Major difference between rat and guinea-pig ureter in the ability of agonists and caffeine to release Ca2+ and influence force

Calcium-induced calcium release in noradrenergic neurons of the locus coeruleus

The locus coeruleus (LC) is a nucleus within the brainstem that consists of norepinephrine-releasing neurons. It is involved in broad processes including cognitive and emotional functions. Understanding the mechanisms that control the excitability of LC neurons is important because they innervate widespread brain regions. One of the key regulators is cytosolic calcium concentration ([Ca²⁺]_c), the increases in which can be amplified by calcium-induced calcium release (CICR) from intracellular calcium stores. Although the electrical activities of LC neurons are regulated by changes in [Ca²⁺]_c, the extent of CICR involvement in this regulation has remained unclear. Here we show that CICR hyperpolarizes acutely dissociated LC neurons of the rat and demonstrate the underlying pathway. When CICR was activated by extracellular application of 10 mM caffeine, LC neurons were hyperpolarized in the current-clamp mode of patch-clamp recording, and the majority of neurons showed an outward current in the voltage-clamp mode. This outward current was accompanied by increased membrane conductance, and its reversal potential was close to the K⁺ equilibrium potential, indicating that it is mediated by opening of K⁺ channels. The outward current was generated in the absence of extracellular calcium and was blocked when the calcium stores were inhibited by applying ryanodine. Pharmacological blockers indicated that it was mediated by Ca²⁺-activated K⁺ channels of the non-small conductance type. The application of caffeine increased [Ca²⁺]_c, as visualized by fluorescence microscopy. These findings show CICR suppresses LC neuronal activity, and indicate its dynamic role in modulating the LC-mediated noradrenergic tone in the brain.

Contribution of Cav1.2 Ca2+ channels and store-operated Ca2+ entry to pig urethral smooth muscle contraction

Urethral smooth muscle (USM) generates tone to prevent urine leakage from the bladder during filling. USM tone has been thought to be a voltage-dependent process, relying on Ca²⁺ influx via voltage-dependent Ca²⁺ channels in USM cells, modulated by the activation of Ca²⁺-activated Cl^- channels encoded by Ano1. However, recent findings in the mouse have suggested that USM tone is voltage independent, relying on Ca²⁺ influx through Orai channels via store-operated Ca²⁺ entry (SOCE). We explored if this pathway also occurred in the pig using isometric tension recordings of USM tone. Pig USM strips generated myogenic tone, which was nearly abolished by the Ca_v1.2 channel antagonist nifedipine and the ATP-dependent K⁺ channel agonist pinacidil. Pig USM tone was reduced by the Orai channel blocker GSK-7975A. Electrical field stimulation (EFS) led to phentolamine-sensitive contractions of USM strips. Contractions of pig USM were also induced by phenylephrine. Phenylephrine-evoked and EFS-evoked contractions of pig USM were reduced by ~50-75% by nifedipine and ~30% by GSK-7975A. Inhibition of Ano1 channels had no effect on tone or EFS-evoked contractions of pig USM. In conclusion, unlike the mouse, pig USM exhibited voltage-dependent tone and agonist/EFS-evoked contractions. Whereas SOCE plays a role in generating tone and agonist/neural-evoked contractions in both species, this dominates in the mouse. Tone and agonist/EFS-evoked contractions of pig USM are the result of Ca²⁺ influx primarily through Ca_v1.2 channels, and no evidence was found supporting a role of Ano1 channels in modulating these mechanisms.

Physiological increases in lactate inhibit intracellular calcium transients, acidify myocytes and decrease force in term pregnant rat myometrium

Key points

Lactate is increased in myometrial capillary blood from women in slow or non‐progressive labour (dystocia), suggesting that it is detrimental to uterine contractions.

There are no studies of the effect of lactate on the myometrium. In the present study, we have investigated its effects and mechanism of action on myometrial strips from term pregnant rats.

We show that lactate significantly decreased spontaneous contractility. Lactatedecreased pH_i and inhibited Ca²⁺ transients. Nulling the decrease in pH_i abolished the effects of lactate effects. If Ca²⁺ entry was enhanced, the effects of lactate were abolished.

The present study suggests that the accumulation of extracellular lactate could contribute to labour dystocia.

Abstract

Lactate is increased in myometrial capillary blood from women in slow or non‐progressive labour (dystocia), suggesting that it is detrimental to uterine contractions. There are, however, no studies of the effect of lactate on the myometrium. We therefore investigated its effects and mechanism of action on myometrial strips from term pregnant rats. The effects on spontaneous and oxytocin‐induced contractility in response to sodium lactate and other weak acids (1–20 mM) were studied. In some experiments, simultaneous force and intracellular Ca²⁺ or pH (pH_i) were measured with Indo‐1 or Carboxy‐SNARF, respectively. Statistical differences were tested using non‐parametric tests. Lactate significantly decreased spontaneous contractility with an EC₅₀ of 3.9 mM. Propionate, butyrate and pyruvate also reduced contractions with similar potency. The effects of lactate were reduced in the presence of oxytocin but remained significant. Lactate decreased pH_i and nulling the decrease in pH_i abolished its effects. We also show that lactate inhibited Ca²⁺ transients, with these changes mirroring those produced on force. If Ca²⁺ entry was enhanced by depolarization (high KCl) or applying the Ca²⁺ channel agonist, Bay K 4644, the effects of lactate were abolished. Taken together, these data show that lactate in the physiological range potently decreases myometrial contractility as a result of its inhibition of Ca²⁺ transients, which can be attributed to the induced acidification. The present study suggests that the accumulation of extracellular lactate will reduce myometrial contractions and could therefore contribute to labour dystocia.

