Depolarisation of guinea-pig visceral smooth muscle causes hydrolysis of inositol phospholipids

Roles of endothelial prostaglandin I2 in maintaining synchronous spontaneous Ca2+ transients in rectal capillary pericytes

In hollow visceral organs, capillary pericytes appear to drive spontaneous Ca2+ transients in the upstream arterioles. Here, mechanisms underlying the intercellular synchrony of pericyte Ca2+ transients were explored. Ca2+ dynamics in NG2 chondroitin sulphate proteoglycan (NG2)-expressing capillary pericytes were examined using rectal mucosa-submucosa preparations of NG2-GCaMP6 mice. Spontaneous Ca2+ transients arising from endoplasmic reticulum Ca2+ release were synchronously developed amongst capillary pericytes in a gap junction blocker (3 μM carbenoxolone)-sensitive manner and could spread into upstream vascular segments. Spontaneous Ca2+ transients were suppressed by the Ca2+ -activated Cl- channel (CaCC) blocker niflumic acid and their synchrony was diminished by a TMEM16A inhibitor (3 μM Ani9) in accordance with TMEM16A immunoreactivity in pericytes. In capillaries where cyclooxygenase (COX)-2 immunoreactivity was expressed in endothelium but not pericytes, non-selective COX inhibitors (1 μM indomethacin or 10 μM diclofenac) or COX-2 inhibitor (10 μM NS 398) disrupted the synchrony of spontaneous Ca2+ transients and raised the basal Ca2+ level. Subsequent prostaglandin I2 (PGI2 ; 100 nM) or the KATP channel opener levcromakalim restored the synchrony with a reduction in the Ca2+ level. PGI2 receptor antagonist (1 μM RO1138452) also disrupted the synchrony of spontaneous Ca2+ transients and increased the basal Ca2+ level. Subsequent levcromakalim restored the synchrony and reversed the Ca2+ rise. Thus, the synchrony of spontaneous Ca2+ transients in pericytes appears to be developed by the spread of spontaneous transient depolarisations arising from the opening of TMEM16A CaCCs. Endothelial PGI2 may play a role in maintaining the synchrony, presumably by stabilising the resting membrane potential in pericytes. KEY POINTS: Capillary pericytes in the rectal mucosa generate synchronous spontaneous Ca2+ transients that could spread into the upstream vascular segment. Spontaneous Ca2+ release from the endoplasmic reticulum (ER) triggers the opening of Ca2+ -activated Cl- channel TMEM16A and resultant depolarisations that spread amongst pericytes via gap junctions, establishing the synchrony of spontaneous Ca2+ transients in pericytes. Prostaglandin I2 (PGI2 ), which is constitutively produced by the endothelium depending on cyclooxygenase-2, appears to prevent premature ER Ca2+ releases in the pericytes allowing periodic, regenerative Ca2+ releases. Endothelial PGI2 may maintain the synchrony of pericyte activity by stabilising pericyte resting membrane potential by opening of KATP channels.

Hypokinetic Activity of Menthofuran on the Gastrointestinal Tract in Rodents

The acute toxicity and hypokinetic activity induced by menthofuran on the gastrointestinal tract of rodents were investigated in the present study. An absence of acute toxicity was observed. Menthofuran delayed gastric emptying at oral doses of 25, 50, and 100 mg/kg in the experimental model of phenol red, as well as it reduced the intestinal transit at oral doses of 50 and 100 mg/kg. Interestingly, a scopolamine-similar hypokinetic effect was observed for menthofuran. In the experimental model of castor oil-induced intestinal hypermotility, menthofuran (50 and 100 mg/kg) reduced the number of loose stools as observed for the normal group. Additionally, menthofuran induced a marked concentration-dependent relaxation in rat ileum segments precontracted with KCl (EC50 = 0.059 ± 0.008 μg/mL) or carbachol (EC50 = 0.068 ± 0.007 μg/mL). These results suggest the possible decrease of calcium influx underlying the effects of menthofuran on the gastrointestinal tract, which opens the door for further study regarding this potential application for the treatment of gastrointestinal disorders, noting possible limitations of its use due to adverse effects in children.

Role of K+ channels in maintaining the synchrony of spontaneous Ca2+ transients in the mural cells of rat rectal submucosal arterioles

Mural cells in precapillary arterioles (PCAs) generate spontaneous Ca²⁺ transients primarily arising from the periodic release of Ca²⁺ from sarcoendoplasmic reticulum (SR/ER). The Ca²⁺ release induces Ca²⁺-activated chloride channel (CaCC)-dependent depolarisations that spread to neighbouring mural cells to develop the synchrony of their Ca²⁺ transients. Here, we explored the roles of K⁺ channels in maintaining the synchrony of spontaneous Ca²⁺ transients. Intracellular Ca²⁺ dynamics in mural cells were visualised by Cal-520 fluorescence Ca²⁺ imaging in the submucosal PCAs of rat rectum. Increasing extracellular K⁺ concentration ([K⁺]_o) from 5.9 to 29.7 mM converted synchronous spontaneous Ca²⁺ transients into asynchronous, high-frequency Ca²⁺ transients. Similarly, the blockade of inward rectifier K⁺ (K_ir) channels with Ba²⁺ (50 μM) or K_v7 voltage-dependent K⁺ (K_v7) channels with XE 991 (10 μM) disrupted the synchrony of spontaneous Ca²⁺ transients, while the blockers for large-, intermediate- or small-conductance Ca²⁺-activated K⁺ channels had no effect. K_ir2.1 immunoreactivity was detected in the arteriolar endothelium but not mural cells. In the PCAs that had been pretreated with XE 991 or Ba²⁺, nifedipine (1 μM) attenuated the asynchronous Ca²⁺ transients but failed to restore their synchrony. In contrast, levcromakalim, an ATP-sensitive K⁺ channel opener, restored the synchronous Ca²⁺ transients. Thus, constitutively active K_v7 and K_ir channels appear to be involved in maintaining the relatively hyperpolarised membrane of mural cells. The hyperpolarised membrane prevents depolarisation-induced 'premature' Ca²⁺ transients to ensure sufficient SR/ER Ca²⁺ refilling that is required for regenerative Ca²⁺ release resulting in synchronous Ca²⁺ transients amongst the mural cells.

Properties of synchronous spontaneous Ca2+ transients in the mural cells of rat rectal arterioles

Synchrony of spontaneous Ca²⁺ transients among venular mural cells (smooth muscle cells and pericytes) in visceral organs relies on the intercellular spread of L-type voltage-dependent Ca²⁺ channel (LVDCC)-dependent depolarisations. However, the mechanisms underlying the synchrony of spontaneous Ca²⁺ transients between arteriolar mural cells are less understood. The spontaneous intracellular Ca²⁺ dynamics of arteriolar mural cells in the rat rectal submucosa were visualised by Cal-520 Ca²⁺ imaging to analyse their synchrony. The mural cells in fine arterioles that had a rounded cell body with several extended processes developed spontaneous 'synchronous' Ca²⁺ transients arising from Ca²⁺ released from sarcoendoplasmic reticulum Ca²⁺ stores. Gap junction blockers (3 μM carbenoxolone, 10 μM 18β-glycyrrhetinic acid), a Ca²⁺-activated Cl^- channel (CaCC) blocker (100 μM 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid) or lowering extracellular Cl^- concentration (from 134.4 to 12.4 mM) disrupted the synchrony of Ca²⁺ transients between arteriolar mural cells. Blockers of T-type voltage-dependent Ca²⁺ channels (TVDCCs, 1 μM mibefradil or ML218) or LVDCCs (1 μM nifedipine) reduced the Ca²⁺ transient frequency or their area under curve (AUC), respectively. However, neither TVDCC nor LVDCC blockers disrupted the synchrony of Ca²⁺ transients among arteriolar mural cells. This is in contrast with rectal venules in which nifedipine disrupted the synchrony of spontaneous Ca²⁺ transients. Thus, spontaneous transient depolarisations arising from the opening of CaCCs may effectively spread to neighbouring arteriolar mural cells via gap junctions to maintain the Ca²⁺ transient synchrony. Activation of TVDCCs appears to accelerate spontaneous Ca²⁺ transients, while LVDCCs predominantly contribute to the duration of Ca²⁺ transients.

Metabotropic regulation of RhoA/Rho-associated kinase by L-type Ca2+ channels

Sustained vascular smooth muscle contraction can be mediated by several mechanisms, including the influx of extracellular Ca(2+) through L-type voltage-gated Ca(2+) channels (LTCCs) and by RhoA/Rho-associated kinase (ROCK)-dependent Ca(2+) sensitization of the contractile machinery. Conformational changes in the LTCC following depolarization can also trigger an ion-independent metabotropic pathway that involves G protein/phospholipase C activation, giving rise to inositol 1,4,5-trisphosphate synthesis and subsequent Ca(2+) release from the sarcoplasmic reticulum (SR) (calcium channel-induced Ca(2+) release or calcium channel-induced calcium release [CCICR]). In this review, we summarize recent data suggesting that LTCC activation and subsequent metabotropic Ca(2+) release from the SR participate in depolarization-evoked RhoA/ROCK activity and sustained arterial contraction. During protracted depolarizations, refilling of the SR stores by a residual influx of extracellular Ca(2+) through LTCCs helps maintain RhoA activity and contractile activation. These findings suggest that CCICR plays a major role in tonic vascular smooth muscle contraction, providing a link between membrane depolarization-induced LTCC activation and metabotropic Ca(2+) release and RhoA/ROCK stimulation.

