Biphasic Way Research Articles

Fine-tuning cardiac insulin-like growth factor 1 receptor signaling to promote health and longevity

Brief Summary: This translational study evaluated cardiac health and lifespan in two cardiomyocyte-specific transgenic mice with either enhanced or reduced IGF-1 signaling and in human cardiac biopsies from failing and nonfailing hearts. Increased IGF1R expression was related to better cardiac performance in young mice but faster decline of cardiac function with aging. Conversely reduced IGF1R signaling was associated improved lifespan and superior cardiac profile during aging. Human failing hearts showed exaggerated IGF1R signaling. These data unveil a novel role of IGF1R signaling in cardiac health, which acts in a biphasic way according to age and suggest that the use of pharmacological inhibitors of IGF1 could be beneficial in elderly adults at risk of heart failure.

Dynamics of the most common pathogenic mtDNA variant m.3243A > G demonstrate frequency-dependency in blood and positive selection in the germline.

The A-to-G point mutation at position 3243 in the human mitochondrial genome (m.3243A > G) is the most common pathogenic mtDNA variant responsible for disease in humans. It is widely accepted that m.3243A > G levels decrease in blood with age, and an age correction representing ~ 2% annual decline is often applied to account for this change in mutation level. Here we report that recent data indicate that the dynamics of m.3243A > G are more complex and depend on the mutation level in blood in a bi-phasic way. Consequently, the traditional 2% correction, which is adequate 'on average', creates opposite predictive biases at high and low mutation levels. Unbiased age correction is needed to circumvent these drawbacks of the standard model. We propose to eliminate both biases by using an approach where age correction depends on mutation level in a biphasic way to account for the dynamics of m.3243A > G in blood. The utility of this approach was further tested in estimating germline selection of m.3243A > G. The biphasic approach permitted us to uncover patterns consistent with the possibility of positive selection for m.3243A > G. Germline selection of m.3243A > G shows an 'arching' profile by which selection is positive at intermediate mutant fractions and declines at high and low mutant fractions. We conclude that use of this biphasic approach will greatly improve the accuracy of modelling changes in mtDNA mutation frequencies in the germline and in somatic cells during aging.

Abstract 14308: Lamotrigine Inhibits the Inward Fast Sodium Current (I Na ) in Epileptic Rat Cardiomyocytes

Introduction: Lamotrigine (LTG) is widely prescribed to treat neurological disorders such as migraine, neurophatic pain, and epilepsy. Recently, LTG has been linked to Brugada síndrome (BrS) ECG associated with a reduction in sodium current (I Na ) amplitude. Experimental studies have revealed that LTG blocks voltage gated sodium channels (VGSC). Hypothesis: Therefore, disrupting the depolarizing fast sodium inward current is a potential cause for arrhythmias. Recently our laboratory has shown that epilepsy induces molecular remodeling of the cardiac ventricle characterized by neuronal sodium channels overexpression, so it would be very plausible that LTG can cause important alterations on I Na in epileptic individuals. Methods: We used Patch-Clamp technique to test the effect of LTG on I Na in sham and epileptic rat ventricular cardiomyocytes. Results: We found that LTG inhibited I Na in a single dose-response manner with an IC 50 =326.35μM in sham rats. In epileptic rat cardiomyocytes the inhibitory effect was biphasic way with an IC 50 (1)=8.09 μM, which is close to the therapeutic range (between 12 and 50 μM), and a IC 50 (2)=333.66 μM. Moreover, LTG also altered sodium channels availability in biphasic manner with Vh(1)=-76.36 ± 0.31 mV and Vh(2)=-121.42 ± 1.38 mV, suggesting that LTG reduced the number of ion channels contributing to I Na . Conclusions: These findings may explain in part how LTG induce a Brugada pattern on the ECG of epileptic patients.

