Changes in the expression of TRPV4 and TRPM8 channels in the colon of rats with 6-OHDA-induced Parkinson’s disease

Parkinson’s disease (PD) is neurodegenerative disease, which is accompanied by degeneration of dopaminergic neurons in subtantia nigra. Non-motor symptoms, in particular, disorders of the gastrointestinal (GI) tract are observed in 20-80% of patients some 15-20 years before clinically diagnosed PD and are not a least important feature of PD pathogenesis. The transient receptor potential (TRP) channels are expressed throughout the GI tract, where they play an important role in taste, thermoregulation, pain, mucosal function and homeostasis, control of interstitial motility etc. The aim of this study was to investigate the contribution of TRPV4 and TRPM8 channels in the GI motor function in the colon of rats with PD, incduced by injection of the 12 μg 6-hydroxydopamine (6-OHDA). The studies were performed on the 4th week and the 7th month after PD induction The rats were randomly divided into: I group – the sham-lesioned rats, 4 μl 0.9% NaCl, autopsy 4 weeks after injection (n = 5); II group – the 6-OHDA-PD rats, 4 μl 12 μg of 6-OHDA, autopsy 4 weeks after injection (n = 5); III group – the sham-lesioned rats, 4 μl 0.9% NaCl, autopsy 7 months after injection (n = 4); IV group – the 6-OHDA-PD rats, 4 μl 12 μg of 6-OHDA, autopsy 7 months after injection (n = 5). We evaluated the body weight of rats, GI transit time, the cecum weight index and immunohistochemical identification of tyrosine hydroxylase (TH) -positive cells, and TRPV4, TRPM8 expression in rat’s colon. We showed that on the 7th month of the experiment, the GI transit time doubles over time; the cecum weight index of 6-OHDA rats increased by 57%; the number of TH-positive cells in colon rats decreased 2-fold, while TRPM8 ion channels were downregulated in PD rats and TRPV4 ion channels were upregulated in the colon of rats with 6-OHDA-PD. It was concluded that TRPV4 and TRPM8 ion channels may be considered pharmacological targets in the progression of PD pathology.

Development of primary human nasal epithelial cell cultures for the study of cystic fibrosis pathophysiology

Cultured primary epithelial cells are used to examine inflammation in cystic fibrosis (CF). We describe a new human model system using cultured nasal brushings. Nasal brushings were obtained from 16 F508del homozygous patients and 11 healthy controls. Cells were resuspended in airway epithelial growth medium and seeded onto collagen-coated flasks and membranes for use in patch-clamp, ion transport, and mediator release assays. Viable cultures were obtained with a 75% success rate from subjects with CF and 100% from control subjects. Amiloride-sensitive epithelial Na channel current of similar size was present in both cell types while forskolin-activated CF transmembrane conductance regulator current was lacking in CF cells. In Ussing chambers, cells from CF patients responded to UTP but not to forskolin. Spontaneous and cytomix-stimulated IL-8 release was similar (stimulated 29,448 ± 9,025 pg/ml; control 16,336 ± 3,308 pg/ml CF; means ± SE). Thus nasal epithelial cells from patients with CF can be grown from nasal brushings and used in electrophysiological and mediator release studies in CF research.

Rho kinase and protein kinase C involvement in vascular smooth muscle myofilament calcium sensitization in arteries from diabetic rats

BACKGROUND AND PURPOSE

Diabetes mellitus (DM) causes multiple dysfunctions including circulatory disorders such as cardiomyopathy, angiopathy, atherosclerosis and arterial hypertension. Rho kinase (ROCK) and protein kinase C (PKC) regulate vascular smooth muscle (VSM) Ca(2+) sensitivity, thus enhancing VSM contraction, and up-regulation of both enzymes in DM is well known. We postulated that in DM, Ca(2+) sensitization occurs in diabetic arteries due to increased ROCK and/or PKC activity.

EXPERIMENTAL APPROACH

Rats were rendered hyperglycaemic by i.p. injection of streptozotocin. Age-matched control tissues were used for comparison. Contractile responses to phenylephrine (Phe) and different Ca(2+) concentrations were recorded, respectively, from intact and chemically permeabilized vascular rings from aorta, tail and mesenteric arteries.

KEY RESULTS

Diabetic tail and mesenteric arteries demonstrated markedly enhanced sensitivity to Phe while these changes were not observed in aorta. The ROCK inhibitor HA1077, but not the PKC inhibitor chelerythrine, caused significant reduction in sensitivity to agonist in diabetic vessels. Similar changes were observed for myofilament Ca(2+) sensitivity, which was again enhanced in DM in tail and mesenteric arteries, but not in aorta, and could be reduced by both the ROCK and PKC blockers.

CONCLUSIONS AND IMPLICATIONS

We conclude that in DM enhanced myofilament Ca(2+) sensitivity is mainly manifested in muscular-type blood vessels and thus likely to contribute to the development of hypertension. Both PKC and, in particular, ROCK are involved in this phenomenon. This highlights their potential usefulness as drug targets in the pharmacological management of DM-associated vascular dysfunction.

Muscarinic receptor-activated cationic channels in murine ileal myocytes

BACKGROUND AND PURPOSE

There is little information about the excitatory cholinergic mechanisms of mouse small intestine although this model is important for gene knock-out studies.

EXPERIMENTAL APPROACH

Using patch-clamp techniques, voltage-dependent and pharmacological properties of carbachol- or intracellular GTPgammaS-activated cationic channels in mouse ileal myocytes were investigated.

