Relationship between force and Ca2+ concentration in smooth muscle as revealed by measurements on single cells

RuBPY decreases intracellular calcium by decreasing influx and increasing storage

RuBPY is a ruthenium complex NO donor that presents a nitrite in its moiety and has been shown to induce vasodilation in various arteries, as well as arterial pressure reduction with no changes in heart rate. Since vascular tone is highly dependent on the cytosolic calcium concentration ([Ca²⁺ ]c), the current study aimed to investigate the effects of RuBPY on the intracellular mobilization of calcium stores of rat aortic vascular smooth muscle cells. Vascular reactivity experiments were performed in isolated aortic rings that were contracted with a high concentration of KCl or phenylephrine (Phe). Moreover, primary cultured vascular smooth muscle cells were used to measure [Ca²⁺ ]c by confocal microscopy. The NO donor RuBPY decreased the [Ca²⁺ ]c and reduced KCl and Phe -induced contractile responses. The selective inhibitor of sarco-endoplasmic Ca-ATPase (SERCA) with thapsigargin impaired the effect of RuBPY on Phe -induced contractile response. RuBPY also reduced caffeine-induced contraction, and the contraction dependent on the capacitive Ca²⁺ influx. Therefore, our results suggest that NO released from RuBPY decreased [Ca²⁺ ]c by calcium influx blockade, and activation of guanylyl-cyclase-cGMP-GK pathway. These results indicate that RuBPY increases Ca²⁺ storage in the sarcoplasmic reticulum by SERCA activation, and also by capacitive Ca²⁺ influx inhibition, which is dependent on the intracellular release of nitric oxide from this compound. This article is protected by copyright. All rights reserved.

On a coupled electro-chemomechanical model of gastric smooth muscle contraction

The stomach is a central organ in the gastrointestinal tract that performs a variety of functions, in which the spatio-temporal organisation of active smooth muscle contraction in the stomach wall (SW) is highly regulated. In the present study, a three-dimensional model of the gastric smooth muscle contraction is presented, including the mechanical contribution of the mucosal and muscular layer of the SW. Layer-specific and direction-dependent model parameters for the active and passive stress-stretch characteristics of the SW were determined experimentally using porcine smooth muscle strips. The electrical activation of the smooth muscle cells (SMC) due to the pacemaker activity of the interstitial cells of Cajal (ICC) is modelled by using FitzHugh-Nagumo-type equations, which simulate the typical ICC and SMC slow wave behaviour. The calcium dynamic in the SMC depends on the SMC membrane potential via a gaussian function, while the chemo-mechanical coupling in the SMC is modelled via an extended Hai-Murphy model. This cascade is coupled with an additional mechano-electrical feedback-mechanism, taking into account the mechanical response of the ICC and SMC due to stretch of the SW. In this way the relaxation responses of the fundus to accommodate incoming food, as well as the typical peristaltic contraction waves in the antrum for mixing and transport of the chyme, have been well replicated in simulations performed at the whole organ level. STATEMENT OF SIGNIFICANCE: In this article, a novel three-dimensional electro-chemomechanical model of the gastric smooth muscle contraction is presented. The propagating waves of electrical membrane potential in the network ofinterstitial cells of Cajal (ICC) and smooth muscle cells (SMC) lead to a global pattern of change in the calciumdynamics inside the SMC. Taking additionally into account the mechanical response of the ICC and SMC due to stretch of the stomach wall, also referred to as mechanical feedback-mechanism, the result is a complex spatio-temporal regulation of the active contraction and relaxation of the gastric smooth muscle tissue. Being a firstapproach, in future view such a three-dimensional model can give an insight into the complexload transferring system of the stomach wall, as well as into the electro-chemomechanicalcoupling process underlying smooth muscle contraction in health and disease.

Stimulation of TRPA1 attenuates ischemia-induced cardiomyocyte cell death through an eNOS-mediated mechanism

The functional expression of transient receptor potential cation channel of the ankyrin-1 subtype (TRPA1) has recently been identified in adult mouse cardiac tissue where stimulation of this ion channel leads to increases in adult mouse ventricular cardiomyocyte (CM) contractile function via a Ca²⁺-Calmodulin-dependent kinase (CaMKII) pathway. However, the extent to which TRPA1 induces nitric oxide (NO) production in CMs, and whether this signaling cascade mediates physiological or pathophysiological events in cardiac tissue remains elusive. Freshly isolated CMs from wild-type (WT) or TRPA1 knockout (TRPA1^-/-) mouse hearts were treated with AITC (100 µM) and prepared for immunoblot, NO detection or ischemia protocols. Our findings demonstrate that TRPA1 stimulation with AITC results in phosphorylation of protein kinase B (Akt) and endothelial NOS (eNOS) concomitantly with NO production in a concentration- and time-dependent manner. Additionally, we found that TRPA1 induced increases in CM [Ca²⁺]_i and contractility occur independently of Akt and eNOS activation mechanisms. Further analysis revealed that the presence and activation of TRPA1 promotes CM survival and viability following ischemic insult via a mechanism partially dependent upon eNOS. Therefore, activation of the TRPA1/Akt/eNOS pathway attenuates ischemia-induced CM cell death.

Calcium sensitization mechanisms in detrusor smooth muscles

The contraction of detrusor smooth muscles depends on the increase in intracellular calcium. The influx of calcium from the plasma membrane calcium channels and calcium release from the sarcoplasmic reticulum give rise to intracellular calcium. Under the pathophysiological conditions, the increased sensitivity of regulatory and contractile proteins to calcium also plays an important role in maintaining the spontaneous detrusor smooth muscle activity. Many proteins have been identified to play a role in calcium sensitization. Both the protein kinase C (PKC) and Rho-kinase (ROCK) signaling pathways are responsible for the induction of calcium sensitization in the detrusor smooth muscles. The balance between the myosin light chain kinase (MLCK) and myosin light chain phosphatase (MLCP) regulates the intracellular calcium-contractile force relationship. The inhibition of MLCP by PKC-mediated phosphatase inhibitor (CPI-17) and myosin phosphatase target subunit (MYPT-1) phosphorylation by both the PKC and ROCK are responsible for calcium sensitization in the detrusor smooth muscles. However, the ROCK pathway predominantly participates in the calcium sensitization induction under pathophysiological situations. Many kinases are well known nowadays to play a role in calcium sensitization. This review aims to enlighten the current understanding of the regulatory mechanisms of calcium sensitization with special reference to the PKC and ROCK pathways in the detrusor smooth muscles. It will also aid in the development of new pharmacological strategies to prevent and treat bladder diseases.

Epithelial Cells Induce a Cyclo-Oxygenase-1-Dependent Endogenous Reduction in Airway Smooth Muscle Contractile Phenotype

Airway smooth muscle cells (ASMCs) are phenotypically regulated to exist in either a proliferative or a contractile state. However, the influence of other airway structural cell types on ASMC phenotype is largely unknown. Although epithelial cells are known to drive ASM proliferation, their effects on the contractile phenotype are uncertain. In the current study, we tested the hypothesis that epithelial cells reduce the contractile phenotype of ASMCs. To do so, we measured force production by traction microscopy, gene and protein expression, as well as calcium release by Fura-2 ratiometric imaging. ASMCs incubated with epithelial-derived medium produced less force after histamine stimulation. We observed reduced expression of myocardin, α-smooth muscle actin, and calponin within ASMCs after coculture with epithelial cells. Peak calcium release in response to histamine was diminished, and depended on the synthesis of cyclo-oxygenase-1 products by ASM and on prostaglandin E receptors 2 and 4. Together, these in vitro results demonstrate that epithelial cells have the capacity to coordinately reduce ASM contraction by functional antagonism and by reduction of the expression of certain contractile proteins.

An effective model of cerebrovascular pressure reactivity and blood flow autoregulation

Understanding cerebral blood flow dynamics is crucial for the care of patients at risk of poor cerebral perfusion. We describe an effective model of cerebral hemodynamics designed to reveal important macroscopic features of cerebral blood flow without having to resolve the detailed microvasculature of the brain. Based on principles of fluid and elastic dynamics and vascular pressure-reactivity, the model quantifies the physical means by which the vasculature executes autoregulatory reflexes. We demonstrate that the frequency response of the proposed model matches experimental measurements and explains the influence of mechanical factors on the autoregulatory performance. Analysis of the model indicates the existence of an optimal mean arterial pressure which minimizes the sensitivity of the flow to changes in perfusion pressure across the frequency spectrum of physiological oscillations. We highlight the simplicity of the model and its potential to improve monitoring of brain perfusion via real-time computational simulations of cerebro- and cardio-vascular interventions.

