Effect of the Ca(2+)-ATPase inhibitor, cyclopiazonic acid, on electromechanical coupling in the guinea-pig ureter

Mediators of human ureteral smooth muscle contraction-a role for erythropoietin, tamsulosin and Gli effectors

Background

Ureteral contractility is a poorly understood process. Contractions have been demonstrated to occur in the smooth muscle layers of the ureter. Previous work suggests the involvement of Gli family proteins and erythropoietin (EPO) in regulating mammalian ureteral smooth muscle contraction. We sought to devise a method by which the effects of these proteins and tamsulosin on distal human ureteral tissue contractility could be investigated to better understand mechanisms regulating human ureteral function.

Methods

IRB approval was obtained to procure portions of extraneous distal ureteral tissue from living donor renal transplants. Contractility was measured by placing the tissue in Krebs buffer and stimulating via a uniform electric current. Contractile force was recorded with each stimulation with and without the presence of a Gli inhibitor (GANT61) or EPO. Each ureteral specimen was subsequently fixed and tested by immunohistochemistry to determine Gli, EPO and alpha-adrenergic receptor activity.

Results

Electrical field stimulation successfully elicited contractions in the ureteral tissue. Administering tamsulosin decreased force and duration of ureteral contractions. Inhibiting Gli signaling decreased contractility and EPO decreased ureteral contractile forces within 5 minutes of administration versus untreated controls. Staining confirmed Gli1 protein and α-adrenergic receptor expression in ureteral smooth muscle and epithelial tissue with EPO receptor expression confined to the epithelial layer.

Conclusions

Distal ureteral contractile forces are decreased by inhibition of Gli family proteins and the α-adrenergic receptor. EPO acts within five minutes, suggesting ion channel involvement instead of changes in gene expression. Continuing work will elucidate the role of these proteins in coordinating ureteral contractions. This has implications for the use of pharmacologic methods to address ureteral contractility and dysfunctional peristalsis during stone passage, ureteroscopy, in transplant patients and potentially to reduce symptoms from ureteral stents.

Differential targeting and signalling of voltage-gated T-type Cav 3.2 and L-type Cav 1.2 channels to ryanodine receptors in mesenteric arteries

KEY POINTS

In arterial smooth muscle, Ca2+ sparks are elementary Ca2+ -release events generated by ryanodine receptors (RyRs) to cause vasodilatation by opening maxi Ca2+ -sensitive K+ (BKCa ) channels. This study elucidated the contribution of T-type Cav 3.2 channels in caveolae and their functional interaction with L-type Cav 1.2 channels to trigger Ca2+ sparks in vascular smooth muscle cells (VSMCs). Our data demonstrate that L-type Cav 1.2 channels provide the predominant Ca2+ pathway for the generation of Ca2+ sparks in murine arterial VSMCs. T-type Cav 3.2 channels represent an additional source for generation of VSMC Ca2+ sparks. They are located in pit structures of caveolae to provide locally restricted, tight coupling between T-type Cav 3.2 channels and RyRs to ignite Ca2+ sparks.

ABSTRACT

Recent data suggest that T-type Cav 3.2 channels in arterial vascular smooth muscle cells (VSMCs) and pits structure of caveolae could contribute to elementary Ca2+ signalling (Ca2+ sparks) via ryanodine receptors (RyRs) to cause vasodilatation. While plausible, their precise involvement in igniting Ca2+ sparks remains largely unexplored. The goal of this study was to elucidate the contribution of caveolar Cav 3.2 channels and their functional interaction with Cav 1.2 channels to trigger Ca2+ sparks in VSMCs from mesenteric, tibial and cerebral arteries. We used tamoxifen-inducible smooth muscle-specific Cav 1.2-/- (SMAKO) mice and laser scanning confocal microscopy to assess Ca2+ spark generation in VSMCs. Ni2+ , Cd2+ and methyl-β-cyclodextrin were used to inhibit Cav 3.2 channels, Cav 1.2 channels and caveolae, respectively. Ni2+ (50 μmol L-1 ) and methyl-β-cyclodextrin (10 mmol L-1 ) decreased Ca2+ spark frequency by ∼20-30% in mesenteric VSMCs in a non-additive manner, but failed to inhibit Ca2+ sparks in tibial and cerebral artery VSMCs. Cd2+ (200 μmol L-1 ) suppressed Ca2+ sparks in mesenteric arteries by ∼70-80%. A similar suppression of Ca2+ sparks was seen in mesenteric artery VSMCs of SMAKO mice. The remaining Ca2+ sparks were fully abolished by Ni2+ or methyl-β-cyclodextrin. Our data demonstrate that Ca2+ influx through CaV 1.2 channels is the primary means of triggering Ca2+ sparks in murine arterial VSMCs. CaV 3.2 channels, localized to caveolae and tightly coupled to RyR, provide an additional Ca2+ source for Ca2+ spark generation in mesenteric, but not tibial and cerebral, arteries.

Review Article: Safety Assessment of the Mycotoxin Cyclopiazonic Acid

Cyclopiazonic acid (CPA) is an indol-tetramic acid mycotoxin and is produced by the nearly ubiquitous molds, Aspergillus and Penicillium. CPA produced by these molds has been identified in a number of food sources (including, but not limited to, grain, legumes, meat, milk, and cheese) and from parasitic infections of man and other animals. Few incidents of CPA mycotoxicoses have been reported because of the benign nature of the intoxication, the small amounts present, and its effects may be disguised with concurrent aflatoxicosis (some toxicity data may have been generated using aflatoxin-contaminated CPA). CPA is absorbed in the gastrointestinal tract and following oral administration; it has a half-life of approximately 30 hours and is excreted largely unchanged in the urine and feces. Cyclopiazonic acid is not considered to be a potent acute toxin as its oral LD50 in rodents is in the range of 30 to 70 mg/kg. Multiple dose studies also show a range of effects in several species and among mammalian models, the pig appears to be the most sensitive with a no-observable-effect level (NOEL) in the range of 1.0 mg/kg/day. The preponderance of evidence from the rat and other test animals supports this dose as a defensible estimate of a no effect level. The target organs of CPA toxicity appear to be muscle, hepatic tissue, and spleen, with a localization in the former, although a more apparent toxic change in the latter two. The toxicity and symptoms of CPA poisoning can be attributed to its ability to alter normal intracellular calcium flux through its inhibition of the reticular form of the Ca2+-ATPase pump. CPA was not teratogenic in mice. CPA is not considered a carcinogen and the weight of evidence militates against its characterization as a mutagen. Despite CPA-induced pathological changes ascribed to the spleen or bursa of Fabricius, there does not appear to be an effect on the immune system. In vitro studies imply a potential immunomodulatory effect of CPA, but in all of those reports very high concentrations of CPA were required and none of these findings have been supported with in vivo studies. Therefore, based on a NOEL of 1 mg/kg/day and accounting for species variation, an appropriate acceptable daily intake (ADI) would be approximately 10 μg/kg/day or 700 μg/day. In the context of human exposure, if the uppermost limit of CPA found in cheese is 4 μg/g and the average individual consumes 50 g of cheese daily, this allows an intake of 200 μg, less than one third of a traditionally established ADI.