Metabotropic regulation of RhoA/Rho-associated kinase by L-type Ca2+ channels: new mechanism for depolarization-evoked mammalian arterial contraction

BACKGROUND

Sustained vascular smooth muscle contraction is mediated by extracellular Ca(2+) influx through L-type voltage-gated Ca(2+) channels (VGCC) and RhoA/Rho-associated kinase (ROCK)-dependent Ca(2+) sensitization of the contractile machinery. VGCC activation can also trigger an ion-independent metabotropic pathway that involves G-protein/phospholipase C activation, inositol 1,4,5-trisphosphate synthesis, and Ca(2+) release from the sarcoplasmic reticulum (calcium channel-induced Ca(2+) release). We have studied the functional role of calcium channel-induced Ca(2+) release and the inter-relations between Ca(2+) channel and RhoA/ROCK activation.

METHODS AND RESULTS

We have used normal and genetically modified animals to study single myocyte electrophysiology and fluorimetry as well as cytosolic Ca(2+) and diameter in intact arteries. These analyses were complemented with measurement of tension and RhoA activity in normal and reversibly permeabilized arterial rings. We have found that, unexpectedly, L-type Ca(2+) channel activation and subsequent metabotropic Ca(2+) release from sarcoplasmic reticulum participate in depolarization-evoked RhoA/ROCK activity and sustained arterial contraction. We show that these phenomena do not depend on the change in the membrane potential itself, or the mere release of Ca(2+) from the sarcoplasmic reticulum, but they require the simultaneous activation of VGCC and the downstream metabotropic pathway with concomitant Ca(2+) release. During protracted depolarizations, refilling of the stores by a residual extracellular Ca(2+) influx through VGCC helps maintaining RhoA activity and sustained arterial contraction.

CONCLUSIONS

These findings reveal that calcium channel-induced Ca(2+) release has a major role in tonic vascular smooth muscle contractility because it links membrane depolarization and Ca(2+) channel activation with metabotropic Ca(2+) release and sensitization (RhoA/ROCK stimulation).

Generation and propagation of gastric slow waves

1. Mechanisms underlying the generation and propagation of gastrointestinal slow wave depolarizations have long been controversial. The present review aims to collate present knowledge on this subject with specific reference to slow waves in gastric smooth muscle. 2. At present, there is strong agreement that interstitial cells of Cajal (ICC) are the pacemaker cells that generate slow waves. What has been less clear is the relative role of primary types of ICC, including the network in the myenteric plexus (ICC-MY) and the intramuscular network (ICC-IM). It is concluded that both ICC-MY and ICC-IM are likely to serve a major role in slow wave generation and propagation. 3. There has been long-standing controversy as to how slow waves 'propagate' circumferentially and down the gastrointestinal tract. Two mechanisms have been proposed, one being action potential (AP)-like conduction and the other phase wave-based 'propagation' resulting from an interaction of coupled oscillators. Studies made on single bundle gastric strips indicate that both mechanisms apply with relative dominance depending on conditions; the phase wave mechanism is dominant under circumstances of rhythmically generating slow waves and the AP-like propagation is dominant when the system is perturbed. 4. The phase wave mechanism (termed Ca(2+) phase wave) uses cyclical Ca(2+) release as the oscillator, with coupling between oscillators mediated by several factors, including: (i) store-induced depolarization; (ii) resultant electrical current flow/depolarization through the pacemaker cell network; and (iii) depolarization-induced increase in excitability of downstream Ca(2+) stores. An analogy is provided by pendulums in an array coupled together by a network of springs. These, when randomly activated, entrain to swing at the same frequency but with a relative delay along the row giving the impression of a propagating wave. 5. The AP-like mechanism (termed voltage-accelerated Ca(2+) wave) propagates sequentially like a conducting AP. However, it is different in that it depends on regenerative store Ca(2+) release and resultant depolarization rather than regenerative activation of voltage-dependent channels in the cell membrane. 6. The applicability of these mechanisms to describing propagation in large intact gastrointestinal tissues, where voltage-dependent Ca(2+) entry is also likely to be functional, is discussed.

A role for membrane potential in regulating GPCRs?

G-protein-coupled receptors (GPCRs) have ubiquitous roles in transducing extracellular signals into cellular responses. Therefore, the concept that members of this superfamily of surface proteins are directly modulated by changes in membrane voltage could have widespread consequences for cell signalling. Although several studies have indicated that GPCRs can be voltage dependent, particularly P2Y(1) receptors in the non-excitable megakaryocyte, the evidence has been mostly indirect. Recent work on muscarinic receptors has stimulated substantial interest in this field by reporting the first voltage-dependent charge movements for a GPCR. An underlying mechanism is proposed whereby a voltage-induced conformational change in the receptor alters its ability to couple to the G protein and thereby influences its affinity for an agonist. We discuss the strength of the evidence behind this hypothesis and include suggestions for future work. We also describe other examples in which direct voltage control of GPCRs can account for effects of membrane potential on downstream signals and highlight the possible physiological consequences of this phenomenon.

Role of intracellular stores in the regulation of rhythmical [Ca2+]i changes in interstitial cells of Cajal from rabbit portal vein

Interstitial cells of Cajal (ICCs) freshly isolated from rabbit portal vein and loaded with the Ca(2+)-sensitive indicator fluo-3 revealed rhythmical [Ca(2+)](i) changes occurring at 0.02-0.1 Hz. Each increase in [Ca(2+)](i) originated from a discrete central region of the ICC and propagated as a [Ca(2+)](i) wave towards the cell periphery, but usually became attenuated before reaching the ends of the cell. In about 40% of ICCs each rhythmical change in [Ca(2+)](i) consisted of an initial [Ca(2+)](i) increase (phase 1) followed by a faster rise in [Ca(2+)](i) (phase 2) and then a decrease in [Ca(2+)](i) (phase 3); the frequency correlated with the rate of rise of [Ca(2+)](i) during phase 1, but not with the peak amplitude. Rhythmical [Ca(2+)](i) changes persisted in nicardipine, but were abolished in Ca(2+)-free solution as well as by SK&F96365, cyclopiazonic acid, thapsigargin, 2-APB, xestospongin C or ryanodine. Intracellular Ca(2+) stores visualised with the low-affinity Ca(2+) indicator fluo-3FF were found to be enriched with ryanodine receptors (RyRs) detected with BODIPY TR-X ryanodine. Rhythmical [Ca(2+)](i) changes originated from a perinuclear S/ER element showing the highest RyR density. Immunostaining with anti-TRPC3,6,7 antibodies revealed the expression of these channel proteins in the ICC plasmalemma. This suggests that these rhythmical [Ca(2+)](i) changes, a key element of ICC pacemaking activity, result from S/ER Ca(2+) release which is mediated via RyRs and IP(3) receptors and is modulated by the activity of S/ER-Ca(2+)-ATPase and TRP channels but not by L-type Ca(2+) channels.

A mathematical model of pacemaker activity recorded from mouse small intestine

The pacemaker activity of interstitial cells of Cajal (ICCs) has been known to initiate the propagation of slow waves along the whole gastrointestinal tract through spontaneous and repetitive generation of action potentials. We studied the mechanism of the pacemaker activity of ICCs in the mouse small intestine and tested it using a mathematical model. The model includes ion channels, exchanger, pumps and intracellular machinery for Ca2+ regulation. The model also incorporates inositol 1,4,5-triphosphate (IP3) production and IP3-mediated Ca2+ release activities. Most of the parameters were obtained from the literature and were modified to fit the experimental results of ICCs from mouse small intestine. We were then able to compose a mathematical model that simulates the pacemaker activity of ICCs. The model generates pacemaker potentials regularly and repetitively as long as the simulation continues. The frequency was set at 20 min(-1) and the duration at 50% repolarization was 639 ms. The resting and overshoot potentials were -78 and +1.2 mV, respectively. The reconstructed pacemaker potentials closely matched those obtained from animal experiments. The model supports the idea that cyclic changes in [Ca2+]i and [IP3] play key roles in the generation of ICC pacemaker activity in the mouse small intestine.

