D-Tubocurarine-Sensitive Component of Calcium-Dependent Potassium Current in Guinea Pig <Emphasis Type="Italic">Taenia Coli</Emphasis> Myocytes

Using a patch-clamp technique in the whole-cell configuration, we studied transmembrane ion currents in isolated single smooth muscle cells of the guinea pig taenia coli. A depolarizing step shift of the membrane potential from −50 mV was accompanied by the appearance of an outward current. Application of d-tubocurarine (d-TK) or a nonselective blocker of voltage-dependent potassium channels, tetraethylammonium (TEA), led to a decrease in the outward current. Application of d-TK against the background of the action of TEA additionally decreased the outward current. Analysis of the current-voltage (I–V) relationships of the d-TK-sensitive current showed that this current is practically voltage-independent. At the same time, an inflection of the I–V curve of the potassium current within the segment of maximum activation of the voltage-dependent potassium current is indicative of the sensitivity of this current to the intracellular Ca²⁺ concentration. Therefore, the calcium-activated potassium current through small-conductance calcium-dependent potassium channels includes a d-TK-sensitive voltage-independent component. Using depolarizing shifts of the membrane potential, we observed high- and low-amplitude spontaneous outward currents (SOCs) in many studied cells, i.e., the effect of an increase in the conductance of calcium-dependent potassium channels as a result of periodic release of Ca²⁺ from the intracellular stores. Application of d-TK led to a decrease in the frequency of low-amplitude SOCs and exerted nearly no influence on the high-amplitude SOCs under study. Neirofiziologiya/Neurophysiology, Vol. 37, No. 3, pp. 271–277, May–June, 2005.     

Sodium Metabisulfite Modulation of Potassium Channels in Pain-Sensing Dorsal Root Ganglion Neurons

The effects of sodium metabisulfite (SMB), a general food preservative, on potassium currents in rat dorsal root ganglion (DRG) neurons were investigated using the whole-cell patch-clamp technique. SMB increased the amplitudes of both transient outward potassium currents and delayed rectifier potassium current in concentration- and voltage-dependent manner. The transient outward potassium currents (TOCs) include a fast inactivating (A-current or I _A) current and a slow inactivating (D-current or I _D) current. SMB majorly increased I_A, and I_D was little affected. SMB did not affect the activation process of transient outward currents (TOCs), but the inactivation curve of TOCs was shifted to more positive potentials. The inactivation time constants of TOCs were also increased by SMB. For delayed rectifier potassium current (I _K), SMB shifted the activation curve to hyperpolarizing direction. SMB differently affected TOCs and I _K, its effects major on A-type K⁺ channels, which play a role in adjusting pain sensitivity in response to peripheral redox conditions. SMB did not increase TOCs and I _K when adding DTT in pipette solution. These results suggested that SMB might oxidize potassium channels, which relate to adjusting pain sensitivity in pain-sensing DRG neurons.     

二氧化硫衍生物对大鼠背根神经元瞬间外向钾电流和延迟整流钾电流的影响

运用全细胞膜片钳技术研究二氧化硫衍生物对大鼠背根神经元瞬间外向钾电流(IA和ID)和延迟整流钾电流(IK)的影响。结果发现二氧化硫衍生物剂量依赖性地增大钾通道的电导,电压依赖性地增大钾电流的幅度,且这种增大作用部分可逆。二氧化硫非常显著地使延迟整流钾电流的激活过程向超极化方向移动,使瞬间外向钾电流的失活过程向去极化方向移动。10μmol/L二氧化硫衍生物作用前后,延迟整流钾电流的半数激活电压分别是(20.3±2.1)mV和(15.0±1.5)mV;IA和ID的半数失活电压分别朝去极化方向移动了6mV和7.4mV。这些结果表明二氧化硫改变了钾通道的特性,改变了神经元的兴奋性。     

Properties of the charibdotoxin-sensitive component of Ca2+-dependent K+ current in smooth muscle cells of the guinea pigTaenia Coli