Pharmacological evidence that potentiation of plasmalemmal Ca(2+)-extrusion is functionally coupled to inhibition of SR Ca(2+)-ATPases in vascular smooth muscle cells

Cyclopiazonic acid (CPA), a specific inhibitor of sarcoplasmic reticulum (SR) Ca(2+)-ATPases, causes slowly developing and subsequently diminishing characteristic contractions in vascular smooth muscle, and the second application of CPA has incompletely repeatable effects, depending on the vessel type. The objective of the present study was to examine the mechanisms underlying the significant decrease of CPA-induced contractions upon the second application. A pharmacological intervention of Ca(2+) extrusion process as a strategy was performed to modulate vasoconstrictor effects of CPA in rat aortic ring preparations. CPA-induced contractions, expressed as percentages of the contractions induced by KCl (80 mM), were significantly decreased from 44.1 ± 5.7 to 7.6 ± 1.8 % (P < 0.001) upon the second application. The contractions, however, were completely repeatable in the presence of vanadate, an inhibitor of ATPases, but not of ouabain, an inhibitor of Na(+)-pumps. Strikingly, CPA-induced contractions were sustained and completely repeatable in Na(+)-free and low Na(+) medium. Furthermore, we found that the contractions were completely repeatable in the presence of 2',4'-dichlorobenzamil, an inhibitor of the forward mode of Na(+)/Ca(2+) exchangers, but not of KBR7943, an inhibitor of the reverse mode of Na(+)/Ca(2+) exchangers. Our findings indicate that CPA by inducing a transient rise in cytosolic Ca(2+) level causes a long-lasting upregulation of plasma membrane (PM) Ca(2+) extruders and thus leads to a diminished contraction upon its second application in blood vessels. This suggests that there is a functional coupling between PM Ca(2+) extruders and SR Ca(2+)-ATPases in rat aortic smooth muscle cells.

Ureteral motility

The pyeloureteral function is to transport urine from the kidneys into the ureter toward the urinary bladder for storage until micturition. A set of mechanisms collaborates to achieve this purpose: the basic process regulating ureteral peristalsis is myogenic, initiated by active pacemaker cells located in the renal pelvis. Great emphasis has been given to hydrodynamic factors, such as urine flow rate in determining the size and pattern of urine boluses which, in turn, affect the mechanical aspects of peristaltic rhythm, rate, amplitude, and baseline pressure. Neurogenic contribution is thought to be limited to play a modulatory role in ureteral peristalsis. The myogenic theory of ureteral peristalsis can be traced back to Engelmann (1) who was able to localize the peristaltic pressure wave's origin in the renal pelvis and suggested that the ureteral contraction impulse passes from one ureteral cell to another, the whole ureter working as a functional syncitium. Recent studies of ureteral biomechanics, smooth muscle cell electrophysiology, membrane ionic currents, cytoskeletal components and pharmacophysiology much improved our understanding of the mechanism of how the urine bolus is propelled, how this process is disturbed in pathological states, and what could be done to improve it.

Unrepeatable extracellular Ca2+-dependent contractile effects of cyclopiazonic acid in rat vascular smooth muscle

Cyclopiazonic acid (CPA), a specific reversible inhibitor of Ca(2+)-pumps in sarcoplasmic reticulum, causes a slowly developing and subsequently diminishing characteristic contraction in endothelium-denuded rat vascular smooth muscle. We recently found that CPA-induced contractions were not completely repeatable in endothelium-denuded rat aorta and superior mesenteric artery. 10 microM CPA-induced contractions expressed as a percentage of 80 mM KCl-induced contraction were significantly decreased from 51.4+/-5.7% to 11.8+/-2.6% (P<0.0001) upon the second application in endothelium-denuded rat aorta, and this was not due to any irreversible cytotoxic effects of CPA. The decrease of CPA-induced contractile responses upon the second application was dependent on both types of blood vessels and doses of CPA upon the first application. CPA upon the second application in Ca(2+)-containing solutions did induce its characteristic contractions in the rings pretreated with Ca(2+)-free solutions or Ca(2+) entry blockers before and during its first application, suggesting that capacitative mode of Ca(2+) influx during the application of CPA might be responsible for the diminishment of contractions upon the second application. These data suggest that CPA by inducing a transient rise in cytosolic Ca(2+) level might cause a long-lasting upregulation of Ca(2+) extrusion across the plasma membrane in vascular smooth muscle cells and thus accelerate Ca(2+) efflux over a prolonged period, leading to unrepeatable contractile effects of CPA. Such long-lasting upregulation of Ca(2+) extrusion may contribute to the regulation of excitability of vascular smooth muscle cells and protect the cells against excitotoxic injury.

The role of Ca2+influx and intracellular Ca2+release in the muscarinic-mediated contraction of mammalian urinary bladder smooth muscle

OBJECTIVE To study the involvement of extracellular Ca2+ and the properties of the intracellular Ca2+ ([Ca2+]i) stores on the carbachol-induced contraction of mammalian urinary bladder smooth muscle strips under polarized and depolarized conditions.

MATERIALS AND METHODS

Strips of bladder were suspended between platinum ring electrodes in a cylindrical organ bath (0.2 mL) and continuously superfused with Krebs' solution at 1 mL/min. The effect of nifedipine, cyclopiazonic acid (CPA), thapsigargin, procaine, ryanodine and caffeine before and during a 10-s application of 100 microm carbachol under polarized conditions were studied. The effect of these drugs was also assessed under depolarized conditions using a protocol that allowed a more detailed assessment of the role of [Ca2+]i stores, consisting of emptying the stores by exposure to Ca2+-free solution, rapidly refilling them by a 10-s application of 81.5 mm Ca2+ (priming), returning to the Ca2+-free solution for 3 min and then applying 100 microm carbachol (10 s) in Ca2+-free solution (store release).

RESULTS

Under polarized conditions, nifedipine and Ca2+ removal almost completely inhibited the carbachol-induced contractions. CPA increased the amplitude and duration of both carbachol- and electrical field stimulation-induced contractions. Although ryanodine had no inhibitory effect, caffeine and procaine significantly inhibited the carbachol-induced contraction. Under depolarized conditions nifedipine blocked both priming and store release contractions. CPA, thapsigargin, procaine and ryanodine significantly increased the priming and inhibited the store release contractions. However, caffeine virtually abolished both priming and store release contractions.

CONCLUSION

These results suggest that in guinea-pig urinary bladder smooth muscle the Ca2+ necessary for contraction enters the cell through voltage-dependent dihydropyridine-sensitive Ca2+ channels and is pumped into an intracellular store that is released by carbachol. Under polarized conditions, the blockade of sarco-endoplasmic reticulum calcium ATP-ase (SERCA) with CPA increases [Ca2+]i and carbachol-induced contractions. The effects of caffeine and procaine suggest that store release involves ryanodine receptors and calcium-induced calcium release. Under depolarized conditions, Ca2+ entry is blocked by nifedipine and the stores diminish. Stored Ca2+ is also greatly reduced by the blockade of SERCA with either CPA or thapsigargin. Procaine, ryanodine and caffeine blocked the store release contractions, suggesting that this involves ryanodine receptors and calcium-induced calcium release.

Action potential refractory period in ureter smooth muscle is set by Ca sparks and BK channels

In excitable tissues the refractory period is a critical control mechanism preventing hyperactivity and undesirable tetani, by preventing subsequent stimuli eliciting action potentials and Ca2+ entry. In ureteric smooth muscle, peristaltic waves that occur as invading pacemaker potentials produce long-lasting action potentials (300-800 ms) and extraordinarily long (more than 10 s) refractory periods, which prevent urine reflux and kidney damage. For smooth muscles neither the mechanisms underlying the refractory period nor the link between excitability and refractoriness are properly understood. Here we show that a negative feedback process, which depends on Ca2+ loading the sarcoplasmic reticulum (SR) during the action potential and on the subsequent activation of local releases of Ca2+ from the SR (sparks), stimulating plasmalemmal Ca2+-sensitive K+ (BK) channels, determines the refractory period of the action potential. As sparks gradually reduce the Ca2+ load in the SR, electrical inhibition is released, the refractory period is terminated and peristaltic contractions occur again. The refractory period can be manipulated, for example from 10 s to 100 s, by altering the Ca2+ content of the SR or release mechanism or by inhibiting BK channels. This insight into the control of excitability and hence function provides a focus for therapies directed at pathologies of smooth muscle.

