Multiple types of voltage-dependent Ca channels in mammalian intestinal smooth muscle cells

The Cav1.2 N terminus contains a CaM kinase site that modulates channel trafficking and function

The L-type voltage-gated calcium channel Cav1.2 and the calcium-activated CaM kinase cascade both regulate excitation transcription coupling in the brain. CaM kinase is known to associate with the C terminus of Cav1.2 in a region called the PreIQ-IQ domain, which also binds multiple calmodulin molecules. Here we identify and characterize a second CaMKII binding site in the N terminus of Cav1.2 that is formed by a stretch of four amino residues (cysteine-isoleucine-serine-isoleucine) and which regulates channel expression and function. By using live cell imaging of tsA-201 cells we show that GFP fusion constructs of the CaMKII binding region, termed N2B-II co-localize with mCherry-CaMKII. Mutating CISI to AAAA ablates binding to and colocalization with CaMKII. Cav1.2-AAAA channels show reduced cell surface expression in tsA-201 cells, but interestingly, display an increase in channel function that offsets the trafficking deficit. Altogether our data reveal that the proximal N terminus of Cav1.2 contains a CaMKII binding region which contributes to channel surface expression and function.

Gastric emptying and Ca(2+) 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 (Ca(2+) ) and potassium (K(+) ) channels of antral circular smooth muscle cells (SMCs).

DESIGN AND METHODS

Whole-cell patch-clamp technique was used to measure Ca(2+) 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 Ca(2+) current (IBa,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 (IKto ) 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 IKto between two groups. The current density of delayed rectifier K(+) current (IKdr ), 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 IBa,L , depolarizing shift of inactivation curve of IKto and lower density of IKdr observed in the antral SMCs of DIO-P rats.

Muscular effects of orexin A on the mouse duodenum: mechanical and electrophysiological studies

Orexin A (OXA) has been reported to influence gastrointestinal motility, acting at both central and peripheral neural levels. The aim of the present study was to evaluate whether OXA also exerts direct effects on the duodenal smooth muscle. The possible mechanism of action involved was investigated by employing a combined mechanical and electrophysiological approach. Duodenal segments were mounted in organ baths for isometric recording of the mechanical activity. Ionic channel activity was recorded in current- and voltage-clamp conditions by a single microelectrode inserted in a duodenal longitudinal muscle cell. In the duodenal preparations, OXA (0.3 μM) caused a TTX-insensitive transient contraction. Nifedipine (1 μM), as well as 2-aminoethyl diphenyl borate (10 μM), reduced the amplitude and shortened the duration of the response to OXA, which was abolished by Ni(2+) (50 μM) or TEA (1 mM). Electrophysiological studies in current-clamp conditions showed that OXA caused an early depolarization, which paralleled in time the contractile response, followed by a long-lasting depolarization. Such a depolarization was triggered by activation of receptor-operated Ca(2+) channels and enhanced by activation of T- and L-type Ca(2+) channels and store-operated Ca(2+) channels and by inhibition of K(+) channels. Experiments in voltage-clamp conditions demonstrated that OXA affects not only receptor-operated Ca(2+) channels, but also the maximal conductance and kinetics of activation and inactivation of Na(+), T- and L-type Ca(2+) voltage-gated channels. The results demonstrate, for the first time, that OXA exerts direct excitatory effects on the mouse duodenal smooth muscle. Finally, this work demonstrates new findings related to the expression and kinetics of the voltage-gated channel types, as well as store-operated Ca(2+) channels.

The alpha1H Ca2+ channel subunit is expressed in mouse jejunal interstitial cells of Cajal and myocytes

T-type Ca²⁺ currents have been detected in cells from the external muscular layers of gastrointestinal smooth muscles and appear to contribute to the generation of pacemaker potentials in interstitial cells of Cajal from those tissues. However, the Ca²⁺ channel α subunit responsible for these currents has not been determined. We established that the α subunit of the α_1H Ca²⁺ channel is expressed in single myocytes and interstitial cells of Cajal using reverse transcription and polymerase chain reaction from whole tissue, laser capture microdissected tissue and single cells isolated from the mouse jejunum. Whole-cell voltage clamp recordings demonstrated that a nifedipine and Cd²⁺ resistant, mibefradil-sensitive current is present in myocytes dissociated from the jejunum. Electrical recordings from the circular muscle layer demonstrated that mibefradil reduced the frequency and initial rate of rise of the electrical slow wave. Gene targeted knockout of both alleles of the cacna1h gene, which encodes the α_1H Ca²⁺ channel subunit, resulted in embryonic lethality because of death of the homozygous knockouts prior to E13.5 days in utero. We conclude that a channel with the pharmacological and molecular characteristics of the α_1H Ca²⁺ channel subunit is expressed in interstitial cells of Cajal and myocytes from the mouse jejunum, and that ionic conductances through the α_1H Ca²⁺ channel contribute to the upstroke of the pacemaker potential. Furthermore, the survival of mice that do not express the α_1H Ca²⁺ channel protein is dependent on the genetic background and targeting approach used to generate the knockout mice.