Calcium Sensitization Mechanisms in Gastrointestinal Smooth Muscles

An increase in intracellular Ca²⁺ is the primary trigger of contraction of gastrointestinal (GI) smooth muscles. However, increasing the Ca²⁺ sensitivity of the myofilaments by elevating myosin light chain phosphorylation also plays an essential role. Inhibiting myosin light chain phosphatase activity with protein kinase C-potentiated phosphatase inhibitor protein-17 kDa (CPI-17) and myosin phosphatase targeting subunit 1 (MYPT1) phosphorylation is considered to be the primary mechanism underlying myofilament Ca²⁺ sensitization. The relative importance of Ca²⁺ sensitization mechanisms to the diverse patterns of GI motility is likely related to the varied functional roles of GI smooth muscles. Increases in CPI-17 and MYPT1 phosphorylation in response to agonist stimulation regulate myosin light chain phosphatase activity in phasic, tonic, and sphincteric GI smooth muscles. Recent evidence suggests that MYPT1 phosphorylation may also contribute to force generation by reorganization of the actin cytoskeleton. The mechanisms responsible for maintaining constitutive CPI-17 and MYPT1 phosphorylation in GI smooth muscles are still largely unknown. The characteristics of the cell-types comprising the neuroeffector junction lead to fundamental differences between the effects of exogenous agonists and endogenous neurotransmitters on Ca²⁺ sensitization mechanisms. The contribution of various cell-types within the tunica muscularis to the motor responses of GI organs to neurotransmission must be considered when determining the mechanisms by which Ca²⁺ sensitization pathways are activated. The signaling pathways regulating Ca²⁺ sensitization may provide novel therapeutic strategies for controlling GI motility. This article will provide an overview of the current understanding of the biochemical basis for the regulation of Ca²⁺ sensitization, while also discussing the functional importance to different smooth muscles of the GI tract.

Respiratory modulated sympathetic activity: a putative mechanism for developing vascular resistance?

KEY POINTS

Sympathetic activity exhibits respiratory modulation that is amplified in hypertensive rats. Respiratory modulated sympathetic activity produces greater changes in vascular resistance than tonic stimulation of the same stimulus magnitude in normotensive but not hypertensive rats. Mathematical modelling demonstrates that respiratory modulated sympathetic activity may fail to produce greater vascular resistance changes in hypertensive rats because the system is saturated as a consequence of a dysfunctional noradrenaline reuptake mechanism. Respiratory modulated sympathetic activity is an efficient mechanism to raise vascular resistance promptly, corroborating its involvement in the ontogenesis of hypertension.

ABSTRACT

Sympathetic nerve activity (SNA) exhibits respiratory modulation. This component of SNA is important - being recruited under cardiorespiratory reflex conditions and elevated in the spontaneously hypertensive (SH) rat - and yet the exact influence of this modulation on vascular tone is not understood, even in normotensive conditions. We constructed a mathematical model of the sympathetic innervation of an arteriole, and used it to test the hypothesis that respiratory modulation of SNA preferentially increases vasoconstriction compared to a frequency-matched tonic pattern. Simulations supported the hypothesis, where respiratory modulated increases in vasoconstriction were mediated by a noradrenergic mechanism. These predictions were tested in vivo in adult Wistar rats. Stimulation of the sympathetic chain (L3) with respiratory modulated bursting patterns, revealed that bursting increases vascular resistance (VR) more than tonic stimulation (57.8 ± 3.3% vs. 44.8 ± 4.2%; P < 0.001; n = 8). The onset of the VR response was also quicker for bursting stimulation (rise time constant = 1.98 ± 0.09 s vs. 2.35 ± 0.20 s; P < 0.01). In adult SH rats (n = 8), the VR response to bursting (44.6 ± 3.9%) was not different to tonic (37.4 ± 3.5%; P = 0.57). Using both mathematical modelling and in vivo techniques, we have shown that VR depends critically on respiratory modulation and revealed that this pattern dependency in Wistar rats is due to a noradrenergic mechanism. This respiratory component may therefore contribute to the ontogenesis of hypertension in the pre-hypertensive SH rat - raising VR and driving vascular remodelling. Why adult SH rats do not exhibit a pattern-dependent response is not known, but further modelling revealed that this may be due to dysfunctional noradrenaline reuptake.

Modelling the vascular response to sympathetic postganglionic nerve activity

This paper explores the influence of burst properties of the sympathetic nervous system on arterial contractility. Specifically, a mathematical model is constructed of the pathway from action potential generation in a sympathetic postganglionic neurone to contraction of an arterial smooth muscle cell. The differential equation model is a synthesis of models of the individual physiological processes, and is shown to be consistent with physiological data.

The model is found to be unresponsive to tonic (regular) stimulation at typical frequencies recorded in sympathetic efferents. However, when stimulated at the same average frequency, but with repetitive respiratory-modulated burst patterns, it produces marked contractions. Moreover, the contractile force produced is found to be highly dependent on the number of spikes in each burst. In particular, when the model is driven by preganglionic spike trains recorded from wild-type and spontaneously hypertensive rats (which have increased spiking during each burst) the contractile force was found to be 10-fold greater in the hypertensive case. An explanation is provided in terms of the summative increased release of noradrenaline. Furthermore, the results suggest the marked effect that hypertensive spike trains had on smooth muscle cell tone can provide a significant contribution to the pathology of hypertension.

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We model the sympathetic-driven contraction of a vascular smooth muscle cell.

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The cell is unresponsive to tonic stimulation at typical sympathetic frequencies.

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We quantify the force produced by the cell in response to sympathetic bursting.

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The response of the cell is strongly dependent on burst amplitude and duration.

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Recordings from hypertensive animals produce significant contractile forces.

Pimaradienoic acid inhibits vascular contraction and induces hypotension in normotensive rats

The present investigation was designed to investigate the effect of the diterpene ent-pimara-8(14),15-dien-19-oic acid (pimaradienoic acid, PA) on smooth muscle extracellular Ca2+ influx. To this end, the effect of PA on phenylephrine- and KCl-induced increases in cytosolic calcium concentration ([Ca2+]c), measured by the variation in the ratio of fluorescence intensities (R340/380 nm) of Fura-2, was analysed. Whether bolus injection of PA could induce hypotensive responses in conscious normotensive rats was also evaluated. PA inhibited the contraction induced by phenylephrine (0.03 or 10 μmol L−1) and KCl (30 or 90 μmol L−1) in endothelium-denuded rat aortic rings in a concentration dependent manner. Pre-treatment with PA (10, 100, 200 μmol L−1) attenuated the contraction induced by CaCl2 (0.5 nmol L−1 or 2.5 μmol L−1) in denuded rat aorta exposed to Ca2+-free medium containing phenylephrine (0.1 μmol L−1) or KCl (30 μmol L−1). Interestingly, the inhibitory effect displayed by PA on CaCl2-induced contraction was more pronounced when KCl was used as the stimulant. Phenylephrine- and KCl-induced increases in [Ca2+]c were inhibited by PA. Similarly, verapamil, a Ca2+-channel blocker, also inhibited the increase in [Ca2+]c induced by either phenylephrine or KCl. Finally, bolus injection of PA (1–15 mg kg−1) produced a dose-dependent decrease in mean arterial pressure in conscious normotensive rats. The results provide the first direct evidence that PA reduces vascular contractility by reducing extracellular Ca2+ influx through smooth muscle cellular membrane, a mechanism that could mediate the hypotensive response induced by this diterpene in normotensive rats.

Ca2+ sparks act as potent regulators of excitation-contraction coupling in airway smooth muscle

Ca2+ sparks are short lived and localized Ca2+ transients resulting from the opening of ryanodine receptors in sarcoplasmic reticulum. These events relax certain types of smooth muscle by activating big conductance Ca2+-activated K+ channels to produce spontaneous transient outward currents (STOCs) and the resultant closure of voltage-dependent Ca2+ channels. But in many smooth muscles from a variety of organs, Ca2+ sparks can additionally activate Ca2+-activated Cl(-) channels to generate spontaneous transient inward current (STICs). To date, the physiological roles of Ca2+ sparks in this latter group of smooth muscle remain elusive. Here, we show that in airway smooth muscle, Ca2+ sparks under physiological conditions, activating STOCs and STICs, induce biphasic membrane potential transients (BiMPTs), leading to membrane potential oscillations. Paradoxically, BiMPTs stabilize the membrane potential by clamping it within a negative range and prevent the generation of action potentials. Moreover, blocking either Ca2+ sparks or hyperpolarization components of BiMPTs activates voltage-dependent Ca2+ channels, resulting in an increase in global [Ca2+](i) and cell contraction. Therefore, Ca2+ sparks in smooth muscle presenting both STICs and STOCs act as a stabilizer of membrane potential, and altering the balance can profoundly alter the status of excitability and contractility. These results reveal a novel mechanism underlying the control of excitability and contractility in smooth muscle.

