Ca2+ pumps in smooth muscle cells

Involvement of myosin regulatory light chain diphosphorylation in sustained vasoconstriction under pathophysiological conditions

Smooth muscle contraction is activated primarily by phosphorylation at Ser19 of the regulatory light chain subunits (LC₂₀) of myosin II, catalysed by Ca²⁺/calmodulin-dependent myosin light chain kinase. Ca²⁺-independent contraction can be induced by inhibition of myosin light chain phosphatase, which correlates with diphosphorylation of LC₂₀ at Ser19 and Thr18, catalysed by integrin-linked kinase (ILK) and zipper-interacting protein kinase (ZIPK). LC₂₀ diphosphorylation at Ser19 and Thr18 has been detected in mammalian vascular smooth muscle tissues in response to specific contractile stimuli (e.g. endothelin-1 stimulation of rat renal afferent arterioles) and in pathophysiological situations associated with hypercontractility (e.g. cerebral vasospasm following subarachnoid hemorrhage). Comparison of the effects of LC₂₀ monophosphorylation at Ser19 and diphosphorylation at Ser19 and Thr18 on contraction and relaxation of Triton-skinned rat caudal arterial smooth muscle revealed that phosphorylation at Thr18 has no effect on steady-state force induced by Ser19 phosphorylation. On the other hand, the rates of dephosphorylation and relaxation are significantly slower following diphosphorylation at Thr18 and Ser19 compared to monophosphorylation at Ser19. We propose that this diphosphorylation mechanism underlies the prolonged contractile response of particular vascular smooth muscle tissues to specific stimuli, e.g. endothelin-1 stimulation of renal afferent arterioles, and the vasospastic behavior observed in pathological conditions such as cerebral vasospasm following subarachnoid hemorrhage and coronary arterial vasospasm. ILK and ZIPK may, therefore, be useful therapeutic targets for the treatment of such conditions.

Effect of diabetes on the ion pumps of the bladder

OBJECTIVE

To establish whether the activities of Na+/K+-adenosine triphosphatase (ATPase) and Ca2+-ATPases ion pumps in bladder smooth muscle are altered as a consequence of diabetes and, if so, how this might contribute to bladder cystopathy. Urinary bladder dysfunction is a common occurrence in patients with diabetes. Pressure generation requires calcium and cytosolic ATP. Activities of these pumps are responsible for calcium homeostasis.

METHODS

Rat urinary detrusor muscle strips were suspended in organ baths containing Krebs solution for isometric tension recording. Tissue responses to the Na+/K+-ATPase pump inhibitor, ouabain, the plasma membrane Ca2+ ATPase inhibitor, vanadate, and the sarcoplasmic reticulum Ca2+ ATPase inhibitor, cyclopiazonic acid (CPA), were examined from normal and streptozocin-induced diabetic rats for 2, 4, and 12 weeks.

RESULTS

Ouabain, vanadate, and CPA caused concentration-dependent contractions of bladder strips from diabetic and normal rats. The degree of contraction of diabetic bladder muscle was lower than that of controls. This reduction was a function of duration of diabetes. For ouabain, the reduction peaked at 2 weeks, with partial restoration to normal after diabetes induction. For vanadate and CPA, the reduction increased with the duration of diabetes.

CONCLUSION

The ion pumps are important modulators of bladder smooth muscle tone, and in a rat model of streptozotocin-induced diabetes, the activity of these pumps is impaired. Although this is only a single model of diabetes, these findings suggest that a defect in these pumps may be an important component of the development of diabetic bladder cystopathy.

Calcium pumps in the central nervous system

Two families of Ca2+ transport ATPases are involved in the maintenance of Ca2+ homeostasis in the nervous system, the plasma membrane Ca2+-ATPase that pumps Ca2+ to the extracellular medium and the intracellular sarco/endoplasmic reticulum Ca2+-ATPase that transports Ca2+ from the cytosol to the endoplasmic reticulum. Both types of calcium pumps show precise regulatory properties and they are localized in specific subcellular regions. In this review, we describe the functional and regulatory properties of both families of calcium pumps, their distribution in nerve cells, and their involvement in neurological disorders. The functional characterization of neuronal calcium pumps is very important in order to understand the biochemical processes involved in the maintenance of intracellular calcium in synaptic terminals.

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.