Progress in understanding electro-mechanical signalling in the myometrium

In this review, we give a state-of-the-art account of uterine contractility, focussing on excitation-contraction (electro-mechanical) coupling (ECC). This will show how electrophysiological data and intracellular calcium measurements can be related to more modern techniques such as confocal microscopy and molecular biology, to advance our understanding of mechanical output and its modulation in the smooth muscle of the uterus, the myometrium. This new knowledge and understanding, for example concerning the role of the sarcoplasmic reticulum (SR), or stretch-activated K channels, when linked to biochemical and molecular pathways, provides a clearer and better informed basis for the development of new drugs and targets. These are urgently needed to combat dysfunctions in excitation-contraction coupling that are clinically challenging, such as preterm labour, slow to progress labours and post-partum haemorrhage. It remains the case that scientific progress still needs to be made in areas such as pacemaking and understanding interactions between the uterine environment and ion channel activity.

The effects of Ginseng Java root extract on uterine contractility in nonpregnant rats

Ginseng Java or Talinum paniculatum (Jacq.) Geartn has long been used in herbal recipes because of its various therapeutic properties. Ginseng Java is believed to be beneficial to the female reproductive system by inducing lactation and restoring uterine functions after the postpartum period. There are, however, no scientific data on verifying the effects on the uterus to support its therapeutic relevance. Therefore, the purpose of this study was to investigate the effects of Ginseng Java root extract and its possible mechanism(s) of action on uterine contractility. Female virgin rats were humanely killed by CO₂ asphyxia and uteri removed. Isometric force was measured in strips of longitudinal myometrium. The effects of Ginseng Java root extract at its IC₅₀ concentration (0.23 mg/mL) on spontaneous, oxytocin‐induced (10 nmol/L), and depolarized (KCl 40 mmol/L) contraction were investigated. After establishing regular phasic contractions, the application of Java root extract significantly inhibited spontaneous uterine contractility (n =5). The extract also significantly inhibited the contraction induced by high KCl solution (n =5) and oxytocin (n =5). The extract also inhibited oxytocin‐induced contraction in the absence of external Ca entry (n =7) and the tonic force induced by oxytocin in the presence of high KCl solution. Taken together, the data demonstrate a potent and consistent ability of extract from Ginseng Java root to reduce myometrial contractility. The tocolytic effects were demonstrated on both spontaneous and agonist‐induced contractions. The fact that force was inhibited in depolarized conditions suggests that the possible mechanisms may be blockade of Ca influx via L‐type Ca channels. The data in Ca‐free solutions suggest that the extract also reduces IP₃‐induced Ca release from the internal store. These tocolytic effects do not support the use of ginseng to help with postpartum contractility, but instead suggest it may be helpful in reducing inappropriate uterine contractions, such as in threatened preterm delivery.

The data demonstrate a potent and consistent ability of extract from Ginseng Java root to reduce myometrial contractility. The tocolytic effects were demonstrated on both spontaneous and agonist‐induced contractions.

Abnormal tracheal smooth muscle function in the CF mouse

Increased airway smooth muscle (ASM) contractility is thought to underlie symptoms of airway hyperresponsiveness (AHR). In the cystic fibrosis (CF) airway, ASM anomalies have been reported, but have not been fully characterized and the underlying mechanisms are largely unknown. We examined ASM in an adult CF mouse tracheal ring preparation, and determined whether changes in contractility were associated with altered ASM morphology. We looked for inherent changes in the cellular pathways involved in contractility, and characterized trachea morphology in the adult trachea and in an embryonic lung culture model during development. Results showed that that there was a reduction in tracheal caliber in CF mice as indicated by a reduction in the number of cartilage rings; proximal cross-sectional areas of cftr (-/-) tracheas and luminal areas were significantly smaller, but there was no difference in the area or distribution of smooth muscle. Morphological differences observed in adult trachea were not evident in the embryonic lung at 11.5 days gestation or after 72 h in culture. Functional data showed a significant reduction in the amplitude and duration of contraction in response to carbachol (CCh) in Ca-free conditions. The reduction in contraction was agonist specific, and occurred throughout the length of the trachea. These data show that there is a loss in the contractile capacity of the CF mouse trachea due to downregulation of the pathway specific to acetylcholine (ACh) activation. This reduction in contraction is not associated with changes in the area or distribution of ASM.

Hydrogen sulphide inhibits carbachol-induced contractile responses in β-escin permeabilized guinea-pig taenia caecum

Hydrogen sulphide (H(2)S) is an endogenous mediator producing a potent relaxation response in vascular and non-vascular smooth muscles. While ATP-sensitive potassium channels are mainly involved in this relaxant effect in vascular smooth muscle, the mechanism in other smooth muscles has not been revealed yet. In the present study, we investigated how H(2)S relaxes non-vascular smooth muscle by using intact and β-escin permeabilized guinea-pig taenia caecum. In intact tissues, concentration-dependent relaxation response to H(2)S donor NaHS in carbachol-precontracted preparations did not change in the presence of a K(ATP) channel blocker glibenclamide, adenylate cyclase inhibitor SQ-22536, guanylate cyclase inhibitor ODQ, protein kinase A inhibitor KT-5720, protein kinase C inhibitor H-7, tetrodotoxin, apamin/charybdotoxin, NOS inhibitor L-NAME and cyclooxygenase inhibitor indomethacin. We then studied how H(2)S affected carbachol- or Ca(2+)-induced contractions in permeabilized tissues. When Ca(2+) was clamped to a constant value (pCa6), a further contraction could be elicited by carbachol that was decreased by NaHS. This decrease in contraction was reversed by catalase but not by superoxide dismutase or N-acetyl cysteine. The sarcoplasmic reticulum Ca(2+)-ATPase pump inhibitor, cyclopiazonic acid, also decreased the carbachol-induced contraction that was further inhibited by NaHS. Mitochondrial proton pump inhibitor carbonyl cyanide p-trifluromethoxyphenylhydrazone also decreased the carbachol-induced contraction but this was not additionally changed by NaHS. The carbachol-induced Ca(2+) sensitization, calcium concentration-response curves, IP(3)- and caffeine-induced contractions were not affected by NaHS. In conclusion, we propose that hydrogen peroxide and mitochondria may have a role in H(2)S-induced relaxation response in taenia caecum.