Cyclopiazonic acid decreases spontaneous transient depolarizations in guinea pig mesenteric lymphatic vessels in endothelium-dependent and -independent manners

Guinea pig mesenteric lymphatic vessels exhibit vasomotion through a pacemaker mechanism that involves intracellular Ca2+release and resultant spontaneous transient depolarizations (STDs) of the smooth muscle membrane potential. This study presents a detailed characterization of the effects of cyclopiazonic acid (CPA) on this pacemaker activity. Microelectrode recordings from smooth muscle in vessel segments revealed that application of CPA (1–10 μM) caused a hyperpolarization accompanied by a decrease in the frequency and amplitude of STDs. The CPA-induced hyperpolarization was abolished after destruction of the endothelium and in the presence of NG-nitro-l-arginine (100 μM) or 1 H-[1,2,4]oxadiazolol-[4,3- a]quinoxaline-1-one (10 μM), which suggests a contribution of endothelium-derived nitric oxide (EDNO) in this response. In the absence of EDNO-induced effects, CPA decreased the frequency and amplitude of STDs recorded before and in the presence of the thromboxane A2mimetic U-46619, norepinephrine, or thimerosal. CPA abolished U-46619-induced vasomotion as determined by measurement of constriction-associated intracellular Ca2+concentration using the ratiometric Ca2+indicator fura-2. The endothelial actions of CPA were compared with those of ACh, which is known to cause EDNO release in this preparation. Although CPA and ACh both increased endothelial intracellular Ca2+concentration and depolarized the membrane potential, the kinetics of action for both parameters were markedly slower for CPA than ACh. These results suggest that CPA first hyperpolarizes the lymphatic smooth muscle and decreases STD frequency and amplitude through endothelial release of EDNO, and second, consistent with the action of CPA to inhibit sarcoplasmic reticulum Ca2+-ATPase and deplete Ca2+stores, it further reduces STD activity. Inhibition of the lymphatic smooth muscle pacemaker mechanism is thought to abolish agonist-induced vasomotion.

Modulators of internal Ca2+ stores and the spontaneous electrical and contractile activity of the guinea-pig renal pelvis

1. The role of internal Ca(2+) stores in the generation of the rhythmic electrical and contractile activity in the guinea-pig proximal renal pelvis was examined using intracellular microelectrode and muscle tension recording techniques. 2. Ryanodine (30 microM) transiently increased contraction amplitude, while caffeine (0.5 - 3 mM) reduced contraction amplitude and frequency. Contractility was also reduced by 2-aminoethoxy-diphenylborate (2-APB 60 microM), xestospongin C (1 microM), U73122 (5 microM) and neomycin (4 mM), blockers of IP(3)-dependent release from Ca(2+) stores. 3. 60 mM K(+) saline-evoked contractions were reduced by caffeine (1 mM), U73122 (5 microM) and neomycin (4 mM), but little affected by ryanodine or 2-APB (60 microM). 4. Spontaneous action potentials consisting of an initial spike followed by a long plateau were recorded (frequency 8.6+/-1.0 min(-1)) in small urothelium-denuded strips of proximal renal pelvis. 5. Action potential discharge was blocked in 75 and 35% of cells by 2-APB (60 microM) and caffeine (1 mM), respectively. In the remaining cells, only a truncation of the plateau phase was observed. 6. Cyclopiazonic acid (CPA 10 microM for 10 - 180 min), blocker of CaATPase, transiently increased contraction frequency and amplitude. Action potential durations were increased 3.6 fold. Contraction amplitude and frequency slowly declined during a prolonged (>60 min) CPA exposure. 7. We conclude that the action potential in caffeine-sensitive cells and the shoulder component of caffeine-insensitive action potential arise from the entry of Ca(2+) through Ca(2+) channels. The inhibitory actions of modulators of internal Ca(2+) release were partially explained by a blockade of Ca(2+) entry.

Properties of a native cation channel activated by Ca2+ store depletion in vascular smooth muscle cells

Depletion of intracellular Ca(2+) stores activates capacitative Ca(2+) influx in smooth muscle cells, but the native store-operated channels that mediate such influx remain unidentified. Recently we demonstrated that calcium influx factor produced by yeast and human platelets with depleted Ca(2+) stores activates small conductance cation channels in excised membrane patches from vascular smooth muscle cells (SMC). Here we characterize these channels in intact cells and present evidence that they belong to the class of store-operated channels, which are activated upon passive depletion of Ca(2+) stores. Application of thapsigargin (TG), an inhibitor of sarco-endoplasmic reticulum Ca(2+) ATPase, to individual SMC activated single 3-pS cation channels in cell-attached membrane patches. Channels remained active when inside-out membrane patches were excised from the cells. Excision of membrane patches from resting SMC did not by itself activate the channels. Loading SMC with BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid), which slowly depletes Ca(2+) stores without a rise in intracellular Ca(2+), activated the same 3-pS channels in cell-attached membrane patches as well as whole cell nonselective cation currents in SMC. TG- and BAPTA-activated 3-pS channels were cation-selective but poorly discriminated among Ca(2+), Sr(2+), Ba(2+), Na(+), K(+), and Cs(+). Open channel probability did not change at negative membrane potentials but increased significantly at high positive potentials. Activation of 3-pS channels did not depend on intracellular Ca(2+) concentration. Neither TG nor a variety of second messengers (including Ca(2+), InsP3, InsP4, GTPgammaS, cyclic AMP, cyclic GMP, ATP, and ADP) activated 3-pS channels in inside-out membrane patches. Thus, 3-pS nonselective cation channels are present and activated by TG or BAPTA-induced depletion of intracellular Ca(2+) stores in intact SMC. These native store-operated cation channels can account for capacitative Ca(2+) influx in SMC and can play an important role in regulation of vascular tone.

Effect of lovastatin on cerebral circulation in spontaneously hypertensive rats

Statins, which are often given to hypertensive patients, reduce the incidence of stroke. However, their effects on the cerebral circulation have been scarcely studied, although lovastatin has been reported to reduce hypertension-induced renal arteriolar hypertrophy. We examined the structure and mechanics of cerebral arterioles and the lower limit of cerebral blood flow (CBF) autoregulation in spontaneously hypertensive rats (SHR) that were untreated (n=9) or treated for 1 month with lovastatin (n=12; 20 mg x kg(-1) x d(-1)) and in untreated Wistar-Kyoto rats (WKY; n=8). We studied the lower limit of CBF autoregulation by repeated measurement of CBF (arbitrary units; laser Doppler) and internal arteriolar diameter (microm; cranial window) at baseline and during stepwise hypotension. Stress-strain relationships were calculated from repeated measurement of internal arteriolar diameter during stepwise hypotension and cross-sectional area (CSA) of the vessel wall in maximally dilated cerebral arterioles (EDTA, 67 mmol/L). Lovastatin slightly reduced mean arterial pressure (treated, 153+/-3 versus untreated, 171+/-5 mm Hg, P<0.05; WKY, 106+/-3 mm Hg) and normalized CSA (treated, 826+/-52 versus untreated, 1099+/-16 microm(2), P<0. 05; WKY, 774+/-28 microm(2)). Stress-strain curves show that lovastatin also attenuated the increase in passive distensibility. Lovastatin had no effect on the external diameter of cerebral arterioles or the lower limit of CBF autoregulation. Our results show that although lovastatin has substantial effects on arteriolar mechanics and wall CSA, it has little effect on internal diameter. This phenomenon may explain its lack of effect on CBF autoregulation.

The relationship between the action potential, intracellular calcium and force in intact phasic, guinea-pig uretic smooth muscle

1. We investigated the relationship between the action potential, Ca2+ and phasic force in intact guinea-pig ureter, following physiological activation. 2. The action potential elicited a Ca2+ transient consisting of three components: a fast increment, associated with the first action potential spike, a slower increment, associated with subsequent spikes and the initial part of the plateau component, and a steady-state phase associated with the plateau. 3. Prolongation of the plateau, by agonists, prolonged the third component of the Ca2+ transient and increased force amplitude and duration. 4. The force-Ca2+ relationship during phasic contractions showed hysteresis; more force was produced as Ca2+ declined than when it rose. Paired pulse stimuli suggested that the delay between Ca2+ and force was not due to mechanical properties. Wortmannin, which has been shown to selectively inhibit force and myosin light chain (MLC) phosphorylation in the guinea-pig ureter, did not affect electrical activity or Ca2+ but significantly increased the delay, suggesting that myosin phosphorylation is a major contributor to it. 5. Prolongation of the duration of the [Ca2+]i transient, at unchanged amplitude, increased force. The rise of [Ca2+]i did not limit the rate of contraction. Slowing of the rate of [Ca2+]i rise abolished the hysteresis between Ca2+ and force. 6. Cooling reduced force, increased the delay and hysteresis between Ca2+ and force, but did not affect the rate of rise of Ca2+. The reduction in force could be compensated, by increasing the duration of the Ca2+ transient. 7. We suggest that in vivo, steady-state force-Ca2+ relationships are not applicable in phasic smooth muscles. Furthermore, agonists increase force mainly by prolonging the action potential, which increases the duration of the [Ca2+] signal.