KEY RESULTS

Three types of cation channels were identified in outside-out patches (17, 70 and 140 pS). The voltage-dependent behaviour of the 70 pS channel, which was also the most abundantly expressed channel (approximately 0.35 micro(-2)) was most consistent with the properties of the whole-cell muscarinic current (half-maximal activation at -72.3+/-9.3 mV, slope of -9.1+/-7.4 mV and mean open probability of 0.16+/-0.01 at -40 mV; at near maximal activation by 50 microM carbachol). Both channel conductance and open probability depended on the permeant cation in the order: Cs+ (70 pS) >Rb+ (66pS) >Na+ (47 pS) >Li+ (30 pS). External application of divalent cations, quinine, SK&F 96365 or La3+ strongly inhibited the whole-cell current. At the single channel level the nature of the inhibitory effects appeared to be very different. Either reduction of the open probability (quinine and to some extent SK&F 96365 and La3+) or of unitary current amplitude (Ca2+, Mg2+, SK&F 96365, La3+) was observed implying significant differences in the dissociation rates of the blockers.

CONCLUSIONS AND IMPLICATIONS

The muscarinic cation current of murine small intestine is very similar to that in guinea-pig myocytes and murine genetic manipulation should yield important information about muscarinic receptor transduction mechanisms.

Regulation of muscarinic cationic current in myocytes from guinea-pig ileum by intracellular Ca2+ release: a central role of inositol 1,4,5-trisphosphate receptors

The dynamics of carbachol (CCh)-induced [Ca(2+)](i) changes was related to the kinetics of muscarinic cationic current (mI(cat)) and the effect of Ca(2+) release through ryanodine receptors (RyRs) and inositol 1,4,5-trisphosphate receptors (IP(3)Rs) on mI(cat) was evaluated by fast x-y or line-scan confocal imaging of [Ca(2+)](i) combined with simultaneous recording of mI(cat) under whole-cell voltage clamp. When myocytes freshly isolated from the longitudinal layer of the guinea-pig ileum were loaded with the Ca(2+)-sensitive indicator fluo-3, x-y confocal imaging revealed CCh (10 microM)-induced Ca(2+) waves, which propagated from the cell ends towards the myocyte centre at 45.9 +/- 8.8 microms(-1) (n = 13). Initiation of the Ca(2+) wave preceded the appearance of any measurable mI(cat) by 229 +/- 55 ms (n = 7). Furthermore, CCh-induced [Ca(2+)](i) transients peaked 1.22 +/- 0.11s (n = 17) before mI(cat) reached peak amplitude. At -50 mV, spontaneous release of Ca(2+) through RyRs, resulting in Ca(2+) sparks, had no effect on CCh-induced mI(cat) but activated BK channels leading to spontaneous transient outward currents (STOCs). In addition, Ca(2+) release through RyRs induced by brief application of 5 mM caffeine was initiated at the cell centre but did not augment mI(cat) (n = 14). This was not due to an inhibitory effect of caffeine on muscarinic cationic channels (since application of 5 mM caffeine did not inhibit mI(cat) when [Ca(2+)](i) was strongly buffered with Ca(2+)/BAPTA buffer) nor was it due to an effect of caffeine on other mechanisms possibly involved in the regulation of Ca(2+) sensitivity of muscarinic cationic channels (since in the presence of 5 mM caffeine, photorelease of Ca(2+) upon cell dialysis with 5 mM NP-EGTA/3.8 mM Ca(2+) potentiated mI(cat) in the same way as in control). In contrast, IP(3)R-mediated Ca(2+) release upon flash photolysis of "caged" IP(3) (30 microM in the pipette solution) augmented mI(cat) (n = 15), even though [Ca(2+)](i) did not reach the level required for potentiation of mI(cat) during photorelease of Ca(2+) (n = 10). Intracellular calcium stores were visualised by loading of the myocytes with the low-affinity Ca(2+) indicator fluo-3FF AM and consisted of a superficial sarcoplasmic reticulum (SR) network and some perinuclear formation, which appeared to be continuous with the superficial SR. Immunostaining of the myocytes with antibodies to IP(3)R type 1 and to RyRs revealed that IP(3)Rs are predominant in the superficial SR while RyRs are confined to the central region of the cell. These results suggest that IP(3)R-mediated Ca(2+) release plays a central role in the modulation of mI(cat) in the guinea-pig ileum and that IP(3) may sensitise the regulatory mechanisms of the muscarinic cationic channels gating to Ca(2+).

[Single nonselective cation channels activated by muscarinic agonists in smooth muscle cells of the guinea-pig small intestine]

The carbachol-evoked inward cationic current in guinea-pig ileum smooth muscle cells is comprised of three types nonselective cationic channels (NSCC) with small (10 +/- 2 pS), medium (56 +/- 8pS) and large (135 +/- 14 pS) unitary conductance. All three types of NSCC could be activated by external application of carbachol as well as by internal application of GTPgS. It was found that behavior of carbachol- and GTPgammaS-evoked whole-cell current is mainly determined by the properties of medium conductance channels. The U-shaped I-V relationship of the whole-cell cationic current at negative potentials range arrives from voltage-dependence of its Po of this channel.

