane potentials (i.e., -60 mV) that was accompanied by a shift in the reversal potential for I(CPA) toward more negative membrane potentials. In addition, I(CPA) was markedly inhibited by 10 microM Gd3+ and La3+ but was largely insensitive to 1 microM nifedipine. We conclude that CPA induces capacitative Ca2+ entry in porcine and bovine tracheal smooth muscle via a Gd3+- and La3+-sensitive, nonselective cation conductance.     

Ca2+ entry, efflux and release in smooth muscle

Control of smooth muscle is vital for health. The major route to contraction is a rise in intracellular [Ca2+], determined by the entry and efflux of Ca2+ and release and re-uptake into the sarcoplasmic reticulum (SR). We review these processes in myometrium, to better understand excitation-contraction coupling and develop strategies for preventing problematic labours. The main mechanism of elevating [Ca2+] is voltage-gated L-type channels, due to pacemaker activity, which can be modulated by agonists. The rise of [Ca2+] produces Ca-calmodulin and activates MLCK. This phosphorylates myosin and force results. Without Ca2+ entry uterine contraction fails. The Na/Ca exchanger (NCX) and plasma membrane Ca-ATPase (PMCA) remove Ca2+, with contributions of 30% and 70% respectively. Studies with PMCA-4 knockout mice show that it contributes to reducing [Ca2+] and relaxation. The SR contributes to relaxation by vectorially releasing Ca2+ to the efflux pathways, and thereby increasing their rates. Agonists binding produces IP3 which can release Ca from the SR but inhibition of SR Ca2+ release increases contractions and Ca2+ transients. It is suggested that SR Ca2+ targets K+ channels on the surface membrane and thereby feedback to inhibit excitability and contraction.     

Simultaneous measurement of Ca2+ release and influx into smooth muscle cells in response to caffeine. A novel approach for calculating the fraction of current carried by calcium

Activation of ryanodine receptors on the sarcoplasmic reticulum of single smooth muscle cells from the stomach muscularis of Bufo marinus by caffeine is accompanied by a rise in cytoplasmic [Ca2+] ([Ca2+]i), and the opening of nonselective cationic plasma membrane channels. To understand how each of these pathways contributes to the rise in [Ca2+]i, one needs to separately monitor Ca2+ entry through them. Such information was obtained from simultaneous measurements of ionic currents and [Ca2+]i by the development of a novel and general method to assess the fraction of current induced by an agonist that is carried by Ca2+. Application of this method to the currents induced in these smooth muscle cells by caffeine revealed that approximately 20% of the current passing through the membrane channels activated following caffeine application is carried by Ca2+. Based on this information we found that while Ca2+ entry through these channels rises slowly, release of Ca2+ from stores, while starting at the same time, is much faster and briefer. Detailed quantitative analysis of the Ca2+ release from stores suggests that it most likely decays due to depletion of Ca2+ in those stores. When caffeine was applied twice to a cell with only a brief (30 s) interval in between, the amount of Ca2+ released from stores was markedly diminished following the second caffeine application whereas the current carried in part by Ca2+ entry across the plasma membrane was not significantly affected. These and other studies described in the preceding paper indicate that activation of the nonselective cation plasma membrane channels in response to caffeine was not caused as a consequence of emptying of internal Ca2+ stores. Rather, it is proposed that caffeine activates these membrane channels either by direct interaction or alternatively by a linkage between ryanodine receptors on the sarcoplasmic reticulum and the nonselective cation channels on the surface membrane.     

pH effects on high conductance Ca2+-activated K+ channels (BK(Ca)) in human internal mammary artery smooth muscle cells