Caloxin: a novel plasma membrane Ca2+ pump inhibitor

Plasma membrane (PM) Ca2+ pump is a Ca+-Mg2+-ATPase that expels Ca2+ from cells to help them maintain low concentrations of cytosolic Ca2+ . There are no known extracellularly acting PM Ca2+ pump inhibitors, as digoxin and ouabain are for Na+ pump. In analogy with digoxin, we define caloxins as extracellular PM Ca2+ pump inhibitors and describe caloxin 2A1. Caloxin 2A1 is a peptide obtained by screening a random peptide phage display library for binding to the second extracellular domain (residues 401-413) sequence of PM Ca2+ pump isoform 1b. Caloxin 2A1 inhibits Ca2+-Mg2+-ATPase in human erythrocyte leaky ghosts, but it does not affect basal Mg2+-ATPase or Na+-K+-ATPase in the ghosts or Ca2+-Mg2+-ATPase in the skeletal muscle sarcoplasmic reticulum. Caloxin 2A1 also inhibits Ca2+-dependent formation of the 140-kDa acid-stable acylphosphate, which is a partial reaction of this enzyme. Consistent with inhibition of the PM Ca2+ pump in vascular endothelium, caloxin 2A1 produces an endothelium-dependent relaxation that is reversed by N(G)-nitro-L-arginine methyl ester. Thus caloxin 2A1 is a novel PM Ca2+ pump inhibitor selected for binding to an extracellular domain.

Role of alternative splicing in generating isoform diversity among plasma membrane calcium pumps

Calcium pumps of the plasma membrane (also known as plasma membrane Ca(2+)-ATPases or PMCAs) are responsible for the expulsion of Ca(2+) from the cytosol of all eukaryotic cells. Together with Na(+)/Ca(2+) exchangers, they are the major plasma membrane transport system responsible for the long-term regulation of the resting intracellular Ca(2+) concentration. Like the Ca(2+) pumps of the sarco/endoplasmic reticulum (SERCAs), which pump Ca(2+) from the cytosol into the endoplasmic reticulum, the PMCAs belong to the family of P-type primary ion transport ATPases characterized by the formation of an aspartyl phosphate intermediate during the reaction cycle. Mammalian PMCAs are encoded by four separate genes, and additional isoform variants are generated via alternative RNA splicing of the primary gene transcripts. The expression of different PMCA isoforms and splice variants is regulated in a developmental, tissue- and cell type-specific manner, suggesting that these pumps are functionally adapted to the physiological needs of particular cells and tissues. PMCAs 1 and 4 are found in virtually all tissues in the adult, whereas PMCAs 2 and 3 are primarily expressed in excitable cells of the nervous system and muscles. During mouse embryonic development, PMCA1 is ubiquitously detected from the earliest time points, and all isoforms show spatially overlapping but distinct expression patterns with dynamic temporal changes occurring during late fetal development. Alternative splicing affects two major locations in the plasma membrane Ca(2+) pump protein: the first intracellular loop and the COOH-terminal tail. These two regions correspond to major regulatory domains of the pumps. In the first cytosolic loop, the affected region is embedded between a putative G protein binding sequence and the site of phospholipid sensitivity, and in the COOH-terminal tail, splicing affects pump regulation by calmodulin, phosphorylation, and differential interaction with PDZ domain-containing anchoring and signaling proteins. Recent evidence demonstrating differential distribution, dynamic regulation of expression, and major functional differences between alternative splice variants suggests that these transporters play a more dynamic role than hitherto assumed in the spatial and temporal control of Ca(2+) signaling. The identification of mice carrying PMCA mutations that lead to diseases such as hearing loss and ataxia, as well as the corresponding phenotypes of genetically engineered PMCA "knockout" mice further support the concept of specific, nonredundant roles for each Ca(2+) pump isoform in cellular Ca(2+) regulation.

Calcium homeostatic pathways change with gestation in human myometrium

A rise in intracellular calcium is the primary trigger for contractile activity in pregnant human myometrium. It is hypothesized that key proteins involved in myometrial calcium homeostasis are gestationally regulated and play an important role in the preparation for labor. The aims of the study were to investigate the role of sarcoplasmic reticulum Ca ATPases (SERCAs) in regulating spontaneous contractile activity in myometrium, and to determine the expression of SERCA isoforms 2a and 2b, and the plasma membrane Ca ATPase (PMCA), at term and during labor. Western blot analysis demonstrated that the expression of SERCA 2a and 2b significantly increased in myometrium of women in labor compared with those not in labor. The augmentation of contractile activity in laboring myometrium in the presence of a SERCA 2 inhibitor, cyclopiazonic acid (CPA), demonstrated the functional significance of this observation. It is interesting that the application of CPA in the presence of a calcium-activated potassium channel inhibitor to term nonlabor myometrium mimicked the response of myometrium from women in active labor to CPA alone. We conclude that the activity of SERCA isoforms becomes increasingly important in the maintenance of regular contractile activity during labor and may compensate for the functional loss of other calcium control pathways at term.