The effect of cyclopiazonic acid on excitation-contraction coupling in guinea-pig ureteric smooth muscle: role of the sarcoplasmic reticulum

1. We have investigated the effect of cyclopiazonic acid (CPA), an inhibitor of the sarcoplasmic reticulum (SR) Ca2+-ATPase on excitation-contraction (EC) coupling in guinea-pig ureter, by measuring membrane currents, action potentials, intracellular [Ca2+] and force. 2. CPA (20 micrometers) significantly enhanced the amplitude and duration of phasic contractions of ureteric smooth muscle associated with action potentials. This was accompanied by an increase in the duration of the intracellular Ca2+ transient in intact tissue and single cells but not their amplitude. However, CPA also slowed the rate of rise, and fall, of the force 1|1|Phiand1Phi Ca2+ transients. 3. Membrane potential recordings showed that CPA produced a small depolarization and a large increase in the duration of the plateau phase of the action potential. 4. Patch-clamp studies showed marked inhibition of outward potassium current in the presence of CPA and an inhibition of spontaneous transient outward currents (STOCs). CPA had no effect on inward Ca2+ current. 5. These data suggest that the SR plays a major role in modulating the excitability of the ureter, particularly via curtailing the action potential duration. This in turn will shorten the Ca2+ transient and decrease force. This negative action on developed force predominates over any small role it may play in initiating force in the guinea-pig ureter.

Exogenous reactive oxygen and nitric oxide alter intracellular oxidant status of skeletal muscle fibres

To test whether exogenous oxidants alter intracellular oxidant levels in skeletal muscle fibres, we exposed rat diaphragm to donors of nitric oxide (NOx), reactive oxygen species (ROS) or hyperoxia, and monitored intracellular oxidant levels using a fluorescent probe. Fibre bundles were dissected from the diaphragm and loaded with 2', 7'-dichlorodihydrofluorescein (DCFH); emissions were monitored using a fluorescence microscope. DCFH-loaded muscles were exposed to either a NOx donor (1 mM S-nitroso-N-acetyl penicillamine, SNAP; 1 mM sodium nitroprusside, SNP; 400 microM 1-hydroxy-2-oxo-3-(N-3-methyl-aminopropyl)-3-methyl-1-triazen, NOC-7), an ROS donor (100 microM hydrogen peroxide, H2O2; 100 microM tert-butyl hydroperoxide; 1 mM hypoxanthine plus 0.01 U mL-1 xanthine oxidase, HXXO) or a range of PO2s (25, 60 or 95% O2 oxygenating Krebs-Ringer solution) for 40 min; time-matched control bundles remained in Krebs-Ringer solution. Control muscles oxidized DCFH at a rate of 0.32 +/- 0.1 greyscale units min-1. SNAP (766%), SNP (1244%), NOC-7 (851%), H2O2 (543%), and HXXO (541%) increased DCFH oxidation from control levels. The increase in emissions caused by NOC-7 and SNP were blunted by the NOx scavenger haemoglobin (1 microM). DCFH oxidation by HXXO was unaffected by 1000 U mL-1 superoxide dismutase but was significantly decreased by 1000 U mL-1 catalase and 1 mM salicylate. PO2 had no effect on intracellular oxidant levels. Therefore, extracellular NOx and ROS can alter intracellular oxidant status in skeletal muscle fibres. These observations suggest that intrafibre oxidant levels could be the result of both intracellular and extracellular oxidant production.

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.

Excitatory motor and electrical effects produced by tachykinins in the human and guinea-pig isolated ureter and guinea-pig renal pelvis

1. In isolated tissue experiments, neurokinin A (NKA) produced concentration-dependent contraction of human and guinea-pig ureter (pD2 = 6.7 and 7.2, respectively); an effect greatly reduced (>80% inhibition) by the tachykinin NK2 receptor-selective antagonist MEN 11420 (0.1 microM). The tachykinin NK1 and NK3 receptor agonists septide and senktide, respectively, were ineffective. 2. Electrical field stimulation (EFS) of the guinea-pig isolated renal pelvis produced an inotropic response blocked by MEN 11420 (0.01-1 microM). In the same preparation MEN 11420 (0.1 microM) blocked (apparent pK(B) = 8.2) the potentiation of spontaneous motor activity produced by the NK2 receptor-selective agonist [betaAla8]NKA(4-10). 3. In sucrose-gap experiments, EFS evoked action potentials (APs) accompanied by phasic contractions of human and guinea-pig ureter, which were unaffected by tetrodotoxin or MEN 11420 (3 microM), but were blocked by nifedipine (1-10 microM). NKA (1-3 microM) produced a slow membrane depolarization with superimposed APs and a tonic contraction with superimposed phasic contractions. NKA prolonged the duration of EFS-evoked APs and potentiated the accompanying contractions. MEN 11420 completely prevented the responses to NKA in both the human and guinea-pig ureter. 4. Nifedipine (1-10 microM) suppressed the NKA-evoked APs and phasic contractions in both human and guinea-pig ureter, and slightly reduced the membrane depolarization induced by NKA. A tonic-type contraction of the human ureter in response to NKA persisted in the presence of nifedipine. 5. In conclusion, tachykinins produce smooth muscle excitation in both human and guinea-pig ureter by stimulating receptors of the NK2 type only. NK2 receptor activation depolarizes the membrane to trigger the firing of APs from latent pacemakers.

Possible mechanism underlying the potent vasoconstrictor actions of cyclopiazonic acid on dog cerebral arteries

A sustained Ca2+ influx via L-type Ca2+ channels has been shown in the resting state of dog cerebral arteries. Sarcoplasmic reticulum is now recognized to serve as a buffer barrier to Ca2+ entry in vascular smooth muscle cells. To clarify whether sarcoplasmic reticulum of the cerebral arteries can buffer the sustained Ca2+ influx, effects of cyclopiazonic acid (CPA), an inhibitor of sarcoplasmic reticulum Ca2+-ATPase, were determined in endothelium-denuded strips of the cerebral (basilar, posterior communicating, middle cerebral), mesenteric and coronary arteries of the dog. The addition of CPA (0.1-10 microM) during the resting state of the strips caused a concentration-dependent contraction in the three cerebral arteries. The CPA-induced contraction was extremely small in the mesenteric or coronary artery. The CPA-induced contractions in the cerebral arteries were inhibited concentration-dependently by nifedipine (1-100 nM). Nifedipine itself induced relaxation from the resting state of cerebral arteries, suggesting a maintenance of basal tone. The CPA-induced potent contraction seen in the cerebral arteries could be mimicked in the mesenteric artery by elevating the extracellular K+ concentration (14.9 mM) or adding Bay K 8644 [methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-pyr idine-5-carboxylate] (100 nM) to produce an increase in Ca2+ influx via L-type Ca2+ channels. We conclude that, in the resting state of dog cerebral arteries, (1) the greater part of the sustained Ca2+ influx is buffered by Ca2+ uptake into the sarcoplasmic reticulum, (2) therefore, the inhibition of sarcoplasmic reticulum Ca2+-ATPase by CPA causes a potent contraction, and (3) the maintenance of basal tone suggests that some Ca2+ that entered via L-type Ca2+ channels always reaches the myofilaments in the resting state.