Sarcoplasmic reticulum function in smooth muscle

The sarcoplasmic reticulum (SR) of smooth muscles presents many intriguing facets and questions concerning its roles, especially as these change with development, disease, and modulation of physiological activity. The SR's function was originally perceived to be synthetic and then that of a Ca store for the contractile proteins, acting as a Ca amplification mechanism as it does in striated muscles. Gradually, as investigators have struggled to find a convincing role for Ca-induced Ca release in many smooth muscles, a role in controlling excitability has emerged. This is the Ca spark/spontaneous transient outward current coupling mechanism which reduces excitability and limits contraction. Release of SR Ca occurs in response to inositol 1,4,5-trisphosphate, Ca, and nicotinic acid adenine dinucleotide phosphate, and depletion of SR Ca can initiate Ca entry, the mechanism of which is being investigated but seems to involve Stim and Orai as found in nonexcitable cells. The contribution of the elemental Ca signals from the SR, sparks and puffs, to global Ca signals, i.e., Ca waves and oscillations, is becoming clearer but is far from established. The dynamics of SR Ca release and uptake mechanisms are reviewed along with the control of luminal Ca. We review the growing list of the SR's functions that still includes Ca storage, contraction, and relaxation but has been expanded to encompass Ca homeostasis, generating local and global Ca signals, and contributing to cellular microdomains and signaling in other organelles, including mitochondria, lysosomes, and the nucleus. For an integrated approach, a review of aspects of the SR in health and disease and during development and aging are also included. While the sheer versatility of smooth muscle makes it foolish to have a "one model fits all" approach to this subject, we have tried to synthesize conclusions wherever possible.

Modulation of ureteric Ca signaling and contractility in humans and rats by uropathogenic E. coli

Ascending urinary tract infections, a significant cause of kidney damage, are predominantly caused by uropathogenic Escherichia coli (UPEC). However, the role and mechanism of changes in ureteric function during infection are poorly understood. We therefore investigated the effects of UPEC on Ca signaling and contractions in rat (n = 17) and human (n = 6) ureters. Ca transients and force were measured and effects of UPEC on the urothelium were monitored in live tissues. In both species, luminal exposure of ureters to UPEC strains J96 and 536 caused significant time-dependent decreases in phasic and high K depolarization-induced contractility, associated with decreases in the amplitude and duration of the Ca transients. These changes were significant after 3-5 h and irreversible over the next 5 h. The infection causes increased activity of K channels, causing inhibition of voltage-gated Ca entry, and K channel blockers could reverse the effects of UPEC on ureteric function. A smaller direct effect on Ca entry also occurs. Nonpathogenic E. coli (TG2) or abluminal application of UPEC did not produce changes in Ca signaling or contractility. UPEC exposure also caused significant impairment of urothelial barrier function; luminal application of the Ca channel blocker nifedipine caused a reduction in contractions as it entered the tissue, an effect not observed in untreated ureters. Thus, UPEC impairs ureteric contractility in a Ca-dependent manner, largely caused by stimulation of potassium channels and this mechanism is dependent on host-urothelium interaction.

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.

A review of recent insights into the role of the sarcoplasmic reticulum and Ca entry in uterine smooth muscle

The uterine sacroplasmic reticulum (SR) takes up and stores calcium [Ca], using an ATPase (SERCA) and the Ca-buffering proteins, calsequestrin and calreticulin. This stored Ca can be released via IP(3)-gated Ca channels. Decreases in luminal Ca concentration [Ca] have been directly measured following agonist stimulation. During spontaneous contractions however, there appears to be no involvement of the SR, as Ca entry and efflux across the plasma membrane account for these phasic contractions. After over-viewing current knowledge concerning SR structure and function, we highlight three areas of research which suggest new ways of looking at the role of the SR in the uterus, although they may be controversial or speculative at the moment. Firstly, we review the evidence for the function, if any, of Ca-induced SR Ca release channels, the ryanodine receptor (RyR) and the lack of Ca sparks (the elemental release events from RyRs), in the uterus. Secondly, we ask does regulation of SERCA by the accessory protein, phospholamban, occur in the uterus and what is the effect of knocking out phospholamban on uterine activity? Thirdly, we address the question of when and how store-operated Ca entry occurs in the myometrium. By analogy with other, usually less excitable tissues, is there a mechanism that links store Ca depletion to plasma membrane Ca entry in smooth muscle cells within intact uterus and is it physiologically relevant and regulated? Are the recently described proteins ORAI and STIM-1 involved in uterine store-operated Ca entry? We end the review by integrating these new insights with previous data to present a new working model of the SR in the uterus.

Calcium signalling in smooth muscle

Calcium signalling in smooth muscles is complex, but our understanding of it has increased markedly in recent years. Thus, progress has been made in relating global Ca2+ signals to changes in force in smooth muscles and understanding the biochemical and molecular mechanisms involved in Ca2+ sensitization, i.e. altering the relation between Ca2+ and force. Attention is now focussed more on the role of the internal Ca2+ store, the sarcoplasmic reticulum (SR), global Ca2+ signals and control of excitability. Modern imaging techniques have shown the elaborate SR network in smooth muscles, along with the expression of IP3 and ryanodine receptors. The role and cross-talk between these two Ca(2+) release mechanisms, as well as possible compartmentalization of the SR Ca2+ store are discussed. The close proximity between SR and surface membrane has long been known but the details of this special region to Ca2+ signalling and the role of local sub-membrane Ca2+ concentrations and membrane microdomains are only now emerging. The activation of K+ and Cl- channels by local Ca2+ signals, can have profound effects on excitability and hence contraction. We examine the evidence for both Ca2+ sparks and puffs in controlling ion channel activity, as well as a fundamental role for Ca2+ sparks in governing the period of inexcitability in smooth muscle, i.e. the refractory period. Finally, the relation between different Ca2+ signals, e.g. sparks, waves and transients, to smooth muscle activity in health and disease is becoming clearer and will be discussed.