Preparation and characterization of amphiphilic nanoparticles based on chondroitin sulfate A conjugated with hydrophobic drug for enhanced doxorubicin delivery

Novel chondroitin sulfate A conjugated doxorubicin (CSA-DOX) was synthesized and characterized. The mean diameter of CSA-DOX was 152 nm analyzed by dynamic light scattering (DLS). Further, DOX was physically encapsulated into the CSA-DOX nanoparticles. Total amounts of DOX in three kinds of DOX-loaded CSA-DOX (CSA-DOX/DOX) nanoparticles were from 21.8 to 24.2%. The mean diameters of three CSA-DOX/DOX nanoparticles were in the range of 167–177 nm. The shape of CSA-DOX/DOX nanoparticles was almost spherical. DOX released from DOX-loaded nanoparticles exhibited a biphasic way in phosphate buffered saline (PBS, pH 7.4). Cellular uptake studies exhibited that CSA-DOX and CSA-DOX/DOX nanoparticles could enhance the uptake of DOX into CD44 receptor-positive A549 cells. Moreover, the cytotoxicity of CSA-DOX/DOX nanoparticles against A549 cells significantly improved in contrast with free DOX and CSA-DOX nanoparticles. These results suggested that CSA-DOX copolymer could be a potential carrier for antitumor drug delivery.

Extracellular cGMP Modulates Learning Biphasically by Modulating Glycine Receptors, CaMKII and Glutamate-Nitric Oxide-cGMP Pathway.

It has been proposed that extracellular cGMP modulates the ability to learn a Y maze task, but the underlying mechanisms remained unknown. Here we show that extracellular cGMP, at physiological concentrations, modulates learning in the Y maze in a biphasic way by modulating the glutamate-nitric oxide-cGMP pathway in cerebellum. Extracellular cGMP reduces glycine receptors activation inducing a voltage-dependent calcium-channels-mediated increase of calcium in Purkinje neurons. This calcium increase modulates CaMKII phosphorylation in a biphasic way. When basal calcium concentration is low extracellular cGMP reduces CaMKII phosphorylation, increasing nitric oxide synthase activity, the glutamate-NO-cGMP pathway function and learning ability. When basal calcium is normal extracellular cGMP increases CaMKII phosphorylation, reducing nitric oxide synthase activity, the pathway function and learning. These data unveil new mechanisms modulating learning in the Y maze and likely other learning types which may be therapeutic targets to improve learning in pathological situations associated with altered cGMP levels.

Modulation of cardiac pacemaker inter beat intervals by sinoatrial fibroblasts -a numerical study.

The potential effect of sinoatrial fibroblasts on beat rate and variability of the cardiac pacemakers is not yet fully understood. Heterocellular coupling formation and fibroblast proliferation during diseased conditions may further signify the impact of those cells on sinoatrial node function. In this study we numerically modeled the impact of varying numbers of fibroblasts that are electrically coupled to a single pacemaker cell on several electrophysiological parameters. We employed cellular kinetics of the rabbit sinoatrial myocyte, and employed a range of potential gap junctional coupling between fibroblasts and myocytes. We show that increasing numbers of attached and coupled fibroblasts result in depolarization of the resting membrane potential of the pacemaker cell, as well as in attenuation in its action potential magnitude. We also demonstrate that the mean pacemaker inter-beat interval (IBI) was modulated in a non-linear, bi-phasic way by increasing numbers of attached fibroblasts, whereby an initial phase of decreasing IBIs was followed by a significant phase of exponentially increasing IBIs. These observations were more substantial for increased gap junctional coupling between the two cell types. We finally show that IBI variability exponentially increased with increasing numbers of attached and electrically coupled fibroblasts. Again, this effect was stronger with higher values of gap junctional coupling. We postulate that the last observation is related to the role of fibroblasts in amplifying membrane voltage fluctuations of attached myocytes.

Conditions for Waveblock Due to Anisotropy in a Model of Human Ventricular Tissue.

Waveblock formation is the main cause of reentry. We have performed a comprehensive numerical modeling study of block formation due to anisotropy in Ten Tusscher and Panfilov (2006) ionic model for human ventricular tissue. We have examined the border between different areas of myocardial fiber alignment and have shown that blockage can occur for a wave traveling from a transverse fiber area to a longitudinal one. Such blockage occurs for reasonable values of the anisotropy ratio (AR): from 2.4 to 6.2 with respect to propagation velocities. This critical AR decreases by the suppression of I Na and I Ca, slightly decreases by the suppression of I Kr and I Ks, and substantially increases by the suppression of I K1. Hyperkalemia affects the block formation in a complex, biphasic way. We provide examples of reentry formation due to the studied effects and have concluded that the suppression of I K1 should be the most effective way to prevent waveblock at the areas of abrupt change in anisotropy.