INTRODUCTION: In vascular smooth muscle cells, different types of K+ channels participate in the regulation of membrane potential and consequently in the contractile behavior of the vessel. There is little information about the properties and role of K+ channels in human internal mammary artery (HIMA), the vessel of choice for coronary revascularization. METHODS: Patch-clamp technique on isolated HIMA smooth muscle cells was used. RESULTS: This work presents for the first time single-channel properties of the high conductance Ca2+-activated K+ channel (BK(Ca)) of HIMA. It presents a single-channel conductance of 228+/-4 pS (n=44, 8 cells), is sensitive to 100 nM iberiotoxin, and its open probability is Ca2+- and voltage-dependent. Inside-out results show that BK(Ca) channels in HIMA are directly activated by increasing the pH of intracellular media (NPo=0.096+/-0.032 at pH 7.4 and NPo=0.459+/-0.111 at pH 7.6, n=12 cells, p<0.05) and inhibited by lowering this pH (NPo=0.175+/-0.067 at pH 7.4 and NPo=0.051+/-0.019 at pH 6.8, n=13 cells, p<0.05). CONCLUSIONS: The evidences presented about single-channel properties and intracellular pH sensitivity of BK(Ca) from HIMA smooth muscle cells provide useful information to elucidate physiological or pathological mechanisms in this vessel, as well as for future studies where drugs could have BK(Ca) channels as targets for pharmacological therapies.     

Signal transduction in smooth muscle as studied by the subcellular membrane approach.

Many of the contractile regulatory events in smooth muscle reside in various cellular membrane components as functional membrane constituents that interact in a variably complex manner. The physiological handling of ionized calcium (Ca2+), which serves multiple roles as an extracellular signal, a second messenger, and an activator interacting directly with myofilaments to effectuate contractile responses, referred to as Ca2+ signalling processes, represents an integral part of a more complicated membrane transduction mechanism. The subcellular membrane approach toward the understanding of Ca2+ signalling as well as the transduction mechanisms involving membrane receptors, GTP binding proteins, ion channels, membrane-bound enzymes, and the production of intracellular second messengers has made a significant contribution in smooth muscle research for the past decade. This review summarizes the current state of knowledge about the multiplicity of interactions between Ca2+ and various membrane constituents in the surface membranes and sarcoplasmic reticulum, such as Ca2+ binding, Ca2+ ATPase pumps, Ca2+ channels, and Ca2+Na+ or related ion exchangers. A number of recent novel findings from this laboratory have also been discussed. First of all, the technical refinement of membrane separation and characterization, which permits better identification of neuronal membranes in highly innervated smooth muscle tissues, led to the distinction of prejunctional and postjunctional membrane receptors. Secondly, unlike the Ca(2+)-release channels labelled with [3H]inositol 1,4,5-trisphosphate, the other type of internal membrane Ca(2+)-release channels labelled by [3H]ryanodine has been identified only recently in smooth muscle.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cyclic nucleotide regulation of store-operated Ca2+ influx in airway smooth muscle

Sarcoplasmic reticulum (SR) Ca2+ release and plasma membrane Ca2+ influx are key to intracellular Ca2+ ([Ca2+]i) regulation in airway smooth muscle (ASM). SR Ca2+ depletion triggers influx via store-operated Ca2+ channels (SOCC) for SR replenishment. Several clinically relevant bronchodilators mediate their effect via cyclic nucleotides (cAMP, cGMP). We examined the effect of cyclic nucleotides on SOCC-mediated Ca2+ influx in enzymatically dissociated porcine ASM cells. SR Ca2+ was depleted by 1 microM cyclopiazonic acid in 0 extracellular Ca2+ ([Ca2+]o), nifedipine, and KCl (preventing Ca2+ influx through L-type and SOCC channels). SOCC was then activated by reintroduction of [Ca2+]o and characterized by several techniques. We examined cAMP effects on SOCC by activating SOCC in the presence of 1 microM isoproterenol or 100 microM dibutryl cAMP (cell-permeant cAMP analog), whereas we examined cGMP effects using 1 microM (Z)-1-[N-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (DETA-NO nitric oxide donor) or 100 microM 8-bromoguanosine 3',5'-cyclic monophosphate (cell-permeant cGMP analog). The role of protein kinases A and G was examined by preexposure to 100 nM KT-5720 and 500 nM KT-5823, respectively. SOCC-mediated Ca2+ influx was dependent on the extent of SR Ca2+ depletion, sensitive to Ni2+ and La3+, but not inhibitors of voltage-gated influx channels. cAMP as well as cGMP potently inhibited Ca2+ influx, predominantly via their respective protein kinases. Additionally, cAMP cross-activation of protein kinase G contributed to SOCC inhibition. These data demonstrate that a Ni2+/La3+-sensitive Ca2+ influx in ASM triggered by SR Ca2+ depletion is inhibited by cAMP and cGMP via a protein kinase mechanism. Such inhibition may play a role in the bronchodilatory response of ASM to clinically relevant drugs (e.g., beta-agonists vs. nitric oxide).     