Calcium events in smooth muscles and their interstitial cells; physiological roles of sparks

The observation of spontaneous sporadic releases of packets of stored calcium made 20 years ago has opened up a number of new concepts in smooth muscle physiology: (1) the calcium release sites are ryanodine and inositol 1,4,5-trisphosphate (IP3) receptor channels which contribute to cell-wide increases in [Ca2+]i in response to cell depolarization, activation of IP3-generating receptors, or other stimuli; (2) changes in [Ca2+]i act back on the cell membrane to activate or modulate K+, Cl- and cation channel activity so affecting contraction, in arterial smooth muscle for example affecting blood pressure; (3) IP3 production is voltage dependent and is believed to contribute to pacemaker potentials and to refractory periods which control the rhythmical motility of many hollow organs. Most smooth muscle tissues contain interstitial cells (ICs) in addition to contractile smooth muscle cells (SMCs). The interactions of these internal mechanisms, and in turn the interactions of SMCs and ICs in various smooth muscle tissues, are major factors in determining the unique physiological profiles of individual smooth muscles.

Role of voltage-dependent modulation of store Ca2+ release in synchronization of Ca2+ oscillations

Slow waves are rhythmic depolarizations that underlie mechanical activity of many smooth muscles. Slow waves result through rhythmic Ca(2+) release from intracellular Ca(2+) stores through inositol 1,4,5-trisphosphate (IP(3)) sensitive receptors and Ca(2+)-induced Ca(2+) release. Ca(2+) oscillations are transformed into membrane depolarizations by generation of a Ca(2+)-activated inward current. Importantly, the store Ca(2+) oscillations that underlie slow waves are entrained across many cells over large distances. It has been shown that IP(3) receptor-mediated Ca(2+) release is enhanced by membrane depolarization. Previous studies have implicated diffusion of Ca(2+) or the second messenger IP(3) across gap junctions in synchronization of Ca(2+) oscillations. In this study, a novel mechanism of Ca(2+) store entrainment through depolarization-induced IP(3) receptor-mediated Ca(2+) release is investigated. This mechanism is significantly different from chemical coupling-based mechanisms, as membrane potential has a coupling effect over distances several orders of magnitude greater than either diffusion of Ca(2+) or IP(3) through gap junctions. It is shown that electrical coupling acting through voltage-dependent modulation of store Ca(2+) release is able to synchronize oscillations of cells even when cells are widely separated and have different intrinsic frequencies of oscillation.

Direct voltage control of signaling via P2Y1 and other Galphaq-coupled receptors

Emerging evidence suggests that Ca2+ release evoked by certain G-protein-coupled receptors can be voltage-dependent; however, the relative contribution of different components of the signaling cascade to this response remains unclear. Using the electrically inexcitable megakaryocyte as a model system, we demonstrate that inositol 1,4,5-trisphosphate-dependent Ca2+ mobilization stimulated by several agonists acting via Galphaq-coupled receptors is potentiated by depolarization and that this effect is most pronounced for ADP. Voltage-dependent Ca2+ release was not induced by direct elevation of inositol 1,4,5-trisphosphate, by agents mimicking diacylglycerol actions, or by activation of phospholipase Cgamma-coupled receptors. The response to voltage did not require voltage-gated Ca2+ channels as it persisted in the presence of nifedipine and was only weakly affected by the holding potential. Strong predepolarizations failed to affect the voltage-dependent Ca2+ increase; thus, an alteration of G-protein betagamma subunit binding is also not involved. Megakaryocytes from P2Y1(-/-) mice lacked voltage-dependent Ca2+ release during the application of ADP but retained this response after stimulation of other Galphaq-coupled receptors. Although depolarization enhanced Ca2+ mobilization resulting from GTPgammaS dialysis and to a lesser extent during AlF4- or thimerosal, these effects all required the presence of P2Y1 receptors. Taken together, the voltage dependence to Ca2+ release via Galphaq-coupled receptors is not due to control of G-proteins or down-stream signals but, rather, can be explained by a voltage sensitivity at the level of the receptor itself. This effect, which is particularly robust for P2Y1 receptors, has wide-spread implications for cell signaling.

Sensitivity limits for voltage control of P2Y receptor-evoked Ca2+ mobilization in the rat megakaryocyte

G-protein-coupled receptor signalling has been suggested to be voltage dependent in a number of cell types; however, the limits of sensitivity of this potentially important phenomenon are unknown. Using the non-excitable rat megakaryocyte as a model system, we now show that P2Y receptor-evoked Ca2+ mobilization is controlled by membrane voltage in a graded and bipolar manner without evidence for a discrete threshold potential. Throughout the range of potentials studied, the peak increase in intracellular Ca2+ concentration ([Ca2+]i) in response to depolarization was always larger than the maximal reduction in [Ca2+]i following an equivalent amplitude hyperpolarization. Significant [Ca2+]i increases were observed in response to small amplitude (< 5 mV, 5 s duration) or short duration (25 ms, 135 mV) depolarizations. Individual cardiac action potential waveforms were also able to repeatedly potentiate P2Y receptor-evoked Ca2+ release and the response to trains of normally paced stimuli fused to generate prolonged [Ca2+]i increases. Furthermore, elevation of the temperature to physiological levels (36 degrees C) resulted in a more sustained depolarization-evoked Ca2+ increase compared with more transient or oscillatory responses at 20-24 degrees C. The ability of signalling via a G-protein-coupled receptor to be potentiated by action potential waveforms and small amplitude depolarizations has broad implications in excitable and non-excitable tissues.

Ca2+ phase waves: a basis for cellular pacemaking and long-range synchronicity in the guinea-pig gastric pylorus

Ca2+ imaging and multiple microelectrode recording procedures were used to investigate a slow wave-like electrical rhythmicity in single bundle strips from the circular muscle layer of the guinea-pig gastric pylorus. The 'slow waves' (SWs) consisted of a pacemaker and regenerative component, with both potentials composed of more elementary events variously termed spontaneous transient depolarizations (STDs) or unitary potentials. STDs and SW pacemaker and regenerative potentials exhibited associated local and distributed Ca2+ transients, respectively. Ca2+ transients were often larger in cellular regions that exhibited higher basal Ca2+ indicator-associated fluorescence, typical of regions likely to contain intramuscular interstitial cells of Cajal (ICCIM). The emergence of rhythmicity arose through entrainment of STDs resulting in pacemaker Ca2+ transients and potentials, events that exhibited considerable spatial synchronicity. Application of ACh to strips exhibiting weak rhythmicity caused marked enhancement of SW synchronicity. SWs and underlying Ca2+ increases exhibited very high 'apparent conduction velocities' ('CVs') orders of magnitude greater than for sequentially conducting Ca2+ waves. Central interruption of either intercellular connectivity or inositol 1,4,5-trisphosphate receptor (IP3R)-mediated store Ca2+ release in strips caused SWs at the two ends to run independently of each other, consistent with a coupled oscillator-based mechanism. Central inhibition of stores required much wider regions of blockade than inhibition of connectivity indicating that stores were voltage-coupled. Simulations, made using a conventional store array model but now including depolarization coupled to IP3R-mediated Ca2+ release, predicted the experimental findings. The linkage between membrane voltage and Ca2+ release provides a means for stores to interact as strongly coupled oscillators, resulting in the emergence of Ca2+ phase waves and associated pacemaker potentials. This distributed pacemaker triggers regenerative Ca2+ release and resultant SWs.

A theoretical model of slow wave regulation using voltage-dependent synthesis of inositol 1,4,5-trisphosphate

A qualitative mathematical model is presented that examines membrane potential feedback on synthesis of inositol 1,4,5-trisphosphate (IP(3)), and its role in generation and modulation of slow waves. Previous experimental studies indicate that slow waves show voltage dependence, and this is likely to result through membrane potential modulation of IP(3). It is proposed that the observed response of the tissue to current pulse, pulse train, and maintained current injection can be explained by changes in IP(3), modulated through a voltage-IP(3) feedback loop. Differences underlying the tissue responses to current injections of opposite polarities are shown to be due to the sequence of events following such currents. Results from this model are consistent with experimental findings and provide further understanding of these experimental observations. Specifically, we find that membrane potential can induce, abolish, and modulate slow wave frequency by altering the excitability of the tissue through the voltage-IP(3) feedback loop.

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.

Resting potentials and potassium currents during development of pulmonary artery smooth muscle cells

The pulmonary circulation changes rapidly at birth to adapt to extrauterine life. The neonate is at high risk of developing pulmonary hypertension, a common cause being perinatal hypoxia. Smooth muscle K+ channels have been implicated in hypoxic pulmonary vasoconstriction in adults and O2-induced vasodilation in the fetus, channel inhibition being thought to promote Ca2+ influx and contraction. We investigated the K+ currents and membrane potentials of pulmonary artery myocytes during development, in normal pigs and pigs exposed for 3 days to hypoxia, either from birth or from 3 days after birth. The main finding is that cells were depolarized at birth and hyperpolarized to the adult level of -40 mV within 3 days. Hypoxia prevented the hyperpolarization when present from birth and reversed it when present from the third postnatal day. The mechanism of hyperpolarization is unclear but may involve a noninactivating, voltage-gated K+ channel. It is not caused by increased Ca2+-activated or delayed rectifier current. These currents were small at birth compared with adults, declined further over the next 2 wk, and were suppressed by exposure to hypoxia from birth. Hyperpolarization could contribute to the fall in pulmonary vascular resistance at birth, whereas the low K+-current density, by enhancing membrane excitability, would contribute to the hyperreactivity of neonatal vessels. Hypoxia may hinder pulmonary artery adaptation by preventing hyperpolarization and suppressing K+ current.