Involvement of non-selective Ca2+ channels in the contraction induced by alkalinization of rat anococcygeus muscle cells

Intracellular pH is a modulator of cellular functions such as smooth muscle contraction. Changes in cytosolic Ca(2+) concentration ([Ca(2+)](c)) associated with contraction are brought about by Ca(2+) influx and release from the sarcoplasmic reticulum, and alterations in the intracellular pH can affect both processes. In this work, therefore, we have investigated the Ca(2+) influx pathway that contributes to the contraction induced by the alkalinizing agent NH(4)Cl in the rat anococcygeus smooth muscle. For this purpose, we measured the isometric tension in muscle preparations, and [Ca(2+)](c) was measured on isolated cells loaded with 5 micromol/l FURA2/AM by using the ratio 340/380 nm. NH(4)Cl (10 mmol/l) induced a larger increase in [Ca(2+)](c) (100%) when compared with the [Ca(2+)](c) increase induced by 0.1 micromol/l phenylephrine (57.0+/-12.3% n=4). Incubation of the muscle preparations for 1 min in Ca(2+)-free medium reduced the contractions induced by 10 mmol/l NH(4)Cl to 11.5+/-5.1% (n=5), when compared with the contractions induced in 2.5 mmol/l Ca(2+) solution (100%). After 3 min in Ca(2+) free medium, contractions stimulated with NH(4)Cl were almost abolished (0.6+/-0.4%, n=5). In the same way, incubation with 10 micromol/l 1-[beta-[3[(4-methoxyphenyl)propoxyl]-4-methoxy-phenetyl]-1H-imidazole hydrochloride (SKF96365), a non-selective Ca(2+) channels, reduced the contractions stimulated with NH(4)Cl to 47.6+/-6.7% (n=7). On the other hand, 1 micromol/l verapamil, a voltage-operated Ca(2+) channel blocker and 0.05 micromol/l calphostin C, a protein kinase-C inhibitor, did not alter the contractions induced by NH(4)Cl. On isolated cells, [Ca(2+)](c) was reduced to 72.2+/-1.7% (n=4) by 10 micromol/l SKF96365. Taken together, our results suggest that NH(4)Cl induces contraction of rat anococcygeus smooth muscle cells, as well as [Ca(2+)](c) increase due to Ca(2+) influx through non-selective Ca(2+) channels.

Cross-talk between the sarcoplasmic reticulum and the mitochondrial calcium handling systems may play an important role in the regulation of contraction in anococcygeus smooth muscle

Mitochondrial Ca(2+) and its relation with the contraction induced by phenylephrine was investigated. In normal Ca(2+), carbonyl cyanide p-(trifluoro-methoxy)phenyl-hydrazone (FCCP) and oligomycin produced contraction similar to that promoted by phenylephrine. Phenylephrine-induced contraction was reduced by FCCP+oligomycin. In Ca(2+)-free, FCCP+oligomycin did not induce contraction. Response to FCCP+oligomycin was reduced upon Ca(2+) repletion and this response was lower than that to phenylephrine. Ca(2+) concentration was increased by FCCP+oligomycin. Since a profuse net of sarcoplasmic reticulum encloses mitochondria, a cross-talk between the two organelles may play an important role in the phenylephrine-induced contraction in presence of Ca(2+) encountered in both sarcoplasmic reticulum and extracellular medium of anococcygeus cells.

A quantitative description of the contraction of blood vessels following the release of noradrenaline from sympathetic varicosities

A model is presented that highlights the principal factors determining the form and extent of contraction in arteries upon stimulation of their sympathetic nerve supply. This model incorporates a previous quantitative model of the process of noradrenaline (NAd) diffusion into the vascular media and reuptake into sympathetic varicosities during nerve stimulation (J. Theor. Biol. 226 (2004) 359). It is also dependent on a model of how the subsequent activation of metabotropic receptors initiates a G-protein cascade, resulting in the production of inositol trisphosphate (IP3) and an increase in intracellular calcium concentration, [Ca2+]i, in the smooth muscle cells (J. Theor. Biol. 223 (2003) 93). In the present work we couple this rise in [Ca2+]i to the increase in phosphorylated myosin bound to actin in the cells and hence determine the force development in arteries due to nerve stimulation. The model accounts for force development as a function of [Ca2+]i and for the rate of change of force as a function of the rate of change of [Ca2+]i in single smooth muscle cells. It also accounts for the characteristic time course of the force developed by the media of the rat-tail artery upon nerve stimulation. This consists of a rapid rise to a transient peak followed by a sustained plateau of contraction during the stimulation period, after which the contraction slowly decays back to baseline at a rate dependent on the strength of the stimulation. The model indicates that the transient peak is primarily due to the partial block of the IP3 receptor by the rise in [Ca2+]i and that the main determinant of the equilibrium condition indicated by the plateau phase is the rate of pumping of calcium into the sarcoplasmic reticulum. The relatively slow decline of contraction at the end of nerve stimulation is primarily a consequence of the slow rates of removal of NAd from the media by diffusion and reuptake into the sympathetic varicosities. The model thus provides a quantitative account of vascular smooth muscle contraction upon sympathetic nerve stimulation.

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.

The myogenic response in isolated rat cerebrovascular arteries: smooth muscle cell model

Previous models of the cerebrovascular smooth muscle cell have not addressed the interaction between the electrical, chemical, and mechanical components of cell function during the development of active tension. These models are primarily electrical, biochemical or mechanical in their orientation, and do not permit a full exploration of how the smooth muscle responds to electrical or mechanical forcing. To address this issue, we have developed a new model that consists of two major components: electrochemical and chemomechanical subsystem models of the smooth muscle cell. Included in the electrochemical model are models of the electrophysiological behavior of the cell membrane, fluid compartments, Ca2+ release and uptake by the sarcoplasmic reticulum (SR), and cytosolic Ca2+ buffering, particularly by calmodulin (CM). With this subsystem model, we can study the mechanics of the production of intracellular Ca2+ transient in response to membrane voltage clamp pulses. The chemomechanical model includes models of: (a) the chemical kinetics of myosin phosphorylation, and the formation of phosphorylated (cycling) myosin cross-bridges with actin, as well as attached (non-cycling) latch-type cross-bridges; and (b) a model of force generation and mechanical coupling to the contractile filaments and their attachments to protein structures and the skeletal framework of the cell. The two subsystem models are tested independently and compared with data. Likewise, the complete (combined) cell model responses to voltage pulse stimulation under isometric and isotonic conditions are calculated and compared with measured single cell length-force and force-velocity data obtained from literature. This integrated cell model provides biophysically based explanations of electrical, chemical, and mechanical phenomena in cerebrovascular smooth muscle, and has considerable utility as an adjunct to laboratory research and experimental design.

Coupling of a P2Z-like purinoceptor to a fatty acid-activated K(+) channel in toad gastric smooth muscle cells

1. Extracellular application of ATP generates two whole-cell currents in toad gastric smooth muscle cells: an immediate inward non-selective cation current (due to the activation of a P2X or P2Z-like receptor) and a slowly developing outward K(+) current. The inward non-selective cation current depends on the continuous presence of ATP while the outward K(+) current can last for minutes after ATP application ceases. 2. In cell-attached patches, application of ATP to the extra-patch membrane can activate K(+) channels in the patch indicating that a diffusible cellular messenger may be involved. The characteristics of these K(+) channels are similar to those of a previously described fatty acid-activated K(+) channel that is also a stretch-activated channel. 3. This whole-cell K(+) current can be induced by ATP in the absence of extracellular Ca(2+) (with EGTA present to chelate trace amounts). However, the current generated in the presence of extracellular Ca(2+) is considerably larger. 4. The pharmacological profiles for the activation of the non-selective cation current and the K(+) current are similar, suggesting that the same P2Z-like receptor could be mediating both responses. This type of plasma membrane receptor/channel-channel coupling by a process that does not appear to involve Ca(2+) flow through the receptor/channel or a subsequent membrane potential change may be representative of a new class of signalling mechanisms.

Multiple pathways responsible for the stretch-induced increase in Ca2+ concentration in toad stomach smooth muscle cells

1. A digital imaging microscope with fura-2 as the Ca2+ indicator was used to determine the sources for the rise in intracellular calcium concentration ([Ca2+]i) that occurs when the membrane in a cell-attached patch is stretched. Unitary ionic currents from stretch-activated channels and [Ca2+]i images were recorded simultaneously. 2. When suction was applied to the patch pipette to stretch a patch of membrane, Ca2+-permeable cation channels (stretch-activated channels) opened and a global increase in [Ca2+]i occurred, as well as a greater focal increase in the vicinity of the patch pipette. The global changes in [Ca2+]i occurred only when stretch-activated currents were sufficient to cause membrane depolarization, as indicated by the reduction in amplitude of the unitary currents. 3. When Ca2+ was present only in the pipette solution, just the focal change in [Ca2+]i was obtained. This focal change was not seen when the contribution from Ca2+ stores was eliminated using caffeine and ryanodine. 4. These results suggest that the opening of stretch-activated channels allows ions, including Ca2+, to enter the cell. The entry of positive charge triggers the influx of Ca2+ into the cell by causing membrane depolarization, which presumably activates voltage-gated Ca2+ channels. The entry of Ca2+ through stretch-activated channels is also amplified by Ca2+ release from internal stores. This amplification appears to be greater than that obtained by activation of whole-cell Ca2+ currents. These multiple pathways whereby membrane stretch causes a rise in [Ca2+]i may play a role in stretch-induced contraction, which is a characteristic of many smooth muscle tissues.

