The effect of L-type calcium channel modulators on the mobilization of intracellular calcium stores in guinea-pig intestinal smooth muscle

In vitro fertilization with frozen embryo transfer increased histamine-mediated contractile sensitivity via PKCβ in human umbilical vein

OBJECTIVE

In vitro fertilization-embryo transfer (IVF-ET) technologies (especially frozen ET) have been widely used, which might affect maternal and fetal health. Information regarding influence of IVF-ET on the vasoconstriction of human umbilical vein (HUV) is limited. This study determined effects of frozen ET on histamine-mediated vascular responses in HUV and related mechanisms.

METHODS AND RESULTS

HUVs were collected from frozen ET conceived pregnancy and spontaneously conceived pregnancy (control). Histamine concentration in umbilical plasma was higher in frozen ET group than the control. Histamine-mediated contractile response curve was left-shifted in the frozen ET group when comparing with the control. In isolated HUV rings, H1R showed a critical role in regulating vascular constriction, while H2R played little roles in regulating vessel tone. Iberiotoxin and 4-aminopyridine didn't significantly change histamine-mediated constriction in HUVs. Histamine-induced vasoconstrictions were significantly decreased by nifedipine, KN93, or GF109203X, while the inhibitory effects were significantly greater in the frozen ET group in comparison to the control. The constrictions by Bay K8644, phenylephrine, or PDBu were stronger in frozen ET, respectively. There was a decrease in the protein expressions of H1R and H2R, an increase in protein expressions of BK_Caα and PKCβ.

CONCLUSIONS

Histamine-induced constriction in HUV was mainly via H1R. The increased sensitivity to histamine in HUV following frozen ET cycles were linked to the enhanced PKCβ protein expression and function. The new data and findings in this study provide important insight into influences of frozen ET on fetal vessel development and potential influence in long-term.

Nimodipine inhibits intestinal and aortic smooth muscle contraction by regulating Ca2+-activated Cl- channels

Nimodipine is a clinically used dihydropyridine L-type calcium channel antagonist that effectively inhibits transmembrane Ca2+ influx following the depolarization of smooth muscle cells, but the detailed effect on smooth muscle contraction is not fully understood. Ca2+-activated Cl- channels (CaCCs) in vascular smooth muscle cells (VSMCs) may regulate vascular contractility. We found that nimodipine can inhibit transmembrane protein 16A (TMEM16A) activity in a concentration-dependent manner by cell-based fluorescence-quenching assay and short-circuit current analysis, with an IC50 value of ~5 μM. Short-circuit current analysis also showed that nimodipine prevented Ca2+-activated Cl- current in both HT-29 cells and mouse colonic epithelia accompanied by significantly decreased cytoplasmic Ca2+ concentrations. In the absence of extracellular Ca2+, nimodipine still exhibited an inhibitory effect on TMEM16A/CaCCs. Additionally, the application of nimodipine to CFTR-expressing FRT cells and mouse colonic mucosa resulted in mild activation of CFTR-mediated Cl- currents. Nimodipine inhibited basolateral CCh-activated K+ channel activity with no effect on Na+/K+-ATPase activity. Evaluation of intestinal smooth muscle contraction showed that nimodipine inhibits intestinal smooth muscle contractility and frequency, with an activity pattern that was similar to that of non-specific inhibitors of CaCCs. In aortic smooth muscle, the expression of TMEM16A in thoracic aorta is higher than that in abdominal aorta, corresponding to stronger maximum contractility in thoracic aorta smooth muscle stimulated by phenylephrine (PE) and Eact. Nimodipine completely inhibited the contraction of aortic smooth muscle stimulated by Eact, and partially inhibited the contraction stimulated by PE. In summary, the results indicate that nimodipine effectively inhibits TMEM16A/CaCCs by reduction transmembrane Ca2+ influx and directly interacting with TMEM16A, explaining the mechanisms of nimodipine relaxation of intestinal and aortic smooth muscle contraction and providing new targets for pharmacological applications.

Necrotising enterocolitis in newborns receiving diazoxide

BACKGROUND

Frequent and severe gastrointestinal disturbances have been reported with the use of diazoxide in adults and older children. However, no studies have investigated the incidence of necrotising enterocolitis (NEC) in diazoxide-exposed newborns.