Effects of G-protein-specific antibodies and G beta gamma subunits on the muscarinic receptor-operated cation current in guinea-pig ileal smooth muscle cells

(1) The effects on the whole-cell carbachol-induced muscarinic cationic current (mIcat) of antibodies against the alpha-subunits of various G proteins, as well as the effect of a Gbetagamma subunit, were studied in single guinea-pig ileal smooth muscle cells voltage-clamped at -50 mV. Ionized intracellular calcium concentration, [Ca(2+)](i), was clamped at 100 nM using a 1,2-bis(2-aminophenoxyl-ethane-N,N,N',N'-tetraacetic acid)/Ca(2+) mixture. (2) Application of ascending concentrations of carbachol (1-300 micro M) activated mIcat (mean amplitude 0.83 nA at 300 micro M carbachol; EC(50) 8 micro M; Hill slope 1.0). A 20 min or longer intracellular application via the pipette solution of G(i3)/G(o) or G(o) antibodies resulted in about a 70% depression of the maximum response without change in the EC(50) value. In contrast, antibodies against alpha-subunits of G(i1), G(i1)/G(i2), G(i3), G(q)/G(11) or G(s) protein over a similar or longer period did not significantly reduce mIcat. Antibodies to common Gbeta or infusion of the Gbetagamma subunit itself had no effect on mIcat. (3) If cells were exposed briefly to carbachol (50 or 100 micro M) at early times (<3 min) after infusion of antibodies to Galpha(i3)/Galpha(o) or to Galpha(o) had begun, carbachol responses remained unchanged even after 20-60 min; that is, the depression of mIcat by these antibodies was prevented. (4) These data show that Galpha(o) protein couples the muscarinic receptor to the cationic channel in guinea-pig ileal longitudinal smooth muscle and that Gbetagamma is not involved. They also show that prior activation of the muscarinic receptor presumably causes a long-lasting postactivation change of the G protein, which is not reflected in mIcat, but acts to hinder antibody binding.

Membrane currents in cultured human intestinal smooth muscle cells

Using whole-cell patch-clamp recording techniques, we have examined voltage-gated ion currents in a cultured human intestinal smooth muscle cell line (HISM). Experiments were performed at room temperature on cells after passages 16 and 17. Two major components of the whole-cell current were a tetraethylammonium-sensitive (IC50 = 9 mM), iberiotoxin-resistant, delayed rectifier K+ current and a Na+ current inhibited by tetrodotoxin (IC50 A 100 nM). No measurable inward current via voltage-gated Ca2+ channels could be detected in these cells even with 10 mM Ca2+ or Ba2+ in the external solution. No current attributable to calcium-activated K+ channels was found and no cationic current in response to muscarinic receptor activation was present. In divalent cation-free external solution two additional currents were activated: an inwardly rectifying hyperpolarization-activated current, I(HA), and a depolarization-activated current, I(DA) x I(HA) and I(DA) could be carried by several monovalent cations; the sizes of currents in descending order were: K+ > Cs+ > Na+ for I(HA) and Na+ > K+ >> Cs+ for I(DA). I(HA) was activated and deactivated instantaneously and showed no inactivation whereas I(DA) was activated, inactivated and deactivated within tens of milliseconds. These currents were inhibited by external calcium with an IC50 of 0.3 microM for I(DA) and an IC50 of 20 microM for I(HA). Cyclopiazonic acid (CPA) induced an outward, but not an inward current. SK&F 96365, a blocker of store-operated Ca2+ channels, suppressed I(DA) with a half-maximal inhibitory concentration of 9 microM but was ineffective in inhibiting I(HA) at concentrations up to 100 microM. Gd3+ and La3+ strongly suppressed I(DA) at 1 and 10 microM, respectively and were less effective in blocking I(HA) (complete inhibition required a concentration of 100 microM for both). Carbachol at 10-100 microM evoked about a 3-fold increase in I(HA) amplitude and completely abolished I(DA). We conclude that I(HA) and I(DA) are Ca2+-blockable cationic currents with different ion selectivity profiles that are carried by different channels. I(DA) shows novel voltage-dependent properties for a cationic current.

Voltage-dependent inhibition of the muscarinic cationic current in guinea-pig ileal cells by SK&F 96365

The effects of SK&F 96365 on cationic current evoked either by activating muscarinic receptors with carbachol or by intracellularly applied GTPgammaS (in the absence of carbachol) were studied using patch-clamp recording techniques in single guinea-pig ileal smooth muscle cells. SK&F 96365 reversibly inhibited the muscarinic receptor cationic current in a concentration-, time- and voltage-dependent manner producing concomitant alteration of the steady-state I-V relationship shape which could be explained by assuming that increasing membrane positivity increased the affinity of the blocker. The inhibition was similar for both carbachol- and GTPgammaS-evoked currents suggesting that the cationic channel rather than the muscarinic receptor was the primary site of the SK&F 96365 action. Increased membrane positivity induced additional rapid inhibition of the cationic current by SK&F 96365 which was more slowly relieved during membrane repolarization. Both the inhibition and disinhibition time course could be well fitted by a single exponential function with the time constants decreasing with increasing positivity for the inhibition (e-fold per about 12 mV) and approximately linearly decreasing with increasing negativity for the disinhibition. At a constant SK&F 96365 concentration, the degree of cationic current inhibition was a sigmoidal function of the membrane potential with a potential of half-maximal increase positive to about +30 mV and a slope factor of about -13 mV. Increasing the duration of voltage steps at -80 or at 80 mV, increased the percentage inhibition; the degree of inhibition was almost identical at both potentials providing evidence that the same cationic channel was responsible for the cationic current both at negative and at positive potentials. It is concluded that the distinctive and unique mode of SK&F 96365 action on the muscarinic receptor cationic channel is a valuable tool in future molecular biology studies of this channel.