Using the voltage-clamp technique, we investigated transmembrane ion currents in isolated smooth muscle cells of the guinea pigtaenia coli. In our study, we identified and studied a charibdotoxin-sensitive component of Ca²⁺-dependent K⁺ current carried through the channels of high conductance (in most publications called “big conductance,”I _BK(Ca)). This component was completely blocked by 100 nM charibdotoxin and by tetraethylammonium in concentrations as low as 1 mM.I _BK(Ca) demonstrated fast kinetics of inactivation, which nearly coincided with that of Ca²⁺ current. In addition to the dependence on Ca²⁺ concentration, this current also showed voltage-dependent properties: with a rise in the level of depolarization its amplitude increased. In many cells, depolarizing shifts in the membrane potential evoke spontaneous outward currents. Such currents probably represent the secondary effect of cyclic Ca²⁺ release from the caffeine-sensitive intracellular stores that result in short-term activation of charibdotoxin-sensitive Ca²⁺-dependent K⁺ channels.     

Ionic currents in cardiac myocytes of squid, Alloteuthis subulata

This paper provides the first study of voltage-sensitive membrane currents present in heart myocytes from cephalopods. Whole cell patch clamp recordings have revealed six different ionic currents in myocytes freshly dissociated from squid cardiac tissues (branchial and systemic hearts). Three types of outward potassium currents were identified: first, a transient outward voltage-activated A-current (I_A), blocked by 4-aminopyridine, and inactivated by holding the cells at a potential of −40 mV; second, an outward, voltage-activated, delayed rectifier current with a sustained time course (I_K); and third, an outward, calcium-dependent, potassium current (I_K(Ca)) sensitive to Co²⁺ and apamin, and with the characteristic N-shaped current voltage relationship. Three inward voltage-activated currents were also identified. First, a rapidly activating and inactivating, sodium current (I_Na), blocked by tetrodotoxin, inactivated at holding potentials more positive than −40 mV, and abolished when external sodium was replaced by choline. Second, an L-type calcium current (I_Ca,L) with a sustained time course, suppressed by nifedipine or Co²⁺, and enhanced by substituting Ca²⁺ for Ba²⁺ in the external medium. The third inward current was also carried by calcium ions, but could be distinguished from the L-type current by differences in its voltage dependence. It also had a more transient time course, was activated at more negative potentials, and resembled the previously described low-voltage-activated, T-type calcium current. Accepted: 24 September 1999     

Nitric oxide as a nonadrenergic inhibitory transmitter in smooth muscle cells of the guinea pig gastro-intestinal tract: Mechanisms of action

The role of nitric oxide (NO) as a possible transmitter for nonadrenergic inhibitory transmission was studied on isolated muscle strips of the guinea pig gastro-intestinal tract (GIT) using sucrose-gap technique. In addition, the voltage clamp and intracellular dialysis techniques were employed to study the effects of sodium nitroprusside (NP) on isolated smooth muscle (SM) cells of thetaenia coli. N-nitro-L-arginine methyl ester (L-NAME), a blocker of NO synthesis from L-arginine (0.1 mM), was shown to selectively suppress the apamin-resistant component of nonadrenergic inhibitory junctional (synaptic) potentials (IJP) in the guinea pig GIT SM cells. At the same time, L-NAME did not affect the vasoactive intestinal polypeptide (VIP)- and NP-evoked hyperpolarization in SM cells of the colon. The NP-induced hyperpolarization (0.1 mM) was accompained by a decrease in the SM cell membrane resistance. Application of NP to isolated SM cells activated a small outward current and increased the frequency of spontaneous transient calcium-dependent outward currents. NP increased the Ca-dependent potassium current evoked in SM cells by step depolarization, but did not affect the potassium currents of delayed rectification. Our results suggest that NO is involved in generation of nonadrenergic IJP in SM cells of the guinea pig GIT. The action of NP on SM cells is complex and results in hyperpolarization and relaxation (partially through the activation of Ca-dependent potassium channels in SM cell membrane).     