OBJECTIVE

To evaluate a possible association between diazoxide treatment for neonatal hypoglycaemia and the occurrence of NEC.

DESIGN

Multicentre retrospective cohort study.

SETTING

Three tertiary neonatal intensive care units in Toronto, Canada.

PATIENTS

All patients treated with diazoxide for persistent hypoglycaemia between July 2012 and June 2017 were included. Overall incidence of NEC during those years on the participating units was obtained for comparison from the Canadian Neonatal Network database.

MAIN OUTCOME

Incidence of NEC after diazoxide exposure.

RESULTS

Fifty-five neonates were exposed to diazoxide during the study period. Eighteen patients (33%) showed signs of feeding intolerance, and 7 developed NEC (13%). A diagnosis of NEC was more prevalent in the diazoxide-exposed, as compared with non-exposed infants of similar gestational age (OR 5.07, 95% CI 2.27 to 11.27; p<0.001), and greatest among infants born at 33-36 weeks' gestation (OR 13.76, 95% CI 3.77 to 50.23; p<0.001). All but one of the neonates diagnosed with NEC developed the disease within 7 days from initiation of diazoxide treatment.

CONCLUSION

The present data suggest a possible association between diazoxide exposure and the development of NEC in neonates. Further evaluation of the diazoxide-associated risk of NEC in neonates treated for persistent hypoglycaemia is warranted.

Potential L-Type Voltage-Operated Calcium Channel Blocking Effect of Drotaverine on Functional Models

Drotaverine is considered an inhibitor of cyclic-3',5'-nucleotide-phophodiesterase (PDE) enzymes; however, published receptor binding data also support the potential L-type voltage- operated calcium channel (L-VOCC) blocking effect of drotaverine. Hence, in this work, we focus on the potential L-VOCC blocking effect of drotaverine by using L-VOCC-associated functional in vitro models. Accordingly, drotaverine and reference agents were tested on KCl-induced guinea pig tracheal contraction. Drotaverine, like the L-VOCC blockers nifedipine or diltiazem, inhibited the KCl-induced inward Ca²⁺- induced contraction in a concentration- dependent fashion. The PDE inhibitor theophylline had no effect on the KCl-evoked contractions, indicating its lack of inhibition on inward Ca²⁺ flow. Drotaverine was also tested on the L-VOCC-mediated resting Ca²⁺ refill model. In this model, the extracellular Ca²⁺ enters the cells to replenish the emptied intracellular Ca²⁺ stores. Drotaverine and L-VOCC blocker reference molecules inhibited Ca²⁺ replenishment of Ca²⁺-depleted preparations detected by agonist-induced contractions in post-Ca²⁺ replenishment Ca²⁺-free medium. Theophylline did not modify the Ca²⁺ store replenishment after contraction. It seems that drotaverine, but not theophylline, inhibits inward Ca²⁺ flux. The addition of CaCl₂ to Ca²⁺-free medium containing the agonist induced inward Ca²⁺ flow and subsequent contraction of Ca²⁺-depleted tracheal preparations. Drotaverine, similar to the L-VOCC blockers, inhibited inward Ca²⁺ flow and blunted the slope of CaCl₂-induced contraction in agonist containing Ca²⁺-free medium with Ca²⁺-depleted tracheal preparations. These results show that drotaverine behaves like L-VOCC blockers but, unlike PDE inhibitors using L-VOCC associated in vitro experimental models.

Regulation of calcium channels in smooth muscle: new insights into the role of myosin light chain kinase

Smooth muscle myosin light chain kinase (MLCK) plays a crucial role in artery contraction, which regulates blood pressure and blood flow distribution. In addition to this role, MLCK contributes to Ca(2+) flux regulation in vascular smooth muscle (VSM) and in non-muscle cells, where cytoskeleton has been suggested to help Ca(2+) channels trafficking. This conclusion is based on the use of pharmacological inhibitors of MLCK and molecular and cellular techniques developed to down-regulate the enzyme. Dissimilarities have been observed between cells and whole tissues, as well as between large conductance and small resistance arteries. A differential expression in MLCK and ion channels (either voltage-dependent Ca(2+) channels or non-selective cationic channels) could account for these observations, and is in line with the functional properties of the arteries. A potential involvement of MLCK in the pathways modulating Ca(2+) entry in VSM is described in the present review.