Ablation of the SERCA3 gene alters epithelium-dependent relaxation in mouse tracheal smooth muscle

Sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase 3 (SERCA3), an isoform of the intracellular Ca(2+) pump that has been shown to mediate endothelium-dependent relaxation of vascular smooth muscle, is also expressed in tracheal epithelium. To determine its possible role in regulation of airway mechanical function, we compared tracheal contractility in gene-targeted mice deficient in SERCA3 (SERCA3(-)) with that in wild-type tracheae. Cumulative addition of ACh elicited concentration-dependent increases in isometric force (ED(50) = 2 microM, maximum force = 8 mN/mm(2)) that were identical in SERCA3(-) and wild-type tracheae. After ACh stimulation, substance P (SP) elicited a transient relaxation (42.6 +/- 3.2%, n = 28) in both tracheae. However, the rate of relaxation was significantly (P < 0.04, n = 9) more rapid in the wild-type [half-time (t(1/2)) = 34.3 s] than in the SERCA3(-) (t(1/2) = 61.6 s) trachea. The SP relaxation was reduced by rubbing the trachea, indicative of epithelial cell involvement. This was verified using a perfused trachea preparation. SP in the outside medium had no effect, whereas SP in the perfusate bathing the epithelial side elicited a relaxation. Nitric oxide synthase inhibition (0.2 mM N(omega)-nitro-L-arginine) reduced the SP relaxation by 36.5 +/- 12.5%, whereas the SP effect was abolished by eicosanoid inhibition (10 microM indomethacin). ATP also elicited an epithelium-dependent relaxation similar to SP but with a more rapid relaxation in the SERCA3(-) trachea than in the wild-type trachea. Our results indicate that SERCA3 gene ablation does not directly affect smooth muscle, which is consistent with the distribution of the isoform, but suggest that SERCA3 plays a role in epithelial cell modulation of airway smooth muscle function.

Sarcoplasmic reticulum in vascular cells in hypertension and during proliferation

1. Multiple sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) and two types of sarcoplasmic reticulum Ca2+ channels, the ryanodine receptor and the inositol 1,4,5 triphosphate (IP3) receptor are expressed. The heterogeneity of the Ca2+ pumps and Ca2+ channels in vascular cells will be discussed. 2. An age-related change in expression of the SERCA isoforms is observed in smooth muscle cells. 3. The sarcoplasmic reticulum Ca(2+)-uptake rate and the level of SERCA 2 mRNA are different in thoracic than in abdominal aortas and in aortas from spontaneously hypertensive rats than from normotensive rats. 4. Proliferation of vascular smooth muscle cells is associated with major changes in intracellular Ca(2+)-handling mechanisms.

Excitation-contraction coupling in gastrointestinal and other smooth muscles

The main contributors to increases in [Ca2+]i and tension are the entry of Ca2+ through voltage-dependent channels opened by depolarization or during action potential (AP) or slow-wave discharge, and Ca2+ release from store sites in the cell by the action of IP3 or by Ca(2+)-induced Ca(2+)-release (CICR). The entry of Ca2+ during an AP triggers CICR from up to 20 or more subplasmalemmal store sites (seen as hot spots, using fluorescent indicators); Ca2+ waves then spread from these hot spots, which results in a rise in [Ca2+]i throughout the cell. Spontaneous transient releases of store Ca2+, previously detected as spontaneous transient outward currents (STOCs), are seen as sparks when fluorescent indicators are used. Sparks occur at certain preferred locations--frequent discharge sites (FDSs)--and these and hot spots may represent aggregations of sarcoplasmic reticulum scattered throughout the cytoplasm. Activation of receptors for excitatory signal molecules generally depolarizes the cell while it increases the production of IP3 (causing calcium store release) and diacylglycerols (which activate protein kinases). Activation of receptors for inhibitory signal molecules increases the activity of protein kinases through increases in cAMP or cGMP and often hyperpolarizes the cell. Other receptors link to tyrosine kinases, which trigger signal cascades interacting with trimeric G-protein systems.