Gastric emptying and Ca2+ and K+ channels of circular smooth muscle cells in diet‐induced obese prone and resistant rats

Objective:

Accelerated gastric emptying that precipitates hunger and frequent eating could be a potential factor in the development of obesity. The aim of this study was to study gastric emptying in diet‐induced obese‐prone (DIO‐P) and DIO‐resistant (DIO‐R) rats and explore possible differences in electrical properties of calcium (Ca²⁺) and potassium (K⁺) channels of antral circular smooth muscle cells (SMCs).

Design and Methods:

Whole‐cell patch‐clamp technique was used to measure Ca²⁺ and K⁺ currents in single SMCs. Gastric emptying was evaluated 90 min after the ingestion of a solid meal.

Results:

Solid gastric emptying in the DIO‐P rats was significantly faster compared with that in the DIO‐R rats. The peak amplitude of L‐type Ca²⁺ current (I_Ba,L) at 10 mV in DIO‐P rats was greater than that in DIO‐R rats without alternation of the current–voltage curve and voltage‐dependent activation and inactivation. The half‐maximal inactivation voltage of transient outward K⁺ current (I_Kto) was more depolarized (～4 mV) in DIO‐P rats compared with that in DIO‐R rats. No difference was found in the current density or recovery kinetics of I_Kto between two groups. The current density of delayed rectifier K⁺ current (I_Kdr), which was sensitive to tetraethylammonium chloride but not 4‐aminopyridine, was lower in DIO‐P rats than that in DIO‐R rats.

Conclusion:

The accelerated gastric emptying in DIO‐P rats might be attributed to a higher density of I_Ba,L, depolarizing shift of inactivation curve of I_Kto and lower density of I_Kdr observed in the antral SMCs of DIO‐P rats.     

Blocking of Ca-dependent outward current in molluscan neuron somatic membrane by raised intracellular pH

The action of a raised intracellular pH (pH_i) on transmembrane ionic currents was investigated on isolated unidentified neurons ofHelix pomatia under intracellular dialysis and membrane voltage clamping conditions. With a rise in pH_i from 7.3 to 9.0 and in the simultaneous presence of an inward calcium current, the outward potassium current was considerably reduced and the current-voltage characteristic curve was shifted toward more positive membrane potential values. The inward calcium current was practically unchanged in this case. If, however, the calcium current was inhibited by the action of cadmium ions, no decrease in the outward current was observed, only a shift of the I_K(V) curve toward more positive values of membrane potential. It is suggested that an increase in pH_i selectively blocks the Ca-dependent component of the outward potassium current.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 14, No. 4, pp. 426–430, July–August, 1982.     

G-Protein Regulation of an L-Type Calcium Channel Current in Canine Jejunal Circular Smooth Muscle

Calcium entry into smooth muscle cells is essential to maintain contractility. In canine jejunal circular smooth muscle cells the predominant calcium entry pathway is through L-type calcium channels. The aim of this study was to determine the G-protein regulation of L-type calcium channel current (I _CaL) in isolated canine jejunal circular smooth muscle cells. Barium (80 mm) was used as the charge carrier. GTP-γS and GTP increased maximal inward current from 118.7 ± 12 pA to 227.5 ± 21.5 pA (n= 8) and 174.6 ± 10.1 pA (n= 6) respectively. The increase in inward current was blocked by nifedipine suggesting it was through L-type calcium channels. Pertussis toxin did not alter baseline I _CaL while cholera toxin increased I _CaL from 125 ± 19 pA in controls (n= 6) to 347 ± 30 pA (n= 4). Staurosporine inhibited the increase in current evoked by GTP-γS and calyculin further increased I _CaL over the increase evoked by GTP-γS. The results suggest that cholera toxin sensitive G-proteins activate L-type calcium channels in isolated canine jejunal circular smooth muscle cells through protein phosphorylation. Received: 27 March 1997/Revised: 3 July 1997     

Electrophysiological characterization of left ventricular myocytes from obese Sprague-Dawley rat