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.

Kinetics of contraction in depolarized smooth muscle from guinea-pig taenia coli after photodestruction of nifedipine

1. The time course and kinetics of force development following activation by opening of L-type Ca2+ channels was investigated using photodestruction of the Ca2+ channel blocker nifedipine in smooth muscle from the guinea-pig taenia coli. 2. In muscles activated using high K+ and Ca2+ and subsequently inhibited with nifedipine, photodestruction of the drug using a strong ultraviolet light flash initiated a rapid contraction. The force initiated by photodestruction of nifedipine reached near-maximal levels. This procedure eliminates diffusional delays and can thus be used to investigate the kinetics of depolarization-induced contractions. 3. The rate of force development of contractions initiated by photodestruction of nifedipine was slower than that observed in maximally thiophosphorylated skinned fibres. This suggests the rate of force development is limited by activation steps in the activation cascade prior to the force generation of the cross-bridge system. 4. The rate of force development and the plateau force were dependent on the extracellular [CaCl2] suggesting that the intracellular [Ca2+] determines the rate of phosphorylation and force development. The delay between illumination and increase in force was about 300 ms. The delay was similar at low and high extracellular [CaCl2] indicating that buffering by superficial sarcoplasmatic reticulum does not introduce a delay in force development following activation of Ca2+ channels in this muscle.

Calcium dependence of C-type natriuretic peptide-formed fast K(+) channel

The lipid bilayer technique was used to characterize the Ca(2+) dependence of a fast K(+) channel formed by a synthetic 17-amino acid segment [OaCNP-39-(1-17)] of a 39-amino acid C-type natriuretic peptide (OaCNP-39) found in platypus (Ornithorhynchus anatinus) venom (OaV). The OaCNP-39-(1-17)-formed K(+) channel was reversibly dependent on 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-buffered cis (cytoplasmic) Ca(2+) concentration ([Ca(2+)](cis)). The channel was fully active when [Ca(2+)](cis) was >10(-4) M and trans (luminal) Ca(2+) concentration was 1.0 mM, but not at low [Ca(2+)](cis). The open probability of single channels increased from zero at 1 x 10(-6) M cis Ca(2+) to 0.73 +/- 0.17 (n = 22) at 10(-3) M cis Ca(2+). Channel openings to the maximum conductance of 38 pS were rapidly and reversibly activated when [Ca(2+)](cis), but not trans Ca(2+) concentration (n = 5), was increased to >5 x 10(-4) M (n = 14). Channel openings to the submaximal conductance of 10.5 pS were dominant at >/=5 x 10(-4) M Ca(2+). K(+) channels did not open when cis Mg(2+) or Sr(2+) concentrations were increased from zero to 10(-3) M or when [Ca(2+)](cis) was maintained at 10(-6) M (n = 3 and 2). The Hill coefficient and the inhibition constant were 1 and 0.8 x 10(-4) M cis Ca(2+), respectively. This dependence of the channel on high [Ca(2+)](cis) suggests that it may become active under 1) physiological conditions where Ca(2+) levels are high, e.g., during cardiac and skeletal muscle contractions, and 2) pathological conditions that lead to a Ca(2+) overload, e.g., ischemic heart and muscle fatigue. The channel could modify a cascade of physiological functions that are dependent on the Ca(2+)-activated K(+) channels, e.g., vasodilation and salt secretion.

Bradykinin modulates arginine vasopressin-induced calcium influx in vascular myocytes

We investigated direct, endothelium-independent effects of bradykinin on arginine vasopressin-induced calcium influx in vascular smooth muscle cells. We studied cultured rat vascular smooth muscle cells by using the whole-cell voltage-clamp and calcium fluorescence imaging methods. Exposing cultured vascular smooth muscle cells (A7r5 cell line) to arginine vasopressin (100 nM) produced a transient increase in [Ca2+]i, followed by a sustained increase in [Ca2+]i. This was readily reversible (n=28). At a holding potential of -40 to -60 mV, arginine vasopressin induced a sustained inward current correlated with a sustained increase in [Ca2+]i. Bradykinin (30 nM to 30 microM) had no effect on arginine vasopressin-induced [Ca2+]i transients. However, during the sustained phase of increased [Ca2+]i, bradykinin reversibly attenuated relative fluorescence and inward current in the presence of arginine vasopressin (n=14). This was concentration dependent and inhibited by [D-Phe7]-bradykinin (30 microM), a kinin receptor antagonist. Also, sustained arginine vasopressin-mediated increases in [Ca2+]i and inward current were attenuated by Ca2+-free or La3+-supplemented perfusate but not by nifedipine (n=5).

CONCLUSIONS

(1) Bradykinin can attenuate arginine vasopressin-induced and sustained Ca2+ influx and sustained inward current through a novel endothelium-independent process. (2) The direct effect of bradykinin on arginine vasopressin-induced increases in [Ca2+]i sustained Ca2+ influx in vascular smooth muscle cells is concentration dependent and kinin-receptor mediated. (3) Arginine vasopressin-induced sustained [Ca2+]i elevation correlates with the activation of a dihydropyridine-insensitive, Ca2+-conducting inward current.

Tension oscillation in arteries and its abnormality in hypertensive animals

1. The mechanisms of oscillatory contraction of arterial smooth muscle in vitro are discussed. 2. The membrane potential and cytoplasmic free Ca2+ concentration in smooth muscle cells oscillate in the presence of agonists. 3. The oscillatory change in the membrane potential of smooth muscle cells is related to Ca2+ release from intracellular stores. 4. Gap junctions between smooth muscle cells play important roles in the synchronized oscillation of the cytoplasmic free Ca2+ concentration in this population of cells. 5. Endothelial cells may increase or decrease the tension oscillation of smooth muscle cells. 6. In arteries from hypertensive rats, an increase in membrane excitability and the number of gap junctions between smooth muscle cells and impaired endothelial function are the main factors responsible for the modulation of tension oscillation.

Oxytocin increases the [Ca2+]i sensitivity of human myometrium during the falling phase of phasic contractions

Oxytocin is commonly used to induce or augment labor, but its mode of action is uncertain. To address the issue, isometric tension and the intracellular free Ca2+ concentration ([Ca2+]i) were simultaneously recorded from isolated strips of pregnant human myometrium loaded with fura 2. The changes in [Ca2+]i and tension during phasic contractions were indistinguishable in myometrium taken before or after the onset of labor, enabling samples to be pooled. Oxytocin (10 nM) had no effect on basal [Ca2+]i or tension, but it increased both the [Ca2+]i and the tension recorded during phasic contractions. Analysis of the [Ca2+]i-tension relationship revealed that during the falling (relaxation) phase of the contractile response, oxytocin increased the tension recorded at each [Ca2+]i. By manipulating extracellular Ca2+ during phasic contractions, it was possible to ensure that the [Ca2+]i signals were similar in the presence and absence of oxytocin, yet oxytocin still improved the [Ca2+]i-tension relationship. We conclude that 10 nM oxytocin increases the [Ca2+]i sensitivity of the contractile proteins only after a contraction has begun, possibly by causing inhibition of myosin light chain phosphatase.

Regulation of smooth muscle actin-myosin interaction and force by calponin

Smooth muscle contraction is regulated primarily by the reversible phosphorylation of myosin triggered by an increase in sarcoplasmic free Ca2+ concentration ([Ca2+]i). Contraction can, however, be modulated by other signal transduction pathways, one of which involves the thin filament-associated protein calponin. The h1 (basic) isoform of calponin binds to actin with high affinity and is expressed specifically in smooth muscle at a molar ratio to actin of 1:7. Calponin inhibits (i) the actin-activated MgATPase activity of smooth muscle myosin (the cross-bridge cycling rate) via its interaction with actin, (ii) the movement of actin filaments over immobilized myosin in the in vitro motility assay, and (iii) force development or shortening velocity in permeabilized smooth muscle strips and single cells. These inhibitory effects of calponin can be alleviated by protein kinase C (PKC)-catalysed phosphorylation and restored following dephosphorylation by a type 2A phosphatase. Three physiological roles of calponin can be considered based on its in vitro functional properties: (i) maintenance of relaxation at resting [Ca2+]i, (ii) energy conservation during prolonged contractions, and (iii) Ca(2+)-independent contraction mediated by phosphorylation of calponin by PKC epsilon, a Ca(2+)-independent isoenzyme of PKC.