Sex hormones and excitation-contraction coupling in the uterus: the effects of oestrous and hormones

In this review, we examine how far the increased understanding that we have of the events in excitation contraction can explain the effects of the oestrous cycle and sex hormones on uterine function. Observational studies of electrical and mechanical activity in the rat myometrium have shown a relative quiescence during pro-oestrous, with little propagation of any electrical events. Thus, uterine activity can be said to approximately inversely reflect plasma 17beta-oestradiol concentrations. We show that Ca(2+) signalling and mechanical activity are greatest in metoestrous and dioestrous compared to pro-oestrous and oestrous. These data are discussed in terms of hormonal effects on Ca(2+) and K(+) channels. Finally, the influence of sex hormones on lipid rafts and caveolae are considered and discussed in relation to recent findings on their role in uterine signalling and contractility, and cholesterol levels and obesity.

FK506-binding protein (FKBP12) regulates ryanodine receptor-evoked Ca2+ release in colonic but not aortic smooth muscle

In smooth muscle, the ryanodine receptor (RyR) mediates Ca(2+) release from the sarcoplasmic reticulum (SR) Ca(2+) store. Release may be regulated by the RyR accessory FK506-binding protein (FKBP12) either directly, as a result of FKBP12 binding to RyR, or indirectly via modulation of the activity of the phosphatase calcineurin or kinase mTOR. Here we report that RyR-mediated Ca(2+) release is modulated by FKBP12 in colonic but not aortic myocytes. Neither calcineurin nor mTOR are required for FKBP12 modulation of Ca(2+) release in colonic myocytes to occur. In colonic myocytes, co-immunoprecipitation techniques established that FKBP12 and calcineurin each associated with the RyR2 receptor isoform (the main isoform in this tissue). Single colonic myocytes were voltage clamped in the whole cell configuration and cytoplasmic Ca(2+) concentration ([Ca(2+)](c)) increases evoked by the RyR activator caffeine. Under these conditions FK506, which displaces FKBP12 (to inhibit calcineurin) and rapamycin, which displaces FKBP12 (to inhibit mTOR), each increased the [Ca(2+)](c) rise evoked by caffeine. Notwithstanding, neither mTOR nor calcineurin are required to potentiate caffeine-evoked Ca(2+) increases evoked by each drug. Thus, the mTOR and phosphatidylinositol 3-kinase inhibitor, LY294002, which directly inhibits mTOR without removing FKBP12 from RyR, did not alter caffeine-evoked [Ca(2+)](c) transients. Nor did inhibition of calcineurin by cypermethrin, okadaic acid or calcineurin inhibitory peptide block the FK506-induced increase in RyR-mediated Ca(2+) release. In aorta, although RyR3 (the main isoform), FKBP12 and calcineurin were each present, RyR-mediated Ca(2+) release was unaffected by either FK506, rapamycin or the calcineurin inhibitors cypermethrin and okadaic acid in single voltage clamped aortic myocytes. Presumably failure of FKBP12 to associate with RyR3 resulted in the immunosuppressant drugs (FK506 and rapamycin) being unable to alter the activity of RyR. The effects of these drugs are therefore, apparently dependent on an association of FKBP12 with RyR. Together, removal of FKBP12 from RyR augmented Ca(2+) release via the channel in colonic myocytes. Neither calcineurin nor mTOR are required for the FK506- or rapamycin-induced potentiation of RyR Ca(2+) release to occur. The results indicate that FKBP12 directly inhibits RyR channel activity in colonic myocytes but not in aorta.

Evidence that a Ca2+ sparks/STOCs coupling mechanism is responsible for the inhibitory effect of caffeine on electro-mechanical coupling in guinea pig ureteric smooth muscle

Recent studies have highlighted the role of the sarcoplasmic reticulum (SR) in controlling excitability, Ca2+ signalling and contractility in smooth muscle. Caffeine, an agonist of ryanodine receptors (RyRs) on the SR has been previously shown to effect Ca2+ signalling but its effects on excitability and contractility are not so clear. We have studied the effects of low concentration of caffeine (1 mM) on Ca2+ signalling, action potential and contractility of guinea pig ureteric smooth muscle. Caffeine produced reversible inhibition of the action potentials, Ca2+ transients and phasic contractions evoked by electrical stimulation. It had no effect on the inward Ca2+ current or Ca2+ transient but increased the amplitude and the frequency of spontaneous transient outward currents (STOCs) in voltage clamped ureteric myocytes, suggesting Ca2+-activated K+ channels (BK) are affected by it. In isolated cells and cells in situ caffeine produced an increase in the frequency and the amplitude of Ca2+ sparks as well the number of spark discharging sites per cell. Inhibition of Ca2+ sparks by ryanodine (50 microM) or SR Ca2+-ATPase (SERCA) cyclopiazonic acid (CPA, 20 microM) or BKCa channels by iberiotoxin (200 nM) or TEA (1 mM), fully reversed the inhibitory effect of caffeine on Ca2+ transients and force evoked by electrical field stimulation (EFS). These data suggest that the inhibitory effect of caffeine on the action potential, Ca2+ transients and force in ureteric smooth muscle is caused by activation of Ca2+ sparks/STOCs coupling mechanism.