Objective: Obesity is a complex multifactorial disease that is often associated with cardiac arrhythmias. Various animal models have been used extensively to study the effects of obesity on physiological functions, but, to our knowledge, no study related to ionic membrane currents has been performed on isolated cardiac myocytes. Therefore, we examined the electrophysiological characteristics of four ionic currents from isolated left ventricular myocytes of a high‐energy (HE)‐induced obesity rat model. Research Methods and Procedures: Male Sprague‐Dawley rats were fed with either a control diet or a diet containing 33% kcal as fat (HE) for 14 weeks starting at 6 weeks of age. Voltage‐clamp experiments were performed on ventricular myocytes. Leptin receptor (ObR) expression was measured using ObR enzyme‐linked immunosorbent assay. Results: In the HE group, rats designated as obese did not develop a cardiac hypertrophy, either at the organ level or at the cellular level. Densities and kinetics of the L‐type calcium current, the transient outward potassium current, the delayed rectifier potassium current, and the sodium‐calcium exchange current (I_NCX) were not significantly different between control and obese rats. A down‐regulation of ObR expression was evidenced in the heart of obese rats compared with controls. Acute exposure (5 minutes) of leptin (100 nM) did not induce a significant modification in the current densities either in control or in obese rats, except for I_NCX density measured in control rats. Discussion: The absence of effect of leptin on I_NCX in obese rats could be a potential arrhythmogenic substrate in obesity.     

Effects of Estradiol on Voltage-Gated Potassium Channels in Mouse Dorsal Root Ganglion Neurons

Voltage-gated potassium channels are regulators of membrane potentials, action potential shape, firing adaptation, and neuronal excitability in excitable tissues including in the primary sensory neurons of dorsal root ganglion (DRG). In this study, using the whole-cell patch-clamp technique, the effect of estradiol (E2) on voltage-gated total outward potassium currents, the component currents transient “A-type” current (I _A) currents, and “delayed rectifier type” (I _KDR) currents in isolated mouse DRG neurons was examined. We found that the extracellularly applied 17β-E2 inhibited voltage-gated total outward potassium currents; the effects were rapid, reversible, and concentration-dependent. Moreover, the membrane impermeable E2-BSA was as efficacious as 17β-E2, whereas 17α-E2 had no effect. 17β-E2-stimulated decrease in the potassium current was unaffected by treatment with ICI 182780 (classic estrogen receptor antagonist), actinomycin D (RNA synthesis inhibitor), or cycloheximide (protein synthesis inhibitor). We also found that I _A and I _KDR were decreased after 17β-E2 application. 17β-E2 significantly shifted the activation curve for I _A and I _KDR channels in the hyperpolarizing direction. In conclusion, our results demonstrate that E2 inhibited voltage-gated K⁺ channels in mouse DRG neurons through a membrane ER-activated non-genomic pathway.     

Human Adipose Cells Have Voltage-dependent Potassium Currents

The whole-cell patch-clamp method was used to study the membrane electrical properties of human adipocyte cells obtained by differentiating from precursors of human abdominal and mammary tissues. All differentiated cells exhibited outward currents with sigmoidal activation kinetics. The outward currents showed activation thresholds between –20 to –30 mV and slow inactivation. The ionic channels underlying the macroscopic current were highly selective for K⁺. Their selectivity was for typical K⁺ channels with relative permeabilities of K⁺>NH ₄ ⁺ >Cs⁺>Na⁺. No evidence of any other type of voltage-gated channel was found. The potassium currents (I _KV) were blocked reversibly by tetraethylammonium and barium. The IC ₅₀ value and Hill coefficient of tetraethylammonium inhibition of I _KV were 0.56 mM and 1.17 respectively. These results demonstrate that human adipose cells have voltage-dependent potassium currents.     

Differential contributions of somatic and dendritic calcium-dependent potassium currents to the control of motoneuron excitability following spinal cord injury