Membrane ion channels as physiological targets for local Ca2+ signalling

Ionized calcium plays a central role as a second messenger in a number of physiologically important processes determining smooth muscle function. To regulate a wide range of cellular activities the mechanisms of subcellular calcium signalling should be very diverse. Recent progress in development of visible light-excitable fluorescent dyes with high affinity for Ca2+ (such as oregon green 488 BAPTA indicators, fluo-3 and fura red) and confocal laser scanning microscopy provides an opportunity for direct visualization of subcellular Ca2+ signalling and reveals that many cell function are regulated by the microenvironment within small regions of the cytoplasm ('local control' concept). Here confocal imaging is used to measure and locate changes in [Ca2+]i on a subcellular level in response to receptor stimulation in visceral myocytes. We show that stimulation of muscarinic receptors in ileal myocytes with carbachol leading to activation of inositol 1,4,5-trisphosphate receptors (IP3Rs) accelerates the frequency of spontaneous calcium sparks (discharged via ryanodine receptors, RyRs) and gives rise to periodic propagating Ca2+ waves oscillating with a frequency similar to that of carbachol-activated cationic current oscillations. Furthermore, by combining the whole-cell patch clamp technique with simultaneous confocal imaging of [Ca2+]i in voltage-clamped vascular myocytes we demonstrate that calcium sparks may lead to the opening of either Ca2+-activated Cl- channels or Ca2+-activated K+ channels, and the discharge of a spontaneous transient inward current (STIC) or a spontaneous transient outward current (STOC), respectively.

Excitation-contraction coupling in gastrointestinal and other smooth muscles

The main contributors to increases in [Ca2+]i and tension are the entry of Ca2+ through voltage-dependent channels opened by depolarization or during action potential (AP) or slow-wave discharge, and Ca2+ release from store sites in the cell by the action of IP3 or by Ca(2+)-induced Ca(2+)-release (CICR). The entry of Ca2+ during an AP triggers CICR from up to 20 or more subplasmalemmal store sites (seen as hot spots, using fluorescent indicators); Ca2+ waves then spread from these hot spots, which results in a rise in [Ca2+]i throughout the cell. Spontaneous transient releases of store Ca2+, previously detected as spontaneous transient outward currents (STOCs), are seen as sparks when fluorescent indicators are used. Sparks occur at certain preferred locations--frequent discharge sites (FDSs)--and these and hot spots may represent aggregations of sarcoplasmic reticulum scattered throughout the cytoplasm. Activation of receptors for excitatory signal molecules generally depolarizes the cell while it increases the production of IP3 (causing calcium store release) and diacylglycerols (which activate protein kinases). Activation of receptors for inhibitory signal molecules increases the activity of protein kinases through increases in cAMP or cGMP and often hyperpolarizes the cell. Other receptors link to tyrosine kinases, which trigger signal cascades interacting with trimeric G-protein systems.

Potassium channels of myenteric neurons in guinea-pig small intestine

Patch-clamp recording was used to study rectifying K+ currents in myenteric neurons in short-term culture. In conditions that suppressed Ca2+ -activated K+ current, three kinds of voltage-activated K+ currents were identified by their voltage range of activation, inactivation, kinetics and pharmacology. These were A-type current, delayed outwardly rectifying current (I(K),dr) and inwardly rectifying current (I(K),ir). I(K),ir consisted of an instantaneous component followed by a time-dependent current that rapidly increased at potentials negative to -80 mV. Time-constant of activation was voltage-dependent with an e-fold decrease for a 31-mV hyperpolarization amounting to a decrease from 800 to 145 ms between -80 and -100 mV. I(K),ir did not inactivate. I(K),ir was abolished in K+ -free solution. Increases in external K+ increased I(K),ir conductance in direct relation to the square root of external K+ concentration. Activation kinetics were accelerated and the activation range shifted to more positive K+ equilibrium potentials. I(K),ir was suppressed by external Cs+ and Ba2+ in a concentration-dependent manner. Ca2+ and Mg+ were less effective than Ba2+. I(K),ir was unaffected by tetraethylammonium ions. I(K),dr was activated at membrane potentials positive to - 30 mV with an e-fold decrease in time-constant of activation from 145 to 16 ms between -20 and 30 mV. It was half-activated at 5 mV and fully activated at 50 mV. Inactivation was indiscernible during 2.5 s test pulses. I(K),dr was suppressed in a concentration-, but not voltage-dependent manner by either tetraethylammonium or 4-aminopyridine and was insensitive to Cs+. The results suggest that I(K),ir may be important in maintaining the high resting membrane potentials found in afterhyperpolarization-type enteric neurons. They also suggest importance of I(K),ir channels in augmentation of the large hyperpolarizing after-potentials in afterhyperpolarization-type neurons and the hyperpolarization associated with inhibitory postsynaptic potentials. I(K),dr in afterhyperpolarization-type enteric neurons has overall kinetics and voltage behaviour like delayed rectifier currents in other excitable cells where the currents can also be distinguished from A-type and Ca2+ -activated K+ current.

Muscarinic cation current and suppression of Ca2+ current in guinea pig ileal smooth muscle cells

Cationic current (Icat) and inhibition of the voltage-dependent Ca2+ current (ICa) evoked by muscarinic receptor activation with carbachol were studied using whole-cell patch clamp technique in smooth muscle cells isolated from longitudinal muscle of guinea pig small intestine. With low buffering of [Ca2+]i (0.1 mM BAPTA [1,2-bis-(2-aminophenoxy)-ethane-N,N, N', N'-tetraacetic acid] in pipette solution) Icat and ICa inhibitory responses had a rapid onset to an initial peak followed by a sustained phase. The sustained phase of ICa suppression was bigger than in the case when [Ca2+]i was clamped to 100 nM, but decreased with repeated stimulation. Upon repeated stimulation with 50 microM carbachol in cells where [Ca2+]i was clamped to 100 nM and when GTP was absent, Icat amplitude decreased strongly and more substantially compared to ICa inhibition, but both responses declined only slightly when 1 mM GTP was present in the pipette solution. GDP-betaS (1 or 5 mM) in pipette solution or pre-treatment of cells with pertussis toxin (6 microg/ml, for 4 h or longer) blocked Icat more than ICa suppression by carbachol, whereas L-NAME (N-omega-nitro-L-arginine methyl ester hydrochloride) (100 microM in pipette solution) affected neither of them significantly. We conclude that the cationic current and the suppression of the voltage-dependent Ca2+ current evoked by muscarinic receptor activation are mediated by pertussis toxin-sensitive G-protein(s) but the latter response was less sensitive to blockade by GDP-betaS and to GTP deficiency in the cell.