Characterization of single L-type Ca2+ channels in myocytes isolated from the cricket lateral oviduct

The single Ca2+ channel activity was obtained from cell-attached patch recordings with the use of pipettes filled with 100 mM Ba2+ as the charge carrier in myocytes isolated from the lateral oviduct of cricket Gryllus bimaculatus. The following results were obtained. (1) The channel had a unitary conductance of 18 pS. (2) The open time histogram of the channel could be fitted with a single exponential while the closed time histogram could be fitted with the sum of two exponentials, suggesting that there are at least one open state and two closed states for this channel. (3) The open probability of the channel increased with increasing membrane depolarization. (4) The mean current reconstructed by averaging individual current trace responses inactivated slowly and the current-voltage relationship for the peak mean current showed a bell-shaped relation. (5) The dihydropyridine (DHP) Ca2+ antagonist, nifedipine, reduced the mean current by increasing the proportion of "blank" sweeps. On the other hand, the DHP Ca2+ agonist, Bay K 8644, increased the mean current by increasing the mean open-times of the channel. These results confirm a presence of DHP-sensitive L-type Ca2+ channel in myocytes isolated from the lateral oviduct of cricket G. bimaculatus.

Mechanism of active repolarization of inhibitory junction potential in murine colon

Enteric inhibitory responses in gastrointestinal (GI) smooth muscles involve membrane hyperpolarization that transiently reduce the excitability of GI muscles. We examined the possibility that an active repolarization mechanism participates in the restoration of resting membrane potential after fast inhibitory junction potentials (IJPs) in the murine colon. Previously, we showed these cells express a voltage-dependent nonselective cation conductance (NSCC) that might participate in active repolarization of IJPs. Colonic smooth muscle cells were impaled with micro-electrodes and voltage responses to nerve-evoked IJPs, and locally applied ATP were recorded. Ba2+ (500 muM), a blocker of the NSCC, slowed the rate of repolarization of IJPs. We also tested the effects of Ba2+, Ni2+, and mibefradil, all blockers of the NSCC, on responses to locally applied ATP. Spritzes of ATP caused transient hyperpolarization, and the durations of these responses were significantly increased by the blockers of the NSCC. We considered whether NSCC blockers might affect ATP metabolism and found that Ni2+ decreased ATP breakdown in colonic muscles. Mibefradil had no effect on ATP metabolism. Because both Ni2+ and mibefradil had similar effects on prolonging responses to ATP, it appears that restoration of resting membrane potential after ATP spritzes is not primarily due to ATP metabolism. Neurally released enteric inhibitory transmitter and locally applied ATP resulted in transient hyperpolarizations of murine colonic muscles. Recovery of membrane potential after these responses appears to involve an active repolarization mechanism due to activation of the voltage-dependent NSCC expressed by these cells.

Contraction of human colonic circular smooth muscle cells is inhibited by the calcium channel blocker pinaverium bromide

The effects of L-type calcium channel blockers (CCBs) selective for the gastrointestinal tract (pinaverium) or non-selective (nicardipine and diltiazem), were investigated on CCK-, CCh- or KCl-induced contraction of smooth muscle cells (SMC) isolated from the circular muscle layer of normal or of inflamed human colons. In the normal tissue colon, whatever the contractile agent used, CCK-8 (1nM), CCh (1nM) or KCl (20mM), a micromolar concentration of pinaverium significantly inhibited contraction (88.36%, 93.10%, 93.92% inhibition respectively); this effect was concentration-dependent for CCh (IC50 = 0.73 +/- 0.08nM) and for CCK (IC50 = 0.92 +/- 0.12nM). In parallel, both nicardipine and diltiazem inhibit significantly contraction of isolated SMC. In inflamed colons, pinaverium (1 microM) display a significant higher efficacy than diltiazem or nicardipine to reduce cell contraction induced by CCK-8 or by KCl. In addition, RT-PCR experiments were performed to evidence tissue specificity of the L-type calcium channel. They revealed the expression of the messenger of the a-1 subunit L-type calcium channel (binding site of such CCBs), consistent with the expression of the rbC-2 splice variant of the alpha1-C gene.

IN CONCLUSION

(i) the inhibition by calcium channel blockers of agonist-induced contractile activity suggest a modulation of SMC contraction upon extracellular calcium via 'L-type' voltage-dependent calcium channel; (ii) this study provides a rationale for the clinical use of pinaverium in colonic motor disoders affecting the contractility of SMC, since it appeared to decrease the contraction even in pathological situation; and (iii) RT-PCR experiments confirms the presence in human colon SMC of the alpha-1 subunit mRNA of calcium channel.