The hyperexcitability of alpha-motoneurons and accompanying spasticity following spinal cord injury (SCI) have been attributed to enhanced persistent inward currents (PICs), including L-type calcium and persistent sodium currents. Factors controlling PICs may offer new therapies for managing spasticity. Such factors include calcium-activated potassium (KCa) currents, comprising in motoneurons an after-hyperpolarization-producing current (I _KCaN) activated by N/P-type calcium currents, and a second current (I _KCaL) activated by L-type calcium currents (Li and Bennett in J neurophysiol 97:767–783, 2007). We hypothesize that these two currents offer differential control of PICs and motoneuron excitability based on their probable somatic and dendritic locations, respectively. We reproduced SCI-induced PIC enhancement in a two-compartment motoneuron model that resulted in persistent dendritic plateau potentials. Removing dendritic I _KCaL eliminated primary frequency range discharge and produced an abrupt transition into tertiary range firing without significant changes in the overall frequency gain. However, I _KCaN removal mainly increased the gain. Steady-state analyses of dendritic membrane potential showed that I _KCaL limits plateau potential magnitude and strongly modulates the somatic injected current thresholds for plateau onset and offset. In contrast, I _KCaN had no effect on the plateau magnitude and thresholds. These results suggest that impaired function of I _KCaL may be an important intrinsic mechanism underlying PIC-induced motoneuron hyperexcitability following SCI.     

Maternal diabetes increases large conductance Ca2+-activated K+ outward currents that alter action potential properties but do not contribute to attenuated excitability of parasympathetic cardiac motoneurons in the nucleus ambiguus of neonatal mice

Previously, we demonstrated that maternal diabetes reduced the excitability and increased small-conductance Ca(2+)-activated K(+) (SK) currents of parasympathetic cardiac motoneurons (PCMNs) in the nucleus ambiguus (NA). In addition, blockade of SK channels with apamin completely abolished this reduction. In the present study, we examined whether maternal diabetes affects large-conductance Ca(2+)-activated K(+) (BK) channels and whether BK channels contribute to the attenuation of PCMN excitability observed in neonates of diabetic mothers. Neonatal mice from OVE26 diabetic mothers (NMDM) and normal FVB mothers (control) were used. The pericardial sac of neonatal mice at postnatal days 7-9 was injected with the tracer X-rhodamine-5 (and 6)-isothiocyanate 2 days prior to the experiment to retrogradely label PCMNs in the NA. Whole cell current- and voltage-clamps were used to measure spike frequency, action potential (AP) repolarization (half-width), afterhyperpolarization potential (AHP), transient outward currents, and afterhyperpolarization currents (I(AHP)). In whole cell voltage clamp mode, we confirmed that maternal diabetes increased transient outward currents and I(AHP) compared with normal cells. Using BK channel blockers charybdotoxin (CTx) and paxilline, we found that maternal diabetes increased CTx- and paxilline-sensitive transient outward currents but did not change CTx- and paxilline-sensitive I(AHP). In whole cell current-clamp mode, we confirmed that maternal diabetes increased AP half-width and AHP, and reduced excitability of PCMNs. Furthermore, we found that after blockade of BK channels with CTx or paxilline, maternal diabetes induced a greater increase of AP half-width but similarly decreased fast AHP without affecting medium AHP. Finally, blockade of BK channels decreased spike frequency in response to current injection in both control and NMDM without reducing the difference of spike frequency between the two groups. Therefore, we conclude that although BK transient outward currents, which may alter AP repolarization, are increased in NMDM, BK channels do not directly contribute to maternal diabetes-induced attenuation of PCMN excitability. In contrast, based on evidence from our previous and present studies, reduction of PCMN excitability in neonates of diabetic mothers is largely dependent on altered SK current associated with maternal diabetes.     