Muscarinic receptor subtypes controlling the cationic current in guinea-pig ileal smooth muscle

1. The effects of muscarinic antagonists on cationic current evoked by activating muscarinic receptors with the stable agonist carbachol were studied by use of patch-clamp recording techniques in guinea-pig single ileal smooth muscle cells. 2. Ascending concentrations of carbachol (3-300 microM) activated the cationic conductance in a concentration-dependent manner with conductance at a maximally effective carbachol concentration (Gmax) of 27.4+/-1.4 nS and a mean -log EC50 of 5.12+/-0.03 (mean+/-s.e.mean) (n=114). 3. Muscarinic antagonists with higher affinity for the M2 receptor, methoctramine, himbacine and tripitramine, produced a parallel shift of the carbachol concentration-effect curve to the right in a concentration-dependent manner with pA2 values of 8.1, 8.0 and 9.1, respectively. 4. All M3 selective muscarinic antagonists tested, 4-DAMP, p-F-HHSiD and zamifenacin, reduced the maximal response in a concentration-dependent and non-competitive manner. This effect could be observed even at concentrations which did not produce any increase in the EC50 for carbachol. At higher concentrations M3 antagonists shifted the agonist curve to the right, increasing the EC50, and depressed the maximum conductance response. Atropine, a non-selective antagonist, produced both reduction in Gmax (M3 effect) and significant increase in the EC50 (M2 effect) in the same concentration range. 5. The depression of the conductance by 4-DAMP, zamifenacin and atropine could not be explained by channel block as cationic current evoked by adding GTPgammaS to the pipette (without application of carbachol) was unaffected. 6. The results support the hypothesis that carbachol activates M2 muscarinic receptors so initiating the opening of cationic channels which cause depolarization; this effect is potentiated by an unknown mechanism when carbachol activates M3 receptors. As an increasing fraction of M3 receptors are blocked by an antagonist, the effects on cationic current of an increasing proportion of activated M2 receptors are disabled.

Effects of protons on muscarinic receptor cationic current in single visceral smooth muscle cells

Effects of extracellular protons on muscarinic receptor-evoked cationic current (Icat) in single guinea pig ileal smooth muscle cells were studied by use of patch-clamp techniques: intracellular pH and pCa were buffered to 7.4 and 7.0, respectively, symmetrical 124 mM Cs+ solutions were used, and divalent cations were removed from the bathing solution. Increasing extracellular pH (pHo) from 7.4 to 8.4 caused a 16-mV parallel negative shift of the activation curve for Icat evoked by guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) with an increase in the maximal conductance and slowed Icat relaxation on hyperpolarization; acidification to pHo 6.4 produced equivalent but opposite effects. Carbachol- and GTPgammaS-activated Icat behaved similarly, suggesting that the cationic channel rather than the muscarinic receptor was the major site of action. From 11.4 to 4.4 pHo, maximal cationic conductance was reduced progressively, and the activation curve shifted positively. Na+, K+, Ca2+, and Mg2+ had complex interactions with pHo-induced effects considered to be attributable to interaction of protons with fixed negative surface charges and, at positive potentials, to channel block.

Activation of M2 muscarinic receptors in guinea-pig ileum opens cationic channels modulated by M3 muscarinic receptors

In longitudinal muscle of guinea-pig ileum, activation of muscarinic receptors causes contraction antagonised by M3 receptor subtype antagonists despite a preponderance of M2 receptor subtype binding sites. Experiments on single smooth muscle cells under voltage-clamp described here show that the cationic current evoked by carbachol which normally causes depolarization of the muscle is inhibited competitively by M2 antagonists with affinities typical of antagonism at a M2 receptor. However, M3 antagonists strongly reduced the maximum cationic current which could be evoked by carbachol in a non-competitive manner with affinities typical for an action at M3 receptors. Thus cation channels are gated by M2 receptor activation but strongly modulated by activation of M3 receptors.

A novel GTP-dependent mechanism of ileal muscarinic metabotropic channel desensitization

1. Cationic current (Icat) was evoked in single isolated smooth muscle cells either by activating muscarinic receptors with the stable muscarinic agonist, carbachol (CCh), or by dialysing cells with GTP-gamma S. It was studied using patch-clamp recording techniques in cells obtained by enzymatic digestion from the longitudinal muscle layer of the guinea-pig small intestine. 2. Icat appears only when muscarinic receptors or G-proteins are activated, but it is strongly voltage-dependent. Its activation could be described by the Boltzmann equation. During desensitization of Icat evoked by 50 microM CCh, the slope factor, k, remained constant whereas the maximal conductance, Gmax, slowly decreased and the potential of half-maximal activation, V1/2, shifted positively by 32 mV during 4 min. 3. At peak response either to extracellular application of CCh (GTP-free, or 1 mM GTP-containing, pipette solution) or to intracellular application of GTP-gamma S (no CCh), the size and voltage-dependent properties of Icat were similar. However, Icat desensitization was slower in the presence of GTP (CCh applied) in the pipette solution and much slower with GTP-gamma S in the pipette (no CCh) compared to GTP-free pipette solution (CCh applied); the decrease in Gmax with time was much delayed and the positive shift of the activation curve was inhibited. GDP-beta S added to the pipette solution at 2 mM abolished Icat in response to applied CCh; 50 microM did not prevent Icat generation but significantly accelerated desensitization. 4. It was concluded that the rate of desensitization of the carbachol-evoked cationic current was due to a decline in the concentration of activated G-protein in the cell, which reduced the maximum number of channels which could be opened and shifted their activation range to less negative potentials.