Mechanisms involved in the cellular calcium homeostasis in vascular smooth muscle: calcium pumps

The regulation of cytosolic Ca2+ homeostasis is essential for cells, and particularly for vascular smooth muscle cells. In this regulation, there is a participation of different factors and mechanisms situated at different levels in the cell, among them Ca2+ pumps play an important role. Thus, Ca2+ pump, to extrude Ca2+; Na+/Ca2+ exchanger; and different Ca2+ channels for Ca2+ entry are placed in the plasma membrane. In addition, the inner and outer surfaces of the plasmalemma possess the ability to bind Ca2+ that can be released by different agonists. The sarcoplasmic reticulum has an active role in this Ca2+ regulation; its membrane has a Ca2+ pump that facilitates luminal Ca2+ accumulation, thus reducing the cytosolic free Ca2+ concentration. This pump can be inhibited by different agents. Physiologically, its activity is regulated by the protein phospholamban; thus, when it is in its unphosphorylated state such a Ca2+ pump is inhibited. The sarcoplasmic reticulum membrane also possesses receptors for 1,4,5-inositol trisphosphate and ryanodine, which upon activation facilitates Ca2+ release from this store. The sarcoplasmic reticulum and the plasmalemma form the superficial buffer barrier that is considered as an effective barrier for Ca2+ influx. The cytosol possesses different proteins and several inorganic compounds with a Ca2+ buffering capacity. The hypothesis of capacitative Ca2+ entry into smooth muscle across the plasma membrane after intracellular store depletion and its mechanisms of inhibition and activation is also commented.

Myorelaxant activity of 2-t-butyl-4-methoxyphenol (BHA) in guinea pig gastric fundus

This study investigates the mechanism whereby the antioxidant 2-t-butyl-4-methoxyphenol (BHA) relaxes guinea pig gastric fundus smooth muscle. In circular smooth muscle strips, 10 microM cyclopiazonic acid, a specific inhibitor of sarcoplasmic reticulum Ca2+-ATPase, induced a prolonged rise in tension which depended on the presence of extracellular Ca2+. BHA (pIC50 = 5.83), sodium nitroprusside (6.85), isoproterenol (7.69) and nifedipine (8.02), but not 2,6-di-t-butyl-4-methoxyphenol (DTBHA) (up to 30 microM), relaxed muscle strips contracted with cyclopiazonic acid. Methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-pyri dine-5-carboxylate (Bay K 8644) (1 microM) antagonised the nifedipine- but not the BHA-induced relaxation. Nifedipine and isoproterenol (10 microM) caused a decrease in spontaneous tone, but did not counteract the subsequent rise in tension elicited by 10 microM cyclopiazonic acid. Conversely, 100 microM BHA and 100 microM sodium nitroprusside not only significantly reduced spontaneous tone but also markedly impaired the response of the muscles to cyclopiazonic acid. DTBHA failed to show either effect. When added to preparations completely relaxed by 100 microM BHA, 10 mM tetraethylammonium still elicited nifedipine-sensitive tonic and phasic contractions in the presence or absence of 10 microM cyclopiazonic acid. BHA and DTBHA inhibited, in a concentration-dependent manner, the Ca2+-promoted contraction of strips depolarised by 10 mM tetraethylammonium. The BHA antagonism showed a non-competitive profile while that of DTBHA was competitive. In muscle strips at rest, 10 microM BHA caused a significant increase in tissue cAMP concentration, leaving cGMP unmodified. To conclude, the myorelaxant action of BHA on gastric fundus smooth muscle appears to be mediated partly by an increase in cAMP levels and partly by inhibition of Ca2+ influx from the extracellular space.

Ca2+ transport by reconstituted synaptosomal ATPase is associated with H+ countertransport and net charge displacement

The synaptosomal plasma membrane Ca2+-ATPase (PMCA) purified from pig brain was reconstituted with liposomes prepared by reverse phase evaporation at a lipid to protein ratio of 150/1 (w/w). ATP-dependent Ca2+ uptake and H+ ejection by the reconstituted proteoliposomes were demonstrated by following light absorption and fluorescence changes undergone by arsenazo III and 8-hydroxy-1,3, 6-pyrene trisulfonate, respectively. Ca2+ uptake was increased up to 2-3-fold by the H+ ionophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone, consistent with relief of an inhibitory transmembrane pH gradient (i.e. lumenal alkalinization) generated by H+ countertransport. The stoichiometric ratio of Ca2+/H+ countertransport was 1.0/0.6, and the ATP/Ca2+ coupling stoichiometry was 1/1 at 25 degrees C. The electrogenic character of the Ca2+/H+ countertransport was demonstrated by measuring light absorption changes undergone by oxonol VI. It was shown that a 20 mV steady state potential (positive on the lumenal side) was formed as a consequence of net charge transfer associated with the 1/1 Ca2+/H+ countertransport. Calmodulin stimulated ATPase activity, Ca2+ uptake, and H+ ejection, demonstrating that these parameters are linked by the same mechanism of PMCA regulation.