Electromechanical coupling in human vas deferens: effects of agents that modulate intracellular release of calcium

1. The effects of ryanodine, cyclopiazonic acid (CPA) and caffeine on electromechanical coupling in human vas deferens were investigated. 2. High [K+]o (120 mM) evoked nifedipine-sensitive contractions of longitudinal and circular muscle which consisted of initial and secondary components. 3. Exposures to ryanodine (< or =10 microM) or CPA (< or = 3 microM) induced a change of basal tension, and higher doses (30 microM) induced intermittent rhythmic contractions of both muscle types in the quiescent tissue. In the presence of the drugs, contraction to high [K+]o was preceded by marked rhythmic activity. 4. In circular muscle, ryanodine (1-30 microM) or CPA (1-30 microM) reduced both components of contractions to high [K+]o. In longitudinal muscle, the drugs enhanced the initial component and prolonged the secondary component. High doses (> or = 10 microM) produced variable effects on the initial component. 5. Caffeine (20 mM) reliably contracted longitudinal, but not circular muscle. Pre-exposures to caffeine enhanced both components in the post-caffeine contractions of circular muscle to high [K+]o. In longitudinal muscle, only the initial component (post-caffeine) was enhanced. 6. Contractions evoked in longitudinal muscle by caffeine were not blocked by ryanodine (30 microM) or CPA (30 microM). However, the enhancement of post-caffeine contractions to high [K+]o was inhibited. 7. These results show that ryanodine and CPA produced comparable effects on the excitability of longitudinal and circular muscle in the quiescent tissue, but electromechanical coupling was affected differently. The findings suggest that the muscle types utilize different mechanisms to regulate elevations in cytosolic Ca2+ during stimulation. 8. Electromechanical coupling in both muscle types involves Ca2+ influx via nifedipine-sensitive voltage-operated calcium channels and activation of ryanodine-sensitive calcium-induced calcium release from the sarcoplasmic reticulum (SR). In longitudinal muscle, the SR also buffers increases in cytosolic Ca2+ via a pharmacologically distinct Ca2+ compartment (caffeine releasable but ryanodine/CPA-insensitive). In circular muscle, the SR (ryanodine/CPA-sensitive) serves mainly in the regulation of excitability of the quiescent tissue.

Cyclopiazonic acid-induced changes in the contraction and Ca2+ transient of frog fast-twitch skeletal muscle

The effects of cyclopiazonic acid (CPA) were investigated on isolated skeletal muscle fibers of frog semitendinosus muscle. CPA (0.5-10 microM) enhanced isometric twitch but produced little change in resting tension. At higher concentrations (10-50 microM), CPA depressed twitch and induced sustained contracture without affecting resting and action potentials. In Triton-skinned fibers, CPA had no significant effect on myofibrillar Ca2+ sensitivity but decreased maximal activated force at concentrations > 5 microM. In intact cells loaded with the Ca2+ fluorescence indicator indo 1, CPA (2 microM) induced an increase in Ca(2+)-transient amplitude (10 +/- 2.5%), which was associated with an increase in time to peak and in the time constant of decay. Consequently, peak force was increased by 35 +/- 4%, and both time to peak and the time constant of relaxation were prolonged. It is concluded that CPA effects, at a concentration of up to 2 microM, were associated with specific inhibition of sarcoplasmic reticulum Ca(2+)-adenosinetriphosphatase in intact skeletal muscle and that inhibition of the pump directly affected the handling of intracellular Ca2+ and force production.

Actions of 4-chloro-3-ethyl phenol on internal Ca2+ stores in vascular smooth muscle and endothelial cells

1. Recently, 4-chloro-3-ethyl phenol (CEP) has been shown to cause the release of internally stored Ca2+ apparently through ryanodine-sensitive Ca2+ channels, in fractionated skeletal muscle terminal cisternae and in a variety of non-excitable cell types. Its action on smooth muscle is unknown. In this study, we characterized the actions of CEP on vascular contraction in endothelium-denuded dog mesenteric artery. We also determined its ability to release Ca2+, by use of Ca2+ imaging techniques, on dog isolated mesenteric artery smooth muscle cells and on bovine cultured pulmonary artery endothelial cells. 2. After phenylephrine-(PE, 10 microM) sensitive Ca2+ stores were depleted by maximal PE stimulation in Ca2+-free medium, the action of CEP on refilling of the emptied PE stores was tested, by first pre-incubating the endothelium-denuded artery in CEP for 15 min before Ca2+ was restored for a 30 min refilling period. At the end of this period, Ca2+ and CEP were removed, and the arterial ring was tested again with PE to assess the degree of refilling of the internal Ca2+ store. 3. In a concentration-dependent manner (30, 100 and 300 microM), CEP significantly reduced the size of the post-refilling PE contraction (49.4, 28.9 and 5.7% of control, respectively) in Ca2+-free media. This suggests that Ca2+ levels are reduced in the internal stores by CEP treatment. CEP alone did not cause any contraction either in Ca2+-containing or Ca2+-free Krebs solution. 4. Restoring Ca2+ in the presence of PE caused a large contraction, which reflects PE-induced influx of extracellular Ca2+. The contraction of tissues pretreated with 300 microM CEP was significantly less compared with controls. However, tissues pretreated with 30 and 100 microM CEP were unaffected. Washout of CEP over 30 min produced complete recovery of responses to PE in Ca2+-free and Ca2+-containing medium suggesting a rapid reversal of CEP effects. 5. Concentration-response curves were constructed for PE, 5-hydroxytryptamine (5-HT) and K+ in the absence of and after 30 min pre-incubation with 30, 100 and 300 microM CEP. In all cases, CEP caused a concentration-dependent depression of the maximum response to PE (84.8, 43.4 and 11.6% of control), 5-HT (65.4, 25.7 and 6.9% of control) and K+ (77.6, 41.1 and 10.8% of control). 6. Some arterial rings were pre-incubated with ryanodine (30 microM) for 30 min before the construction of PE concentration-response curves. In Ca2+-free Krebs solution, ryanodine alone did not cause any contraction. However, 58% (11 out of 19) of the tissues tested with ryanodine developed contraction (6.9+/-1.2% of 100 mM K+ contraction, n=11) in the presence of external Ca2+. EC50 values for PE in ryanodine-treated tissues (1.7+/-0.25 microM, n=16) were not significantly different from controls (2.5+/-0.41 microM, n=22). Maximum contractions to PE (118.5+/-4.4% of 100 mM K+ contraction, n=16) were also unaffected by ryanodine when compared to controls (129+/-4.2%, n=23). 7. When fura-2 loaded smooth muscle cells (n=13) and endothelial cells (n=27) were imaged for Ca2+ distribution, it was observed that 100 and 300 microM CEP in Ca2+-free medium caused Ca2+ release in both cell types. Smooth muscle cells showed a small decrease in cell length. Addition of EGTA (5 mM) reversed the effect of CEP on intracellular Ca2+ to control values. 8. These data show, for the first time in vascular smooth muscle and endothelial cells, that CEP releases Ca2+ more rapidly than ryanodine. Unlike ryanodine, CEP caused no basal contraction but depressed contractions to PE, 5-HT and K+. The lack of basal contraction may result from altered responsiveness of the contractile system to intracellular Ca2+ elevation.

Cyclopiazonic acid-induced changes in contractile activity of smooth muscle strips isolated from cat and guinea-pig stomach

The effects of cyclopiazonic acid (CPA), a specific inhibitor of sarcoplasmic reticulum Ca(2+)-ATPase, on contractile activity of circular smooth muscle strips isolated from the antrum, corpus and fundus regions of the cat and guinea-pig stomach were studied. Contractile activity was recorded under isometric conditions, in organ baths. CPA, concentration dependently (3 x 10(-7)-3 x 10(-5) M) increased the tone of the cat and guinea-pig gastric fundus and corpus as well as the amplitude of the phasic contractions of the cat corpus and antrum, affecting their frequency. CPA had a dual action on the phasic contractions of the guinea-pig antrum: an increase at low concentrations (up to 10(-6) M) and inhibition at high concentrations (10(-6)-3 x 10(-5) M). Tetrodotoxin (10(-6) M), atropine (10(-6) M) and N omega-nitro-L-arginine (10(-4) M) did not change significantly the effects of CPA. Nifedipine completely inhibited the CPA-induced phasic contractions and partly inhibited the CPA-induced tonic contractions. The nitric oxide-releasing agents, sodium nitroprusside (10(-3) M) and 3-morpholino-sydnonimine (10(-3) M), completely inhibited the CPA-induced tonic and phasic contractions. CPA induced tonic contractions in the cat and guinea-pig gastric fundus precontracted by acetylcholine (10(-5) M) and inhibited the acetylcholine (10(-6) M)-induced phasic contractions in the guinea-pig gastric antrum and corpus. The results suggest multiple roles for sarcoplasmic reticulum Ca2+ stores and sarcoplasmic reticulum Ca(2+)-ATPase in the shaping of spontaneous and evoked tonic and phasic contractions of the stomach, and highlight important species and tissue differences.