Modulation of calcium currents by G-proteins and adenosine receptors in myenteric neurones cultured from adult guinea-pig small intestine

1. Whole-cell patch clamp methods were used to analyse voltage-dependent calcium currents in cultured myenteric neurones enzymatically isolated from adult guinea-pig small intestine. 2. Activation of G-proteins by intracellular administration of GTP-gamma-S (100-200 microM in pipette) decreased the amplitude of high voltage activated Ca2+ current (ICa) by more than 50%. Residual ICa was activated more slowly and was non-inactivating during 500 ms test pulses when GTP-gamma-S was included in the pipette solution. 3. Inclusion of 500 microM GDP-beta-S in the patch pipettes increased the amplitude of ICa by over 30% without altering the voltage-dependency. 4. Extracellular application of 2-chloroadenosine suppressed ICa dose-dependently by reducing both transient and sustained components of the current. 5. Pretreatment of the neurones with cholera toxin or forskolin did not alter the actions of GTP-gamma-S or GDP-beta-S or 2-chloroadenosine. 6. The results suggest that high threshold calcium channels in myenteric neurones are influenced by G-proteins and that the inhibitory action of 2-chloroadenosine on ICa involves G-protein coupling of the adenosine receptors to the Ca2+ channel.

Effects of divalent cations on muscarinic receptor cationic current in smooth muscle from guinea-pig small intestine

1. Effects of Mg2+ and Ca2+ on muscarinic receptor cationic current (Icat) in guinea-pig ileal smooth muscle cells have been studied using patch-clamp techniques (whole-cell recording). Icat was activated either by externally applied carbachol or, to bypass receptors, by intracellular GTP-gamma-S. 2. Independently of the main permeant cation the current-voltage (I-V) relation for Icat was U-shaped between the reversal potential (usually 0 mV) and very negative potentials such as -120 mV where current could be virtually lost. Adding Ca2+ to Ca(2+)- and Mg(2+)-free external solution reduced inward current and made it less U-shaped whereas adding Mg2+ reduced inward current and shifted more positively the potential at which maximum inward current occurred. 3. Activation of the conductance underlying Icat could be described by the Boltzmann relation which was shifted positively by adding Ca2+ or Mg2+. Extracellular Ca2+ also distorted the relation by increasing the slope factor; maximal conductance was reduced in all cases. Icat relaxation at negative potentials was accelerated by increasing Mg2+ and slowed down by Ca2+. 4. These data suggest the presence of fixed negative surface charges on or near the muscarinic receptor cationic channel, which allow its modulation through alteration of surface potential. Additional more direct ion binding to and blocking of the channel cannot be ruled out. Some additional effects of Ca2+ (if compared with Mg2+) could be explained on the assumption that the Ca(2+)-binding activation site known to be present on the internal side of the channel can be accessible to Ca2+ entering through the open channel during muscarinic receptor stimulation, as Ca2+ ions contribute to a limited extent to Icat. 5. We conclude that voltage-dependent gating of muscarinic receptor cationic channels is an intrinsic channel property and that Ca2+ and Mg2+ have strong modulatory effects.

Ca2+ inhibition of inositol trisphosphate-induced Ca2+ release in single smooth muscle cells of guinea-pig small intestine

1. Single smooth muscle cells from the longitudinal muscle layer of guinea-pig small intestine were voltage clamped using patch pipettes in the whole-cell mode. 2. When D-myo-inositol 1,4,5-trisphosphate (InsP3) was released at intervals, by photolysis of 'caged' InsP3 within the cell, increases in [Ca2+]i in many cells, as judged from Ca(2+)-activated K(+)-current, were all-or-none; release of InsP3 before a critical interval had elapsed, which was quite stable for an individual cell, resulted in no response. After Ca(2+)-induced Ca2+ release had been evoked by depolarization, the InsP3 response was inhibited. Oscillations in [Ca2+]i evoked by muscarinic receptor activation were unaffected by Ruthenium Red; during these oscillations exogenous InsP3 was not effective close to, or shortly after, peak [Ca2+]i but was effective at other times. 3. Reproducible release of Ca2+ and elevation of [Ca2+]i could be produced by brief (up to 0.5 s) pressure applications of 10 mM caffeine at intervals of 10 s or greater but caffeine itself rarely evoked oscillations in [Ca2+]i. Responses to flash release of InsP3 were reduced after caffeine-induced responses and recovery of caffeine-induced Ca2+ release was faster than recovery of InsP3-induced Ca2+ release. 4. The results support the idea that InsP3-induced Ca(2+)-store release can be inhibited by a certain level of [Ca2+]i at a time when Ca2+ stores have refilled and can be released by caffeine; they also support the suggestion that during oscillations of [Ca2+]i evoked by muscarinic receptor activation, Ca2+ inhibition of InsP3-induced Ca2+ release at some critical level of [Ca2+]i allows Ca2+ stores to refill and leads to a fall in [Ca2+]i so contributing to the oscillations which are observed.