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.

Effects of the potassium channel openers KRN4884 and levcromakalim on the contraction of rat aorta induced by A23187, compared with nifedipine

We examined the different vasodilatory effects of the K+ channel openers levcromakalim and 5-amino-N- [2-(2-chlorophenyl)ethyl]-N'-cyano-3-pyridinecarboxamidine (KRN4884), and the Ca2+ channel blocker nifedipine in the rat aorta. KRN 4884 (10(-10)-10(-5) M) and nifedipine (10(-10)-10(-5) M) produced concentration-dependent relaxation in the rat aorta precontracted by 25 mM KCl. The K+ channel blocker glibenclamide (1 microM) inhibited the relaxation induced by KRN4884 but did not influence nifedipine-induced relaxation. KRN4884 had almost no effect on contraction induced by 80 mM KCl, whereas nifedipine completely relaxed the muscle precontracted by 80 mM KCl, whereas nifedipine completely relaxed the muscle precontracted by 80 mM KCl. These results indicate that KRN4884 is a K+ channel opener. We investigated the relaxant effects of KRN4884 (10(-10)-10(-5) M), levcromakalim (10(-9)-10(-5) M) and nifedipine (10(-9)-10(-5) M) on A23187 (1 microM)-induced contraction. KRN4884 and levcromakalim had a potent relaxant effect but nifedipine only a weak effect on the smooth muscle contracted by A23187. Glibenclamide (1 microM) inhibited the relaxation induced by KRN4884 and levcromakalim, but did not influence the nifedipine-induced relaxation. KRN4884 (1 microM) produced a larger relaxation of A23187-induced contraction but had little effect on the increase in intracellular [Ca2+] induced by A23187. These results suggest that KRN4884 is a specific K+ channel opener and its vasodilating mechanisms involve not only deactivation of Ca2+ channels but also a decrease in the Ca2+ sensitivity of contractile elements.

Ca(2+)-induced inhibition of 45Ca2+ influx and Ca2+ current in smooth muscle of the rat vas deferens

The present study investigates how changes in intracellular Ca2+ concentration modulate the influx of 45Ca2+ in isolated rat vasa deferentia. Raising extracellular K+ concentration ([K+]0) to > or = 32 mM increased 45Ca2+ influx during the 1st min in solutions containing 0.03-1.5 mM extracellular Ca2+ concentration ([Ca2+]0). During the 6th min in [K+]0 > or = 50 mM, 45Ca2+ influx was less than during the 1st min. This decline in 45Ca2+ influx occurred for [Ca2+]0 > or = 0.4 mM. Procaine potentiated K(+)-stimulated 45Ca2+ influx in 1.5 mM [Ca2+]0 and eliminated the decline of 45Ca2+ influx in low [Ca2-]0. Ryanodine and norepinephrine reduced K(+)-stimulated 45Ca2+ influx. 45Ca2+ content changed with time in accordance with the changes observed in 45Ca2+ influx. In isolated cells, voltage-dependent inward currents inactivated more rapidly with 1.5 mM Ca2+ as the charge carrier than with 1.5 mM Ba2+, and the steady-state inactivation relationship was shifted in the hyperpolarizing direction. Inward current was reduced with either caffeine, ryanodine, or norepinephrine. The inhibitory effects of norepinephrine were abolished by depletion of intracellular Ca2+ stores. These results are compatible with the hypothesis that K(+)-stimulated 45Ca2+ influx declines with time due to Ca(2+)-induced inhibition of Ca2- channels. Ca(2+)- and inositol 1,4,5-trisphosphate-induced releases of Ca2+ from the sarcoplasmic reticulum appear to play an important role in this process.

Effect of membrane potential on the initiation of acetylcholine-induced Ca2+ transients in isolated guinea pig coronary myocytes

The muscarinic stimulation of single voltage-clamped coronary arterial smooth muscle cells of the guinea pig was used to evaluate the effect of membrane potential on the inositol 1,4,5-tris-phosphate (IP3)-mediated changes of ionized [Ca2+] in the cytoplasm (Ca2+ transient) measured with indo 1. When applied at the membrane potential of -50 mV, 10 micromol/L acetylcholine (ACh) induced a [Ca2+]i increase after the mean latency of 2.6+/-0.9 s. The latency was reduced to 1.1 +/- 0.3 s when the same dose was applied at a holding potential of +50 mV. In paired experiments in the same cells, the latency of response at +50 mV was reduced by a factor of 2.2 +/- 0.3 compared with the response at -50 mV. Supramaximal [ACh] (100 micromol/L) induced Ca2+ transients with a 0.4 +/- 0.1-s latency, which was independent of membrane potential. When applied repetitively at -50 mV, ACh induced Ca2+ transients with a progressively reduced amplitude and slower rate of rise. Depolarization to +50 mV accelerated the rate of rise of the Ca2+ transient by a factor of 3.4 +/- 0.4 without affecting the amplitude. The modulation of the initiation of Ca2+ transient by a 100-mV depolarization can be explained by an approximately threefold increase in the rate of IP3 accumulation.

Effects of GTP gamma S on muscarinic receptor-stimulated inositol phospholipid hydrolysis in permeabilized smooth muscle from the small intestine

1. Smooth muscle fragments from the longitudinal layer of the small intestine of the guinea-pig were permeabilized with Staphylococcus aureus alpha toxin (alpha-toxin) and used to investigate the role of G-protein activation in the regulation of muscarinic acetylcholine receptor (AChR)-stimulated inositol phospholipid hydrolysis. 2. The efficiency of alpha-toxin permeabilization was estimated by the release of [3H]-2-deoxyglucose ([3H]-2DG) after prior loading or lactate dehydrogenase (LDH) enzyme release from the smooth muscle fragments. 3. In alpha-toxin-permeabilized smooth muscle, but not in non-permeabilized muscle, GTP gamma S induced time- and concentration-dependent increases in labelled inositol phosphates. Carbachol (CCh) increased labelled inositol phosphates in both permeabilized and non-permeabilized muscle, although the increases were greater in non-permeabilized smooth muscle. The response to 100 microM CCh was severely reduced by 0.5 microM atropine. 4. In permeabilized muscle the effects of GTP gamma S or CCh on inositol phosphate levels were reduced by treatment with pertussis toxin (PTX) and completely inhibited by GDP beta S. 5. GTP gamma S caused a concentration-dependent inhibition of the CCh-induced increases in the levels of labelled inositol phosphates. Dibutyryl cyclic AMP or Sp-cAMPs (adenosine-3',5'-cyclic phosphorothiolate-Sp) reduced the effects of CCh on inositol phosphate levels. 6. The results suggest that muscarinic AChR activation induces inositol phospholipid hydrolysis via more than one G-protein in this smooth muscle and that several mechanisms may contribute to the modulation of both stimulatory and inhibitory responses observed.

Inhibition of muscarinic receptor-induced inositol phospholipid hydrolysis by caffeine, beta-adrenoceptors and protein kinase C in intestinal smooth muscle

1. The effects of caffeine, isoprenaline, dibutyryl cyclic AMP, isobutylmethylxanthine (IBMX), 12-O-tetradecanoylphorbol-13-acetate (TPA) or 1-oleoyl-2-acetylglycerol (OAG), (protein kinase C (PKC) activators), 2-methoxy verapamil (D600), thapsigargin and ryanodine on muscarinic acetylcholine receptor (AChR)-stimulated inositol phospholipid hydrolysis were studied in smooth muscle fragments from the longitudinal layer of the small intestine of the guinea-pig. 2. Incubation of the fragments with the muscarinic agonist, carbachol (CCh) (100 microM) resulted in rapid increases in the levels of all the inositol phosphate isomers with maximal increases in the [3H]-inositol (1,4,5) trisphosphate ([3H]-Ins(1,4,5)P3) isomer occurring 10 s following incubation. 3. The beta-adrenoceptor agonist, isoprenaline (10 microM) and dibutyryl cyclic AMP (10 microM), a membrane permeant analogue of cyclic AMP both reduced the CCh stimulation, but not the basal levels of [3H]-inositol phosphates. This inhibition by dibutyryl cyclic AMP was enhanced in the presence of the phosphodiesterase inhibitor, IBMX. CCh inhibited the isoprenaline-induced increases in the levels of cyclic AMP and this was via a pertussi toxin (PTX)-sensitive G-protein mechanism. 4. TPA (1 microM) and OAG (100 microM) a 1,2-diacylglycerol (DAG) analogue both reduced the CCh-induced increases in [3H]-inositol phosphates levels but neither affected basal values nor the basal levels of cyclic AMP. 5. D600 (10 microM), which blocks voltage-dependent Ca2+ channels, also reduced the CCh-stimulated levels of [3H]-inositol phosphates suggesting that some of the agonist-induced increases are due to a potentiating effect of Ca2+ entering the cell. 6. Caffeine (0.5-30 mM) significantly inhibited both the basal and CCh-induced increases in all the [3H]-inositol phosphate isomers. Its inhibitory action was not due to increases in cyclic AMP since caffeine had no effect on the levels of cyclic AMP at concentrations up to 30 mM. 7. Incubation with thapsigargin (1 microM) and ryanodine (10 microM) had no effect on either basal or CCh-induced inositol phospholipid hydrolysis or cyclic AMP levels. 8. The results indicate a reciprocal inhibition by beta-adrenoceptors and muscarinic AChRs of their effects on cyclic AMP and inositol phosphate levels respectively. Ca2+ entering the cell (but not the action of ryanodine or thapsigargin) potentiates while caffeine inhibits muscarinic AChR-induced rises in inositol phosphate levels. Diacylglycerols may exert a negative feedback inhibition on inositol phosphate production.