Myosin light chain kinase controls voltage-dependent calcium channels in vascular smooth muscle

The Ca(2+)-dependent kinase myosin light chain kinase (MLCK) is the activator of smooth muscle contraction. In addition, it has been reported to be involved in Ca(2+) channel regulation in cultured cells, and we previously showed that the MLCK inhibitor ML-7 decreases arginine vasopressin (AVP)-induced Ca(2+) influx in rat aorta. This study was designed to investigate whether MLCK is involved in Ca(2+) regulation in resistance artery smooth muscle cell, which plays a major role in the control of blood pressure. As ML compounds were shown to have off-target effects, MLCK was downregulated by transfection with a small interfering RNA targeting MLCK (MLCK-siRNA) in rat small resistance mesenteric artery (RMA) and in the rat embryonic aortic cell line A7r5. Noradrenaline-induced contraction and Ca(2+) signal were significantly depressed in MLCK-siRNA compared to scramble-siRNA-transfected RMA. Contraction and Ca(2+) signal induced by high KCl and voltage-activated Ca(2+) current were also significantly decreased in MLCK-siRNA-transfected RMA, suggesting that MLCK depletion modifies voltage-operated Ca(2+) channels. KCl- and AVP-induced Ca(2+) signals and voltage-activated Ca(2+) current were decreased in MLCK-depleted A7r5 cells. Eventually, real-time quantitative PCR analysis indicated that in A7r5, MLCK controlled mRNA expression of CaV1.2 (L-type) and CaV3.1 (T-type) voltage-dependent Ca(2+) channels. Our results suggest that MLCK controls the transcription of voltage-dependent Ca(2+) channels in vascular smooth muscle cells.

Pharmacological preconditioning by diazoxide downregulates cardiac L-type Ca(2+) channels

BACKGROUND AND PURPOSE

Pharmacological preconditioning (PPC) with mitochondrial ATP-sensitive K(+) (mitoK(ATP) ) channel openers such as diazoxide, leads to cardioprotection against ischaemia. However, effects on Ca(2+) homeostasis during PPC, particularly changes in Ca(2+) channel activity, are poorly understood. We investigated the effects of PPC on cardiac L-type Ca(2+) channels.

EXPERIMENTAL APPROACH

PPC was induced in isolated hearts and enzymatically dissociated cardiomyocytes from adult rats by preincubation with diazoxide. We measured reactive oxygen species (ROS) production and Ca(2+) signals associated with action potentials using fluorescent probes, and L-type currents using a whole-cell patch-clamp technique. Levels of the α(1c) subunit of L-type channels in the cellular membrane were measured by Western blot.

KEY RESULTS

PPC was accompanied by a 50% reduction in α(1c) subunit levels, and by a reversible fall in L-type current amplitude and Ca(2+) transients. These effects were prevented by the ROS scavenger N-acetyl-L-cysteine (NAC), or by the mitoK(ATP) channel blocker 5-hydroxydecanoate (5-HD). PPC significantly reduced infarct size, an effect blocked by NAC and 5-HD. Nifedipine also conferred protection against infarction when applied during the reperfusion period. Downregulation of the α(1c) subunit and Ca(2+) channel function were prevented in part by the protease inhibitor leupeptin.

CONCLUSIONS AND IMPLICATIONS

PPC downregulated the α(1c) subunit, possibly through ROS. Downregulation involved increased degradation of the Ca(2+) channel, which in turn reduced Ca(2+) influx, which may attenuate Ca(2+) overload during reperfusion.

Aging-Induced Decrease of Cholinergic Response and Calcium Sensitivity on Rat Jejunum Contractions

The role of aging on contraction or relaxation through muscarinic or alpha-adrenergic receptors, respectively, was studied in isolated rat jejunum. Furthermore, the influence of extracellular calcium was analyzed, through functional and radioligand binding assays. The rank order of potency for selective muscarinic antagonists for M(1), M(2), and M(3) receptor subtypes, measured from affinity (pA(2)) values, was p-fluorohexahydrosiladifenidol (pFHHSiD) (M(3)) > pirenzepine (M(1)) > methoctramine (M(2)), indicating a predominance of M(3) subtype. This order was unchanged with age. Contractions by muscarinic agonist methacholine (MCh) were diminished in aged rats, resulting in lower apparent affinity (pD(2)) values, compared with adult controls. A larger decrease of MCh contractions occurred in aged rats after Ca(2+) withdrawal or after the calcium channel blocker isradipine. Changes were not detected for relaxation by adrenergic agonists. In conclusion, aging caused a decrease of MCh potency, which is probably related to the reduction of calcium sensitivity in jejunum.