Calcium transients and the effect of a photolytically released calcium chelator during electrically induced contractions in rabbit rectococcygeus smooth muscle

Intracellular Ca2+ was determined with the fura-2 technique during electrically induced contractions in the rabbit rectococcygeus smooth muscle at 22 degreesC. The muscles were electrically activated to give short, reproducible contractions. Intracellular [Ca2+] increased during activation; the increase in [Ca2+] preceded force development by approximately 2 s. After cessation of stimulation Ca2+ fell, preceding the fall in force by approximately 4 s. The fluorescence properties of fura-2 were determined with time-resolved spectroscopy using synchrotron light at the MAX-storage ring, Lund, Sweden. The fluorescence decay of free fura-2 was best described by two exponential decays (time constants approximately 0.5 and 1.5 ns) at low Ca2+ (pCa 9). At high Ca2+ (pCa 4.5), fluorescence decay became slower and could be fitted by one exponential decay (1.9 ns). Time-resolved anisotropy of free fura-2 was characteristic of free rotational motion (correlation time 0.3 ns). Motion of fura-2 could be markedly inhibited by high concentrations of creatine kinase. Time-resolved spectroscopy measurements of muscle fibers loaded with fura-2 showed that the fluorescence lifetime of the probe was longer, suggesting an influence of the chemical environment. Anisotropy measurements revealed, however, that the probe was mobile in the cells. The Ca2+-dependence of contraction and relaxation was studied using a photolabile calcium chelator, diazo-2, which could be loaded into the muscle cells in a similar manner as fura-2. Photolysis of diazo-2 leads to an increase in its Ca2+-affinity and a fall in free Ca2+. When muscles that had been loaded with diazo-2 were illuminated with UV light flashes during the rising phase of contraction, the rate of contraction became slower, suggesting a close relation between intracellular Ca2+ and the cross-bridge interaction. In contrast, photolysis during relaxation did not influence the rate of force decay, suggesting that relaxation of these contractions is not determined by the rate of Ca2+ removal or due to an increased Ca2+ sensitivity, but instead is limited by other processes such as deactivation by dephosphorylation or detachment of tension-bearing cross-bridges, possibly regulated by thin filament systems.

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

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

The temporal profile of calcium transients in voltage clamped gastric myocytes from Bufo marinus

1. Decay in intracellular calcium concentration ([Ca2+]i) was recorded following step depolarizations in voltage clamped gastric myocytes from Bufo marinus. 2. Depolarizations (300 ms) to +10 mV were followed by three phases of [Ca2+]i decay with repolarization to both -110 and -50 mV. The decline was initially rapid (mean fractional decay rate = 81 +/- 11%s-1 at -110 mV), then slowed (decay rate = 14 +/- 2%s-1) and finally accelerated again (decay rate = 24 +/- 3%s-1; n = 19). 3. The initial phase of rapid decay became shorter as the length of the depolarizing pulse increased but was unaffected by changes in pulse voltage. 4. The delayed acceleration in [Ca2+]i decay was no longer seen when the duration of the depolarizing pulses was reduced to 100 ms, but was clearly evident following 500 ms pulses. This phase was abolished when the depolarizing voltage was altered to minimize the rise in [Ca2+]i. 5. Ryanodine and caffeine had no effect on the temporal profile of [Ca2+]i decay. 6. Removal of extracellular Na+ decreased the decay rate during all three phases at -110 mV, but this effect was particularly marked for the initial rapid phase of decay, the rate of which was reduced by 75%. A delayed increase in decay rate was still seen. 7. Inhibition of mitochondrial Ca2+ uptake with cyanide, carbonyl cyanide p-trifluoromethoxy-phenylhydrazone or Ruthenium Red had no effect on the initial rate of [Ca2+]i decay but blocked the delayed acceleration. 8. These results are discussed in terms of a model in which rapid influx of Ca2+ produces a high subsarcolemmal [Ca2+], favouring rapid Ca2+ removal by near-membrane mechanisms, particularly Na(+)-Ca2+ exchange. Mitochondrial Ca2+ removal produces a delayed increase in [Ca2+]i decay if the global [Ca2+]i is raised high enough for long enough.

Near-membrane [Ca2+] transients resolved using the Ca2+ indicator FFP18

(Ca2+)-sensitive processes at cell membranes involved in contraction, secretion, and neurotransmitter release are activated in situ or in vitro by Ca2+ concentrations ([Ca2+]) 10-100 times higher than [Ca2+] measured during stimulation in intact cells. This paradox might be explained if the local [Ca2+] at the cell membrane is very different from that in the rest of the cell. Soluble Ca2+ indicators, which indicate spatially averaged cytoplasmic [Ca2+], cannot resolve these localized, near-membrane [Ca2+] signals. FFP18, the newest Ca2+ indicator designed to selectively monitor near-membrane [Ca2+], has a lower Ca2+ affinity and is more water soluble than previously used membrane-associating Ca2+ indicators. Images of the intracellular distribution of FFP18 show that >65% is located on or near the plasma membrane. [Ca2+] transients recorded using FFP18 during membrane depolarization-induced Ca2+ influx show that near-membrane [Ca2+] rises faster and reaches micromolar levels at early times when the cytoplasmic [Ca2+], recorded using fura-2, has risen to only a few hundred nanomolar. High-speed series of digital images of [Ca2+] show that near-membrane [Ca2+], reported by FFP18, rises within 20 msec, peaks at 50-100 msec, and then declines. [Ca2+] reported by fura-2 rose slowly and continuously throughout the time images were acquired. The existence of these large, rapid increases in [Ca2+] directly beneath the surface membrane may explain how numerous (Ca2+)-sensitive membrane processes are activated at times when bulk cytoplasmic [Ca2+] changes are too small to activate them.

Depolarization-evoked increases in cytosolic calcium concentration in isolated smooth muscle cells of rat portal vein

1. Ca2+ current through voltage-dependent Ca2+ channels (ICa) and intracellular free Ca2+ concentration ([Ca2+]i) were measured simultaneously in rat portal vein smooth muscle cells using conventional whole-cell voltage clamp technique and high temporal resolution microfluorimetry. 2. The relationship between depolarization-evoked ICa and rise in [Ca2+]i was examined. The extracellular Ca2+ concentration dependence and the voltage dependence of the depolarization-evoked increases in ICa and [Ca2+]i were similar. Both ICa and increased [Ca2+]i were blocked to a similar extent by nimodipine and cadmium and augmented by Bay K 8644. Furthermore, the time course of the measured increase in [Ca2+]i, closely followed the increase in [Ca2+]i expected from the time-integrated ICa. These observations suggest that the depolarization-evoked rise in [Ca2+]i was tightly coupled to ICa. 3. The cytosolic Ca2+ buffering capacity, determined as the ratio of the expected increase in [Ca2+]i (from ICa) divided by the measured increase in [Ca2+]i, was over 100. Therefore, less than 1 out of 100 Ca2+ ions entering the cell appears as a free Ca2+. 4. Ryanodine (30 microM), a blocker of the Ca(2+)-induced Ca2+ release mechanism, had little effect on buffering capacity measured over the first 200 ms of the depolarizing voltage clamp pulse. Ryanodine also had little effect on the buffering capacity during 800-1000 ms of the depolarizing voltage clamp pulse. Therefore, it was concluded that there is little Ca(2+)-induced Ca2+ release from the stores in rat portal vein smooth muscle cells during depolarization-evoked Ca2+ entry. 5. During brief depolarizations, the largest [Ca2+]i increase and ICa occurred at 0 mV. However, during steady-state depolarization, the largest increase in [Ca2+]i occurred around -30 mV, and we estimate the peak steady-state ICa to be about 0.6 pA.

Mitochondria contribute to Ca2+ removal in smooth muscle cells

Recent evidence, from a variety of cell types, suggests that mitochondria play an important role in shaping the change in intracellular calcium concentration ([Ca2+]i) that occurs during physiological stimulation. In the present study, using a range of inhibitors of mitochondrial Ca2+ uptake, we have examined the contribution of mitochondria to Ca2+ removal from the cytosol of smooth muscle cells following stimulation. In voltage-clamped single smooth muscle cells, we found that following a 8-s train depolarizing pulses, the rate of Ca2+ extrusion from the cytosol was reduced by more than 50% by inhibitors of cytochrome oxidase or exposure of cells to the protonophore carbonyl cyanide P-trifluoromethoxy-phenylhydrazone. Using the potential-sensitive indicator-tetramethyl rhodamine ethyl ester, we confirmed that the effect of these agents was associated with depolarization of the mitochondrial membrane. Since, the primary function of the mitochondria is to provide the cell's ATP, it could be argued that it is the ATP supply to the ion pumps which is limiting the rate of Ca2+ removal. However, experiments carried out with the mitochondrial Ca2+ uniporter inhibitor ruthenium red produced similar results, while the ATP synthetase inhibitor oligomycin had no effect, suggesting that the effect was not due to ATP insufficiency. These results establish that mitochondria in smooth muscle cells play a significant role in removing Ca2+ from the cytosol following stimulation. The uptake of Ca2+ into mitochondria is proposed to stimulate mitochondrial ATP production, thereby providing a means for matching increased energy demand, following the cell's rise in [Ca2+]i, with increased cellular ATP production.