Effects of membrane depolarization and changes in intra- and extracellular calcium concentration on phosphoinositide hydrolysis in bovine tracheal smooth muscle

Agonist-stimulated phosphoinositide metabolism plays a central role in pharmacomechanical coupling in airways smooth muscle (ASM). In many other tissues and cells, most noteably excitable cells, membrane depolarization or an increase in intracellular Ca2+ ([Ca2+]i) generated by inositol 1,4,5-trisphosphate (Ins(1,4,5)P3)-induced Ca2+ release or agonist-mediated Ca2+ influx is able to trigger or augment phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) hydrolysis and/or initiate PtdIns4P/PtdIns hydrolysis by direct stimulation of PIC. To assess the importance of these mechanisms in ASM the effects of KCl-induced membrane depolarization, extracellular Ca2+ ([Ca2+]e) chelation, and addition of ionomycin to elevate [Ca2+]i on basal and agonist-stimulated Ins(1,4,5)P3 concentration and [3H]-InsPx accumulation have been examined. Reducing [Ca2+]e from 1.8 mM to 6 or 0.8 microM caused a progressive inhibition of agonist-stimulated [3H]inositol polyphosphate accumulation over 30 min with the histamine-stimulated response being significantly more sensitive to [Ca2+]e chelation than the response to carbachol. In contrast, the initial accumulation of Ins(1,4,5)P3 was completely unaffected by such reductions in [Ca2+]e. Incubation of [3H]inositol-prelabelled BTSM slices with buffer containing 80 mM KCl failed to stimulate [3H]InsPx accumulation, causing instead a small inhibition of carbachol-stimulated [3H]InsPx accumulation with a similar effect seen with respect to Ins(1,4,5)P3 accumulation. Addition of 5 microM ionomycin to BTSM slices similarly did not stimulate Ins(1,4,5)P3 generation and only increased [3H]InsPx accumulation after prolonged stimulation in the presence of high (mM) [Ca2+]e. These data indicated that in ASM, membrane depolarization or physiological increases in [Ca2+]i did not result in either independent activation of PIC or augmentation of initial agonist-stimulated PtdIns(4,5)P2 hydrolysis. However, while the initial agonist-stimulated generation of Ins(1,4,5)P3 was not dependent on [Ca2+]e, a normal plasmalemmal Ca2+ gradient was required to sustain maximal rates of agonist-stimulated PtdIns(4,5)P2 hydrolysis.

The relative importance of extracellular and intracellular calcium in the responses of the human vas deferens to noradrenaline and potassium: a study using Ca(2+)-deprivation and Ca(2+)-antagonists

1. Mechanical responses of the human vas deferens, activated by noradrenaline (50-100 microM) or high potassium (130 mM), showed either biphasic shortening or lengthening or a combination of initial shortening and lengthening. These are interpreted as representing the contractions of longitudinal and circular muscle respectively. 2. Caffeine (10-20 mM) induced only shortening responses which were 86% (SE 34, n = 7) of that caused by noradrenaline (100 microM). 3. The calcium channel antagonists, nifedipine, verapamil and diltiazem (0.01-10 microM), inhibited responses to high potassium, and the initial phase of shortening and lengthening responses to noradrenaline. However, the secondary phase of the shortening response to noradrenaline (100 microM) was relatively insensitive to these antagonists. 4. In calcium-free (1 mM EGTA) media, noradrenaline (100 microM) could repeatedly induce both shortening and lengthening tonic responses which were 39 +/- 13% (n = 4) and 40 +/- 16% (n = 5) of their values in Krebs media. Except for a small shortening, responses to high potassium were abolished. Calcium-free media also blocked phasic bursts of mechanical activity. 5. Calcium removal during a prolonged exposure to noradrenaline (50 microM) caused a fall of tension of the lengthening but not the shortening response. Depletion of intracellular stores caused an inhibition of the responses to noradrenaline. Recovery of responses following restoration of calcium was blocked by nifedipine for the shortening response but not for lengthening. 6. We conclude that longitudinal and circular muscle rely to different degrees on activation by extracellular calcium and intracellular mechanisms. The longitudinal, but not circular, muscle appears mainly to use an intracellular calcium pool which is replenished via nifedipine-sensitive calcium channels and has a caffeine sensitive store. Evidence suggesting differences in the role of calcium and processes controlling its entry during activation of both muscle types is presented and discussed.

Effects of muscarinic agonists and depolarizing agents on inositol monophosphate accumulation in the rabbit vagus nerve

The effects of muscarinic agonists and depolarizing agents on inositol phospholipid hydrolysis in the rabbit vagus nerve were assessed by the measurement of [3H]inositol monophosphate production in nerves that had been preincubated with [3H]inositol. After 1 h of drug action, carbachol, oxotremorine, and arecoline increased the inositol monophosphate accumulation, though the maximal increase induced by these agonists differed. Addition of the muscarinic antagonists atropine or pirenzepine shifted the carbachol dose-response curves to the right, without decreasing the carbachol maximal stimulatory effects. The KB for pirenzepine was 35 nM, which is characteristic of muscarinic high-affinity binding sites coupled to phosphoinositide turnover and often associated with the M1 receptor subtype. On the other hand, agents known to depolarize or to increase the intracellular Ca2+ concentration, e.g., elevated extracellular K+, ouabain, Ca2+, and the Ca2+ ionophore A23187, also increased inositol monophosphate accumulation. These effects were not mediated by the release of acetylcholine, as suggested by the fact that they could not be potentiated by the addition of physostigmine nor inhibited by the addition of atropine. The Ca(2+)-channel antagonist Cd2+, also known to inhibit the Na+/Ca2+ exchanger, was able to block the effects of K+ and ouabain, but did not alter those of carbachol. These results suggest that depolarizing agents increase inositol monophosphate accumulation in part through elevation of the intracellular Ca2+ concentration and that muscarinic receptors coupled to phosphoinositide turnover are present along the trunk of the rabbit vagus nerve.

Depolarization and agonist-stimulated changes in inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate mass accumulation in rat cerebral cortex

Muscarinic receptor stimulation or depolarization with elevated extracellular K+ induced rapid and sustained increases in mass accumulations of myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and myo-inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] in cerebral cortex slices. Synergistic but transient responses of both inositol polyphosphate second messengers were observed when slices were stimulated with carbachol under depolarizing conditions; this synergy was observed as an increase in the maximal responsiveness, with no significant change in EC50 values for carbachol. Omission of buffer Ca2+ ([Ca2+]e 10-20 microM) reduced basal Ins(1,4,5)P3 and Ins(1,3,4,5)P4 concentrations; the relative stimulatory effects of muscarinic receptor stimulation were maintained, but the effects of depolarization were markedly attenuated under these conditions. A component of the response to depolarization appeared to be indirectly mediated by the release of acetylcholine, because the K(+)-evoked increase in Ins(1,3,4,5)P4 was enhanced by the cholinesterase inhibitor physostigmine, and was partially attenuated by atropine. An additive suppression by nitrendipine suggests that entry of Ca2+ through L-type Ca2+ channels may serve to accelerate phosphorylation of Ins(1,4,5)P3 by 3-kinase. Norepinephrine did not significantly increase Ins(1,4,5)P3 or Ins(1,3,4,5)P4 accumulation; however, in the presence of depolarizing K+, norepinephrine caused a dramatic increase in Ins(1,3,4,5)P4 mass accumulation. In contrast, the excitatory amino acid quisqualate caused significant increases in the mass accumulations of both inositol polyphosphates measured, with no further increase being observed under depolarizing conditions. The results are discussed with respect to the interactive effects of agonist and depolarization stimuli on inositol polyphosphate accumulation which might more accurately reflect the conditions pertaining in vivo.

