Intracellular application of calmidazolium increases Ca2+ current through activation of protein kinase A in cultured vascular smooth muscle cells

Involvement of Ca(2+) channel activity in proliferation of vascular smooth muscle cells

Proliferation of vascular smooth muscle (VSM) cells is a crucial step for developing vascular diseases such as atherosclerosis, hypertension and vascular restenosis after angioplasty. Proliferation of VSM cells is regulated by many intracellular signals: second messengers (e.g. Ca(2+), phosphatydylinositol, cAMP/cGMP), protein kinases and transcription factors. Although Ca(2+) regulation of cell proliferation is very important, there is rarely any informative review paper about the topic. Increase in cytosolic intracellular Ca(2+) concentration ([Ca(2+)](i)) due to Ca(2+) entry is necessary for proliferation of VSM cells. Elevation of [Ca(2+)](i) is needed for both cell cycle progressions at G(1)/S phase and the cell division in M phase. Intracellular Ca(2+) is regulated by the balance between Ca(2+)-elevating machinery such as Ca(2+) influx through voltage-dependent Ca(2+) channels (VDCC), Ca(2+) release from stored Ca(2+) in sarcoplasmic reticulum and Ca(2+)-lowering machinery such as Ca(2+) transport ATPases. In this review paper, we focus on the role of VDCC in the regulation of cell proliferation, especially in VSM cells. We also described significant roles of VDCC in pathophysiological conditions such as atherosclerosis, stroke and renal dysfunction.

Changes in calmodulin immunocytochemical localization associated with capacitation and acrosomal exocytosis of ram spermatozoa

The aim of this study was to determine the localization of calmodulin (CaM) in ram sperm and the possible changes during in vitro capacitation (CA) and the ionophore-induced acrosome reaction (AR). Likewise, changes in intracellular calcium levels ([Ca(2+)](i)) were also analysed by using flow cytometry. CA was induced in vitro in a medium containing BSA, CaCl(2), NaHCO(3), and AR by the addition of the calcium ionophore A23187. The acrosomal status was assessed by the chlortetracycline-fluorescence (CTC) assay. Flow cytometry (FC) analyses were performed by loading samples with Fluo-3 AM, that emits fluorescence at a high [Ca(2+)](i), combined with propidium iodide (PI) that allowed us to discriminate sperm with/without an integral plasma membrane both with high/low [Ca(2+)](i). Immunocytochemistry localized CaM to the flagellum, and some sperm also contained CaM in the head (equatorial and post-acrosomal regions). CA and AR resulted in a slight increase in the post-acrosomal labelling. The treatment of sperm with increasing concentrations of two CaM antagonists, W7 and calmidazolium (CZ), accounted for an increase in capacitated and acrosome-reacted CTC-sperm patterns. CZ induced a significant reduction in the content of three protein tyrosine-phosphorylated bands of approximately of 30, 40 and 45kDa. However, W7 showed no significant effect at any of the studied concentrations. Neither of them significantly influenced protein serine and threonine phosphorylation. FC analysis revealed that the main subpopulation in the control samples contained 70% of the total sperm with integral plasma membrane and a medium [Ca(2+)](i). After CA, 67.1% of the sperm preserved an integral membrane with a higher [Ca(2+)](i). After AR, only 7.2% of the total sperm preserved intact membranes with a very high [Ca(2+)](i). These results imply that CaM appears to be involved in ram sperm capacitation, and both treatments increased its localization in the post-acrosomal region.

WNT signaling promotes Nkx2.5 expression and early cardiomyogenesis via downregulation of Hdac1

The cardiac transcription factor NKX2.5 plays a crucial role in cardiomyogenesis, but its mechanism of regulation is still unclear. Recently, epigenetic regulation has become increasingly recognized as important in differentiation and development. In this study, we used P19CL6 cells to investigate the regulation of Nkx2.5 expression by methylation and acetylation during cardiomyocyte differentiation. During the early stage of differentiation, Nkx2.5 expression was upregulated, but the methylation status of the Nkx2.5 promoter did not undergo significant change; while the acetylation levels of histones H3 and H4 were increased, accompanied by a significant reduction in Hdac1 expression. Suppression of Hdac1 activity stimulated cardiac differentiation accompanied by increased expression of cardiac-specific genes and cell cycle arrest. Overexpression of Hdac1 inhibited cardiomyocyte formation and downregulated the expressions of Gata4 and Nkx2.5. Mimicking induction of the WNT pathway inhibited Hdac1 expression with upregulated Nkx2.5 expression. WNT3a and WNT3 downregulated the expression of Hdac1, contrary to the effect of SFRP2 and GSK3beta. Cotransfection of beta-catenin and Lef1 significantly downregulated the expression of Hdac1. Our data suggest that WNT signaling pathway plays important roles in the regulation of Hdac1 during the early stage of cardiomyocyte differentiation and that the downregulation of Hdac1 promotes cardiac differentiation.