Physiological features of visceral smooth muscle cells, with special reference to receptors and ion channels

Visceral smooth muscle cells (VSMC) play an essential role, through changes in their contraction-relaxation cycle, in the maintenance of homeostasis in biological systems. The features of these cells differ markedly by tissue and by species; moreover, there are often regional differences within a given tissue. The biophysical features used to investigate ion channels in VSMC have progressed from the original extracellular recording methods (large electrode, single or double sucrose gap methods), to the intracellular (microelectrode) recording method, and then to methods for recording from membrane fractions (patch-clamp, including cell-attached patch-clamp, methods). Remarkable advances are now being made thanks to the application of these more modern biophysical procedures and to the development of techniques in molecular biology. Even so, we still have much to learn about the physiological features of these channels and about their contribution to the activity of both cell and tissue. In this review, we take a detailed look at ion channels in VSMC and at receptor-operated ion channels in particular; we look at their interaction with the contraction-relaxation cycle in individual VSMC and especially at the way in which their activity is related to Ca2+ movements and Ca2+ homeostasis in the cell. In sections II and III, we discuss research findings mainly derived from the use of the microelectrode, although we also introduce work done using the patch-clamp procedure. These sections cover work on the electrical activity of VSMC membranes (sect. II) and on neuromuscular transmission (sect. III). In sections IV and V, we discuss work done, using the patch-clamp procedure, on individual ion channels (Na+, Ca2+, K+, and Cl-; sect. IV) and on various types of receptor-operated ion channels (with or without coupled GTP-binding proteins and voltage dependent and independent; sect. V). In sect. VI, we look at work done on the role of Ca2+ in VSMC using the patch-clamp procedure, biochemical procedures, measurements of Ca2+ transients, and Ca2+ sensitivity of contractile proteins of VSMC. We discuss the way in which Ca2+ mobilization occurs after membrane activation (Ca2+ influx and efflux through the surface membrane, Ca2+ release from and uptake into the sarcoplasmic reticulum, and dynamic changes in Ca2+ within the cytosol). In this article, we make only limited reference to vascular smooth muscle research, since we reviewed the features of ion channels in vascular tissues only recently.

Characterization of the 3' end of the mouse SERCA 3 gene and tissue distribution of mRNA spliced variants

The sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA) type 1 and 2 genes are alternatively spliced at their 3' end. We hypothesized that similar mechanism may occur for SERCA 3. Two spliced variants were identified by RNase protection analysis. We then isolated and sequenced the 3' end portion of the mouse SERCA 3 gene, and confirmed the presence of an alternative mRNA transcript by sequencing a cDNA fragment obtained by RT-PCR. Tissue distribution of the alternatively spliced mRNAs was studied by RT-PCR: SERCA 3b was the only isoform expressed in endothelial cells from aorta and heart and also was the major isoform in lung and kidney whereas SERCA 3a and 3b were coexpressed in trachea, intestine, thymus, spleen, and fetal liver.

A role for the sarcoplasmic reticulum in Ca2+ extrusion from rabbit inferior vena cava smooth muscle

Ca2+ extrusion from rabbit inferior vena cava smooth muscle was studied using ratiometric fura 2 fluorimetry. Concomitant blockade of the plasma membrane Ca(2+)-adenosinetriphosphatase (ATPase; PCMA), Na(+)-Ca2+ exchanger, and sarcoendoplasmic reticulum Ca(2+)-ATPase (SERCA) completely prevented the decline in intracellular Ca2+ concentration ([Ca2+)]i) normally observed when Ca2+ is removed from the extracellular space (ECS) after stimulated Ca2+ influx. Blockade of the Na(+)-Ca2+ exchanger by removal of external Na+ reduced the rate of [Ca2+]i decline by 47%. Blockade of SERCA with cyclopiazonic acid reduced it by 23%, and this was not additive to the effects of Na+ removal. Exposure to nominally Ca(2+)-free solution prevented the sarcoplasmic reticulum (SR) from reloading only if the Na(+)-Ca2+ exchanger was operational. Our results can be explained by an SR contribution to Ca2+ extrusion in which SERCA is arranged in series with Na(+)-Ca2+ exchange.