Role of 4-hydroxylated estradiol in reducing Ca2+ uptake of uterine arterial smooth muscle cells through potential-sensitive channels

Entry of ionic Ca2+ into the vascular smooth muscle cell for contraction is thought to be mediated by two major membrane channels. The first are designated as potential-sensitive channels (PSCs), which are opened by membrane depolarization, and the second, as receptor-operated channels (ROCs), which are activated by alpha 1-receptor-ligand interactions. This study was designed to determine the presence of these 2 distinct populations of Ca2+ entry channels in smooth muscle cells of the uterine arteries in pigs. This was studied by measuring the baseline tone and contractile properties of uterine arteries in in vitro perfusion studies, as well as their specific Ca2+ uptakes. These parameters showed markedly different sensitivities towards two smooth muscle inhibitors used in this study: D-600 and amrinone. D-600 specifically inhibits uptake of extracellular Ca2+ through PSCs, while amrinone specifically inhibits Ca2+ uptake through ROCs. By choosing an appropriate concentration of D-600 or amrinone, Ca2+ uptake and contractions of uterine arterial segments induced by high-K+ (PSC activator) and phenylephrine (ROC activator) could be selectively inhibited. Furthermore, it was demonstrated that the blockade of Ca2+ uptake by D-600 and amrinone was additive, excluding the interpretation of a common Ca2+ pathway with two separate mechanisms for opening it. It was also determined that 4-hydroxylated estradiol (4OH-E2), a compound known to increase uterine blood flow in pigs, decreased Ca2+ uptake through the PSCs and exhibited no effect on ROCs. The presence of separate Ca2+ pathways that can be activated independently by agonists may indicate a refined system for controlling uterine blood flow.     

KCa channel insensitivity to Ca2+ sparks underlies fractional uncoupling in newborn cerebral artery smooth muscle cells

In smooth muscle cells, localized intracellular Ca2+ transients, termed "Ca2+ sparks," activate several large-conductance Ca2+-activated K+ (KCa) channels, resulting in a transient KCa current. In some smooth muscle cell types, a significant proportion of Ca2+ sparks do not activate KCa channels. The goal of this study was to explore mechanisms that underlie fractional Ca2+ spark-KCa channel coupling. We investigated whether membrane depolarization or ryanodine-sensitive Ca2+ release (RyR) channel activation modulates coupling in newborn (1- to 3-day-old) porcine cerebral artery myocytes. At steady membrane potentials of -40, 0, and +40 mV, mean transient KCa current frequency was approximately 0.18, 0.43, and 0.26 Hz and KCa channel activity [number of KCa channels activated by Ca2+ sparksxopen probability of KCa channels at peak of Ca2+ sparks (NPo)] at the transient KCa current peak was approximately 4, 12, and 24, respectively. Depolarization between -40 and +40 mV increased KCa channel sensitivity to Ca2+ sparks and elevated the percentage of Ca2+ sparks that activated a transient KCa current from 59 to 86%. In a Ca2+-free bath solution or in diltiazem, a voltage-dependent Ca2+ channel blocker, steady membrane depolarization between -40 and +40 mV increased transient KCa current frequency up to approximately 1.6-fold. In contrast, caffeine (10 microM), an RyR channel activator, increased mean transient KCa current frequency but did not alter Ca2+ spark-KCa channel coupling. These data indicate that coupling is increased by mechanisms that elevate KCa channel sensitivity to Ca2+ sparks, but not by RyR channel activation. Overall, KCa channel insensitivity to Ca2+ sparks is a prominent factor underlying fractional Ca2+ spark uncoupling in newborn cerebral artery myocytes.     