Effect of sodium channel blocker, pilsicainide hydrochloride, on net inward current of atrial myocytes in thyroid hormone toxicosis rats

To investigate effect of pilsicainide hydrochloride (pilsicainide) on electrocardiogram (ECG) signals and action potentials (APs) of atrial myocytes, levo-thyroxine (T4, 500 microg/kg body weight) was daily injected into peritoneal cavity of Sprague-Dawley rats for 14 days. T4-treatment significantly shortened RR interval, P wave, and QRS complex durations on ECG. Although pilsicainide did not affect the heart rate, P wave and corrected QT interval (QTc) was increased in T4-treated rats. AP recordings revealed that AP durations at 20%, 50%, and 90% repolarization were significantly shortened and maximal rate of rise (Max dV/dt) was significantly increased in T4-treated rat atrial cells. Pilsicainide significantly decreased AP amplitude (APA) and Max dV/dt in both control and T4-treated rat atrial cells. Concentration-inhibition study demonstrated that pilsicainide significantly inhibited net inward current of T4-treated rats at lower concentration (IC50 of 29.2 microg/mL) than that of control rats (133 microg/mL). In conclusion, pilsicainide could decrease the conduction velocity in T4-treated rat atrium by decreasing the Max dV/dt and net inward current, which could be a possible treatment of thyrotoxicosis-induced arrhythmia.

Electrophysiologic characteristics of atrial myocytes in levo-thyroxine-treated rats

To investigate whether thyroid hormone modulates electrical properties of atrial myocytes, electrocardiogram (ECG), action potentials (APs), and ionic currents were measured. Male Sprague-Dawley rats were randomly divided into control and levo-thyroxine (T4)-treated groups at 6 weeks of age. Levo-thyroxine (500 microg/kg of body weight) was injected daily into the peritoneal cavity for 14 days (T4-treated rats) and the same volume of saline was injected in control rats daily. ECG signals were recorded using apex-base leads. APs, voltage-dependent Na+ and L-type Ca2+ channel current (I(Na) and I(Ca(L))), inwardly rectifying K+ channel current (I(K1)), transient outward K+ channel current (I(to)), and delayed rectifier K+ channel current (I(K(delay))) were measured using patch-clamp techniques. T4 treatment significantly changed electrical properties in rat atrial myocytes, including (1) the increase in heart rate, (2) the increase in cell size, (3) the shortening of action potential duration (APD), (4) the increase in cell membrane capacitance (C(m)), and (5) the decrease in input resistance (R(in)). Although the current densities of I(Na) and I(K1) in T4-treated atrial myocytes did not differ from those in control cells, I(Ca(L)) was significantly decreased and I(K(delay)) was significantly increased in T4-treated rats. Thus, thyrotoxicosis could induce the shortening of APD by alterations in current density of both I(Ca(L)) and I(K(delay)) in rat atrial myocytes.

Calcium-mediated protein secretion potentiates motility in Toxoplasma gondii

Apicomplexans such as Toxoplasma gondii actively invade host cells using a unique parasite-dependent mechanism termed gliding motility. Calcium-mediated protein secretion by the parasite has been implicated in this process, but the precise role of calcium signaling in motility remains unclear. Here we used calmidazolium as a tool to stimulate intracellular calcium fluxes and found that this drug led to enhanced motility by T. gondii. Treatment with calmidazolium increased the duration of gliding and resulted in trails that were twice as long as those formed by control parasites. Calmidazolium also increased microneme secretion by T. gondii, and studies with a deletion mutant of the accessory protein m2AP specifically implicated that adhesin MIC2 was important for gliding. The effects of calmidazolium on gliding and secretion were due to increased release of calcium from intracellular stores and calcium influx from the extracellular milieu. In addition, we demonstrate that calmidazolium-stimulated increases in intracellular calcium were highly dynamic, and that rapid fluxes in calcium levels were associated with parasite motility. Our studies suggest that oscillations in intracellular calcium levels may regulate microneme secretion and control gliding motility in T. gondii.