NO donor sodium nitroprusside inhibits excitation-contraction coupling in guinea pig taenia coli

In guinea pig taenia coli, the nitric oxide (NO) donor sodium nitroprusside (SNP, 1 microM) reduced the carbachol-stimulated increases in muscle force in parallel with a decrease in intracellular Ca(2+) concentration ([Ca(2+)](i)). A decrease in the myosin light chain phosphorylation was also observed that was closely correlated with the decrease in [Ca(2+)](i). With the patch-clamp technique, 10 microM SNP decreased the peak Ba(2+) current, and this effect was blocked by an inhibitor of soluble guanylate cyclase. Carbachol (10 microM) induced an inward current, and this effect was markedly inhibited by SNP. SNP markedly increased the depolarization-activated outward K(+) currents, and this current was completely blocked by 0.3 micorM iberiotoxin. SNP (1 microM) significantly increased cGMP content without changing cAMP content. Decreased Ca(2+) sensitivity by SNP of contractile elements was not prominent in the permeabilized taenia, which was consistent with the [Ca(2+)](i)-force relationship in the intact tissue. These results suggest that SNP inhibits myosin light chain phosphorylation and smooth muscle contraction stimulated by carbachol, mainly by decreasing [Ca(2+)](i), which resulted from the combination of the inhibition of voltage-dependent Ca(2+) channels, the inhibition of nonselective cation currents, and the activation of Ca(2+)-activated K(+) currents.

The alpha 1-subunit of smooth muscle Ca(2+) channel preserves multiple open states induced by depolarization

The cloned alpha 1-subunits of the smooth muscle Ca(2+) channel (alpha (1C-b)) from rabbit lung were expressed in Chinese hamster ovary cells. The effect of large depolarizations was examined using cell-attached patch clamp techniques. After large, long-duration depolarizations (to +80 mV, 4 s), the cloned smooth muscle Ca(2+) channels were still open, and also showed slow channel closure upon repolarization. The sum of unitary channel currents revealed that the tail current seen after large conditioning depolarizations had a slower deactivation time constant compared to that seen when the cell membrane was depolarized briefly with a test step (to +40 mV), suggesting that large depolarizations transform the conformation of the Ca(2+) channels to a second open state. The decay time course of the tail current induced by large conditioning depolarizations was prolonged by reducing the negativity of the repolarization step, and vice versa. Using the slow deactivating characteristic, the current-voltage relationship was directly measured by applying a ramp pulse after a large depolarization. Its slope conductance was approximately 26 pS. Since the patch pipettes contained Ca(2+) agonists, the transition of the Ca(2+) channel conformation to the second, long open state during a large depolarization was distinct from that caused by Ca(2+) agonists, suggesting that the cloned alpha 1-subunits of smooth muscle Ca(2+) channels preserve the characteristic features seen in native smooth muscle Ca(2+) channels. In addition, when skeletal muscle beta-subunits were coexpressed with the alpha 1-subunits, the long channel openings after large, long-duration depolarizations were frequently suppressed. This phenomenon could be explained if the skeletal muscle beta-subunits increased the inactivation rate during the preconditioning depolarization.

Ionic conductances in gastrointestinal smooth muscles and interstitial cells of Cajal

Ion channels are the unitary elements that underlie electrical activity of gastrointestinal smooth muscle cells and of interstitial cells of Cajal. The result of ion channel activity in the gastrointestinal smooth muscle layers is a rhythmic change in membrane potential that in turn underlies events leading to organized motility patterns. Gastrointestinal smooth muscle cells and interstitial cells of Cajal express a wide variety of ion channels that are tightly regulated. This review summarizes 20 years of data obtained from patch-clamp studies on gastrointestinal smooth muscle cells and interstitial cells, with a focus on regulation.

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

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

Purification of a new dimeric protein from Cliona vastifica sponge, which specifically blocks a non-L-type calcium channel in mouse duodenal myocytes

Marine sponges are synthesizing a wide variety of peptidic and organic molecules with biological activities. Multiple-step purification of Cliona vastifica extract led to a new dimeric peptide (mapacalcine; M(r) = 19,064) that is composed of two homologous chains, each containing nine cysteins. This protein has been found to selectively block a new calcium conductance characterized in mouse duodenal myocytes with an IC50 value of approximately 0.2 microM. The mapacalcine-sensitive current was a non-L-type calcium current activated from a holding potential of -80 mV that persisted during stimulation of the cell at high frequencies (0.1-0.2 Hz) within 5-10 min. Time constants of inactivation were similar for both L-type and non-L-type calcium currents. The non-L-type calcium current of duodenal myocytes was not blocked by the pharmacological agents specific for N-, L-, P-, or Q-type calcium channels. Mapacalcine was unable to block T-type calcium current in portal vein myocytes as well as voltage-dependent potassium currents and calcium-activated chloride currents in duodenal and portal vein cells. Mapacalcine did not affect caffeine-induced calcium responses, indicating that it did not interfere with intracellular calcium stores. Competition experiments on mouse intestinal membranes showed that mapacalcine did not interact with dihydropyridines receptors. These data suggest that mapacalcine may be a specific inhibitor of a new type of calcium current, first identified in duodenal myocytes.