Induction of hippocampal long-term depression requires release of Ca2+ from separate presynaptic and postsynaptic intracellular stores

Studies have suggested that an increase in intracellular [Ca2+] is necessary for the induction of both long-term potentiation (LTP) and long-term depression (LTD) of synaptic transmission, and that release of Ca2+ from intracellular storage pools can be necessary to induce LTP. We investigated whether release of Ca2+ from intracellular stores also is required for the induction of LTD at Schaffer collateral-CA1 synapses in hippocampal slices. Both thapsigargin (1 microM) and cyclopiazonic acid (1 microM), compounds that deplete all intracellular Ca2+ pools by blocking LTP-dependent Ca2+ uptake into intracellular compartments, blocked the induction, but not maintenance, of LTD by low-frequency stimulation (LFS) (1 Hz/15 min) without affecting baseline synaptic transmission. Washout of the reversible inhibitor cyclopiazonic acid restored the ability to induce LTD. In contrast, thapsigargin did not block depotentiation of LTP by 1 Hz LFS, suggesting that LTP causes a reduction in the threshold [Ca2+] necessary for LTD. Selective depletion of the ryanodine receptor-gated Ca2+ pool by bath application of ryanodine (10 microM) also blocked the induction of LTD, indicating a requirement for Ca(2+)-induced Ca2+ release. Impalement of CA1 pyramidal neurons with microelectrodes containing thapsigargin (500 nM to 200 microM) prevented the induction of LTD at synapses on that neuron without blocking LTD in the rest of the slice. In contrast, similar filling of CA1 pyramidal neurons with ryanodine (2 microM to 5 mM) did not block the induction of LTD. From these data, we conclude that the induction of LTD requires release of Ca2+ both from a presynaptic ryanodine-sensitive pool and from postsynaptic (presumably IP3-gated) stores.

Potent vasoconstrictor actions of cyclopiazonic acid and thapsigargin on femoral arteries from spontaneously hypertensive rats

1. Ca2+ buffering function of sarcoplasmic reticulum (SR) in the resting state of arteries from spontaneously hypertensive rats (SHR) was examined. Differences in the effects of cyclopiazonic acid (CPA) and thapsigargin, agents which inhibit the Ca(2+)-ATPase of SR, on tension and cellular Ca2+ level were assessed in endothelium-denuded strips of femoral arteries from 13-week-old SHR and normotensive Wistar-Kyoto rats (WKY). 2. In resting strips preloaded with fura-PE3, the addition of CPA (10 microM) or thapsigargin (100 nM) caused an elevation of cytosolic Ca2+ level ([Ca2+]i) and a contraction. These responses were significantly greater in SHR than in WKY. 3. The additional of verapamil (3 microM) to the resting strips caused a decrease in resting [Ca2+]i, which was significantly greater in SHR than in WKY. In SHR, but not in WKY, this decrease was accompanied by a relaxation from the resting tone, suggesting the maintenance of myogenic tone in the SHR artery. 4. Verapamil (3 microM) abolished differences between SHR and WKY. The effects of verapamil were much greater on the contraction than on the [Ca2+]i. 5. The resting of Ca2+ influx in arteries measured after a 5 min incubation of the artery with 45Ca was not increased by CPA or thapsigargin in either SHR or WKY. The net Ca2+ entry measured after a 30 min incubation of the artery with 45Ca was decreased by CPA or thapsigargin in both SHR and WKY. The resting Ca2+ influx was significantly higher in SHR than in WKY, and was decreased by nifedipine (100 nM) in the SHR artery, but was unchanged in the WKY artery. 6. The resting 45Ca efflux from the artery was increased during the addition of CPA (10 microM). This increase was less in SHR than in WKY. The resting 45Ca efflux was the same in SHR and WKY. 7. These results suggest that (1) the Ca2+ influx via L-type voltage-dependent Ca2+ channels (VDCCs) was increased in the resting state of the SHR femoral artery, (2) the greater part of the increased Ca2+ influx was buffered by Ca2+ uptake into the SR and some Ca2+ reached the myofilaments resulting in the maintenance of the myogenic tone, and (3) therefore the functional elimination of SR by CPA or thapsigargin caused a large elevation of [Ca2+]i and a potent contraction in this artery. During this process, the contraction was mainly due to the basal Ca2+ influx via L-type VDCCs. The present study also showed the existence of a relatively large compartment of [Ca2+]i which does not contribute to the contraction during the addition of CPA or thapsigargin.

Induction of hippocampal long-term depression requires release of Ca2+ from separate presynaptic and postsynaptic intracellular stores

Studies have suggested that an increase in intracellular [Ca2+] is necessary for the induction of both long-term potentiation (LTP) and long-term depression (LTD) of synaptic transmission, and that release of Ca2+ from intracellular storage pools can be necessary to induce LTP. We investigated whether release of Ca2+ from intracellular stores also is required for the induction of LTD at Schaffer collateral-CA1 synapses in hippocampal slices. Both thapsigargin (1 microM) and cyclopiazonic acid (1 microM), compounds that deplete all intracellular Ca2+ pools by blocking LTP-dependent Ca2+ uptake into intracellular compartments, blocked the induction, but not maintenance, of LTD by low-frequency stimulation (LFS) (1 Hz/15 min) without affecting baseline synaptic transmission. Washout of the reversible inhibitor cyclopiazonic acid restored the ability to induce LTD. In contrast, thapsigargin did not block depotentiation of LTP by 1 Hz LFS, suggesting that LTP causes a reduction in the threshold [Ca2+] necessary for LTD. Selective depletion of the ryanodine receptor-gated Ca2+ pool by bath application of ryanodine (10 microM) also blocked the induction of LTD, indicating a requirement for Ca(2+)-induced Ca2+ release. Impalement of CA1 pyramidal neurons with microelectrodes containing thapsigargin (500 nM to 200 microM) prevented the induction of LTD at synapses on that neuron without blocking LTD in the rest of the slice. In contrast, similar filling of CA1 pyramidal neurons with ryanodine (2 microM to 5 mM) did not block the induction of LTD. From these data, we conclude that the induction of LTD requires release of Ca2+ both from a presynaptic ryanodine-sensitive pool and from postsynaptic (presumably IP3-gated) stores.