Intracellular calcium stores in beta-escin skinned rat and guinea-pig bladders

Intracellular Ca2+ stores in rat and guinea-pig bladders and taenia caecum were studied in beta-escin skinned smooth muscle strips. 30 min of skinning with 40 microM and 80 microM beta-escin were the best parameters found to obtain good calcium response curves (10(-7)-10(-4) M) in rat and guinea pig, respectively. Calmodulin (1 microM) increased the calcium contractions significantly. pCa 6 was used to load intracellular stores and application of carbachol (50 microM) in all tissues then only contracted the tissues in the presence of guanosine-5'-triphosphate (GTP; 100 microM). Inositol triphosphate (IP3; 50 microM), applied after pCa 6, contracted all tissues. Carbachol added after IP3 or heparin (1 mg/ml) no longer caused a contraction in any of them. In bladders, caffeine (30 mM) but not ryanodine (5 microM) prevented the subsequent carbachol contraction. A slowly rising contraction with carbachol was elicited after caffeine (30 mM) or ryanodine (5 microM) in the taenia and after ryanodine in the bladders. Caffeine (30 mM) suppressed the calcium response curves in all tissues. Procaine (30 mM) blocked the carbachol (50 microM) contractions in bladders but not in taenia. These results suggest that calcium induced calcium release (CICR) and IP3 induced calcium release (IICR) release calcium from a common store in bladder but two different compartments in taenia.

Ca2+ microdomains in smooth muscle

In smooth muscle, Ca(2+) controls diverse activities including cell division, contraction and cell death. Of particular significance in enabling Ca(2+) to perform these multiple functions is the cell's ability to localize Ca(2+) signals to certain regions by creating high local concentrations of Ca(2+) (microdomains), which differ from the cytoplasmic average. Microdomains arise from Ca(2+) influx across the plasma membrane or release from the sarcoplasmic reticulum (SR) Ca(2+) store. A single Ca(2+) channel can create a microdomain of several micromolar near (approximately 200 nm) the channel. This concentration declines quickly with peak rates of several thousand micromolar per second when influx ends. The high [Ca(2+)] and the rapid rates of decline target Ca(2+) signals to effectors in the microdomain with rapid kinetics and enable the selective activation of cellular processes. Several elements within the cell combine to enable microdomains to develop. These include the brief open time of ion channels, localization of Ca(2+) by buffering, the clustering of ion channels to certain regions of the cell and the presence of membrane barriers, which restrict the free diffusion of Ca(2+). In this review, the generation of microdomains arising from Ca(2+) influx across the plasma membrane and the release of the ion from the SR Ca(2+) store will be discussed and the contribution of mitochondria and the Golgi apparatus as well as endogenous modulators (e.g. cADPR and channel binding proteins) will be considered.

Interactions between inositol 1,4,5-trisphosphate receptors and ryanodine receptors in smooth muscle: one store or two?

This short review proposes a system of simplified functional models describing possible interactions between Ca(2+)-release channels associated with IP(3)Rs and RyRs in smooth muscle, and considers each of these models in the light of the available experimental evidence. Complete separation of IP(3)R- and RyR-gated stores seems to be unusual. Where both receptors release Ca(2+) from a common pool, simple interactions can occur since changes in the activation of one receptor type affects the availability of Ca(2+) for release through the other. Alterations in [Ca(2+)] within the sarcoplasmic reticulum can also affect the open probability of the release channels, and not just the Ca(2+)-flux through the channels when open, e.g., Ca(2+)-release through tonically active IP(3)Rs appears to limit SR Ca(2+)-content in some myocytes, and this modulates RyR activity, as indicated by changes in Ca(2+)-spark frequency. There is also evidence that intracellular release channels may co-operate, leading to positive feedback during activation. In particular, agonist-dependent activation of IP(3)Rs can promote activation of RyRs, amplifying and shaping the resulting Ca(2+)-signal. While there is little direct evidence as to the mechanism responsible for this interaction, some form of Ca(2+)-induced Ca(2+)-release in response to local increases in [Ca(2+)](c) seems likely.

Rho-kinase inhibition and electromechanical coupling in rat and guinea-pig ureter smooth muscle: Ca2+-dependent and -independent mechanisms

Recent data have shown Ca(2+)-dependent activation of Rho-kinase by sustained depolarization of arterial smooth muscle. Visceral smooth muscles, however, contract phasically in response to action potentials and it is unclear whether Ca(2+)-dependent or -independent Rho-kinase activation occurs. We have therefore investigated this, under physiologically relevant conditions, in intact ureter. Action potentials, ionic currents, Ca(2+) transients, myosin light chain (MLC) phosphorylation and phasic contraction evoked by action potentials in guinea-pig and rat ureter were investigated. In rat, but not guinea-pig ureter, three Rho-kinase inhibitors, Y-27632, HA-1077 and H-1152, significantly decreased phasic contractions and Ca(2+) transients. Voltage- and current-clamp data showed that Rho-kinase inhibition reduced the plateau component of the action potential, inhibited Ca(2+)-channels and, indirectly, Ca(2+)-activated Cl(-) channels. The Ca(2+) channel agonist Bay K8644 could reverse these effects. The K(+) channel blocker TEA could also reverse the inhibitory effect of Y-27632 on the action potential and Ca(2+) transient. Ca(2+) transients and inward current, activated by carbachol-induced sarcoplasmic reticulum Ca(2+)release, were not affected by Rho-kinase inhibition. Rho-kinase inhibition produced a Ca(2+)-independent increase in the relaxation rate of contraction, associated with acceleration of MLC dephosphorylation, which was sensitive to calyculin A. These data show for the first time that: (1) Rho-kinase has major effects on Ca(2+) signalling associated with the action potential, (2) this effect is species dependent and (3) Rho-kinase controls relaxation of phasic contraction of myogenic origin. Thus Rho-kinase can modulate phasic smooth muscle in the absence of agonist, and the mechanisms are both Ca(2+)-dependent, involving ion channels, and Ca(2+)-independent, involving MLC phosphorylation activity.