G-protein control of voltage dependence as well as gating of muscarinic metabotropic channels in guinea-pig ileum

1. Voltage-dependent properties of muscarinic receptor cationic current activated by carbachol in single smooth muscle cells have been studied using patch-clamp recording techniques. Cells were obtained by enzymic digestion from the longitudinal muscle layer of guinea-pig small intestine. 2. The inward cationic current showed a pronounced U-shaped current-voltage relationship (inward current negative). The relationship of cationic conductance to voltage could be described by a Boltzman distribution which was shifted 36 mV in the negative direction on the voltage axis by increasing fractional receptor occupancy (by increasing agonist concentration from 3 to 300 microM), and in the positive direction by desensitization during prolonged application of agonist. Cationic channels opened by low and high concentrations of carbachol at the same potential do not have identical properties. 3. Release of GTP within the cell, by flash photolysis of an inert caged precursor, had the same effect on the current-voltage relationship as increasing receptor occupancy by the agonist. Release of GDP beta S by flash photolysis had the opposite effect. 4. These various results could be explained if cationic channel opening upon receptor activation required binding of at least one alpha-GTP subunit, but the position of the activation curve on the voltage axis depended critically on the concentration of activated G-protein alpha-subunits in the cell.

Some properties of Ca(2+)-induced Ca2+ release mechanism in single visceral smooth muscle cell of the guinea-pig

1. Late transient outward Ca(2+)-dependent K+ current (ILTO) correlated with Ca(2+)-induced Ca2+ release mechanism was studied in relation to the calcium inward current (ICa) in single isolated smooth muscle cells of the guinea-pig ileum using the whole-cell patch-clamp technique. 2. The voltage dependencies of peak ICa and ILTO were both bell shaped. However, the I-V curve of the outward current was shifted toward more positive potentials by about 60 mV in comparison to that for ICa. 3. Reduction in the external Ca2+ concentration resulted in a decrease of peak amplitude of both ICa and ILTO. However, caffeine-induced outward current was also decreased abruptly suggesting a rapid loss of stored Ca2+ upon lowering the external Ca2+ concentration. 4. Investigation of the relation of ILTO to partially inactivated ICa showed that inactivation of ICa by approximately 65, 80 or 84% of control (produced by prepulse to -20 mV for 2 s, shifting the holding potential to -20 mV for 30 s or by the ramp voltage command from -50 to +10 mV, respectively) was without detectable effect on the ILTO generation. 5. Bath application of the Ca2+ antagonist nifedipine (300 nM) inhibited ICa by 81% without affecting ILTO peak amplitude (92.0 +/- 5.6% of control in six cells). The mean concentration-response curve for ICa inhibition was sigmoidal with the apparent dissociation constant of 86.9 nM, whereas that for the ILTO had a characteristic sharp transition indicating a definite threshold of Ca2+ influx for ILTO generation. 6. Application of Ca(2+)-free external solution during 500 ms of the time when ICa peaked inhibited the current by about 76% whereas the ILTO during such an intervention remained virtually unchanged. 7. In double-pulse experiments, with conditioning and test pulses to +10 mV from -50 mV and an interpulse interval of 600 ms, most of the cells (about 80%) showed larger outward current at the test pulse suggesting continued Ca2+ release triggered by Ca2+ influx during a short (50-200 ms) depolarizing prepulse. The outward current could also be evoked at large positive potentials (presumably near the calcium equilibrium potential) where it did not occur normally by a prepulse to +10 mV for 50 ms. The charge transferred by Ca2+ current necessary to activate Ca2+ release in most of the cells was estimated to be from 6 to 20 pC. 8. The data are interpreted to suggest that the Ca(2+)-induced Ca2+ release mechanism operates in single ileal cells in a regenerative manner.(ABSTRACT TRUNCATED AT 400 WORDS)

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)

A potential-dependent fast outward current in single smooth muscle cells isolated from the newborn rat ileum

1. Whole-cell outward currents have been studied in single smooth muscle cells isolated from newborn and adult rat ileum, using fire-polished glass micropipettes. 2. Two major outward currents, delayed (I(do)) and fast inactivating potential-dependent (I(fo)), have been observed in the newborn rat ileal cells. I(fo) is activated between -50 and -40 mV from the holding potential of -80 mV, whereas I(do) usually starts to activate at membrane potentials positive to -20 mV. Activation of I(do) was fast, its time-to-peak decreased from 10.8 +/- 0.9 ms (n = 5) at -30 mV to 4.5 +/- 0.7 ms (n = 4) at 20 mV. 3. I(fo) decay was monoexponential and its time constant did not depend on the membrane potential. Dependence of I(fo) inactivation on membrane voltage in normal physiological salt solutions (PSS) could be described by the Boltzmann equation with the following parameters: a half-inactivation potential, V0.5 = -70.8 mV and slope factor, k = 7.7 mV. 4. Recovery of I(fo) from inactivation was fitted by a single exponential and was potential dependent. The average time constant was 28.4 +/- 2.4 ms (n = 11) at -120 mV, 47.7 +/- 3.0 ms (n = 6) at -100 mV and 89.6 +/- 5.3 ms (n = 13) at -80 mV. 5. Removal of Ca2+ ions from the PSS (in the presence of 5 mM-Mg2+) increased I(fo) amplitude by about two times, and shifted its voltage dependence of inactivation towards negative membrane potentials by about 16 mV (V0.5 = -87.2 mV). Removal of Mg2+ from the PSS (in the presence of 2.5 mM-Ca2+) exerted no effects upon either inactivation dependence (V0.5 = -74.2 mV) or I(fo) amplitude. 6. I(do) and I(fo) had different sensitivities to K+ channel blockers. With 10 mM-external TEA+ I(do), was preferentially suppressed, while 5 mM-4-aminopyridine (4-AP) completely blocked I(fo). I(fo) was also partially blocked by a higher TEA+ concentration (30 mM), which suppressed I(fo) to 0.55 +/- 0.02 (n = 9). The blocking effect of 4-AP on I(fo) was potential, use and time dependent. 7. Ileal cells isolated from the adult rat demonstrated the presence of two populations of smooth muscle cells. One has an outward current which seems to be similar to that described in the newborn rat. However, in other cells spontaneous transient outward currents, well described in other single smooth muscle cells, but not found in newborn rat ileal cells, have been observed.