Role of intracellular Ca2+ in the K channel opener action of CGRP in the guinea-pig ureter

1. The aim of this study was to assess the role of sarcoplasmic reticulum (SR) calcium (Ca2+) in the smooth muscle relaxant and hyperpolarizing actions of calcitonin gene-related peptide (CGRP) in the guinea-pig ureter. 2. CGRP (0.1 microM) rapidly and transiently reduced myogenic phasic contractions (twitches) produced by electrical field stimulation (EFS). Approximately 70% of the response to CGRP was antagonized by glibenclamide (1 microM). 3. Cyclopiazonic acid (CPA, 10 microM), ryanodine (100 microM) and thapsigargin (1 microM) reduced only the glibenclamide-sensitive component of the response to CGRP (0.1 microM) but did not modify the mechano-inhibitory effect of cromakalim (3 microM). A low concentration of CPA (1 microM), assumed to produce a limited impairment of Ca2+ uptake from the stores, prolonged the duration of the inhibitory response to CGRP. Pre-exposure to caffeine (5 mM) inhibited the suppression of twitches by CGRP or cromakalim. 4. When the frequency of EFS was increased, the suppression of twitches by CGRP was reduced. Under these conditions, CPA (1 microM) again prolonged the duration of the inhibitory response to CGRP. 5. CGRP (0.1 microM) and cromakalim (3 microM) markedly depressed the phasic component of contractions to 80 mM KCl. CPA (10 microM) antagonized the inhibitory effect of CGRP but not that of cromakalim. Inhibition of the tonic contraction to 80 mM KCl by CGRP was insensitive to CPA. 6. In sucrose gap experiments, a 5 min exposure to CGRP (0.1 microM) or cromakalim (3 microM) produced a sustained membrane hyperpolarization. Caffeine (5 mM) produced a glibenclamide-sensitive transient hyperpolarization followed by a sustained depolarization. When tested in a Ca(2+)-free medium the hyperpolarization produced by CGRP, cromakalim or caffeine was reduced. In normal Krebs, pre-exposure to CPA (10 microM, 60 min) only abolished the hyperpolarization induced by CGRP. In contrast, 5 min after a caffeine challenge (5 mM) the hyperpolarizations induced by CGRP or cromakalim were reduced. The CGRP-induced hyperpolarization was insensitive to apamin (0.1 microM) or charybdotoxin (0.1 microM). 7. We conclude that the K channel-opening action of CGRP in the guinea-pig ureter requires the mobilization of intracellular Ca2+ from a caffeine- and CPA-sensitive store, leading to transient activation of glibenclamide-sensitive K channels. The K channel-opening action of caffeine appears to involve Ca2+ mobilization from a store which is insensitive to depletion by CPA.

An investigation into the mechanism whereby pH affects tension in guinea-pig ureteric smooth muscle

1. We have altered intracellular (pHi) and extracellular pH (pHo) in the smooth muscle of guinea-pig ureter and determined the effects on evoked phasic contractions. In order to investigate the mechanisms underlying the effects of pH alteration, intracellular Ca2+ ([Ca2+]i), pHi, electrical activity and force were measured. 2. Intracellular acidification, produced by the weak acid butyrate, application of CO2 at constant pHo or removal of weak bases, greatly increased phasic contractions. Alkalinization with weak bases or by removal of CO2 inhibited contractions. The results were similar whether Hepes or CO2-HCO3-buffered the solutions. 3. Phasic contractions were preceded by intracellular Ca2+ transients in the ureter. Acidification of the cytoplasm led to an increase in the amplitude of the Ca2+ transient, and alkalinization decreased its magnitude. 4. In the ureter the action potential leads to Ca2+ influx, therefore electrophysiological recordings of its configuration were made during alteration of pHi. Acidification led to the action potential duration and amplitude being increased, whereas alkalinization shortened the action potential and reduced its amplitude. 5. As the effects of acidification on the action potential resembled the effects of blocking of K+ channels, we investigated whether pHi alteration was able to alter tension when K+ channels were blocked by tetraethylammonium. Acidification was unable to potentiate force under these conditions nor did alkalinization decrease force. 6. External pH over the range 6.8-8.0 had little or no effect on pHi, phasic contractions and [Ca2+]i. Tonic contractions were enhanced, however, when pHo was increased. 7. These data suggest that pHi alteration in the guinea-pig ureter modulates the action potential, probably by alteration of K+ currents. Subsequent changes in [Ca2+]i and contraction then occur. A potentiating effect of acidic pH on force is not common in muscle, but may be a characteristic of the smooth muscle of the urinary tract. Changes of pHo had little effect on phasic force or pHi, but modulated tonic contractions. The possible physiological significance of these results is discussed.

Effects of cyclopiazonic acid and thapsigargin on electromechanical activities and intracellular Ca2+ in smooth muscle of carotid artery of hypertensive rats

1. The effects of cyclopiazonic acid (CPA) and thapsigargin (TG), both of which are known to inhibit sarcoplasmic reticular Ca(2+)-ATPase, on the mechanical activities, intracellular Ca2+ level and electrical activities of smooth muscle of the carotid artery of stroke-prone spontaneously hypertensive rats (SHRSP) and Wistar Kyoto rats (WKY) were compared. 2. Both CPA and TG induced elevation of tension of the smooth muscle, which was composed of a phasic and a tonic component. The level of tension attained, especially the tonic component, was greater in the preparation from SHRSP. 3. The elevation of tension was associated with an increased intracellular Ca2+ level. Both the elevation of tension and the increase in intracellular Ca2+ were diminished by the removal of extracellular Ca2+ or by the application of verapamil. 4. The resting membrane potential of the preparations from SHRSP were depolarized to a greater extent than those from WKY.CPA depolarized the smooth muscle from both SHRSP and WKY, and the final level was also more depolarized in the preparation from SHRSP. 5. These results indicate that the elevation of tension induced by these drugs is mainly due to increased Ca2+ influx through voltage-dependent Ca2+ channels, and the difference in the action between the preparation from SHRSP and that from WKY can be explained mainly by the changes in the channels. 6. Thus, differences in the action of these drugs on the tension of smooth muscle between preparations from WKY and SHRSP can mainly be explained by the difference in the membrane potential which is related to the difference in voltage-dependent Ca2+ influx.

The role of sarcoplasmic reticulum and sarcoplasmic reticulum Ca2+-ATPase in the smooth muscle tone of the cat gastric fundus

Circular smooth muscle strips isolated from cat gastric fundus were studied in order to understand whether the sarcoplasmic reticulum (SR) and SR Ca2+-ATPase could play a role in the regulation of the muscle tone. Cyclopiazonic acid (CPA), a specific inhibitor of SR Ca2+-ATPase, caused a significant and sustained increase in muscle tone, depending on the presence of extracellular Ca2+. Nifedipine and cinnarizin only partially suppressed the CPA-induced tonic contraction. Bay K 8644 antagonized the relaxant effect of nifedipine in CPA-contracted fundus. Nitric-oxide-releasing agents sodium nitroprusside and 3-morpholino-sydnonimine completely suppressed the CPA-induced tonic contraction. The blockers of Ca2+-activated K+ channels, tetraethylammonium, charybdotoxin and/or apamin, decreased the contractile effect of CPA. Vanadate increased the tone but did not change significantly the effect of CPA. CPA exerted its contractile effect even when Ca2+ influx was triggered through the Na+/Ca2+ exchanger and the other Ca2+ entry pathways were blocked. Thapsigargin, another specific SR Ca2+-ATPase inhibitor, also increased the muscle tone. The effect of thapsigargin was completely suppressed by sodium nitroprusside and 3-morpholino-sydnonimine and partially by nifedipine. In conclusion, under conditions when the SR Ca2+-ATPase is inhibited, the tissue develops a strong tonic contraction and a large part of this is mediated by Ca2+ influx presumably via nifedipine-sensitive Ca2+ channels. This study suggests the important role of SR Ca2+-ATPase in the modulation of the muscle tone and the function of SR as a "buffer barrier" to Ca2+ entry in the cat gastric fundus smooth muscle.