Ca(2+) regulation in guinea-pig colonic smooth muscle: the role of the Na(+)-Ca(2+) exchanger and the sarcoplasmic reticulum

To study the contribution of the Na(+)-Ca(2+) exchanger to Ca(2+) regulation and its interaction with the sarcoplasmic reticulum (SR), changes in cytoplasmic Ca(2+) concentration ([Ca(2+)](c)) were measured in single, voltage clamped, smooth muscle cells. Increases in [Ca(2+)](c) were evoked by either depolarisation (-70 mV to 0 mV) or by release from the SR by caffeine (10 mM) or flash photolysis of caged InsP(3) (InsP(3)). Depletion of the SR of Ca(2+) (verified by the absence of a response to caffeine and InsP(3)) by either ryanodine (50 microM), to open the ryanodine receptors (RyRs), or thapsigargin (500 nM) or cyclopiazonic acid (CPA, 10 microM), to inhibit the SR Ca(2+) pumps, reduced neither the magnitude of the Ca(2+) transient nor the relationship between the influx of and the rise in [Ca(2+)](c) evoked by depolarisation. This suggested that Ca(2+)-induced Ca(2+) release (CICR) from the SR did not contribute significantly to the depolarisation-evoked rise in [Ca(2+)](c). However, although Ca(2+) was not released from it, the SR accumulated the ion following depolarisation since ryanodine and thapsigargin each slowed the rate of decline of the depolarisation-evoked Ca(2+) transient. Indeed, the SR Ca(2+) content increased following depolarisation as assessed by the increased magnitude of the [Ca(2+)](c) levels evoked each by InsP(3) and caffeine, relative to controls. The increased SR Ca(2+) content following depolarisation returned to control values in approximately 12 min via Na(+)-Ca(2+) exchanger activity. Thus inhibition of the Na(+)-Ca(2+) exchanger by removal of external Na(+) (by either lithium or choline substitution) prevented the increased SR Ca(2+) content from returning to control levels. On the other hand, the Na(+)-Ca(2+) exchanger did not appear to regulate bulk average Ca(2+) directly since the rates of decline in [Ca(2+)](c), following either depolarisation or the release of Ca(2+) from the SR (by either InsP(3) or caffeine), were neither voltage nor Na(+) dependent. Thus, no evidence for short term (seconds) control of [Ca(2+)](c) by the Na(+)-Ca(2+) exchanger was found. Together, the results suggest that despite the lack of CICR, the SR removes Ca(2+) from the cytosol after its elevation by depolarisation. This Ca(2+) is then removed from the SR to outside the cell by the Na(+)-Ca(2+) exchanger. However, the exchanger does not contribute significantly to the decline in bulk average [Ca(2+)](c) following transient elevations in the ion produced either by depolarisation or by release from the store.

On the mechanisms whereby temperature affects excitation-contraction coupling in smooth muscle

Moderate cooling of smooth muscle can modulate force production and may contribute to pathophysiological conditions, but the mechanisms underlying its effects are poorly understood. Interestingly, cooling increases force in rat ureter, but decreases it in guinea pigs. Therefore, this study used ureteric smooth muscle as a model system to elucidate the mechanisms of the effects of cooling on excitation-contraction coupling. Simultaneous recordings of force, intracellular [Ca(2+)], and electrical activity were made in intact ureter and ionic currents measured in isolated cells. The increase in force amplitude in rat ureter with cooling was found to be due to a significant increase in the duration of the Ca(2+) transient. This in turn was due to a marked prolongation of the action potential. In guinea pigs, both these parameters were much less affected by cooling. Examination of membrane currents revealed that differences in ion channel contribution to the action potential underlie these differences. In particular, cooling potentiated Ca(2+)-activated Cl(-) currents, which are present in rat but not guinea pig ureteric smooth muscle, and prolonged the plateau of the action potential and Ca(2+) entry. The force-Ca(2+) relationship revealed that the increased duration of the Ca(2+) transient was sufficient in the rat, but not in the guinea pig, to overcome kinetic lags produced in both species by cooling and potentiate force. Ca(2+) entry and release processes were largely temperature-insensitive, but the rate of relaxation was very temperature-sensitive. Effects of cooling on myosin light chain phosphatase, confirmed in experiments using calyculin A, appear to be the predominant mechanisms affecting relaxation. Thus, smooth muscle is diverse in its response to temperature, even when experimental variables, such as the mode of stimulation, are removed. Although the biochemical and mechanical events accompanying contraction are likely to be affected in similar ways by temperature, differences in electrical events lead to subsequent differences in these processes between smooth muscles.

Ca(2+) signals mediated by Ins(1,4,5)P(3)-gated channels in rat ureteric myocytes

Localized Ca(2+)-release signals (puffs) and propagated Ca(2+) waves were characterized in rat ureteric myocytes by confocal microscopy. Ca(2+) puffs were evoked by photorelease of low concentrations of Ins(1,4,5)P(3) from a caged precursor and by low concentrations of acetylcholine; they were also observed spontaneously in Ca(2+)-overloaded myocytes. Ca(2+) puffs showed some variability in amplitude, time course and spatial spread, suggesting that Ins(1,4,5)P(3)-gated channels exist in clusters containing variable numbers of channels and that within these clusters a variable number of channels can be recruited. Immunodetection of Ins(1,4,5)P(3) receptors revealed the existence of several spots of fluorescence in the confocal cell sections, supporting the existence of clusters of Ins(1,4,5)P(3) receptors. Strong Ins(1,4,5)P(3) photorelease and high concentrations of acetylcholine induced Ca(2+) waves that originated from an initiation site and propagated in the whole cell by spatial recruitment of neighbouring Ca(2+)-release sites. Both Ca(2+) puffs and Ca(2+) waves were blocked selectively by intracellular applications of heparin and an anti-Ins(1,4,5)P(3)-receptor antibody, but were unaffected by ryanodine and intracellular application of an anti-ryanodine receptor antibody. mRNAs encoding for the three subtypes of Ins(1,4,5)P(3) receptor and subtype 3 of ryanodine receptor were detected in these myocytes, and the maximal binding capacity of [(3)H]Ins(1,4,5)P(3) was 10- to 12-fold higher than that of [(3)H]ryanodine. These results suggest that Ins(1,4,5)P(3)-gated channels mediate a continuum of Ca(2+) signalling in smooth-muscle cells expressing a high level of Ins(1,4,5)P(3) receptors and no subtypes 1 and 2 of ryanodine receptors.