Effects of surgical sympathetic denervation on myo-inositol trisphosphate production and contraction in the dilator and sphincter smooth muscles of the rabbit iris: evidence for interaction between the cyclic AMP and calcium signaling systems

The effects of norepinephrine (NE), carbachol (CCh), NaF, 3-isobutyl-1-methylxanthine (IBMX), and high K+ concentration (80 mM) depolarization on inositol trisphosphate (IP3) accumulation, cyclic AMP (cAMP) formation, and contraction were investigated in the dilator and sphincter smooth muscles of the sympathetically denervated as well as the normal rabbit eye. (a) In the denervated dilator muscle, NE-stimulated IP3 production and contraction are enhanced. (b) In the sphincter muscle of rabbits that have undergone sympathetic denervation. CCh-stimulated IP3 production and contraction are attenuated. (c) The increase in tension by a maximal effective dose of NaF (209 mM) in the dilator was 12.5 and 18 mg of tension/mg wet weight in normal and denervated tissue, respectively, and in the sphincter was 33.8 and 15.2 mg of tension/mg wet weight in normal and denervated tissue, respectively. NaF had no effect on cAMP formation. (d) Addition of NE had no effect on cAMP formation in both the normal and denervated dilator, whereas basal and IBMX-induced cAMP formation increased. in the denervated sphincter over that of the normal tissue by 15 and 60%, respectively. (e) Isoproterenol (5 microM) increased cAMP formation in the normal and denervated sphincter by 47 and 91%, respectively. (f) Whereas CCh inhibits cAMP formation in the normal sphincter, it lost its inhibitory effect in the sphincter with denervation. (g) IBMX (0.1 mM) attenuated the CCh-stimulated IP3 production and contraction of the sphincter by approximately 30% of their respective controls. (h) High K+ concentration depolarization attenuated contraction in both dilator and sphincter muscles with denervation. These observations suggest that an increase in the level of cAMP in the iris sphincter due to sympathetic denervation could lead to inhibition of phospholipase C (or other target sites, such as phosphorylation of the muscarinic receptor, Gp protein itself, myosin light chain kinase, or the IP3 receptor), IP3 production, and contraction. In conclusion, we suggest that the supersensitivity and subsensitivity observed after surgical sympathetic denervation of the iris dilator and sphincter muscles, respectively, are caused by alterations in the efficiency of coupling, probably through the Gp proteins, between their respective receptors and the breakdown of polyphosphoinositides by phospholipase C. In addition, we propose that the sympathetic nervous system can regulate, through alterations in cAMP levels, the muscarinic stimulation of IP3 accumulation and contraction in the iris sphincter. These findings add further support to the hypothesis that there are reciprocal interactions between the cAMP and IP3-Ca2+ signaling systems and the contractile response in the iris smooth muscle.

Subtypes and excitation-contraction coupling mechanisms for neurokinin receptors in smooth muscle of the guinea-pig Taenia caeci

This study investigated the subtype and coupling mechanisms mediating the direct contractile response to tachykinins in the guinea-pig Taenia caeci preparation in vitro. Coupling of neurokinin receptors was compared throughout with coupling of muscarinic receptors. The smooth muscle neurokinin receptors seem to be predominantly of the NK-1 subtype. Thus, the relative activities of the common naturally-occurring tachykinins fell within one order of magnitude, and the selective NK-1 receptor agonist substance P methyl ester was high in activity (0.38 relative to substance P). Some contribution from NK-3 receptors is, however, possible in view of the appreciable activity of the selective NK-3 agonist succ-[Asp6, N-MePhe8]-SP(6-11) (senktide; activity 0.004 relative to substance P), and NK-2 or NK-3 receptors in view of the higher activity of the D-isomer of [Glp6, *Pro9]-SP(6-11) as compared to its NK-1 selective L-isomer (D/L-activity ratio 1.53). Contractile actions of tachykinins were compared with carbachol for reliance on membrane-potential dependent (electromechanical) and membrane-potential independent (pharmacomechanical) coupling mechanisms. Log concentration-response curves to carbachol and substance P in normal Krebs' medium were compared with curves obtained in a high-K+ solution where processes dependent on changes in membrane potential could play no part in excitation. In the high-K+ depolarizing solution, a concentration-related relationship was maintained, though with some diminution in the maximal additional tension generated: the maximum tension with carbachol was under both conditions greater than that with substance P. The relative effects of several tachykinins and carbachol in producing receptor-mediated changes in membrane permeability through presumed receptor-operated ion channel opening, was estimated in terms of the ability to increase 86Rb-efflux, as a marker for K+, in a high-K+ depolarizing solution. Carbachol (10 microM) consistently increased 86Rb-efflux. In contrast, no permeability increase could be detected with any tachykinin tested (substance P, eledoisin, substance P methyl ester, neurokinin A, neurokinin B, 1 or 10 microM). Tachykinins and carbachol were compared in terms of ability to increase phosphatidylinositol hydrolysis. Both substance P and carbachol showed a concentration-related increase in accumulation of total inositol phosphates; though the maximal response to carbachol was considerably greater than that to any tachykinin (substance P, eledoisin, substance P methyl ester, senktide, neurokinin A, neurokinin B), or combination of two tachykinins (substance P and eledoisin, senktide and substance P methyl ester).(ABSTRACT TRUNCATED AT 400 WORDS)

Paradoxical effects of thioridazine on electromechanical coupling in the human and rat vas deferens

The effects of thioridazine on the responses of isolated human and rat vas deferens to high [K+]0, A23187 and caffeine were examined. In the presence of Ca2+ (2.5 mM), thioridazine (1-10 microM) induced spontaneous contractions but caused a dose-related inhibition of the phasic and secondary parts of the response to high [K+]0 (136 mM). The relaxation phase of the high [K+]0 response of the human vas deferens was unaffected by thioridazine (up to 10 microM). In Ca2(+)-free/EGTA (0.5 mM) media, thioridazine caused a dose-related potentiation, shortened the latency and prolonged the duration of high [K+]0 responses. Contractions to caffeine (20 mM) and A23187 (20-50 microM) were relatively unchanged by thioridazine (10 microM). The spontaneous activity caused by thioridazine (10 microM) was sensitive to the Ca2(+)-channel blockers nifedipine (10 microM) or verapamil (10 microM). These results indicate that the action of thioridazine during electromechanical coupling in the human and rat vas deferens may involve more than its blockade of voltage gated Ca2+ channels.

Reduction of Na+ enhances phosphoinositide hydrolysis and differentiates the stimulatory and inhibitory responses to quisqualate in rat brain slices

The concentration of Na+ in the incubation medium significantly influenced phosphoinositide hydrolysis induced by some, but not all, agonists in rat cerebral cortical slices. Reductions of the Na+ concentration below 120 mM resulted in incremental increases in basal and norepinephrine-stimulated accumulation of [3H]inositol monophosphate in cortical slices that had been prelabelled with [3H]inositol, and maximal responses were obtained with 0 and 5 mM Na+. In contrast, the responses to carbachol and ibotenate were similar in medium containing 120 or 5 mM Na+. In medium with 120 mM Na+, quisqualate has two effects on phosphoinositide hydrolysis in cortical slices, including a relatively weak stimulatory effect and an inhibitory modulation of the stimulation induced by norepinephrine. These two responses to quisqualate were differentially modulated by Na+; in 5 mM compared with 120 mM Na+ the stimulatory response was greatly increased and the inhibitory effect was mostly eliminated. That these were two separate events was confirmed by the use of L-BOAA (beta-N-oxalyl-L-alpha, beta-diaminopropionic acid), which reproduces the inhibitory, but not the stimulatory effect of quisqualate on phosphoinositide hydrolysis. In 5 mM Na+, inhibition by L-BOAA of norepinephrine-stimulated phosphoinositide hydrolysis was completely eliminated. These results demonstrate that a physiological concentration of Na+ maintains phosphoinositide hydrolysis at a submaximal level of sensitivity to some, but not all, agonists. The differential effects of Na+ on the stimulatory and inhibitory effects of quisqualate further substantiate the suggestion that these are two separate processes and indicate that alterations of the Na+ concentration may influence the effects of quisqualate, and other agonists, on phosphoinositide hydrolysis.

Increased intracellular calcium stimulates 3H-inositol polyphosphate accumulation in rat cerebral cortical slices

Agents that increase the intracellular Ca2+ concentration have been examined for their ability to stimulate 3H-inositol polyphosphate accumulation in rat cerebral cortex slices. Elevated extracellular K+ levels, the alkaloid sodium channel activator veratrine, the calcium ionophore ionomycin, and the marine toxin maitotoxin were all able to stimulate phosphoinositide metabolism. Certain features appear common to the agents studied. Thus, although [3H]inositol monophosphate, [3H]inositol bisphosphate ([3H]InsP2), and [3H]inositol trisphosphate were all stimulated, a proportionally greater effect was observed on [3H]InsP2 in comparison to stimulation by the muscarinic receptor agonist carbachol. However, only an elevated K+ level stimulated [3H]inositol tetrakisphosphate ([3H]InsP4) accumulation alone or produced marked synergy with carbachol on the formation of this polyphosphate. The results suggest that agents that elevate the cytoplasmic Ca2+ concentration in cerebral cells can increase the hydrolysis of membrane polyphosphoinositides. The pattern of the response differs from that produced by muscarinic receptor agonists and indicate that Ca2(+)-dependent hydrolysis may involve different pools of lipids, phosphoinositidase C enzymes, or both. However, clear differences in the ability of these agents to stimulate InsP4, alone or in the presence of muscarinic agonist, suggest that factors other than a simple elevated intracellular Ca2+ concentration are implicated.