Voltage-dependent suppression of calcium current by caffeine in single smooth muscle cells of the guinea-pig urinary bladder

The suppressive action of caffeine on L-type Ca current (Ica) in smooth muscle cells of the guinea-pig urinary bladder was investigated using the whole-cell patch clamp technique. Caffeine (5-30 mM) suppressed Ica, the effect having two phases: a rapid and transient suppression of Ica, which was followed by a sustained suppression. When intracellular Ca2+ was strongly buffered by the Ca2+ chelator EGTA (20 mM) or BAPTA (5 mM) in the patch pipette, the transient suppression of Ica was abolished, whereas the sustained effect remained. Similarly, inclusion of both 10 mM procaine and 1 mg/ml heparin in the patch pipette blocked the transient suppression of Ica, but did not block the sustained effect. The degree of the sustained effect of caffeine on Ica was dose-dependent with a kd of 20 mM. Application of the cyclic AMP analogue, 8-bromo-cyclic AMP (100 microM) or forskolin (10 microM) to the bath failed to mimick the sustained suppression of Ica, suggesting that inhibition of phosphodiesterase activity was not involved in the caffeine action. The steady-state activation curve remained unchanged by 10 mM caffeine but the steady-state inactivation curve was significantly shifted in the negative direction by 15.6 mV in 1.8 mM Ca2+ solution or by 10 mV in 1.8 mM Ba2+ solution. From these results it appears that caffeine inhibits L-type Ica via two mechanisms: (1) it releases Ca2+ from an internal store causing a transient Ca2+ -mediated inactivation of the Ca channel; (2) it inhibits Ca channel via a mechanism that does not require such a Ca2+ release. It is possible that caffeine suppresses Ica through a preferential binding to the inactivated state of L-type Ca channel.

Effects of polyamines on voltage-activated calcium channels in guinea-pig intestinal smooth muscle

Effects of polyamines on the spontaneous mechanical and electrical activity of guinea-pig intestinal smooth muscle were studied. Spermine and spermidine inhibited action potential generation and contractions, while putrescine had no effect. Single smooth muscle cells were isolated from the longitudinal muscle layer of the guinea-pig ileum. Whole-cell voltage-clamp experiments were carried out to investigate the effects of polyamines on current through voltage-activated Ca2+ channels. Spermine and spermidine (0.1-1 mM) reduced the inward current in a concentration-dependent manner. Spermine blocked current activated by the dihydropyridine agonist BAY K 8644 (1 microM), whereas no additional inhibition by spermine was seen after blockage of dihydropyridine-sensitive channels by nifedipine (0.1 microM). Inhibition by spermine or spermidine did not shift the peak of the current voltage relation of the inward current. Steady-state activation and inactivation relationships were not affected and thus the amplitude, but not the voltage dependence, of the window current responsible for Ca2+ inflow during sustained depolarization was affected. Putrescine (1 mM) had no significant effect on the inward current. These results suggest that spermine and spermidine inhibit contraction in spontaneously active intestinal smooth muscle by inhibiting Ca2+ current responsible for generation of action potentials.

Properties of Ca(2+)-mediated inactivation of L-type Ca channel in smooth muscle cells of the guinea-pig urinary bladder

The properties of Ca(2+)-mediated inactivation as revealed by a conventional double-pulse protocol were examined by using the whole-cell patch clamp technique. A U-shaped relationship between the conditioning potential and the Ca2+ current (ICa) inactivation was observed, with a maximum inactivation of 52 +/- 4% (n = 5) at 10 mV with 0.5 mM EGTA in the patch pipettes. The maximum inactivation was reduced significantly, to 31 +/- 5.7% (n = 12) and 32 +/- 7.0% (n = 5), when a high concentration of EGTA (20 mM) or a more efficient Ca2+ chelator, BAPTA, was included in the patch pipettes, respectively. The same double-pulse protocol was applied under conditions where the stored Ca2+ was depleted by using caffeine or the stored Ca2+ release function was blocked by using ryanodine or procaine and heparin. No significant difference in the maximum ICa inactivation before (45%) and after (50%) application of 10 mM caffeine was observed. The maximum ICa inactivations of 48 +/- 3.2% (n = 4) and 52 +/- 8.4% (n = 6) were still observed after treatment of the cell with ryanodine (20 microM) or loading 10 mM procaine and 1 mg/mL heparin in the patch pipettes, respectively. These results suggest that Ca2+ mobilization from an internal Ca2+ store is not essential for the Ca(2+)-mediated inactivation observed in the double-pulse experiment, rather influx of Ca2+ through a voltage-dependent Ca channel seems to be important for ICa inactivation.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of cytochalasin B on intestinal smooth muscle cells