Interactions between Ca(2+) and H(+) and functional consequences in vascular smooth muscle

Ca(2+) and H(+) ions can profoundly alter vascular tone. In many physiological and pathological processes, changes in the concentration of both ions occur. Thus, to understand the processes and mechanisms that modify force, it is necessary to understand what changes occur in these ions and, importantly, how they interact with each other. In this minireview, we highlight the quantitatively important mechanisms involved in the contractile responses of vascular tissues to pH change and discuss the cellular and molecular reasons underlying these responses.

Effect of 18beta-glycyrrhetinic acid on electromechanical coupling in the guinea-pig renal pelvis and ureter

We have tested the effect of the gap junction inhibitor, 18beta-glycyrrhetinic acid (18betaGA) on electromechanical coupling in the guinea-pig renal pelvis and ureter by the sucrose gap technique. In the ureter 18betaGA (3 - 30 microM) produced a concentration-dependent inhibition of the spike component of the action potential (AP) and reduced contraction evoked by electrical stimulation. Neurokinin A (NKA) produced a slow depolarization with superimposed APs and phasic contractions of the ureter. 18betaGA (30 microM) markedly inhibited the depolarization and APs evoked by NKA. However the contractile response was more sustained in the presence than in the absence of 18betaGA. At 100 microM, 18betaGA inhibited the mechanical responses to NKA. KCl (80 mM) produced APs and phasic contractions followed by sustained depolarization and tonic contraction. At 30 microM 18betaGA markedly inhibited the KCl-evoked APs and phasic contractions without affecting the sustained responses. At 100 microM 18betaGA inhibited the tonic contraction to KCl. In the renal pelvis 18betaGA (30 microM) inhibited the amplitude of pacemaker potentials and accompanying contractions and induced the appearance of low-amplitude APs not associated with contraction. We conclude that, up to 30 microM, the action of 18betaGA is consistent with an inhibition of cell-to-cell electrical coupling via gap junctions. The single-unit character of smooth muscles in the guinea-pig upper urinary tract is partly converted to a multi-unit pattern. At high concentrations 18betaGA possesses non specific effects which limit its usefulness as a tool for studying the role of gap junctions in smooth muscles. British Journal of Pharmacology (2000) 129, 163 - 169

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.

Ionic mechanisms underlying spontaneous muscle contractions in the liver fluke, Fasciola hepatica

Spontaneous contractions of liver fluke muscle were abolished in Ca(2+)-free saline and by 100 microM nifedipine and reduced by 5 mM cadmium chloride, suggesting that they are dependent on extracellular Ca(2+). Caffeine (5 mM) significantly increased contraction amplitude and frequency. Ryanodine (100 microM) failed to block the caffeine response but significantly reduced spontaneous contraction frequency, suggesting that intracellular stores have a functional role. Cyclopiazonic acid (5 microM) had no effect on the caffeine response or spontaneous activity. 3-Isobutyl-1-methylxanthine (IBMX), forskolin, and 8-bromoadenosine 3',5'-cyclic monophosphate significantly increased spontaneous contractions, which implies that cAMP has a regulatory function in motility. Caffeine, however, produced no measurable increase in cAMP. The caffeine effect was inhibited by cadmium chloride and nifedipine, whereas IBMX-induced increases in amplitude were reduced by cadmium chloride. Thus caffeine and cAMP appear capable of opening plasma membrane Ca(2+) channels, but the involvement of cAMP in caffeine responses has not been proved.

Intracellular calcium stores and agonist-induced contractions in isolated human myometrium

OBJECTIVE

We hypothesized that the release of calcium from intracellular stores contributes to the contractions produced by the agonists oxytocin, carbachol, and prostaglandin F(2 )(alpha ) in human myometrium.

STUDY DESIGN

Strips of myometrium were obtained at cesarean section and hysterectomy. The strips were loaded with the calcium-sensitive dye Indo-1 to enable simultaneous measurement of tension and intracellular calcium levels. Agonist-induced responses in the presence and absence of extracellular calcium were studied.

RESULTS

Strips of myometrium were obtained from 48 women not in labor undergoing cesarean section and 6 women not pregnant undergoing hysterectomy. An increase in intracellular calcium level after agonist stimulation invariably preceded an increase in tension. Intracellular calcium level returned to baseline before myometrial relaxation. Oxytocin, carbachol, and prostaglandin F(2)(alpha) all gave both force and intracellular calcium responses in the absence of extracellular calcium, although both these responses were only 26% to 40% of the maximal response when extracellular calcium was present.

CONCLUSIONS

Release of calcium from internal stores induced by oxytocin, carbachol, and prostaglandin F(2)(alpha) may contribute to agonist-induced myometrial force production.

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.

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.