The quantal nature of calcium release to caffeine in single smooth muscle cells results from activation of the sarcoplasmic reticulum Ca(2+)-ATPase

Calcium release from intracellular stores occurs in a graded manner in response to increasing concentrations of either inositol 1,4,5-trisphosphate or caffeine. To investigate the mechanism responsible for this quantal release phenomenon, [Ca2+] changes inside intracellular stores in isolated single smooth muscle cells were monitored with mag-fura 2. Following permeabilization with saponin or alpha-toxin the dye, loaded via its acetoxymethyl ester, was predominantly trapped in the sarcoplasmic reticulum (SR). Low caffeine concentrations in the absence of ATP induced only partial Ca2+ release; however, after inhibiting the calcium pump with thapsigargin the same stimulus released twice as much Ca2+. When the SR Ca(2+)-ATPase was rendered non-functional by depleting its "ATP pool," submaximal caffeine doses almost fully emptied the stores of Ca2+. We conclude that quantal release of Ca2+ in response to caffeine in these smooth muscle cells is largely due to the activity of the SR Ca(2+)-ATPase, which appears to return a portion of the released Ca2+ back to the SR, even in the absence of ATP. Apparently the SR Ca(2+)-ATPase is fueled by ATP, which is either compartmentalized or bound to the SR.

Relaxation of arterial smooth muscle by calcium sparks

Local increases in intracellular calcium ion concentration ([Ca2+]i) resulting from activation of the ryanodine-sensitive calcium-release channel in the sarcoplasmic reticulum (SR) of smooth muscle cause arterial dilation. Ryanodine-sensitive, spontaneous local increases in [Ca2+]i (Ca2+ sparks) from the SR were observed just under the surface membrane of single smooth muscle cells from myogenic cerebral arteries. Ryanodine and thapsigargin inhibited Ca2+ sparks and Ca(2+)-dependent potassium (KCa) currents, suggesting that Ca2+ sparks activate KCa channels. Furthermore, KCa channels activated by Ca2+ sparks appeared to hyperpolarize and dilate pressurized myogenic arteries because ryanodine and thapsigargin depolarized and constricted these arteries to an extent similar to that produced by blockers of KCa channels. Ca2+ sparks indirectly cause vasodilation through activation of KCa channels, but have little direct effect on spatially averaged [Ca2+]i, which regulates contraction.

Kinetics of prephosphorylation reactions and myosin light chain phosphorylation in smooth muscle. Flash photolysis studies with caged calcium and caged ATP

The pre-myosin light chain (MLC20) phosphorylation components of the lag phase (td) of contractile activation were determined in permeabilized smooth muscles activated by photolytic release of ATP from caged ATP and/or Ca2+ from 4-(2-nitrophenyl)-EGTA (NP-EGTA). Calmodulin (CaM) shortened the td (470 ms at 0 added CaM) that followed Ca2+ release, but its effect (td = approximately 200 ms) saturated at 40 microM. Photolysis of caged ATP following preequilibration with identical [Ca4CaM] shortened td to 41 ms. The rate of phosphorylation was very fast (3.5 s-1 at 22 degrees C in the presence of 5 microM exogenous CaM) following photolysis of caged ATP, and, following Ca2+ release, phosphorylation was accelerated by CaM. Simultaneous photolysis of caged ATP and NP-EGTA was followed by a td of 194 ms at 5 microM CaM and a rate of MLC20 phosphorylation intermediate between these parameters following photolysis of, respectively, NP-EGTA and caged ATP. In the presence of the normal, total endogenous CaM content (37 +/- 4 microM) of protal vein smooth muscles td was 565 ms. Steady state maximum force at pCa 5.5 was increased by much lower (100 nM) exogenous [CaM] than was required (> 2.5 microM) to shorten the td. We estimate the endogenous CaM available under steady state conditions in vivo to be approximately 0.25 microM and probably less during a rapid Ca2+ transient. We conclude that the [CaM] dependence of the kinetics of MLC20 phosphorylation and force development (t1/2 and td) initiated by Ca2+ reflects the recruitment of a slowly diffusible component of total CaM. The relatively long duration of td (197 ms) at saturating [CaM] suggests the contribution to td of an additional component, possibly a prephosphorylation activation/isomerization of the Ca4CaM myosin light chain kinase complex (Török, K., and Trentham, D. R. (1994) Biochemistry 33, 12807-12820). The relatively short delay (108 ms in the presence of 40 microM CaM) following simultaneous photolysis of NP-EGTA and caged ATP suggests that preincubation with ATP (prior to photolysis of NP-EGTA) may inhibit the formation of a preactive Ca2CaM myosin light chain kinase complex.

Is calpain activity regulated by membranes and autolysis or by calcium and calpastatin?

Although the Ca(2+)-dependent proteinase (calpain) system has been found in every vertebrate cell that has been examined for its presence and has been detected in Drosophila and parasites, the physiological function(s) of this system remains unclear. Calpain activity has been associated with cleavages that alter regulation of various enzyme activities, with remodeling or disassembly of the cell cytoskeleton, and with cleavages of hormone receptors. The mechanism regulating activity of the calpain system in vivo also is unknown. It has been proposed that binding of the calpains to phospholipid in a cell membrane lowers the Ca2+ concentration, [Ca2+], required for the calpains to autolyze, and that autolysis converts an inactive proenzyme into an active protease. Recent studies, however, show that the calpains bind to specific proteins and not to phospholipids, and that binding to cell membranes does not affect the [Ca2+] required for autolysis. It seems likely that calpain activity is regulated by binding of Ca2+ to specific sites on the calpain molecule, with binding to each site eliciting a response (proteolytic activity, calpastatin binding, etc.) specific for that site. Regulation must also involve an, as yet, undiscovered mechanism that increases the affinity of the Ca(2+)-binding sites for Ca2+.

Endothelin-1 and endothelin-3 regulate differently vasoconstrictor responses of smooth muscle of the porcine coronary artery

1. Using front-surface fluorometry of fura-2 and medial strips of the porcine coronary artery, we investigated mechanisms by which endothelin-1 (ET-1) and ET-3 function as vasoconstrictors. 2. In the presence of extracellular Ca2+(1.25 mM), ET-1 (10(-10)-10(-7) M) increased cytosolic Ca2+ concentrations ([Ca2+]i) and tension, in a concentration-dependent manner. ET-1, at concentrations greater than 10(-8) M, induced an abrupt elevation of [Ca2+]i which reached a transient peak (the first component, [Ca2+]i-rising phase) and subsequently declined ([Ca2+]i-declining phase) to reach a lower sustained phase (the second component, steady-state phase), while the tension rose monotonically to reach a peak and then slightly and gradually declined. ET-1, at concentrations lower than 10(-8) M, induced slowly developing and sustained increases in [Ca2+]i and tension ([Ca2+]i-rising phase followed by steady-state phase). All concentrations of ET-1 increased tension more slowly than [Ca2+]i. 3. In the presence of extracellular Ca2+, ET-3 (10(-8)-10(-5) M) induced concentration-dependent increases in [Ca2+]i and tension. However, the maximal elevations of [Ca2+]i and tension induced by ET-3 were substantially smaller than those induced by ET-1, indicating the involvement of an ETA receptor subtype. ET-3, at concentrations greater than 6 x 10(-7) M, caused biphasic slowly developing increases in [Ca2+]i and tension. At concentrations lower than 10(-6) M, ET-3 caused monophasic increases in [Ca2+]i and tension. At all concentrations of ET-3, the time courses of increases in [Ca2+]i and tension were similar. 4. The biphasic increases in [Ca2+]i and tension induced by 10-5 M ET-3 and by 1O-7M ET-1 were significantly inhibited by pretreatment with 10-5 M of the Ca2+ entry blocker, diltiazem, although the inhibition of the first component of ET-l-induced [Ca2+]i increase was partial.5. In the absence of extracellular Ca2+, ET-1 induced a concentration-dependent transient increase in[Ca2+]i, possibly due to release of Ca2+ from intracellular stores, and a sustained contraction. In contrast, ET-3 ( 10-6 M) caused little, if any, transient increase in [Ca2+]i and a small sustained contraction.6. Temporal changes in the relationships between [Ca2+]i and tension ([Ca2+]1-tension relationship)during contractions induced by ET-1 and ET-3 were compared with the [Ca2+]i-tension relationship of Ca2+-induced contractions (Ca2+-contractions) obtained by cumulative applications of extracellular Ca2+(0-7.5 mM) to tissues depolarized in the presence of 118 mMK+. In the [Ca2+]i-rising phase, ET-1 increased tension more slowly than [Ca2+]i, thereby shifting the [Ca2+]i-tension relation to the right from that for Ca2+-contractions. In the [Ca2+I-declining and the steady-state phases, ET-1, at concentrations higher than 10-9 M, produced greater tension development than that expected from a given change in[Ca2+ji, resulting in a leftward shift of the [Ca2+]i-tension relation. During ET-3-induced contractions,([Ca2+]i-rising, [Ca2+]i-declining and steady-state phases), the [Ca2+]i-tension relation was similar to that of Ca2+-contractions.7. BQ-123, a selective ETA receptor antagonist, completely inhibited the increases in [Ca2+1]i and tension induced by ET-1 and ET-3.8. These results suggest: (1) That ET-1 elicits vasoconstriction by increasing [Ca2+]i through the activation of Ca2+ influx from the extracellular space and Ca2+ release from intracellular storage sites,and by increasing the Ca2+ sensitivity of the contractile apparatus, whereas ET-3 induces vasoconstriction by increasing [Ca2+1] mainly through Ca2+ influx from the extracellular space. (2) Distinct mechanisms of time-dependent modulation of the Ca2+ sensitivity function in the vasoconstrictor responses to ET-1 and ET-3. (3) That both ET-1- and ET-3-induced contractions seem to be mediated via ETA-receptors in porcine coronary artery, and that the ETA-receptor-mediated effects of ET-1 and ET-3 can be dissociated at the sub-receptor levels of the signal transduction pathway.