The inhibitory effect of cytochalasin B on contraction of smooth muscle cells isolated from guinea-pig taenia coli was investigated. Cytochalasin B (10-70 microM) inhibited the high K+ (70 mM)-induced contraction in a dose-dependent manner, and the maximum and the half-maximum effects were obtained at 50 and 15 microM, respectively. Cytochalasin B (70 microM) decreased ATPase activity in skinned guinea-pig taenia coli. However, cytochalasin B (50 microM) had no significant effect on the voltage-dependent Ca2+ currents, the passive membrane properties or the membrane potential. Cytochalasin B also had no effect on the phosphorylation of 20 kDa myosin light chain induced by high K+ and cytosolic Ca2+ levels. These results suggest that the inhibition of contraction by cytochalasin B may be due to its effects on actin of microfilaments and contractile filaments of guinea-pig taenia coli smooth muscle cells.

Effect of isoprenaline on Ca2+ channel current in single smooth muscle cells isolated from taenia of the guinea-pig caecum

1. The effects of isoprenaline (Iso) on Ca2+ channel current in enzymatically isolated single cells of the guinea-pig taenia caeci were examined using the standard whole-cell voltage-clamp method. 2. Iso potentiated the voltage-dependent Ca2+ current; the threshold and maximally effective concentration of Iso to increase Ca2+ current were 3-10 nM and 1-3 microM, respectively. The average increase in Ca2+ current produced by 3 microM Iso was 42 +/- 6% (mean +/- S.E.M.) and the response could be obtained repeatedly in the same cell. The concentration-response relationship could be fitted by a binding model with a Hill coefficient of 1 and a dissociation constant of 42 nM. 3. The effect of Iso on Ca2+ current was voltage dependent. Although potentiation of Ca2+ current by Iso was obvious between -30 and +10 mV, it was small or absent around +20 to +30 mV. Iso had little effect on the relationship between inactivation of the Ca2+ current and voltage obtained using a double-pulse protocol. 4. External application of forskolin, an adenylyl cyclase activator, or internal perfusion of cAMP or dibutyryl cAMP from the recording pipette, did not increase Ca2+ current and potentiation of Ca2+ current by Iso was observed repeatedly and was unchanged. 5. Internal perfusion of GTP gamma S or GDP beta S increased or did not affect the Ca2+ current and potentiation of Ca2+ current by Iso was unchanged and could be recorded repeatedly for about 20 min after rupture of the cell membrane. In addition, treatment of cells with the potent protein kinase C inhibitor, chelerythrine, had no effect on Ca2+ current or on potentiation of Ca2+ current by Iso. 6. These results suggest that the Ca2+ current in guinea-pig taenia caeci cells is potentiated by isoprenaline via mechanisms which do not involve either a cAMP pathway, a G-protein pathway or a protein kinase C pathway. The receptor involved appeared to be an atypical adrenoreceptor not blocked by either alpha- or beta-receptor blocking agents.

Inhibition of Ca2+ current in ileal cells by cyclopiazonic acid and ryanodine

The effects of cyclopiazonic acid and its combination with ryanodine on the inward Ca2+ current (ICa) were investigated in smooth muscle cells isolated from the circular layer of guinea-pig ileum. The ICa of these cells exhibited two components: a low-threshold, nicardipine (5 microM)-resistant, fast-inactivating component and a high-threshold, nicardipine-blockable and slowly inactivating component. Neither cyclopiazonic acid (up to 10 microM) nor ryanodine (10 microM) was able to affect both these components of ICa, when applied separately. Cyclopiazonic acid and ryanodine combination led to total abolishment of the high-threshold component, leaving the low-threshold component unaffected. The data presented suggest a process of Ca(2+)-dependent inactivation of the high-threshold component, elicited by an increase in the subplasmalemmal Ca2+ concentration due to Ca2+ released from the sarcoplasmic reticulum. It is considered that the combination of cyclopiazonic acid and ryanodine can be used as a valuable method to study the calcium sensitivity of both components of the ICa.