Involvement of Ca2+/calmodulin-dependent protein kinase II in endothelial NO production and endothelium-dependent relaxation

Nitric oxide (NO) is synthesized from l-arginine by the Ca(2+)/calmodulin-sensitive endothelial NO synthase (NOS) isoform (eNOS). The present study assesses the role of Ca(2+)/calmodulin-dependent protein kinase II (CaMK II) in endothelium-dependent relaxation and NO synthesis. The effects of three CaMK II inhibitors were investigated in endothelium-intact aortic rings of normotensive rats. NO synthesis was assessed by a NO sensor and chemiluminescence in culture medium of cultured porcine aortic endothelial cells stimulated with the Ca(2+) ionophore A23187 and thapsigargin. Rat aortic endothelial NOS activity was measured by the conversion of l-[(3)H]arginine to l-[(3)H]citrulline. Three CaMK II inhibitors, polypeptide 281-302, KN-93, and lavendustin C, attenuated the endothelium-dependent relaxation of endothelium-intact rat aortic rings in response to acetylcholine, A23187, and thapsigargin. None of the CaMK II inhibitors affected the relaxation induced by NO donors. In a porcine aortic endothelial cell line, KN-93 decreased NO synthesis and caused a rightward shift of the concentration-response curves to A23187 and thapsigargin. In rat aortic endothelial cells, KN-93 significantly decreased bradykinin-induced eNOS activity. These results suggest that CaMK II was involved in NO synthesis as a result of Ca(2+)-dependent activation of eNOS.

Actin filament disruption inhibits L-type Ca(2+) channel current in cultured vascular smooth muscle cells

To clarify interactions between the cytoskeleton and activity of L-type Ca(2+) (Ca(L)) channels in vascular smooth muscle (VSM) cells, we investigated the effect of disruption of actin filaments and microtubules on the L-type Ca(2+) current [I(Ba(L))] of cultured VSM cells (A7r5 cell line) using whole cell voltage clamp. The cells were exposed to each disrupter for 1 h and then examined electrophysiologically and morphologically. Results of immunostaining using anti-alpha-actin and anti-alpha-tubulin antibodies showed that colchicine disrupted both actin filaments and microtubules, cytochalasin D disrupted only actin filaments, and nocodazole disrupted only microtubules. I(Ba(L)) was greatly reduced in cells that were exposed to colchicine or cytochalasin D but not to nocodazole. Colchicine even inhibited I(Ba(L)) by about 40% when the actin filaments were stabilized by phalloidin or when the cells were treated with phalloidin plus taxol to stabilize both cytoskeletal components. These results suggest that colchicine must also cause some inhibition of I(Ba(L)) due to another unknown mechanism, e.g., a direct block of Ca(L) channels. In summary, actin filament disruption of VSM cells inhibits Ca(L) channel activity, whereas disrupting the microtubules does not.

Direct block of Ca2+ channels by calmidazolium in cultured vascular smooth muscle cells

We investigated the action of calmidazolium (CMZ), an inhibitor of calmodulin (CaM), on the L-type Ca2+ currents (ICa(L)) of cultured vascular smooth muscle (VSM) cells (A7r5 cell line), by using the whole-cell voltage-clamp method. All experiments were conducted at room temperature (24-25 degrees C). The peak IBa (Ca2+ channel current with 5 mM Ba2+ as charge carrier) was evoked every 15 s by a test potential to +10 mV from a holding potential of -60 mV. To elevate intracellular free Ca2+ concentration ([Ca]i) to pCa 6.5, the pipette solution contained a Ca2+-EGTA buffer (pCa 6.5) to allow equilibration with the cells. Bath application of 1 microM CMZ reduced the peak amplitude of IBa to 36.7+/-4.9% (n = 8); maximal effect occurred within 7-8 min. Peak IBa continued to decrease even after washing out the CMZ. Recovery of IBa was not observed even after 10 min of washout. Even in presence of an peptide inhibitor of CaM-dependent protein kinase-II (5.2 microM) in the pipette solution, CMZ inhibited IBa to 27.8 +/-5.3% (n = 7). To exclude the possibility that other Ca2+/ CaM-dependent kinases and phosphatases may regulate Ca2+ channel activity, we examined the effect of CMZ on IBa when [Ca]i was reduced by use of Ca2+/EGTA-buffered pipette solutions. At pCa approximately equal to 10 (10 mM EGTA and only contaminant Ca2+), CMZ inhibited IBa to 33.4+/-5.9% (n = 14) with a median inhibitory concentration (IC50) value of 0.29 microM. The activation curve (pCa approximately equal to 10) was shifted in the positive direction by 6.3 mV; the inactivation curve was shifted in the negative direction by 5.0 mV. CMZ decreased IBa progressively during repetitive step depolarizations. CMZ did not slow the rate of recovery from inactivation. In conclusion, CMZ inhibits Ca2+ channel current in a use-dependent manner. This inhibition is independent of CaMK-II and other Ca2+/CaM-dependent pathways. Therefore it is likely due to direct blockade of Ca2+ channels by CMZ. CMZ may reduce the outer surface charge and block the open state of the Ca2+ channels.