Signal transduction and regulation in smooth muscle

Smooth muscle cells in the walls of many organs are vital for most bodily functions, and their abnormalities contribute to a range of diseases. Although based on a sliding-filament mechanism similar to that of striated muscles, contraction of smooth muscle is regulated by pharmacomechanical as well as by electromechanical coupling mechanisms. Recent studies have revealed previously unrecognized contractile regulatory processes, such as G-protein-coupled inhibition of myosin light-chain phosphatase, regulation of myosin light-chain kinase by other kinases, and the functional effects of smooth muscle myosin isoforms. Abnormalities of these regulatory mechanisms and isoform variations may contribute to diseases of smooth muscle, and the G-protein-coupled inhibition of protein phosphatase is also likely to be important in regulating non-muscle cell functions mediated by cytoplasmic myosin II.

Evidence for a Ca(2+)-gated ryanodine-sensitive Ca2+ release channel in visceral smooth muscle

Although a role for the ryanodine receptor (RyR) in Ca2+ signaling in smooth muscle has been inferred, direct information on the biochemical and functional properties of the receptor has been largely lacking. Studies were thus carried out to purify and characterize the RyR in stomach smooth muscle cells from the toad Bufo marinus. Intracellular Ca2+ measurements with the Ca(2+)-sensitive fluorescent indicator fura-2 under voltage clamp indicated the presence of a caffeine- and ryanodine-sensitive internal store for Ca2+ in these cells. The (CHAPS)-solubilized, [3H]ryanodine-labeled RyR of toad smooth muscle was partially purified from microsomal membranes by rate density centrifugation as a 30-S protein complex. SDS/PAGE indicated the comigration of a high molecular weight polypeptide with the peak attributed to 30-S RyR, which had a mobility similar to the cardiac RyR and on immunoblots cross-reacted with a monoclonal antibody to the canine cardiac RyR. Following planar lipid bilayer reconstitution of 30-S stomach muscle RyR fractions, single-channel currents (830 pS with 250 mM K+ as the permeant ion) were observed that were activated by Ca2+ and modified by ryanodine. In vesicle-45Ca2+ efflux measurements, the toad channel was activated to a greater extent at 100-1000 microM than 1-10 microM Ca2+. These results suggest that toad stomach muscle contains a ryanodine-sensitive Ca2+ release channel with properties similar but not identical to those of the mammalian skeletal and cardiac Ca(2+)-release channels.

The isoquinoline derivative LOE 908 selectively blocks vasopressin-activated nonselective cation currents in A7r5 aortic smooth muscle cells

The effect of (R,S)-(3,4-dihydro 6,7-dimethoxy-isoquinoline-1-yl)-2-phenyl- N,N-di-[2-(2,3,4-trimethoxyphenyl)ethyl]-acetamide (LOE 908), a cation channel blocker in HL-60 promyeloblasts, was studied in the A7r5 smooth muscle cell line from rat thoracic aorta, using the whole-cell patch-clamp technique. At a holding potential of -60 mV, application of vasopressin induced a nonselective cation conductance in voltage-clamped A7r5 cells. The current-voltage relation was linear, and currents reversed close to 0 mV regardless of the chloride gradient. The activation of the nonselective cation conductance by vasopressin was not affected by dialysing cells with Ca(2+)-free internal solution. LOE 908 blocked this current in a concentration-dependent manner with an IC50 of 560 nM, whereas dihydropyridine-sensitive Ba2+ current through voltage-dependent Ca2+ channels was blocked with an IC50 of 28 microM. Another organic blocker of receptor-mediated Ca2+ entry, 1-beta-[3-(4-methoxyphenyl)-propoxy]-4-methoxyphenethyl-1H-imidazole hydrochloride (SK&F 96365), blocked both, the vasopressin-induced nonselective conductance and the voltage-activated Ba2+ current with similar IC50 values of 13 microM and 8 microM, respectively. The rank order of potency of inorganic blockers on the vasopressin-induced inward current was Gd3+ > La3+ > Cd2+. Vasopressin-induced non-selective cation current was also observed in pertussis toxin-pretreated A7r5 cells but was completely abolished after infusion of the GDP analogue, guanosine 5'-O-[3-thio]diphosphate, from the patch pipette. Furthermore, vasopressin induced a transient outward current, suggesting a Ca(2+)-activated K(+)-current, which overlapped with the nonselective cation conductance.(ABSTRACT TRUNCATED AT 250 WORDS)

On the origin and dynamics of the vasomotion of small arteries

A system of differential equations describing stationary vasomotion is formulated. It incorporates the ionic transports, cell-membrane potential, muscle contraction of the vessel smooth muscle cells, and the mechanics of a thick-walled cylinder. It is shown that the interaction of Ca2+ and K+ fluxes mediated by voltage-gated and voltage-calcium-gated channels, respectively, brings about periodicity of those transports. This results on a time-periodic cytoplasmic calcium concentration, myosin light chains phosphorylation, and crossbridges formation with the attending muscle stress. The vessel's transmural pressure determines a hoop stress. The resultant hoop, elastic, and muscle stresses determine the rate of change of the vessel's diameter: vasomotion. The model results agree with the experimental observations. The sensitivity of the vasomotion's dependence on parameter values and its significance to experimental protocols are examined. Further, it is hypothesized that the dependence of calcium-channel openings on voltage is shifted by changes on transmural pressure. Thus, Harder's experimental results are reproduced, among them the decreasing of vessel diameter with increasing pressure. Those behaviors are associated with a pattern of change of the singularities of the system of equations describing the model. This suggests a functional relationship on the interactions of Ca2+ and K+ fluxes responsible for the myogenic response; it may not result from a single molecular mechanism. The model is constructed so that additional experimental information can be readily incorporated.

Isoproterenol stimulates rapid extrusion of sodium from isolated smooth muscle cells

beta-Agonists cause an inhibition of contractility and a transient stimulation of Na+/K+ pumping in smooth muscle cells of the stomach from the toad Bufo marinus. To determine if the stimulation of Na+/K+ pumping causes changes in intracellular [Na+] ([Na+]i) that might link Na+ pump stimulation to decrease Ca2+ availability for contraction, [Na+]i was measured in these cells with SBFI, a Na(+)-sensitive fluorescent indicator. Basal [Na+]i was 12.8 +/- 4.2 mM (n = 32) and was uniform throughout the cell. In response to isoproterenol, [Na+]i decreased an average of 7.1 +/- 1.1 mM in 3 sec. Since this decrease in [Na+]i could be completely blocked by inhibition of the Na+ pump, or by blockade of the beta-receptor, [Na+]i reduction is the result of occupation of the beta-receptor by isoproterenol and subsequent stimulation of the Na+ pump. 8-Bromoadenosine 3',5'-cyclic monophosphate and forskolin mimicked the effect of isoproterenol, indicating that the sequence of events linking beta-receptor occupation to Na+ pump stimulation most likely includes activation of adenylate cyclase, production of cAMP, and stimulation of cAMP-dependent protein kinase. The decrease in [Na+]i is sufficiently large and fast that it is expected to stimulate turnover of the Na+/Ca2+ exchanger in the Ca2+ extrusion mode, thereby accounting for the observed linkage between stimulation of the Na+/K+ pump and inhibition of contractility in response to beta-adrenergic agonists.

Bilirubin levels in subarachnoid clot and effects on canine arterial smooth muscle cells

BACKGROUND AND PURPOSE

Previous studies have suggested that bilirubin is a potential contributor to cerebral vasospasm. The purpose of this investigation was to determine whether bilirubin accrues in subarachnoid clot, whether its vasoconstrictive effect could involve a direct action on arterial smooth muscle cells, and, if so, whether bilirubin affects their Ca2+ uptake.