Changes in calcium channel current densities in rat colonic smooth muscle cells during development and aging

The age-related changes of Ca2+ channel currents were investigated in freshly isolated single smooth muscle cells from the circular layer of the distal colon from the rat using the whole cell voltage clamp technique. Under physiological conditions (Ca2+ concentration of 2.0 mM), the averaged total Ca2+ current density increased markedly from 1.25 pA/pF in the newborn rat to 6.46 pA/pF in the 60-day-old rat; it then gradually declined with aging. Two types of Ca2+ channel currents seemed to be present; one type possessed more negative threshold potentials (-70 to -60 mV) when the cells were held at -80 or -100 mV and inactivated quickly. The voltage for peak current was -20 to -10 mV, and the reversal potential was +60 to +70 mV. This current was highly sensitive to low concentrations of Ni2+ (30 microM) but was resistant to nifedipine, diltiazem, cadmium, and tetrodotoxin. In contrast, the other type of Ca2+ channel current possessed more positive threshold potential (-40 mV) and inactivated more slowly. The voltage for peak current was 0 mV, and the reversal potential was +60 to +70 mV. This current was insensitive to low concentrations of Ni2+ but highly sensitive to nifedipine, diltiazem, and cadmium. These results suggest that the fast inactivating (transient) current might be T-type Ca2+ current [ICa(T)], and such cells were ICa(T) positive cells; whereas the sustained Ca2+ current was L-type Ca2+ current [ICa(L)], and such cells were ICa(L) positive cells. Our results showed that the fraction of ICa(T) positive cells increased with development; the current densities of both ICa(L) and ICa(T) also increased with development.(ABSTRACT TRUNCATED AT 250 WORDS)

Dihydropyridine-sensitive calcium channels expressed in canine colonic smooth muscle cells

Experiments were performed to identify and characterize the types of calcium channels that regulate inward calcium current in canine colonic smooth muscle. Freshly dispersed smooth muscle cells from the circular layer of the canine proximal colon were used. Single-channel currents were measured with 80 mM Ba2+ as the charge carrier. Small-conductance (10 +/- 2 pS, EBa = 46 +/- 11 mV, n = 9) and large-conductance (21 +/- 1 pS, EBa = 52 +/- 3 mV, n = 19) single-channel currents were observed during depolarizing voltage steps positive to -30 mV. Both types of single-channel currents were inhibited by the addition of 10(-6) M nifedipine to the bath solution. The smaller current was infrequently observed and therefore was not further characterized. Open probability (P(o)) of the larger current amplitude was strongly dependent on voltage. Activation curves were well described by a Boltzmann function with half activation occurring at 4 mV, and a 5-mV increase in membrane potential resulted in an e-fold increase in P(o). BAY K 8644 (1 microM) shifted the activation curve to the left while nifedipine (1 microM) resulted in a right shift. Molecular analysis showed that only the C class of Ca2+ channel alpha 1-subunit is expressed in this tissue. Furthermore, only a single splice variant (rbc-II) was observed. The results suggest that a single class of dihydropyridine-sensitive calcium channels regulates inward calcium current in canine colonic smooth muscle cells.

Potential-dependent inward currents in single isolated smooth muscle cells of the rat ileum

1. Calcium (ICa) and sodium (INa) currents were studied in single smooth muscle cells freshly isolated from both the newborn (1-3 days old) and adult rat ileum, using the patch-clamp technique (whole-cell configuration). 2. Under conditions when INa was blocked, two components of ICa, low-voltage activated or ICa,low and high-voltage activated or ICa,high, were observed in the newborn rat ileal cells. ICa,high and ICa,low have differing voltage ranges of activation and steady-state inactivation and time courses of recovery from inactivation. Potential dependence of ICa,low was much steeper and shifted toward negative membrane potential than that for ICa,high (slope factors and the potential of half-maximal inactivation were 13.6 and -60.6 and 8.8 and -49 mV for ICa,low and ICa,high, correspondingly). 3. Nifedipine at the high concentration of 30 microM exerted no effect on ICa,low and only slightly suppressed ICa,high, decreasing its peak to 0.81 +/- 0.04 (n = 7) at the holding potential of -80 mV and to 0.66 +/- 0.05 (n = 3) at -50 mV. ICa,high was suppressed significantly by Cd2+ ions, while ICa,low was more sensitive to Ni2+ ions. 4. Results presented here suggest that the properties of high-voltage-activated (HVA) Ca2+ channels in the rat small intestine are quite different to those described for L-type Ca2+ channels found in other smooth muscles. It is proposed that HVA Ca2+ channels are similar to N-type Ca2+ channels. 5. Comparison of Ca2+ currents in newborn and adult rat ileal cells showed that the contribution of ICa,low to the net Ca2+ current was negligible in adults, whereas the properties of HVA Ca2+ channels were similar in the neonatal and adult animals. 6. INa, studied in nominally Ca(2+)-free physiological salt solution, activated in the voltage range between -50 and -40 mV and reached its peak at -10 mV. INa was blocked in a dose-dependent manner by TTX with an apparent dissociation constant of 4.5 nM. 7. INa decay was monoexponential in the voltage range studied and its time constant decreased monotonically with membrane depolarization from 4.7 +/- 0.2 ms (n = 6) at -30 mV to 0.51 +/- 0.03 ms (n = 7) at 20 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Erythromycin stimulates ileal motility by activation of dihydropyridine-sensitive calcium channels