The Interrelations among Stochastic Pacing, Stability, and Memory in the Heart

Low pacing variabilty in the heart has been clinically reported as a risk factor for lethal cardiac arrhythmias and arrhythmic death. In a previous simulation study, we demonstrated that stochastic pacing sustains an antiarrhythmic effect by moderating the slope of the action potential duration (APD) restitution curve, by reducing the propensity of APD alternans, converting discordant to concordant alternans, and ultimately preventing wavebreaks. However, the dynamic mechanisms relating pacing stochasticity to tissue stability are not yet known. In this work, we develop a mathematical framework to describe the APD signal using an autoregressive stochastic model, and we establish the interrelations between stochastic pacing, cardiac memory, and cardiac stability, as manifested by the degree of APD alternans. Employing stability analysis tools, we show that increased stochasticity in the ventricular tissue activation sequence works to lower the maximal absolute eigenvalues of the stochastic model, thereby contributing to increased stability. We also show that the memory coefficients of the autoregressive model are modulated by pacing stochasticity in a nonlinear, biphasic way, so that for exceedingly high levels of pacing stochasticity, the antiarrhythmic effect is hampered by increasing APD variance. This work may contribute to establishment of an optimal antiarrhythmic pacing protocol in a future study.

Time-dependent modulation of muscarinic m1/m3 receptor signalling by local anaesthetics

BackgroundSignalling of several G-protein-coupled receptors of the Gq/11 family is time-dependently inhibited by local anaesthetics (LAs). Since LA-induced modulation of muscarinic m1 and m3 receptor function may explain their beneficial effects in clinical practice, such as decreased postoperative cognitive dysfunction or less bronchoconstriction, we studied how prolonged exposure affects muscarinic signalling (Wang D, Wu X, Li J, Xiao F, Liu X, Meng M. The effect of lidocaine on early postoperative cognitive dysfunction after coronary artery bypass surgery. Anesth Analg 2002; 95: 1134–41; Groeben H, Silvanus MT, Beste M, Peters J. Combined lidocaine and salbutamol inhalation for airway anesthesia markedly protects against reflex bronchoconstriction. Chest 2000; 118: 509–15). MethodsA two-electrode voltage clamp was used to assess the effects of lidocaine or its permanently charged analogue QX314 on recombinantly expressed m1 and m3 receptors in Xenopus oocytes. Antisense knock-down of functional Gαq-protein and inhibition of protein kinase C (PKC) served to define mechanisms and sites of action. ResultsLidocaine affected muscarinic signalling in a biphasic way: an initial decrease in methylcholine bromide-elicited m1 and m3 responses after 30 min, followed by a significant increase in muscarinic responses after 8 h. Intracellularly injected QX314 time-dependently inhibited muscarinic signalling, but had no effect in Gαq-depleted oocytes. PKC-antagonism enhanced m1 and m3 signalling, but completely abolished the LA-induced increase in muscarinic responses, unmasking an underlying time-dependent inhibition of m1 and m3 responses after 8 h. ConclusionsLidocaine modulates muscarinic m1 and m3 receptors in a time- and Gαq-dependent manner, but this is masked by enhanced PKC activity. The biphasic time course may be due to interactions of LAs with an extracellular receptor domain, modulated by PKC activity. Prolonged exposure to LAs may not benefit pulmonary function, but may positively affect postoperative cognitive function.

Preparation and characterization of galactosylated glycol chitosan micelles and its potential use for hepatoma-targeting delivery of doxorubicin

This study aimed to develop novel galactosylated cholesterol modified-glycol chitosan (Gal-CHGC) micelles for targeting delivery of doxorubicin (DOX) in live cancer cells. Three kinds of Gal-CHGC conjugates were synthesized and characterized. The mean particle size and critical aggregation concentration of these polymeric micelles increased with the increase of galactose substitution degree. The DOX-loaded micelles were prepared by an o/w method. The mean diameters of DOX-loaded galactosylated micelles were in the range of 387-497nm. DOX released from drug-loaded micelles displayed a biphasic way. Cellular uptake studies demonstrated that DOX-loaded galactosylated micelles could enhance the uptake of DOX into HepG2 cells. Moreover, the cytotoxicity of DOX-loaded galactosylated micelles against HepG2 cells significantly improved in contrast with free DOX and DOX-loaded micelles without galactosylation. These results suggested that Gal-CHGC micelles could be a potential carrier for hepatoma-targeting drug delivery.