K(+)-stimulation of the phosphoinositide pathway in guinea-pig ileum longitudinal smooth muscle is predominantly neuronal in origin and mediated by the entry of extracellular Ca2+

1. K+ and scorpion toxin stimulate formation of inositol phosphates in guinea-pig ileum longitudinal smooth muscle slices. The response to these two agents is not additive. 2. The response to K+ is inhibited partially by nifedipine and partially by omega-conotoxin. When given together the effect of these two Ca2+ channel blockers is additive and the response to K+ is reduced by more than 80%. 3. The response to scorpion toxin is inhibited completely by tetrodotoxin, partially by omega-conotoxin but not by atropine or nifedipine. Scorpion toxin induces a similar formation of inositol phosphates in collagenase-dispersed cells to that seen in cross-chopped slices. 4. The responses to scorpion toxin and K+ are inhibited completely when the extracellular Ca2+ concentration is reduced to below cytosolic levels (less than 100 nM). 5. Neither nifedipine nor omega-conotoxin, either alone or in combination, inhibited formation of inositol phosphates by substance P or carbachol. Both of these agonists induced a significant formation of inositol phosphates even when the extracellular Ca2+ concentration was reduced to 10 nM. 6. These results indicate that K+ and scorpion toxin induce formation of inositol phosphates through the mobilisation of extracellular Ca2+. The response to K+ appears to occur predominantly in neuronal cells.

Calcium uptake-dependent and -independent mechanisms of inositol trisphosphate formation in adrenal chromaffin cells: comparative studies with high K+, carbamylcholine and angiotensin II

When [3H]inositol prelabelled cultured bovine adrenal chromaffin cells were stimulated with 56 mM KCl (high K+), 300 microM carbamylcholine (CCh) or 10 microM angiotensin II (Ang II), a rapid accumulation of [3H]IP3 was observed. At the same time, high K+ or CCh induced rapid increases in 45Ca2+ uptake, but Ang II did not induce a significant 45Ca2+ uptake. The concentration-response curve for KCl-induced [3H]IP3 accumulation coincided well with that for KCl-induced 45Ca2+ uptake into the cells. Nifedipine, a Ca2+ channel antagonist, inhibited the high K(+)-induced [3H]IP3 accumulation and 45Ca2+ uptake with a similar potency. Nifedipine at a similar concentration range also inhibited CCh-induced 45Ca2+ uptake. Although nifedipine inhibited CCh-induced [3H]IP3 accumulation, the potency was approximately 300-fold less than that for the inhibition of 45Ca2+ uptake. Nifedipine failed to affect the Ang II-induced [3H]IP3 accumulation. BAY K 8644 (2 microM), a Ca2+ channel activator, plus partially depolarizing concentration of KCl (14 mM), induced 45Ca2+ uptake and [3H]IP3 accumulation. Ionomycin (1 microM and 10 microM), a Ca2+ ionophore, also induced 45Ca2+ uptake and [3H]IP3 accumulation in a concentration-dependent manner. Pretreatment of the cells with protein kinase C activator, 100 nM 12-O-tetradecanoyl phorbol-13-acetate, for 10 min, partially inhibited CCh and Ang II-induced [3H]IP3 accumulation, but failed to inhibit the high K(+)-induced accumulation. Furthermore, the effects of high K+ and Ang II on the IP3 accumulation was additive. Ang II and CCh induced a rapid and transient increase in inositol 1,4,5-trisphosphate (1,4,5-IP3) accumulation (5 s) followed by a slower accumulation of inositol 1,3,4-trisphosphate (1,3,4-IP3). High K+ evoked an increase in 1,3,4-IP3 accumulation but obvious accumulation of 1,4,5-IP3 could not be detected. In Ca2(+)-depleted medium, high K(+)-induced [3H]IP3 accumulation was completely abolished, whereas [3H]IP3 accumulation induced by CCh and Ang II was partially inhibited. These results demonstrate the existence of the Ca2+ uptake-triggered mechanism of IP3 accumulation represented by high K+, and also the Ca2+ uptake-independent mechanism of IP3 accumulation represented by Ang II in cultured bovine adrenal chromaffin cells. Mechanism of CCh-induced IP3 accumulation has an intermediate property between those of high K+ and Ang II.

Dual effects of K+ depolarisation on inositol polyphosphate production in rat cerebral cortex

Depolarisation of [3H]inositol-prelabelled slices of rat cerebral cortex with elevated extracellular K+ induced a rapid and marked increase in inositol polyphosphate accumulation. Addition of the muscarinic antagonist atropine (10 microM) markedly inhibited the K+-induced accumulation of inositol tetrakisphosphate (InsP4), with only a slight reduction in stimulated inositol bis- and trisphosphate levels. Inhibitory effects on InsP4 were noted at the earliest time period measured (30 s) and suggested the involvement of released endogenous acetylcholine in part of the response. The atropine-insensitive component of depolarisation did not appear to be secondary to release of noradrenaline, histamine, or 5-hydroxytryptamine, because addition of prazosin, mepyramine, or ketanserin was without effect on the K+ response. Furthermore, secretion of a neuropeptide that could stimulate phosphoinositide hydrolysis was unlikely, because the peptidase inhibitor bacitracin was also without effect. The results suggest that endogenous acetylcholine can stimulate phosphoinositide metabolism by interacting with muscarinic receptors and that this is particularly evident on InsP4 accumulation. Atropine-insensitive responses may be secondary to Ca2+ entry via voltage-sensitive channels.

Ketamine-induced relaxation in intact and skinned smooth muscles of the rabbit ear artery

1. The effects of ketamine, an intravenous anaesthetic, on the rabbit ear artery were investigated by measuring the tension in intact and saponin-treated skinned smooth-muscle fibres. 2. Ketamine dose-dependently inhibited contractions of intact smooth-muscle fibres induced by high K+ solution and by noradrenaline (NA) or histamine in Krebs solution. This drug similarly attenuated both phasic and tonic contractions induced by high K+ solution. 3. Ketamine also inhibited NA- or histamine-induced contractions in Ca2+-free solution containing 2mM EGTA, but it did not affect the caffeine-induced contraction in this solution. 4. Because the pCa-tension relationship of saponin-treated skinned smooth-muscle fibres was not affected, it can be proposed that ketamine does not have an effect on the contractile proteins. 5. In the presence of 5mM NaN3, 20 microM inositol 1,4,5-trisphosphate (InsP3) or 25mM caffeine produced a contraction in skinned smooth-muscle fibres after accumulation of Ca2+ by intracellular stores. Analysis of the InsP3- or caffeine-induced contractions indicates that ketamine does not have an effect on the Ca2+ accumulation into and Ca2+ release from the intracellular stores. 6. These results indicate that the relaxant effects produced by ketamine in the rabbit ear artery are not likely to be due to an intracellular action. The inhibitory effects of ketamine could be caused by a decrease of the Ca2+ influx through the plasma membrane or interference with the process of signal transduction between receptors on the plasma membrane and intracellular stores.

The diacylglycerol kinase inhibitor, R 59022, suppresses contractility of intestinal smooth muscle

R 59022, a compound which causes the accumulation of diacylglycerol by inhibition of the enzyme diacylglycerol kinase, was tested for its effect on stimulated motor activity of the longitudinal muscle from the guinea-pig ileum. Motor responses to histamine were very potently inhibited by R 59022 (greater than or equal to 0.03 microM), which confirms its known activity as a histamine receptor antagonist. Contractions elicited by acetylcholine, substance P, or K+ depolarization were also concentration dependently depressed by R 59022 (1-30 microM); further analysis showed that substance P was antagonized in a non-competitive manner. R 59022 was significantly more active to block the tonic than the phasic component of the contractile response to K+ depolarization. The Ca2+-induced activation of the contractile apparatus in chemically skinned muscle strips was depressed by similar concentrations of R 59022 (3-30 microM). These data indicate that R 59022 suppresses the contractile activity of intestinal smooth muscle at an intracellular site of action but it is not yet clear whether this action can be accounted for by the accumulation of diacylglycerol and subsequent stimulation of protein kinase C.