Positive inotropic action of NMDA receptor antagonist (+)-MK801 in rat heart

(+)-MK801, a noncompetitive NMDA receptor antagonist, was reported to exhibit anticonvulsive and neuroprotective activities during the postischemic period. Intravenous administration of (+)-MK801 produced tachycardia in rats, but bradycardia in pigs. We examined the mechanical and electrophysiological effects of (+)-MK801 on rat cardiac tissues. (+)-MK801 dose-dependently increased (3–100 µM) twitch tension in rat atria and ventricular strips. The spontaneous beating rate in rat right atria, however, was dose-dependently decreased by (+)-MK801. The inotropic effect of (+)-MK801 was affected neither by ₁-antagonist (1 µM prazosin) nor by ₁-adrenoceptor antagonist (3 µM atenolol), but significantly by a transient outward K⁺ channel blocker (3 mM 4-aminopyridine). (+)-MK801 did not cause any significant change of intracellular cAMP content. Electrophysiological study in rat ventricular cells revealed that (+)-MK801 concentration-dependently prolonged the action potential duration with a concomitant decrease in the maximum rate of the action potential upstroke (V_max) and an increase in the recovery time constant of V_max. Voltage clamp study showed that (+)-MK801 (3 µM) reduced inward Na⁺ current (I_Na), along with a slowing of its recovery from inactivation and a slight negative shift of its voltage-dependent steady-state inactivation curves. At a much higher concentration (30 µM), (+)-MK801 slightly reduced the amplitude of L-type calcium inward current (I_Ca), although the voltage dependence of its steady-state inactivation was unaffected. For the potassium currents in rat ventricular cells, 3 µM of (+)-MK801 reduced the peak transient outward current (I_to), steady-state outward current (I_ss) and inward current through K₁ channels. The inhibition of I_to was associated with a prominent negative shift in the voltage dependence of its steady-state inactivation curve. The outward current through K₁ channels was unaffected. These results indicate that (+)-MK801 may be a strong I_Na and I_to blocker with some I_Ca blocking activity. The inhibition of I_to and other K⁺ efflux would prolong action potential duration, produce positive inotropic action and contribute to the negative chronotropic effect of (+)-MK801.     

Classification of 2-pore domain potassium channels based on rectification under quasi-physiological ionic conditions

It is generally expected that 2-pore domain K⁺ (K_2P) channels are open or outward rectifiers in asymmetric physiological K⁺ gradients, following the Goldman-Hodgkin-Katz (GHK) current equation. Although cloned K_2P channels have been extensively studied, their current-voltage (I-V) relationships are not precisely characterized and previous definitions are contradictory. Here we study all the functional channels from 6 mammalian K_2P subfamilies in transfected Chinese hamster ovary cells with patch-clamp technique, and examine whether their I-V relationships are described by the GHK current equation. K_2P channels display 2 distinct types of I-V curves in asymmetric physiological K⁺ gradients. Two K_2P isoforms in the TWIK subfamily conduct large inward K⁺ currents and have a nearly linear I-V curve. Ten isoforms from 5 other K_2P subfamilies conduct small inward K⁺ currents and exhibit open rectification, but fits with the GHK current equation cannot precisely reveal the differences in rectification among K_2P channels. The Rectification Index, a ratio of limiting I-V slopes for outward and inward currents, is used to quantitatively describe open rectification of each K_2P isoform, which is previously qualitatively defined as strong or weak open rectification. These results systematically and precisely classify K_2P channels and suggest that TWIK K⁺ channels have a unique feature in regulating cellular function.     

Classification of 2-pore domain potassium channels based on rectification under quasi-physiological ionic conditions

It is generally expected that 2-pore domain K⁺ (K_2P) channels are open or outward rectifiers in asymmetric physiological K⁺ gradients, following the Goldman-Hodgkin-Katz (GHK) current equation. Although cloned K_2P channels have been extensively studied, their current-voltage (I-V) relationships are not precisely characterized and previous definitions are contradictory. Here we study all the functional channels from 6 mammalian K_2P subfamilies in transfected Chinese hamster ovary cells with patch-clamp technique, and examine whether their I-V relationships are described by the GHK current equation. K_2P channels display 2 distinct types of I-V curves in asymmetric physiological K⁺ gradients. Two K_2P isoforms in the TWIK subfamily conduct large inward K⁺ currents and have a nearly linear I-V curve. Ten isoforms from 5 other K_2P subfamilies conduct small inward K⁺ currents and exhibit open rectification, but fits with the GHK current equation cannot precisely reveal the differences in rectification among K_2P channels. The Rectification Index, a ratio of limiting I-V slopes for outward and inward currents, is used to quantitatively describe open rectification of each K_2P isoform, which is previously qualitatively defined as strong or weak open rectification. These results systematically and precisely classify K_2P channels and suggest that TWIK K⁺ channels have a unique feature in regulating cellular function.     