Abstract 2957: Cellular level of HSP27 phosphorylation plays a key switch from cell survival to death induced by TRAIL or gemcitabine.

Abstract TRAIL-induced activation of Akt catalytic activity and phosphorylation were highly correlated with p38/HSP27 phosphorylation, whereas the phosphorylation of p38/HSP27 increased further during incubation with curcumin and TRAIL, which caused significant apoptotic cell death in DU-145 cells. Next, we investigated whether the cellular level of HSP27 phosphorylation acts as a determinant of cell fate: from cell survival to cell death by biphasic way using another anticancer agent, gemcitabine an antitumor drug currently used for the treatment of advanced pancreatic cancer. We have observed that gemcitabine induced p38/HSP27 phosphorylation and acquired resistance at lower concentration as TRAIL did. After the acquisition of TRAIL or gemcitabine resistance, cancer cells (DU-145 or Mia-PaCa) with acquired resistance to these drugs expressed higher amount of vimentin as well as various MMPs. However, the expression level of these proteins even in TRAIL or gemcitabine-resistant cells was significantly decreased by the incorporation of constitutively phospho-mimic HSP27 with concurrent cell death, providing a new potential strategy to overcome the drug resistance and ultimate metastasis. Citation Format: Dongxu Kang, Hye J. Choi, Jina Kim, Joo-Hang Kim, Jae Jin Song. Cellular level of HSP27 phosphorylation plays a key switch from cell survival to death induced by TRAIL or gemcitabine. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2957. doi:10.1158/1538-7445.AM2013-2957

Modulation of Spiral-Wave Dynamics and Spontaneous Activity in a Fibroblast/Myocyte Heterocellular Tissue–-A Computational Study

Fibroblasts make for the most common nonmyocyte cells in the human heart and are known to play a role in structural remodeling caused by aging and various pathological states, which can eventually lead to cardiac arrhythmias and fibrillation. Gap junction formed between fibroblasts and myocytes have been recently described and were shown to alter the cardiac electrical parameters, such as action potential duration and conduction velocity, in various manners. In this study, we employed computational modeling to examine the effects of fibroblast-myocyte coupling and ratio on automaticity and electrical wave conduction during reentrant activity, with specific emphasis on dynamic phenomena and stability. Our results show that fibroblast density and coupling impact wave frequency in a biphasic way, first increasing wave frequency and then decreasing it. This can be explained by the dual role of the fibroblast cell as a current sink or a current source, depending on the coupled myocytes intracellular potential. We have also demonstrated that wave stability as manifested by the spiral-wave tip velocity and reentrant activity lifespan depends on fibroblast-myocyte coupling and ratio in a complex way. Finally, our study describes the required conditions in which spontaneous activity can occur, as a result of the fibroblasts depolarizing the myocytes' resting potential sufficiently to induce rhythmic pulses without any stimulation applied.

Development of the anticitrullinated peptide antibody repertoire prior to the onset of rheumatoid arthritis

BackgroundAnticitrullinated protein antibodies (ACPA) probably play a pathogenic role in rheumatoid arthritis (RA). The ACPA response is already expanded before the first symptoms of RA and hardly changes thereafter.ObjectivesTo explore...

Identification of sites responsible for the potentiating effect of niflumic acid on ClC-Ka kidney chloride channels.

ClC-K kidney Cl(-) channels are important for renal and inner ear transepithelial Cl(-) transport, and are potentially interesting pharmacological targets. They are modulated by niflumic acid (NFA), a non-steroidal anti-inflammatory drug, in a biphasic way: NFA activates ClC-Ka at low concentrations, but blocks the channel above approximately 1 mM. We attempted to identify the amino acids involved in the activation of ClC-Ka by NFA. We used site-directed mutagenesis and two-electrode voltage clamp analysis of wild-type and mutant channels expressed in Xenopus oocytes. Guided by the crystal structure of a bacterial CLC homolog, we screened 97 ClC-Ka mutations for alterations of NFA effects. Mutations of five residues significantly reduced the potentiating effect of NFA. Two of these (G167A and F213A) drastically altered general gating properties and are unlikely to be involved in NFA binding. The three remaining mutants (L155A, G345S and A349E) severely impaired or abolished NFA potentiation. The three key residues identified (L155, G345, A349) are localized in two different protein regions that, based on the crystal structure of bacterial CLC homologs, are expected to be exposed to the extracellular side of the channel, relatively close to each other, and are thus good candidates for being part of the potentiating NFA binding site. Alternatively, the protein region identified mediates conformational changes following NFA binding. Our results are an important step towards the development of ClC-Ka activators for treating Bartter syndrome types III and IV with residual channel activity.