Novel role for K+-dependent Na+/Ca2+ exchangers in regulation of cytoplasmic free Ca2+ and contractility in arterial smooth muscle

Cytoplasmic free Ca2+ ([Ca2+]cyt) is essential for the contraction and relaxation of blood vessels. The role of plasma membrane Na+/Ca2+ exchange (NCX) activity in the regulation of vascular Ca2+ homeostasis was previously ascribed to the NCX1 protein. However, recent studies suggest that a relatively newly discovered K+-dependent Na+/Ca2+ exchanger, NCKX (gene family SLC24), is also present in vascular smooth muscle. The purpose of the present study was to identify the expression and function of NCKX in arteries. mRNA encoding NCKX3 and NCKX4 was demonstrated by RT-PCR and Northern blot in both rat mesenteric and aortic smooth muscle. NCXK3 and NCKX4 proteins were also demonstrated by immunoblot and immunofluorescence. After voltage-gated Ca2+ channels, store-operated Ca2+ channels, and Na+ pump were pharmacologically blocked, when the extracellular Na+ was replaced with Li+ (0 Na+) to induce reverse mode (Ca2+ entry) activity of Na+/Ca2+ exchangers, a large increase in [Ca2+]cyt signal was observed in primary cultured aortic smooth muscle cells. About one-half of this [Ca2+]cyt signal depended on the extracellular K+. In addition, after the activity of NCX was inhibited by KB-R7943, Na+ replacement-induced Ca2+ entry was absolutely dependent on extracellular K+. In arterial rings denuded of endothelium, a significant fraction of the phenylephrine-induced and nifedipine-resistant aortic or mesenteric contraction could be prevented by removal of extracellular K+. Taken together, these data provide strong evidence for the expression of NCKX proteins in the vascular smooth muscle and their novel role in mediating agonist-stimulated [Ca2+]cyt and thereby vascular tone.     

Fatty acid modifies Ca2+-dependent potassium channel activity in smooth muscle cells from the human aorta

By using the patch-clamp technique the effect of 2-decenoic acid (DA) on Ca2+-activated potassium (K+) channels in the membrane of smooth muscle cells from the human aorta was studied. In the presence of 0.5 microM Ca2+ and 2 mM Mg2+ on the cytoplasmic side of the membrane, a more than tenfold elevation in the probability of the channels being open (po) was observed under the effect of DA. With divalent cation concentrations of less than 1 nM DA caused a more than twofold elevation in po. In the DA-treated membranes Mg2+ ions, which normally fail to activate the channels, brought about a nearly threefold increase in the channel activity when applied to the inner membrane surface. Channel sensitivity to the activating effect of cytoplasmic Ca2+ ions did not increase with the application of DA. Single-channel conductance was unchanged by DA exposure. We suggest that DA alters the Ca2+-binding mechanism of the channel, increasing its sensitivity to Mg2+ ions, presumably owing to membrane fluidization.     