Caffeine- and noradrenaline-induced contractions of human vas deferens: contrasting effects of procaine, ryanodine and W-7

1. The effects of ryanodine, procaine, and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) on noradrenaline (NA)- and caffeine-induced contractions of human vas deferens were investigated. 2. In the presence of nifedipine (1 microM), NA ( 100 microM) evoked biphasic contractions. Caffeine (20 mM) evoked repeatable tonic contractions. 3. Ryanodine (30 microM) inhibited the initial but not the secondary component of NA contractions. Procaine (1 and 10 mM) inhibited both components. Contractions induced by caffeine were unaffected by ryanodine or procaine. 4. The calmodulin antagonist W-7 (100 microM) reduced, in a reversible manner, both components of NA-induced response. Caffeine-induced contractions were also reduced in most preparations (8 of 11). In all preparations, contractions induced by caffeine were markedly inhibited after the washout of W-7. Higher doses of W-7 (300 microM) induced an increase in basal tension. 5. These results indicate that NA contracts the longitudinal muscle of human vas deferens by a ryanodine-sensitive calcium-induced calcium release (CICR) mechanism and, in addition, a ryanodine-insensitive pathway: both are sensitive to procaine. In contrast, contraction induced by caffeine is mediated by a pathway that is atypically insensitive to either ryanodine or procaine. The sensitivity of NA- and caffeine-induced contraction to W-7 suggests a role for calcium and its interaction with calmodulin in the response to both agents. The paradoxical action of W-7 is discussed.

Contribution of sarcoplasmic reticular calcium to smooth muscle contractile activation: gestational dependence in isolated rat uterus

1. The contribution of Ca2+ released from the sarcoplasmic reticulum (SR) to smooth muscle contractile activation remains poorly understood. By simultaneously monitoring cytosolic [Ca2+] ([Ca2+]i) and force in isolated rat uterine smooth muscle, we report the influence of SR Ca2+ release on contractility during conditions (a) of altered SR Ca2+ homeostasis and (b) where the only source of activating Ca2+ was derived from the SR. 2. In myometria of non-pregnant rats, ryanodine (1-50 microM), a modulator of calcium-induced calcium release (CICR), had no effect on the spontaneous [Ca2+]i or force transients. However, depletion of SR Ca2+ by inhibiting the SR Ca2+-ATPase (with cyclopiazonic acid (CPA), 20 microM) resulted in an enhancement of spontaneous [Ca2+]i and force transients. 3. In myometria of pregnant rats, although ryanodine had no effect in 40% of tissues studied it produced a small but significant enhancement of the integrated spontaneous [Ca2+]i and force transient in 60% of cases. The potentiating effects of CPA were enhanced in myometria of pregnant rats compared with non-pregnant rats, often resulting in maintained [Ca2+]i increases and contraction. 4. In zero external Ca2+, agonist-induced SR Ca2+ release resulted in transient increases in [Ca2+]i and force. The magnitude of these agonist-induced [Ca2+]i and force changes were significantly enhanced in myometria of pregnant rats. No evidence for agonist-induced Ca2+-independent force production was observed. 5. These results indicate that CICR plays little role in SR Ca2+ release from the myometrium, and that there are gestational-dependent alterations in the ability of SR Ca2+ mobilization to contribute to contractile activation. The implications of these findings for the co-ordination of myometrial [Ca2+]i signalling and contractility are discussed.

Ryanodine- and cyclopiazonic acid-sensitive components in human vas deferens contractions to noradrenaline

1. The role of calcium stores in noradrenaline- (NA) and caffeine-induced contractions of human vas deferens were investigated using ryanodine and cyclopiazonic acid (CPA) in the presence of the calcium antagonist, nifedipine (1 microM) or in calcium-free/EGTA (1 mM) medium. 2. In either media, NA (100 microM) evoked biphasic contractions of longitudinal muscle and tonic circular muscle contractions. Caffeine (20 mM) evoked longitudinal but not circular muscle contractions. 3. Ryanodine (1-30 microM) or CPA (1-30 microM) inhibited contractions of circular muscle, and the initial but not secondary component of longitudinal muscle contraction to NA. 4. In the presence of nifedipine, pre-exposure to caffeine caused a potentiation of circular muscle, and the initial but not secondary longitudinal muscle contractions to NA. The presence of ryanodine or CPA during the caffeine pre-exposures effectively blocked the potentiation of the initial component and reduced the secondary component of the subsequent responses to NA in longitudinal muscle. 5. In calcium-free media, caffeine pre-exposures had little effect on subsequent NA-induced contractions in circular muscle, but reduced both components in longitudinal muscle. The presence of ryanodine or CPA during caffeine pre-exposures produced no further effects on either component of the subsequent NA-induced contraction in longitudinal muscle. 6 In the presence of nifedipine or in calcium-free media, repeated applications of caffeine evoked contractions in longitudinal muscle which were not blocked by either ryanodine or CPA. 7. These results suggest that circular muscle contraction by NA and the initial component of longitudinal muscle to NA both utilize an intracellular pool of calcium that is triggered via a ryanodine-sensitive mechanism and replenished via a CPA-sensitive Ca2+-ATPase. 8. In longitudinal muscle, both the secondary component of its response to NA and contraction to caffeine appear to involve an unusual but pharmacologically distinct (ryanodine- and CPA-insensitive) pathway. 9. The quiescence to caffeine of circular muscle may be caused by a relative absence of the ryanodine- and CPA-insensitive pathway.

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.

The mechanism of Ca2+ release from the SR of permeabilised guinea-pig and rat ureteric smooth muscle

Recent work has indicated that there is a major difference in the Ca2+ store of smooth muscle from rat and guinea-pig ureter; with the rat store being agonist-sensitive but ryanodine insensitive and the guinea-pig store being ryanodine sensitive but agonist insensitive [Th. V. Burdyga, M.J. Taggart, S. Wray, J. Physiol. 489 (1995) 327-335]. We have therefore examined directly the mechanism of Ca2+ release from the internal Ca2+ store (SR). Following permeabilisation with alpha-toxin or beta-escin the SR was Ca(2+)-loaded before application of carbachol or caffeine. Only carbachol evoked a transient contraction in rat ureter. The carbachol-induced contraction was blocked by heparin and cyclopiazonic acid (CPA) but not ryanodine. Only caffeine produced contraction in guinea-pig ureter, and this was blocked by ryanodine. Direct application of IP3 caused a small transient contraction in rat but not guinea-pig ureter. We conclude that rat ureter possesses only an IP3 sensitive store while guinea-pig ureter only has a ryanodine sensitive store.