The Complement System Is Activated in a Biphasic Pattern After Coronary Artery Bypass Grafting

The complement system is a key component in the inflammatory response after coronary artery bypass grafting (CABG). The routes of complement activation and deactivation after cardiac surgery are not clear. The aim of this study was to analyze routes of complement activation after uncomplicated CABG. Complement components and activation products were measured in 20 nondiabetic adult patients undergoing elective CABG at several times postoperatively starting at admission to the intensive care unit. Complement activation after uncomplicated CABG showed a biphasic pattern. In the first 8 hours after admission to the intensive care unit, complement activation was initiated by the classical lectin pathway and augmented by the alternative pathway. Ultimately, this resulted in terminal pathway activation and formation of terminal complement complex. In the second phase, starting at 8 hours after the operation, complement was still activated by the classical lectin pathway, but there was no augmentation by the alternative pathway and no terminal complement complex formation. This implies that during this second stage, inhibitory mechanisms beyond C3b are engaged. Complement activation after cardiac surgery is regulated in a complex biphasic way, with additional inhibitory mechanisms engaged from 8 hours postoperatively onward.

Prospective Evaluation of Symptom Responses to Successful Operation for Mild-Moderate Primary Hyperparathyroidism

Parathyroidectomy is claimed to strongly improve the hypercalcemic symptoms in patients with primary hyperparathyroidism (HPT). The object of this study was to register the patients own experience of symptom changes in response to successful parathyroid surgery. Material one. 44 HPT patients filled in a questionnaire consisting of 8 visual analogue scales referring to "typical" hypercalcemic symptoms before as well as 2 and 12 months after neck surgery. Their results were compared with those obtained from an age- and sex-matched group of patients undergoing surgery for non-toxic thyroid disease. Material two. 25 elderly women with mild-moderate HPT were followed more closely up to 3 years after neck surgery with the same type of questionnaire. HPT patients expressed significantly more of fatigue, muscular weakness, impaired memory, thirst and polyuria than patients with non-toxic thyroid disorders. These differences were eliminated 2 months after surgery. The pattern of response to parathyroidectomy was similar for most symptoms: an impressive improvement after 2 months and then a gradual return towards the preoperative value. However, the response was still significant for fatigue, weakness, thirst, polyuria and inactivity after 1 year. The effect of normalization of serum calcium on symptom scoring varied considerably between different individuals and also between different symptoms in the same individual. The elderly women responded in a similar biphasic way but their improvements were less pronounced. HPT patients experience significant improvements of most "hypercalcemic" symtoms after successful parathyroidectomy, but the effects are transient and vary considerably between different individuals.

Self-aggregated nanoparticles of cholesterol-modified glycol chitosan conjugate: Preparation, characterization, and preliminary assessment as a new drug delivery carrier

Cholesterol-modified glycol chitosan (CHGC) conjugate was synthesized and characterized by FTIR and 1H NMR. The degree of substitution (DS) was 6.7 cholesterol groups per 100 sugar residues of glycol chitosan. CHGC formed self-aggregated nanoparticles with a roughly spherical shape and a mean diameter of 228nm by probe sonication in aqueous medium. The physicochemical properties of the self-aggregated nanoparticles were studied using dynamic light scattering (DLS), transmission electron microscopy (TEM) and fluorescence spectroscopy. The critical aggregation concentration (CAC) of self-aggregated nanoparticles in aqueous solution was 0.1223mg/mL. Indomethacin (IND), as a model drug, was physically entrapped into the CHGC nanoparticles by dialysis method. The characteristics of IND-loaded CHGC (IND–CHGC) nanoparticles was analyzed using DLS, TEM and high performance liquid chromatography (HPLC). The IND–CHGC nanoparticles were almost spherical in shape and their size increased from 275 to 384nm with the IND-loading content increasing from 7.14% to 16.2%. The in vitro release behavior of IND from CHGC nanoparticles was studied by a dialysis method in phosphate buffered saline (PBS, pH 7.4). IND was released in a biphasic way. The initial rapid release in 2h and slower release for up to 12h were observed. The results indicated that CHGC nanoparticles had a potential as a drug delivery carrier.