Protein kinase A inhibitors selectively inhibit the tonic contraction of the guinea pig ureter to high potassium

1. We have investigated the effect of various protein kinase A (PKA) inhibitors on the phasic and tonic components of the response to potassium chloride (KCl) in the guinea pig ureter. All experiments were performed in ureters pretreated with capsaicin (10 microM for 15 min) to prevent the release of sensory neuropeptides and in the presence of 1 microM Bay K 8644 to maximize calcium (Ca) entry via voltage-sensitive channels. The addition of 80 mM hypertonic KCl produced maximal shortening of the ureter with distinct phasic and tonic components, the latter further showing a transient and a sustained component. Nifedipine (30 microM for 120 min) totally abolished all the responses to KCl. 2. The selective PKA inhibitor, H89 (10 microM), abolished the tonic response to KCl in about 30 min with minor inhibitory effect on the phasic contraction. This pattern was unchanged when extending the contact time to 120 min. When added 30 min before the next challenge, H89 (1-30 microM) concentration-dependently inhibited the responses to KCl with a preferential inhibitory effect on the tonic contraction. Another PKA inhibitor, H8, produced similar effects at tenfold higher concentrations (10-300 microM) than H89, consistent with the known potency ratio of these isoquinoline derivatives in inhibiting PKA. 3. The potent and nonselective protein kinase inhibitor, staurosporine (10-100 nM) produced an even depression of the various phases of the response to KCl. The selective protein kinase G inhibitor, KT 5823 (10 microM for 60 min) produced only a slight reduction of the sustained tonic response to KCl. The selective protein kinase C inhibitor GF 109,203X (1-3 microM) and the cAMP analog, Rp-cAMPS (300 microM for 60 min) had no effect on the three components of the response to KCl. 4. In the presence of Bay K 8644, electrical field stimulation (10 Hz for 1 sec, 60 V, pulse width 5 ms) produces direct myogenic phasic contractions (twitches) of the ureter which are suppressed by nifedipine (10-30 microM). H8 (up to 30 microM) and H89 (up to 300 microM) had minor effect on the amplitude of twitches, consistent with their poor inhibitory activity on the phasic responses to KCl. 5. In sucrose gap, superfusion with 80 mM hypertonic KCl produced action potentials followed by a sustained depolarization of the membrane: the two electrical responses underlie the phasic and tonic components of contraction to KCl, respectively. H89 (10 microM for 30 min) did not affect the resting membrane potential nor the KCl-evoked action potentials and sustained depolarization. H89 had no effect on the phasic contraction to KCl but markedly depressed (about 65% inhibition) the tonic contraction. 6. The present findings are consistent with the view that phosphorylation by PKA increases the availability of L-type Ca channels in the ureter smooth muscle. Blockade of PKA dissociates the electromechanical coupling between the sustained membrane depolarization produced by KCl and the corresponding sustained increase in tension. The L-type Ca channel responsible for generating action potentials and phasic contractions to KCl are less sensitive to PKA inhibitors than those responsible for the tonic contraction.

Calcitonin gene-related peptide (CGRP) in the circular muscle of guinea-pig colon: role as inhibitory transmitter and mechanisms of relaxation

In the presence of 1 microM tetrodotoxin (TTX), human alpha calcitonin gene-related peptide (CGRP) produced a concentration-dependent relaxation (EC50 1.1 nM; Emax 86% of the relaxation to 1 microM isoprenaline) of mucosa-free circular muscle strips from the guinea-pig proximal colon. In the presence of TTX, the C-terminal fragment CGRP(8-37) produced a concentration (0.3-3 microM)-dependent rightward shift of the curve to CGRP. The TTX-resistant, receptor-mediated, CGRP-induced relaxation was unaffected by apamin (0.3 microM) and L-nitroarginine (L-NOARG, 100 microM), alone or in combination, as well as by glibenclamide (3 microM) or (S)-ketoprofen (10 microM). Tetraethylammonium (TEA, 1-10 mM) and cyclopiazonic acid (CPA, 3-10 microM) produced a concentration-dependent partial inhibition of the relaxant response to CGRP. The inhibitory effect of TEA on the maximal relaxation produced by CGRP was prevented by nifedipine (1 microM) which did not affect the CGRP-relaxation of its own. In the presence of atropine (1 microM), guanethidine (3 microM), SR 140,333 (0.3 microM), MEN 10,627 (1 microM), apamin (0.3 microM) and L-NOARG (100 microM), the application of 1 microM capsaicin produced a transient relaxation of the strips. This response was not reproduced upon a second application of capsaicin, 60 min later, indicating complete desensitization. CGRP(8-37) (0.3-1.0 microM) produced a partial inhibitory effect (about 50% inhibition) of the relaxant response to capsaicin. In the presence of atropine (1 microM), guanethidine (3 microM), SR 140,333 (0.3 microM), MEN 10,627 (1 microM), apamin (0.3 microM), L-NOARG (100 microM) and after capsaicin in vitro pretreatment (10 microM for 15 min), electrical field stimulation (EFS, 10 Hz for 5 s) produced a transient relaxation which was unchanged by CGRP(8-37) (1 microM) while being abolished by TTX. In sucrose gap, brief superfusion with 0.3 microM CGRP produced a TTX (1 microM)- resistant membrane hyperpolarization and relaxation: the hyperpolarization produced by CGRP was inhibited by about 50% by either TEA (10 mM) or CPA (10 microM), while being unaffected by glibenclamide (3 microM). The combined application of TEA and CPA was not more effective (65% inhibition) in inhibiting the CGRP-induced hyperpolarization than each drug alone. We conclude that CGRP produces a direct relaxation of the circular muscle of the guinea-pig proximal colon by activating receptors sensitive to blockade by CGRP(8-37). Activation of Ca-dependent potassium channels and Ca release/reuptake from internal store(s) appear both to be involved in the action of CGRP. Endogenous CGRP mediates part of the relaxant response evoked by stimulation of capsaicin-sensitive primary afferent nerves in the circular muscle of guinea-pig colon, while it is not involved in the apamin and L-NOARG-resistant nonadrenergic noncholinergic (NANC) relaxation produced by electrical field stimulation of intrinsic inhibitory nerves.

CGRP inhibition of electromechanical coupling in the guinea-pig isolated renal pelvis

We aimed at studying the mechanism(s) of the inhibitory effect exerted by calcitonin gene-related peptide (CGRP) on the spontaneous activity of the guinea-pig isolated renal pelvis. In organ bath experiments, CGRP (1-100 nM) produced a concentration-dependent (EC50 8 nM) partial inhibition (Emax about 35% inhibition of motility index) of spontaneous contractions. The potassium (K) channel opener, cromakalim (3-10 microM) promptly suppressed the spontaneous contractions in a glibenclamide-(10 microM) sensitive manner. Glibenclamide (10 microM) did not affect the inhibitory action of CGRP. The calcium (Ca) channel agonist, Bay K 8644 (1 microM), markedly enhanced the spontaneous activity of the renal pelvis and reduced the inhibitory effect of CGRP. The protein kinase A inhibitors Rp-cAMPS (300 microM), H8 (100 microM) and H89 (10 microM), and the blockers of intracellular Ca handling by sarcoplasmic reticulum, ryanodine (100 microM) and thapsigargin (1 microM) did not affect the response to CGRP. The response to CGRP was likewise unaffected by the nitric oxide synthase inhibitor, L-nitroarginine (30 microM) and by the protein kinase G inhibitor, KT5823 (3 microM). Furthermore, the inhibitory action of CGRP was not modified by lowering the extracellular concentration of K (from 5.9 to 1.2 mM) nor by increasing (from 2.5 to 3.75 mM) or decreasing (from 2.5 to 0.25 mM) the extracellular Ca concentration. Replacement of 80% glucose with 2-deoxyglucose (2-DOG) reduced the amplitude of spontaneous contractions, both in the absence and presence of 10 microM glibenclamide. In the presence of 2-DOG, the inhibitory action of CGRP was enhanced at a similar extent, either in the absence or presence of glibenclamide. In sucrose gap, the effect of CGRP (0.1 microM for 5 min) was separately analyzed in the proximal (close to the kidney) and distal (close to the ureter) regions of the renal pelvis. Both preparations discharged spontaneous (pacemaker) action potentials having different shape, duration and frequently. CGRP had no effect on pacemaker potentials in the proximal renal pelvis while producing about 30% reduction of the frequency of pacemaker potentials and motility index in the distal renal pelvis. Cromakalim (3 microM) abolished pacemaker potentials in both regions of the renal pelvis. In conjunction with the results of previous studies in the guinea-pig ureter, the present findings document the existence of remarkable regional differences in the effector mechanisms initiated by CGRP receptor occupancy in the guinea-pig pyeloureteral tract. CGRP appears to be inherently unable to activate glibenclamide-sensitive K channels in the guinea-pig renal pelvis, a mechanism which is central for its ability to suppress latent pacemakers in the ureter. Within the renal pelvis, the sensitivity to the inhibitory effect of CGRP appears in the more distal region, from which an 'ureter-like' action potential is recorded.