Effect of a 14-day hindlimb suspension on cytosolic Ca2+ concentration in rat portal vein myocytes

Effects of a 14-day hindlimb suspension were examined on increases in cytosolic Ca2+ concentration ([Ca2+]i) evoked by vasoactive compounds and on Ca2+ channels in rat portal vein myocytes. The maximal increases in [Ca2+]i elicited by caffeine, norepinephrine, and angiotensin II were reduced by 30-50% in suspended rats, and complete recovery was obtained 4 days after suspension removal. In contrast, voltage-gated Ca2+ channels were unaffected by hindlimb suspension. Using both confocal microscopy and the patch-clamp technique, we measured local increases in [Ca2+]i which corresponded to activation of a small number of ryanodine-sensitive Ca(2+)-release channels (Ca2+ sparks) and D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]-gated Ca2+ channels. After hindlimb suspension, these local Ca2+ events, as well as the Ca2+ sensitivity of ryanodine-sensitive Ca2+ release channels, remained unchanged. In contrast, the propagated Ca2+ responses (Ca2+ waves) were significantly reduced in parallel with a noticeable inhibition of [3H]ryanodine binding to vascular membranes. Taken together, these results suggest that inhibition of the vasoconstrictor-induced increases in [Ca2+]i during long-term suspension may be related to a reduction of the number of functional ryanodine-sensitive and Ins(1,4,5)P3-gated channels in the sarcoplasmic reticulum of rat portal vein myocytes.

Ca2+ transient, cell volume, and microviscosity of the plasma membrane in smooth muscle

Despite pronounced differences by which membrane-depolarizing or phospholipase C-activating stimuli initiate contractile responses, a rise in [Ca2+]i is considered the primary mechanism for induction of smooth muscle contractions. Subsequent to the formation of the well-characterized Ca(2+)4-calmodulin complex, interaction with the catalytic subunit of myosin light chain kinase triggers phosphorylation of 20 kDa myosin light chain and activates actin-dependent Mg2+-ATPase activity, which ultimately leads to the development of tension. The present article reviews the fundamental mechanisms leading to an increase in [Ca2+]i and discusses the biochemical processes involved in the transient and sustained phases of contraction. Moreover, the commentary summarizes current knowledge on the modulatory effect of changes in the microviscosity of the plasma membrane on the Ca2+ transient as well as the contractile response of smooth muscle. Evidence has accumulated that these changes in microviscosity alter the activity of membrane-bound enzymes and affect the generation of endogenous mediators responsible for the regulation of cytosolic Ca2+ concentrations and for the [Ca2+]i-sensitivity of myosin light chain phosphorylation.

Cytochemical localization of Ca(2+)-ATPases and demonstration of ATP-dependent calcium sequestration in giant smooth muscle fibres of Beroe

A cytochemical analysis of the mechanisms underlying cytosolic calcium regulation was undertaken in the giant smooth muscle fibres of the marine invertebrate Beroe. The ability of the sarcoplasmic reticulum to accumulate Ca2+ was demonstrated on living skinned single cells. In the presence of oxalate, and physiological concentrations of Ca2+, calcium oxalate crystals were formed in the lumen of tubules and cisternae of the sarcoplasmic reticulum. The subcellular distribution of Ca(2+)-ATPase was studied with a cytochemical technique; a dense precipitate resulting from Ca(2+)-ATPase activity was found on the plasma membrane, on the membranes of tubules and cisternae of the sarcoplasmic reticulum, and in mitochondria.

Regulation of cytosolic calcium levels in vascular smooth muscle

Ca2+ plays an important role in the contraction of skeletal, cardiac, and smooth muscle, as well as in a number of important processes, such as secretion and neuronal activity. In this review, I focus on the various mechanisms by which cytosolic Ca2+ concentration is regulated in vascular smooth muscle, in the resting state and during activation. Particular attention is paid to the calcium pumps of the plasmalemma and the sarcoplasmic reticulum, to the inositol 1,4,5-trisphosphate- and ryanodine-sensitive calcium channels of the sarcoplasmic reticulum, and to voltage-dependent and voltage-independent calcium channels of the plasmalemma.