Arachidonic Acid Metabolites Inhibit the Stimulatory Effect of Angiotensin II in Renal Proximal Tubules

Angiotensin II (Ang II) regulates renal proximal transport in a biphasic way via Ang II type 1 receptor (AT1). Whereas extracellular signal-regulated kinase (ERK) activation mediates the stimulatory effect, cytosolic phospholipase A2 (cPLA2) mediates the inhibitory effect independently of ERK. In this study, we tested the hypothesis that the cPLA2/P450 epoxygenase pathway might work to suppress the Ang II-mediated ERK activation. In the presence of arachidonic acid or 5,6-epoxyeicosatrienoic acid (EET), Ang II failed to stimulate the Na-HCO3 cotransporter activity in renal proximal tubules isolated from wild-type, AT1A-deficient, and cPLA2-alpha-deficient mice. In addition, Ang II failed to induce a significant ERK phosphorylation in the presence of arachidonic acid or 5,6-EET. Arachidonic acid or 5,6-EET also suppressed the stimulatory effect of Ang II on net proximal tubule bicarbonate absorption without changing cell Ca2+ concentrations. These results indicate that the cPLA2-alpha/P450/EET pathway blocks the stimulatory effect of Ang II by suppressing the ERK activation. Thus, the cPLA2-alpha/P450/EET pathway may operate as a unique negative feedback mechanism to attenuate excessive Ang II activity in the renal proximal tubules, where extremely high concentrations of Ang II are found.

Ketoprofen poly(lactide-co-glycolide) physical interaction

The aim of this work was to provide an understanding of the interaction occurring between ketoprofen and poly(lactic-co-glycolic acid) (PLGA) that leads to polymer plasticization. Experimental glass transition temperature (Tg) values were fitted with the theoretical ones predicted by the Fox and Gordon-Taylor/Kelley-Bueche equations. PLGA films containing different amounts of ketoprofen (KET) were prepared by solvent casting and characterized by scanning electron microscopy, differential scanning calorimetry, and Fourier transform infrared spectroscopy (FTIR). Differential scanning calorimetry evidenced that KET acted as a plasticizer in a similar biphasic way in both end-capped and uncapped PLGA. At KET contents of 20% to 35%, depending on the investigated polymer, the Tg was around 23 degrees C. Higher KET amounts did not lower further the Tg, and the excess of drug was found to crystallize into the polymeric matrix. Experimental Tg's deviated negatively from the predicted ones probably because of hydrogen bonding. The FTIR spectra of the films, loaded with different amounts of KET, showed a shift to higher wavenumbers for the peaks at 1697 and 1655 cm(-1) confirming the presence of some interactions, probably hydrogen bonds between the ketoprofen carboxylic group and the PLGA carbonyl groups along the polymer backbone. The hydrogen bonding between KET and PLGA is probably responsible for KET plasticizing effect. KET behaving as a lubricant may disrupt polymer chain-chain interactions, removing additional barriers to bond rotation and chain mobility.

Mibefradil-sensitive component involved in the plateau potential in submucosal interstitial cells of the murine proximal colon

Submucosal interstitial cells of Cajal (ICC(SM)) produce plateau potentials comprised of initial fast and subsequent plateau components. The possible involvement of voltage-dependent Ca(2+) channels in plateau potentials was examined in ICC(SM) of the murine proximal colon. Increases in external K(+) concentration ([K(+)](o)) changed the rise rate of the initial component in a biphasic way, an increase in 10.6 or 15.3mM [K(+)](o) and a decrease in 20.0mM [K(+)](o). The rise rate of plateau potentials was significantly reduced by the application of 3 microM mibefradil or 100 microM Ni(2+) but not by 0.3 microM nifedipine. The inhibitory effect of mibefradil on the rise rate of plateau potentials was concentration-dependent with an IC(50) value of 1.0 microM. In conclusion, the initial phase of plateau potentials is partly due to the activation of T-type Ca(2+) channel in ICC(SM) from the murine proximal colon.