Marrubenol interacts with the phenylalkylamine binding site of the L-type calcium channel

Marrubenol inhibits contraction of rat arteries by blocking L-type calcium (Ca(2+)) channels in smooth muscle cells, but its interaction with binding sites for calcium antagonists had never been investigated. Competition binding studies indicated that marrubenol was a weak inhibitor of 1,4-dihydropyridine binding in membranes isolated from rat intestinal smooth muscle but completely displaced specifically bound (-)-[(3)H]desmethoxyverapamil ([(3)H]D888) with an apparent K(i) value of 16 microM (95% confidence interval: 6.5-39.5 microM). As marrubenol inhibited the contraction evoked by KCl depolarization of intestinal smooth muscle half-maximally at a concentration of approximately 12 microM, interaction with the phenylalkylamine binding site seems to account for the inhibition of L-type Ca(2+) channels by marrubenol.

Stimulation of L-type Ca(2+) channels by increase of intracellular InsP3 in rat retinal pigment epithelial cells

The purpose of this study was to investigate the role of voltage-dependent L-type Ca(2+)channels in intracellular Ca(2+)signaling of the retinal pigment epithelium (RPE). Patch-clamp techniques in conjunction with measurements of the intracellular free Ca(2+)using the Ca(2+)-sensitive fluorescence dye fura-2 were performed using cultured rat RPE cells. Intracellular application of inositol-1,4,5-trisphosphate (InsP3; 10 microM) via the patch-pipette during the whole-cell configuration led to an increase in the intracellular free Ca(2+)([Ca(2+)](i)). This effect could be reduced by the L-type Ca(2+)channel blocker nifedipine (2 microM). At the moment of the maximal rise in [Ca(2+)](i)L-type currents displayed an increase in the current density and shifts in the activation curve and of the steady-state inactivation. Comparable changes of L-type channel activity could be observed by induction of capacitative Ca(2+)entry, a maneuver to release Ca(2+)from intracellular Ca(2+)stores independently from InsP3. The increase in L-type Ca(2+)channel activity and [Ca(2+)](i)by intracellular application of InsP3 or induction of capacitative Ca(2+)entry could be inhibited by blocking tyrosine kinase activity using genistein (5 microM) or tyrphostin 51 (10 microM). It is concluded that L-type Ca(2+)channels are involved in the Ca(2+)/InsP3 second messenger system by generating an influx of extracellular Ca(2+)into the cell. This is enabled by depletion of cytosolic Ca(2+)stores and tyrosine kinase-dependent activation of L-type channels.

Relationship between extra and intracellular sources of calcium and the contractile effect of thiopental in rat aorta

To evaluate the relationship between the vasocontractile effect of thiopental and the extra and intracellular sources of Ca2+, we analyzed both the contractile effect of the barbiturate on rat aortic rings and its ability to modify the intracellular calcium concentration in cultured rat aorta smooth muscle cells. Thiopental (10-310 microg/mL) contracted aortic rings only in the presence of extracellular Ca2+, and this effect was not blocked by verapamil or diltiazem. On the contrary, Ca2+ (0.1-3.1 mM) evoked contractions only when thiopental (100 microg/mL) was present. Although in calcium-free solution thiopental (100 microg/mL) did not contract aortic rings, it abolished the contractile effect of either phenylephrine (10(-6) M) or caffeine (10 mM). Finally, thiopental augmented the intracellular calcium concentration in cultured smooth muscle cells incubated either in the presence or absence of calcium. In conclusion, thiopental's vasocontractile effect depends on extracellular calcium influx, which is independent of L-calcium channels. The increase in intracellular Ca2+ concentration elicited by thiopental in Ca2+-free solution and its ability to block the effect of phenylephrine and caffeine suggest that this barbiturate can deplete intracellular pools of calcium. Therefore, the calcium entry pathway associated with the contractile effect of thiopental may correspond to the capacitative calcium entry model.