Erythromycin, a macrolide antibiotic, is a potent stimulant of small bowel motor activity (MA) which may motility either via the peptide motilin receptor or neural mechanisms. We hypothesized that erythromycin stimulates directly stimulates smooth muscle cells by a calcium-mediated event. Thus, we evaluated the effect of neuronal blockade with tetrodotoxin, muscarinic blockade with atropine, and opiate blockade with naloxone on erythromycin-stimulated MA in isolated perfused segments of rabbit terminal ileum. We also tested the effect of nonspecific calcium channel blockade (verapamil and cadmiun) and specific blockade (dihydroxypyridine and nichol) on erythromycin-stimulated MA. MA was measured with a multichannel continuous perfusion manometry catheter. Erythromycin caused a concentration-dependent increase in MA (ED100 5 x 10(-4) M). Tetrodotoxin, atropine, and naloxone did not effect erythromycin-stimulated MA (P greater than 0.05). Both verapamil (10(-7) M) and cadmium (10(-2)-10(-4) M) inhibited erythromycin-stimulated MA. Selective blockade of "l" type calcium channels using dihydropyridine (10(-6) M) and "t" channels with nickel (10(-2)-10(-4) M) both reversed erythromycin-stimulated MA. Since the isolated segments of terminal ileum were free of exogenous humoral and neural effects, these studies indicated that erythromycin directly stimulated MA in the terminal ileum. Furthermore, since tetrodotoxin, atropine, and naloxone did not inhibit this increase in MA, erythromycin acted by a mechanism which was independent of the intrinsic nervous and opiate systems. In conclusion, these data are consistent with the model that erythromycin stimulates ileal motility by a mechanism involving activation of dihydroxypyridine and nickel-sensitive calcium channels.

Calyculin A increases voltage-dependent inward current in smooth muscle cells isolated from guinea pig taenia coli

The effects of a potent phosphatase inhibitor, calyculin A (CL-A), on inward currents in guinea pig taenia coli smooth muscle cells were examined. CL-A increased the inward current, and this effect of CL-A was inhibited by a protein kinase C inhibitor, H-7, and by nifedipine. Phorbol 12,13-dibutyrate, an activator of protein kinase C, also increased the inward current and this effect was antagonized by H-7. These results suggest that in guinea pig taenia coli smooth muscle cells CL-A may facilitate the opening of the L-type Ca2+ channels through the protein kinase C-dependent phosphorylation system.

Multiple types of Ca2+ channels in visceral smooth muscle cells

Single-channel currents were recorded from two classes of Ca2+ channels in visceral smooth muscle cells isolated from the stomach of the toad, Bufo marinus: a class of small-conductance channels (approximately 11 pS) and a class of large-conductance channels (approximately 26 pS). Small-conductance channels were present in a majority of patches and gave rise to a slowly inactivating current (t1/2 approximately 250 ms at 0 mV). Openings of large-conductance channels could be unequivocally resolved only in the presence of the dihydropyridine Ca2+ agonist Bay K 8644. Two subtypes of the large-conductance channels were found--those with a very slow rate of decay (greater than 500 ms) and those with a faster one (less than 100 ms). Large-conductance channels resemble L-type Ca2+ channels of other preparations. Small-conductance channels do not fit unambiguously into the other existing categories (i.e., N or T). Correspondence between single-channel and macroscopic Ca2+ currents is discussed.

Effects of 2,3-butanedione monoxime on whole-cell Ca2+ channel currents in single cells of the guinea-pig taenia caeci