Patch-clamp recording in myenteric neurons of guinea pig small intestine

The results of our research established the feasibility of applying patch-clamp methods in the study of the cellular neurophysiology of myenteric neurons enzymatically dissociated from adult guinea pig small intestine. Recording in current-clamp mode revealed two populations of neurons. One population discharged repetitively during depolarizing current pulses and displayed anodal-break excitation reminiscent of S/type 1 myenteric neurons. In the second population, spike discharge was limited to one or two spikes at the onset of depolarizing pulses and was similar to the behavior of AH/type 2 neurons. Recording in voltage-clamp mode revealed a complex of overlapping inward and outward whole cell currents. Fast and slow components of inward current were interpreted as sodium and calcium currents, respectively. Outward currents were blocked by cesium and consisted of components with properties of delayed rectifier current and A-type potassium current.

Properties of the late transient outward current in isolated intestinal smooth muscle cells of the guinea-pig

1. Whole-cell membrane currents in voltage-clamped single isolated cells of longitudinal smooth muscle of guinea-pig ileum were studied at room temperature using patch pipettes filled with either high-K+ solution or high-Cs+ solution, to suppress K+ outward current, and containing 0.3 mM-EGTA. 2. In the presence of high-K+ solution in the pipette, membrane depolarization from the holding potential of -50 mV evoked an initial inward calcium current (ICa) followed by a large initial transient outward current and a sustained outward current with spontaneous oscillations superimposed. Prolonged depolarization above -20 mV produced a late transient outward current which reached a maximum (up to several nanoamps at +10 mV) within approximately 1 s and lasted several seconds. 3. The late outward current (ILTO) was voltage dependent and reversed at the EK (potassium equilibrium potential) in cells exposed to high-K+ external solution. It was blocked by TEA+ (tetraethylammonium) or Ba2+ applied externally (calculated Kd (dissociation constant) values were 0.67 and 4.43 mM, respectively) or by high-Cs+ solution perfusing the cell. The removal of extracellular Ca2+, application of Ca2+ channel blockers (3 mM-Co2+, 0.2 mM-Cd2+ or 1 microM-nifedipine) or perfusion of 5 mM-EGTA inside the cell also abolished the current. Thus, the current seems to be a Ca(2+)-activated K+ current. 4. There is a great discrepancy between the time course of the ICa and that of the late ILTO, which suggests that Ca2+ release from intracellular storage sites may contribute to the generation of the ILTO. 5. Bath application of caffeine (10 mM) during the development of ILTO enhanced the current. However, in the presence of caffeine ILTO was inhibited. Moderate inhibition of ICa by caffeine was also observed. 6. Ryanodine (5 microM) applied to the bathing solution completely inhibited ILTO within 3.5 min; however, it had no or little effect on the ICa. 7. Ruthenium Red (10 microM) completely blocked the ILTO and slightly and more slowly inhibited the ICa. 8. Increasing Mg2+ concentration in the pipette solution from 1 to 6 mM abolished the ILTO. 9. It was concluded that the ILTO was activated mainly by Ca2+ released from the intracellular storage sites following Ca2+ entry, presumably by a Ca(2+)-induced Ca2+ release mechanism.

The inhibitory action of caffeine on calcium currents in isolated intestinal smooth muscle cells

The patch-clamp method has been used to investigate the action of caffeine on the calcium current (ICa) in single isolated smooth muscle cells of the guinea-pig ileum. Caffeine (10 mM) substantially inhibited ICa. This effect occurred in a biphasic manner and it was not due either to activation of additional ionic currents of opposite direction nor to inhibition of phosphodiesterase activity. It strongly depended upon the ethylenebis-(oxonitrilo)tetraacetate (EGTA) concentration in the pipette solution. When there was K+ in the pipette solution, application of caffeine evoked a transient Ca-dependent K+ current and an abrupt and transient increase in the frequency of channel openings. Such well-known blockers of Ca release as procaine and ruthenium red strongly decreased ICa. Ryanodine had only little effect on ICa, but application of caffeine in the presence of ryanodine led to a complete and irreversible inhibition of ICa. The results of experiments involving different EGTA concentrations and comparison of the time courses of all caffeine-induced phenomena clearly indicated that only the initial, transient component of the ICa inhibition by caffeine was related to a Ca-dependent inactivation of Ca channels, evoked as a result of Ca release from intracellular stores. The tonic component of ICa inhibition was probably due to a direct blocking action of caffeine on Ca channels.

[Electrical properties and transmembrane ion currents of single smooth-muscle cells]

Current and voltage clamp investigations of freshly isolated smooth muscle cells from guinea-pig ileum and taenia coli were performed using single suction micropipette technique. Specific membrane capacity of smooth muscle cells was calculated and accounted for 1.6 microF/cm2, with specific resistance varying from 50 to 150 k omega X cm2. Transmembrane currents consisted of two inward components, inactivating and noninactivating ones, carried by Ca2+ ions, overlapping with early activated potassium outward current. Time constant of inward current activation was not only voltage-sensitive but also ion-dependent. When Ca2+ ions in Krebs solution were replaced by Ba2+, both the rate of activation and inactivation of inward current were significantly reduced. Estimation of intracellular Ca2+ concentration increase has indicated that inward calcium current transports enough Ca2+ for direct contraction activation.