Effects of zinc ions on Ca2+ uptake during histamine-induced contraction in guinea-pig taenia coli

The present study was undertaken to clarify the effects of zinc ions (Zn2+) on Ca2+ uptake during histamine-induced contraction in guinea-pig taenia coli. Zn2+ (0.3 mM) had smaller effect on the initial phasic response to 10(-5) M histamine accompanied with decreased Ca uptake at the high affinity sites, while it inhibited the tonic response by inhibiting Ca2+ uptake at the low affinity sites. After the first stimulation with histamine in the presence of Zn2+, the Ca2+ binding at high affinity sites was not significantly recovered in the presence of Zn2+. A second stimulation with histamine in the presence of Zn2+ did not elicit any phasic response. Furthermore, after beta-escin treatment of the fibres, which leaves receptor-coupled signal transduction including the Ca2+ storage sites intact, the contraction due to 10(-5) M histamine was not so affected by 0.3 mM Zn2+, but, after resuspension in a solution of pCa 5, 10(-5) M histamine of second application did not elicit the contraction in the presence of Zn2+. These results suggested that Zn2+ does not affect the histamine-induced Ca2+ release from the intracellular storage sites in taenia coli. However, when Zn2+ is present, Ca2+ is not supplied to the storage sites for the phasic response due to second stimulation with histamine. In addition, Zn2+ reduced the tonic response to histamine mainly by inhibiting Ca2+ influx through receptor-operated and/or voltage-dependent Ca2+ channels.

Amlodipine inhibits thapsigargin-sensitive CA(2+) stores in thrombin-stimulated vascular smooth muscle cells

Ca(2+) channel blockers, such as amlodipine, inhibit vascular smooth muscle cell (VSMC) growth through interactions with targets other than L-type Ca(2+) channels. The effects of amlodipine on Ca(2+) movements in thrombin- and thapsigargin-stimulated VSMCs were therefore investigated by determining the variations of intracellular free Ca(2+) concentration in fura 2-loaded cultured VSMCs. Results indicated that 10-1,000 nM amlodipine inhibited 1) thrombin-induced Ca(2+) mobilization from a thapsigargin-sensitive pool and 2) thapsigargin-induced Ca(2+) responses, including Ca(2+) mobilization from internal stores and store-operated Ca(2+) entry. These effects of amlodipine do not involve L-type Ca(2+) channels and could not be reproduced with 100 nM isradipine, diltiazem, or verapamil. The inhibition by amlodipine of Ca(2+) mobilization appears therefore to be a specific property of the drug, in addition to its Ca(2+) channel-blocking property. It is suggested that amlodipine acts in this capacity by interacting with Ca(2+)-ATPases of the sarcoplasmic reticulum, thus modulating the enzyme activity. This mechanism might participate in the inhibitory effect of amlodipine on VSMC growth.

Calcium sparks in smooth muscle

Local intracellular Ca(2+) transients, termed Ca(2+) sparks, are caused by the coordinated opening of a cluster of ryanodine-sensitive Ca(2+) release channels in the sarcoplasmic reticulum of smooth muscle cells. Ca(2+) sparks are activated by Ca(2+) entry through dihydropyridine-sensitive voltage-dependent Ca(2+) channels, although the precise mechanisms of communication of Ca(2+) entry to Ca(2+) spark activation are not clear in smooth muscle. Ca(2+) sparks act as a positive-feedback element to increase smooth muscle contractility, directly by contributing to the global cytoplasmic Ca(2+) concentration ([Ca(2+)]) and indirectly by increasing Ca(2+) entry through membrane potential depolarization, caused by activation of Ca(2+) spark-activated Cl(-) channels. Ca(2+) sparks also have a profound negative-feedback effect on contractility by decreasing Ca(2+) entry through membrane potential hyperpolarization, caused by activation of large-conductance, Ca(2+)-sensitive K(+) channels. In this review, the roles of Ca(2+) sparks in positive- and negative-feedback regulation of smooth muscle function are explored. We also propose that frequency and amplitude modulation of Ca(2+) sparks by contractile and relaxant agents is an important mechanism to regulate smooth muscle function.