1. The inhibitory actions of cadmium (Cd2+), nifedipine and 2,3-butanedione monoxime (BDM) on whole-cell Ca2+ channel currents in single cells of the guinea-pig taenia caeci were investigated using a single-electrode whole-cell voltage-clamp technique. 2. Calcium channel currents were isolated using pipette solutions containing Cs+, tetraethylammonium and ATP (3 mM). Ca2+ or Ba2+ (7.5 mM) in the bathing solution acted as the charge carrier during inward current flow. Ca2+ channel currents in 7.5 mM-Ba2+ (IBa) were recorded at potentials positive to -40 mV, were maximal near 0 mV and reversed near +60 mV. Ca2+ channel activation showed a sigmoidal relationship with potential, which was half-maximal at -13 mV. 3. Both the inward and outward flow of current was depressed and eventually blocked by 0.3-100 microM-Cd2+, 0.1-10 microM-nifedipine and 2-20 mM-BDM. Half-maximal blockade of IBa at 0 mV was achieved with approximately 3 microM-Cd2+, 1 microM-nifedipine and 10 microM-BDM. Steady-state activation curves were not affected by Cd2+ or BDM, but were shifted in the hyperpolarizing direction by nifedipine at concentrations > 1 microM. 4. Calcium channel currents in single cells and K+ contractures in intact strips were both blocked in a voltage-dependent manner. Steady-state inactivation curves (f infinity (V)) for IBa were shifted 20 mV in the hyperpolarizing direction by 0.3 microM-nifedipine and 4 mV by 10 mM-BDM. From these shifts a dissociation binding constant to inactivated Ca2+ channels for nifedipine was estimated as 78 nM, and for BDM, 5 mM. 5. At 10 microM Cd2+ produced a 43 +/- 6% (n = 3) block of the inward current at 0 mV when Ca2+ (7.5 mM) was the charge carrier (ICa), compared with the 36 +/- 3% block of IBa induced by 1 microM-Cd2+, consistent with the suggestion that Ca2+, Ba2+ and Cd2+ compete for the same binding site. In contrast, nifedipine (1 microM) and BDM (10 mM) blocked ICa more effectively than IBa. 6. Bay K 8644 (1.0 microM) increased Ca2+ channel currents two- to fourfold at all potentials due to a shift, of approximately 10 mV in the negative direction, of their activation curve and an equal shift in the positive direction of their inactivation curve. BDM (5-10 mM) could antagonize the action of Bay K 8644, shifting both curves back towards their control.(ABSTRACT TRUNCATED AT 400 WORDS)

Properties of sympathetic neuromuscular transmission and smooth muscle cell membranes in vascular beds

In vascular smooth muscle tissues, the cycle of contraction-relaxation is mainly regulated by the cytosolic Ca, and many other factors, such as substances released from endothelial cells and perivascular nerve terminals (mainly sympathetic nerves). In this article, we introduce regional differences in specific features of ionic channels in vascular smooth muscle membranes (mainly on features of Ca, Na and K channels) in relation to mobilization of the cytosolic Ca. In many vascular tissues, neurotransmitters released from sympathetic nerve terminals activate post-junctional receptors, and subsequently modify ion channels (receptor-activated cation channel and voltage-dependent Ca channel), whereas in some tissues, ionic channels are not modified by receptor activations (pharmaco-mechanical coupling). However, activation of receptors, with or without modulation of ionic channels, regulates the cytosolic Ca through synthesis of second messengers. In addition, receptors distributed on prejunctional nerve terminals positively or negatively regulate the release of transmitters. Roles of neurotransmitters (mainly ATP and noradrenaline) are also discussed in relation to the generation of excitatory junction potentials.

Contribution of two types of calcium channels to membrane conductance of single myocytes from guinea-pig coronary artery

1. Whole-cell and single-channel current recordings were used to study calcium channels in single smooth muscle cells isolated from guinea-pig coronary artery. Potassium currents were blocked by intracellular Cs+ ions. 2. Whole-cell currents were recorded with 10 mM-barium in the bath. Step pulses of 200 ms from a holding potential of -90 mV activated calcium channel current when the depolarization reached -55 to -50 mV. All cells showed a current component which inactivated slowly and incompletely. About half of the cells showed an additional current component with a rapid inactivation time course. Both components were abolished by Cd2+ ions (1 mM) and were reduced by changing the holding membrane potential to -40 mV or by addition of 0.1 mM-Ni2+. 3. Single calcium channel currents were measured in cell-attached patches with 110 or 10 mM-Ba2+ as a current carrier. Two different types of single calcium channel activity were observed. 4. A high-conductance calcium channel was activated near -30 mV with 110 mM-Ba2+ and this threshold was changed to about -60 mV with 10 mM-Ba2+ in the patch pipette. The conductance was 28.0 +/- 1.5 pS (mean +/- S.D.) in 110 mM-Ba2+ and 16.0 +/- 1.0 pS in 10 mM-Ba2+. Dependence of the conductance on the concentration of Ba2+ in the patch pipette followed a Langmuir curve: the apparent dissociation constant of Ba2+ was 8 mM. It was concluded that this channel type corresponds to L-type calcium channels. 5. Another calcium channel was found in these experiments. It had a low conductance and was activated at around -50 mV with 110 mM-Ba2+, and this threshold was shifted to about -70 mV when 10 mM-Ba2+ was the charge carrier. The conductance of this calcium channel was 7.5 +/- 0.6 pS in 110 mM-Ba2+ and 5.5 +/- 1.0 pS in 10 mM-Ba2+. With 10 mM-Ba2+, inactivation of the mean current was slow at potentials -70 to -50 mV, but fast and complete (within 100 ms) at more positive potentials. It was concluded that this type of calcium channel corresponds to T-type calcium channels. 6. With the membrane potential continuously held at -50 to -40 mV (with 10 mM-Ba2+ in the patch pipette), i.e. close to the usual resting potential of these cells, T-type calcium channels were completely inactivated whereas rare openings of L-type calcium channels could be detected.(ABSTRACT TRUNCATED AT 400 WORDS)