Fast Na+ channels and slow Ca2+ current in smooth muscle from pregnant rat uterus

Summary Smooth muscle cells normally do not possess fast Na²⁺ channels, but inward current is carried through two types of Ca²⁺ channels: slow (L-type) Ca²⁺ channels and fast (T-type) Ca²⁺ channels. Using whole-cell voltage clamp of single smooth muscle cells isolated from the longitudinal layer of 18-day pregnant rat uterus, depolarizing pusles, applied from a holding potential of –90 mV, evoked two types of inward current, fast and slow [8]. The fast inward current decayed within 30 ms, depended on [Na]₀, and was inhibited by TTX (K_0.5 = 27 nM). The slow inward current decayed slowly, was dependent on [Ca]₀, and was inhibited by nifedipine. These results suggest that the fast inward current is a fast Na²⁺ channel current, and that the slow inward current is a Ca²⁺ channel current was not evident. Thus, the ion channels which generate inward currents in pregnant rat uterine cells are TTX-sensitive fast Na⁺ channels and dihudropuridine-sensitive slow Ca²⁺ channels. The number of fast Na⁺ channels increased during gestation [9]. The averaged current density increased from 0 on day 5, to 0.19 on day 9, to 0.56 on day 14, to 0.90 on day 18, and to 0.86 pA/pF on day 21. This almost linear increase occurs because of an increase in the fraction of cells which possess fast Na²⁺ channels, and it suggested that the fast Na⁺ current may be involved in spread of excitation. The Ca²⁺ channel current density also was higher during the latter half of gestation. These results indicate that the fast Na⁺ channels and Ca²⁺ slow channels in myometrium become more numerous as term approaches, and may facilitate parturition. Isoproterenol (beta-agonist) did not affect either I_Ca(s) or I_Na(f), whereas Mg²⁺ (K_0.5 of 12 mM) and nifedipine (K_0.5 of 3.3 nM) depressed I_Ca(s). Oxytocin had no effect on I_Na(f) and actually depressed I_Ca(s) to a small extect. Therefore, the tocolytic action of beta-agonists cannot be explained by an inhibition of I_Ca(s), whereas that of Mg²⁺ can be so explained. The stimulating action of oxytocin on uterine contractions is not due to stimulation of I_Ca(s).     

L-type calcium current in right ventricular outflow tract myocytes of rabbit heart

The mechanism of idiopathic ventricular tachycardia originating from the right ventricular outflow tract (RVOT) is not clear. Many clinical reports have suggested a mechanism of triggered activity. However, there are few studies investigating this because of the technical difficulties associated with examining this theory. The L-type calcium current (I _Ca-L), an important inward current of the action potential (AP), plays an important role in arrhythmogenesis. The aim of this study was to explore differences in the APs of right ventricular (RV) and RVOT cardiomyocytes, and differences in electrophysiological characteristics of the ICa-L in these myocytes. Rabbit RVOT and RV myocytes were isolated and their AP and I _Ca-L were investigated using the patch-clamp technique. RVOT cardiomyocytes had a wider range of AP duration (APD) than RV cardiomyocytes, with some markedly prolonged APDs and markedly shortened APDs. The markedly shortened APDs in RVOT myocytes were abolished by treatment with 4-AP, an inhibitor of the transient outward potassium current, but the markedly prolonged APDs remained, with some myocytes with a long AP plateau not repolarizing to resting potential. In addition, early afterdepolarization (EAD) and second plateau responses were seen in RVOT myocytes but not in RV myocytes. RVOT myocytes had a higher current density for I _Ca-L than RV myocytes (RVOT (13.16±0.87) pA pF⁻¹, RV (8.59±1.97) pA pF⁻¹; P<0.05). The I _Ca-L and the prolonged APD were reduced, and the EAD and second plateau response disappeared, after treatment with nifedipine (10 μmol L⁻¹), which blocks the I _Ca-L. In conclusion, there was a wider range of APDs in RVOT myocytes than in RV myocytes, which is one of the basic factors involved in arrhythmogenesis. The higher current density for I _Ca-L is one of the factors causing prolongation of the APD in RVOT myocytes. The combination of EAD with prolonged APD may be one of the mechanisms of RVOT-VT generation.