Numerical simulation of motility patterns of the small bowel. II. Comparative pharmacological validation of a mathematical model

A model of a locus of the small bowel, described earlier by the authors (Miftakhov et al., 1999) was validated in a comparison of the results of numerical simulations of pharmacological compounds to their effects in biological studies. The actions of the following four classes of drugs were simulated, those: (i) acting on the sarcoplasmic reticulum, (ii) altering the permeability of L- and T-type Ca(2+)channels on the smooth muscle membrane, (iii) motilides, and (iv) benzodiazepines. The strong qualitative resemblance between the theoretical and experimental results supports the robustness of the model.

Receptor-activated Ca2+ inflow in animal cells: a variety of pathways tailored to meet different intracellular Ca2+ signalling requirements

Receptor-activated Ca2+ channels (RACCs) play a central role in regulation of the functions of animal cells. Together with voltage-operated Ca2+ channels (VOCCs) and ligand-gated non-selective cation channels, RACCs provide a variety of pathways by which Ca2+ can be delivered to the cytoplasmic space and the endoplasmic reticulum (ER) in order to initiate or maintain specific types of intracellular Ca2+ signal. Store-operated Ca2+ channels (SOCs), which are activated by a decrease in Ca2+ in the ER, are a major subfamily of RACCs. A careful analysis of the available data is required in order to discern the different types of RACCs (differentiated chiefly on the basis of ion selectivity and mechanism of activation) and to properly develop hypotheses for structures and mechanisms of activation. Despite much intensive research, the structures and mechanisms of activation of RACCs are only now beginning to be understood. In considering the physiological functions of the different RACCs, it is useful to consider the specificity for Ca2+ of each type of cation channel and the rate at which Ca2+ flows through a single open channel; the locations of the channels on the plasma membrane (in relation to the ER, cytoskeleton and other intracellular units of structure and function); the Ca2+-responsive enzymes and proteins; and the intracellular buffers and proteins that control the distribution of Ca2+ in the cytoplasmic space. RACCs which are non-selective cation channels can deliver Ca2+ directly to specific regions of the cytoplasmic space, and can also admit Na+, which induces depolarization of the plasma membrane, the opening of VOCCs and the subsequent inflow of Ca2+. SOCs appear to deliver Ca2+ specifically to the ER, thereby maintaining oscillating Ca2+ signals.

Comparison of contractions produced by carbachol, thapsigargin and cyclopiazonic acid in the guinea-pig tracheal muscle

1. Thapsigargin (TPG, 3 microM) and cyclopiazonic acid (CPA, 10 microM) slowly increased muscle tone in the guinea-pig isolated tracheal muscle. A large sustained contraction was produced when 2.4 mM Ca2+ was readmitted after 10 min exposure to Ca2+-free solution following 30 min treatment with TPG or CPA. 2. The sustained contraction after Ca2+ readmission was partially inhibited by nifedipine (3 microM) and highly dependent on external Ca2+. The TPG- and CPA-induced sustained contractions were 75% and 67%, respectively, of the sustained contraction produced by carbachol (Cch, 1 microM, EC80) in the presence of nifedipine. 3. The contractions produced by Cch, TPG and CPA were all inhibited by isoprenaline (ISO) and sodium nitroprusside (SNP). In the presence of nifedipine, the IC50 of ISO was 11, 17, and 23 nM and that of SNP was 0.5, 1, 0.8 microM for Cch-, TPG-, and CPA-induced contractions, respectively. The contraction produced by 60 mM K+ was only weakly inhibited by ISO and SNP. As with ISO and SNP, the Cch-, TPG- and CPA-induced contractions were also similarly inhibited by SKF 96365 (100 microM) and cadmium (Cd2+, 100 microM). 4. It was concluded that TPG and CPA increased Ca2+ influx probably via a mechanism activated by Ca2+ depletion of the sarcoplasmic reticulum. The susceptibility of the contraction produced by TPG, CPA and Cch to inhibition by ISO and SNP and also by SKF-96365 and Cd2+ suggests that the contractions use common pathways for increasing intracellular Ca2+, and that the contractions produced by K+ involve a different mechanism.