A Mg2+-independent high-affinity Ca2+-stimulated adenosine triphosphatase in the plasma membrane of rat stomach smooth muscle. Subcellular distribution and inhibition by Mg2+

Plasma membrane enriched fraction isolated from the fundus smooth muscle of rat stomach displayed Ca2+-stimulated ATPase activity in the absence of Mg2+. The Ca2+ dependence of such an ATPase activity can be resolved into two hyperbolic components with a high affinity (Km = 0.4 microM) and a low affinity (Km = 0.6 mM) for Ca2+. Distribution of these high-affinity and low-affinity Ca2+-ATPase activities parallels those of several plasma membrane marker enzyme activities but not those of endoplasmic reticulum and mitochondrial membrane marker enzyme activities. Mg2+ also stimulates the ATPase in the absence of Ca2+. Unlike the Mg2+-ATPase and low-affinity Ca2+-ATPase, the plasmalemmal high-affinity Ca2+-ATPase is not sensitive to the inhibitory effect of sodium azide or Triton X-100 treatment. The high-affinity Ca2+-ATPase is noncompetitively inhibited by Mg2+ with respect to Ca2+ stimulation. Such an inhibitory effect of Mg2+ is potentiated by Triton X-100 treatment of the membrane fraction. Calmodulin has little effect on the high-affinity Ca2+-ATPase activity of the plasma membrane enriched fraction with or without EDTA pretreatment. Findings of this novel, Mg2+-independent, high-affinity Ca2+-ATPase activity in the rat stomach smooth muscle plasma membrane are discussed with those of Mg2+-dependent, high-affinity Ca2+-ATPase activities previously reported in other smooth muscle plasma membrane preparations in relation to the plasma membrane Ca2+-pump.     

Chloride channel blockers inhibit Ca2+ uptake by the smooth muscle sarcoplasmic reticulum.

Despite the fact that Ca2+ transport into the sarcoplasmic reticulum (SR) of muscle cells is electrogenic, a potential difference is not maintained across the SR membrane. To achieve electroneutrality, compensatory charge movement must occur during Ca2+ uptake. To examine the role of Cl- in this charge movement in smooth muscle cells, Ca2+ transport into the SR of saponin-permeabilized smooth muscle cells was measured in the presence of various Cl- channel blockers or when I-, Br-, or SO42- was substituted for Cl-. Calcium uptake was inhibited in a dose-dependent manner by 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) and by indanyloxyacetic acid 94 (R(+)-IAA-94), but not by niflumic acid or 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS). Smooth muscle SR Ca2+ uptake was also partially inhibited by the substitution of SO42- for Cl-, but not when Cl- was replaced by I- or Br-. Neither NPPB nor R(+)-IAA-94 inhibited Ca2+ uptake into cardiac muscle SR vesicles at concentrations that maximally inhibited uptake in smooth muscle cells. These results indicate that Cl- movement is important for charge compensation in smooth muscle cells and that the Cl- channel or channels involved are different in smooth and cardiac muscle cells.     

T-type Ca2+ channels in vascular smooth muscle: multiple functions

Vascular smooth muscle is a major constituent of the blood vessel wall, and its many functions depend on type and location of the vessel, developmental or pathological state, and environmental and chemical factors. Vascular smooth muscle cells (VSMCs) use calcium as a signal molecule for multiple functions. An important component of calcium signaling pathways is the entry of extracellular calcium via voltage-gated Ca2+ channels, which in vascular smooth muscle cells (VSMCs) are of two main types, the high voltage-activated (HVA) L-type and low voltage-activated (LVA) T-type channels. Whereas L-type channels function primarily to regulate Ca2+ entry for contraction, it is generally accepted that T-type Ca2+ channels do not contribute significantly to arterial vasoconstriction, with the possible exception of the renal microcirculation. T-type Ca2+ channels are also present in some veins that display spontaneous contractile activity, where they likely generate pacemaker activity. T-type Ca2+ channel expression has also been associated with normal and pathological proliferation of VSMCs, often stimulated by external cues in response to insult or injury. Expression of T-type channels has been linked to the G1 and S phases of the cell cycle, a period important for the signaling of gene expression necessary for cell growth, progression of the cell cycle and ultimately cell division. To better understand T-type Ca2+ channel functions in VSM, it will be necessary to develop new approaches that are specifically targeted to this class of Ca2+ channels and its individual members.