Factors involved in the generation of tension during contraction to high potassium in the rat vas deferens

A large number of studies indicate that K(+)-induced contractions of smooth muscle depend on extracellular calcium. If these contractions depend exclusively on extracellular calcium then contractile responses to 140 mM K+, which are larger than the response to 35 mM K+, should be associated with a larger influx of 45Ca. This is not the case in the vas deferens from reserpine pretreated rats. During a 2 min interval, 45Ca influx induced by 140 mM K+ was identical to that produced by 35 mM K+. This suggests that a second mechanism may be involved in responses to high K+. Indeed, 140 mM K+ caused an approximately 300% increase above control in the formation of inositol trisphosphate (IP3) in tissues prelabelled with 3H-myoinositol whereas 35 mM K+ did not increase IP3. IP3 is thought to cause the release of calcium from internal stores which is consistent with our finding of an increase in 45Ca efflux into calcium-free medium from tissues prelabelled with 45Ca and stimulated with 140 mM K+. Stimulation with 35 mM K+ did not influence 45Ca efflux. We conclude that in the rat vas deferens high K+ promotes tension development by smooth muscle by a dual mechanism: influx of extracellular calcium and release of calcium from internal stores via an IP3 mechanism.

Lowering of the extracellular Na+ concentration enhances high-K+-induced formation of inositol phosphates in the guinea-pig ileum

1. Formation of inositol phosphates (InsPs) was measured in cross-chopped slices or dispersed cells, isolated by collagenase treatment, of guinea-pig ileum longitudinal smooth muscle pre-labelled with [3H]inositol. 2. Elevation of the extracellular K+ concentration by equimolar replacement of Na+ induced accumulation of InsPs in the dispersed cells and in the tissue slices. These effects were blocked by neither tetrodotoxin (1 microM) nor atropine (10 microM), and were approximately additive with carbachol-induced accumulation. 3. In the tissue slices, the response to K+ was partially inhibited by nifedipine (10 microM) and by CdCl2 (0.3 mM), but the carbachol-induced response was not altered. 4. Accumulation of InsPs induced by KCl-excess solution (high-K+ solution without Na+ replacement) was suppressed strongly by nifedipine and completely by CdCl2. The response to KCl excess was approx. 40% of that to high K+ with Na+ replacement. 5. Low-NaCl solution (replacement of NaCl with equimolar sucrose) also produced InsPs, and this was not blocked by either nifedipine (10 microM) or CdCl2 (0.3 mM). 6. The formation of InsPs by a maximally effective concentration of carbachol (1 mM) in the presence of KCl excess or low NaCl was greater than the additive effect of the two stimuli on their own. Enhancement of the carbachol-induced response by KCl excess disappeared in the presence of CdCl2 (0.3 mM). 7. These data suggest that formation of InsPs induced by high-K+ solution with equimolar replacement of Na+ consists of two components, i.e. high-K+-induced inositol-phospholipid hydrolysis by Ca2+ entry through voltage-sensitive channels, and low-Na+-induced formation of InsPs, insensitive to Ca2+ antagonists, but that both of them do not contribute significantly to the activation of phospholipase C by muscarinic stimuli.

Does the hydrolysis of inositol phospholipids lead to the opening of voltage operated Ca2+ channels in guinea-pig ileum? Studies with fluoride ions and caffeine

Fluoride ions (1-30 mM) stimulate phosphoinositide hydrolysis in guinea-pig ileum longitudinal smooth muscle slices, and this is not inhibited in the presence of indomethacin or nifedipine. This action is associated with a slow contractile response which peaks after approximately five minutes and then declines towards baseline; at this time the contractile response to a maximally effective concentration of carbachol is also inhibited. Fluoride-induced contractions are inhibited completely in the presence of nifedipine. Similarly, contractions induced by caffeine, which releases Ca2+ from intracellular stores, are also inhibited by nifedipine. These data are consistent with a model in which the activation of a G-protein by F- ions leads to the following sequential events: activation of phospholipase C, release of intracellular Ca2+, opening of voltage operated (i.e. dihydropyridine sensitive) Ca2+ channels and contraction. The transient nature of the fluoride contraction and the inhibition of the carbachol contraction may be due to a slow elevation of cAMP levels induced by F-.

Effect of phorbol ester on contractile response of aorta from endotoxic rats

The influence of phorbol ester on the isometric contractile response of aorta from endotoxic rats was examined. In endotoxic rat aorta, the contractile responses to KCl and phorbol 12,13-dibutyrate (PDBu) were both remarkably diminished, compared to those in control rat aorta. Preincubation with PDBu augmented the aortic contractile response to KCl in both control and endotoxic rats. This augmentative effect of PDBu was significantly more pronounced in endotoxic rats than in controls. When the contractile response to 80 mM KCl reached a plateau after PDBu pretreatment, addition of 5 mM CaCl2 (final concentration) to the organ bath completely reversed the diminished contractile response of endotoxic rat aorta to the control level. These results suggest that the hyporesponsiveness of endotoxic rat aorta to KCl may be caused by decreases in both protein kinase C mediated response and calcium sensitivity of vascular smooth muscle cells.

Inositol trisphosphate accumulation by high K+ stimulation in cultured adrenal chromaffin cells

Stimulation with high K+ (KCl, 56 mM) of myo-[3H]inositol-prelabelled cells increased Ca2+ uptake and [3H]inositol trisphosphate (IP3) accumulation in a concentration-dependent manner. Nifedipine, a Ca2+ channel antagonist, inhibited high K+-induced [3H]IP3 accumulation and 45Ca2+ uptake with a similar potency. Furthermore, ionomycin (1 microM), a Ca2+ ionophore, also induced 45Ca2+ uptake and [3H]IP3 accumulation. These results indicate the existence of the Ca2+ uptake-triggered mechanism of IP3 formation in cultured adrenal chromaffin cells.

Effects of potassium and norepinephrine on calcium influx in guinea-pig vas deferens

The effects of potassium (K+) and adrenoceptor agonists on 45Ca influx and contraction of isolated vas deferens from reserpine-treated guinea-pigs has been investigated. K+ (18-84 mM) increased 45Ca influx with the maximum influx occurring at 50 mM K+. The rate of influx upon exposure to K+ was highest in the first 2 min and decreased thereafter. In contrast to K+, norepinephrine, methoxamine and xylazine did not increase the rate of 45Ca influx. Norepinephrine and methoxamine, but not xylazine, inhibited 45Ca influx induced by high K+. The inhibitory effect of norepinephrine was also present in denervated tissues. Prazosin, but not yohimbine or propranolol, blocked the inhibitory effect of norepinephrine. Potassium-induced contractions were significantly potentiated in the presence of norepinephrine, especially at low calcium concentrations. We conclude that K+ contracts the guinea-pig vas deferens by increasing the influx of calcium whereas stimulation of alpha 1-adrenoceptors prevents 45Ca influx elicited by high potassium.

Increased reactivity in the mesenteric artery of spontaneously hypertensive rats to phorbol ester

The contraction responses of mesenteric artery from 10 week old spontaneously hypertensive rats (SHRs) and normotensive Wistar Kyoto controls (WKYs) to phorbol 12, 13 - dibutyrate (PDBu) and agents acting on the potential-operated calcium channels were compared. The vessels from the SHR were significantly more sensitive to PDBu than those from the WKY. The PDBu-induced contractions were inhibited by nifedipine. The vessels from the SHR were also more sensitive to Bay K 8644 and KCl than the WKY. Low concentrations of PDBu (1 nM) potentiated the KCl contraction significantly more in the SHR than the WKY. It is suggested that the increased reactivity to PDBu in the SHR may in part be related to changes in the activity of the potential-operated calcium channels.

Interaction of phorbol esters with Ca2+ channels in smooth muscle

The phorbol ester 12-O-tetradecanoyl phorbol-13-acetate (TPA), a selective activator of protein kinase C, had no effect on the sensitivity to Ca2+ or verapamil of K+-depolarized taenia preparations from the guinea-pig caecum, despite the use of high concentrations (1 microM for 3 h); this preparation is sensitive to Ca2+ channel activators and antagonists. TPA (0.03-3 microM) caused a slow contraction of rat aorta preparations; the contractions were resistant to the calcium-antagonists nifedipine (0.01 microM), verapamil (10 microM), diltiazem (10 microM) and cinnarizine (10 microM), but were antagonized by N-(6-aminohexyl)-5-chloro-1-naphthalensulphonamide (W-7, 50-200 microM). Prolonged exposure to TPA (greater than 2 h) resulted in spontaneous contractions which were sensitive to verapamil (1 microM). Isoprenaline and sodium nitroprusside relaxed phenylephrine-induced contractions in rat aorta preparations. TPA (0.3 microM) blocked the maximal response to isoprenaline but not to sodium nitroprusside indicating that TPA did selectively activate protein kinase C under these experimental conditions. These findings indicate that protein kinase C activation does not result in direct effects on Ca2+ channel function, but may exert effects indirectly (e.g. by modifying intracellular sensitivity to Ca2+, Ca2+ extrusion, or cellular depolarization).