METHODS

Subarachnoid clots were analyzed for bilirubin using high-performance liquid chromatography. The length and 45Ca2+ uptake of vascular smooth muscle cells enzymatically dissociated from canine carotid arteries were measured before and after exposure to bilirubin solution. Additional experiments were conducted on cultured smooth muscle cells from canine basilar artery and on ATP-depleted cardiac myocytes.

RESULTS

Mean +/- SE bilirubin concentration in experimental clot was 263 +/- 35.7 mumol/L. Vascular smooth muscle cells exposed to bilirubin showed progressive shortening (P < .01) and an increased uptake of 45Ca2+ (P < .001). Contraction was prevented by Ca(2+)-free media but not by verapamil. Experiments with heart myocytes showed that bilirubin caused an increased uptake of 45Ca2+ but not of [14C]sucrose.

CONCLUSIONS

The results indicate that bilirubin accrues in subarachnoid clot, that it exerts a direct constrictive effect on arterial smooth muscle cells, and that this effect is associated with an increased uptake of Ca2+. Studies on heart myocytes suggest that the Ca2+ uptake induced by bilirubin could be due to a selective increase in membrane permeability to Ca2+.

Mechanisms of oxygen-induced contraction of ductus arteriosus isolated from the fetal rabbit

The present study was designed to investigate the effect of O2 on intracellular Ca concentration ([Ca]i) in the ductus arteriosus and the mechanisms for O2-induced ductal contraction. The force of isometric contraction of the ring of the ductus arteriosus isolated from fetal rabbits at 30 days of gestation (term, 31 days) was measured. The ductus arteriosus was loaded with fura 2, a calcium-sensitive dye, and [Ca]i was determined from the ratio of fluorescence intensity at 340 and 380 nm excitation wavelengths. The ductus arteriosus was initially superfused with hypoxic control solutions and contraction was induced by application of oxygenated solutions. The O2-induced contraction of the ductus arteriosus was associated with increases in [Ca]i and was eliminated in the absence of extracellular calcium. An increase in [K]o from 5 to 50 mM, which causes membrane depolarization, induced ductal contraction. The calcium channel blockers verapamil, diltiazem, and nickel caused a similar inhibition of O2-induced contraction as well as KCl-induced contraction. The role of intracellular calcium stores in O2-induced ductal contraction was examined using ryanodine, an inhibitor of calcium uptake and release from the sarcoplasmic reticulum. The inhibition of O2-induced contraction by ryanodine was minimal. Infusion of glibenclamide, an inhibitor for opening the ATP-sensitive potassium channel, caused contraction of the ductus arteriosus in the hypoxic solution. Cromakalim, an opener of ATP-sensitive potassium channels, completely relaxed the contraction induced by O2. These data suggest that O2 increases [Ca]i and causes contraction in the ductus arteriosus. Application of O2 may change from anaerobic to aerobic metabolism and depolarize membrane potential by closing the ATP-sensitive potassium channel, which in turn increases calcium influx via the voltage-dependent calcium channel. Mechanisms other than the ATP-sensitive potassium channel may also be involved in the O2-induced contraction and remain to be studied.

Ca(2+)-dependent K+ channels of high conductance in smooth muscle cells isolated from rat cerebral arteries

1. Cerebrovascular smooth muscle cells (CVSMCs) were dispersed from cerebral arteries of adult rats using collagenase and trypsin. The extracellular patch clamp technique was used to study single calcium-activated potassium channels, KCa+ channels, in these cells at 21-23 degrees C. 2. Whole-cell, current clamp recordings showed that isolated CVSMCs possessed a mean resting potential of -41 +/- 7.4 mV (n = 69), an input resistance of 3.2 +/- 0.49 G omega (n = 20) and a capacitance of 24 +/- 2.3 pF (n = 7). 3. Inside-out patches displayed a calcium-dependent potassium channel, KCa+ channel, of mean conductance 207 +/- 10 pS (n = 16) and potassium permeability 3.9 x 10(-13) cm s-1 (n = 16) in symmetrical 140 mM K+ solutions. No substate conductance level was evident. 4. This channel was highly selective for K+ over Na+ or Cs+ (permeability ratio PNa/PK < 0.05; PCs/PK < 0.05, n = 5 patches in each case). Cs+ caused a voltage-dependent block of the open channel. 5. Channel openings were detected at a threshold level of free internal calcium, [Ca2+]i = 10(-8) M, and channels were open half of the time at [Ca2+]i = 2.3 x 10(-5) M (membrane potential, Vm = +40 mV, n = 5). Over the probable physiological range of [Ca2+]i, the open probability of the KCa+ channel increased with the second power of calcium concentration. 6. Open time distributions were well fitted by the sum of two exponential terms, showing the occurrence of at least two kinetically distinguishable open states. Raising [Ca2+]i increased the time constant of the slow exponential component, but had no effect on that of the fast component. 7. At [Ca2+]i < 5 x 10(-5) M, a 14 mV depolarization in membrane potential resulted in an e-fold increase in the probability of KCa+ channels adopting an open state (n = 5). The slow time constant of the open time distributions also increased on membrane depolarization. 8. Tetraethylammonium ions applied to the cytoplasmic membrane face caused a reversible, dose-dependent reduction in current flow through the KCa+ channel. This block was characterized by a dissociation constant of 0.83 +/- 0.09 mM at Vm = +40 mV and [Ca2+]i = 10(-4) M (n = 5). 9. The lower calcium sensitivity and higher sensitivity to tetraethylammonium block distinguish this from other large conductance KCa+ channels in smooth muscle cells.

Ca-EGTA affects the relationship between [Ca2+] and tension in alpha-toxin permeabilized rat anococcygeus smooth muscle

The relationship between calcium concentration ([Ca2+]) and force in smooth muscle can be studied by permeabilizing the sarcolemma and bathing the preparation in a mock intracellular solution. Normally [Ca2+] is set in these solutions using the Ca2+ chelator EGTA in the concentration range of 4-10 mM. This study shows that lowering total EGTA concentration ([EGTA]t) below 10 mM depresses Ca(2+)-activated force generated in 0.1 microM Ca2+. The observed threshold for the effect of EGTAt is 0.2 mM, and the effect is maximal at approximately 10 mM. BAPTA, another Ca2+ chelator, also produces this effect. Tension production in smooth muscle is controlled by acto-myosin interaction. This in turn is mediated by the relative activities of myosin light chain kinase (MLCK) and phosphatase (MLCP). Inhibiting MLCP with Microcystin LR (10 microM), an increase [EGTAt] from 0.2 mM to 10 mM still enhanced force. This suggests that EGTA promotes phosphorylation of myosin by the activation of MLCK and not by inhibition of MLCP.

Ca2+ regulation of the contractile apparatus in canine gastric smooth muscle

1. The relationships between cytosolic Ca2+ ([Ca2+]cyt; expressed as a fluorescence ratio at 400 nm and 500 nm using Indo-1) and contractile force was examined in strips of circular smooth muscles of canine gastric antrum. Rhythmic increases in [Ca2+]cyt were observed and contractions were biphasic. 2. In most muscles (70%), the amplitude of the second phase of the Ca2+ transient was less than or equal to the first phase of the Ca2+ transient, but the second phase of the contraction was much smaller than the first phase, suggesting a decrease in Ca2+ sensitivity during the second contractile phase. In 30% of muscles, the amplitude of the second phase of the Ca2+ transient was 2- to 3-fold greater than the first phase. In these muscles, the second phase of contraction was 10-fold greater than the first phase of contraction. Thus, a non-linear relationship between [Ca2+]cyt and force greatly amplifies force development when [Ca2+]cyt exceeds a threshold level. 3. Acetylcholine (ACh, 0.3-1 microM) increased the amplitudes of Ca2+ transients and basal [Ca2+]cyt between phasic contractions. The increase in basal [Ca2+]cyt did not cause tone to develop. ACh increased the amplitude of Ca2+ transients 2- to 3-fold and this was associated with a 15 to 20-fold increase in the force of phasic contractions. Pentagastrin (0.5 nM) and cholecystokinin octapeptide (CCK, 40 nM) had similar effects on Ca2+ transients and phasic contractions. 4. Bay K 8644 (0.1 microM) and TEA (5 mM) also increased the amplitudes of Ca2+ transients by 2- to 3-fold and phasic contractions by 15- to 30-fold. There was no significant difference observed between the [Ca2+]cyt-force relationships in the presence of agonists (i.e. ACh, pentagastrin and CCK) or when [Ca2+]cyt was increased by Bay K 8644 or TEA. These data suggest that agonist-dependent increases in Ca2+ sensitivity may not significantly regulate the [Ca2+]cyt-force relationship in antral muscles. 5. D600 (5 microM), added during stimulation with ACh (0.3 M), decreased [Ca2+]cyt and force without affecting the [Ca2+]cyt-force relationship. 6. Mechanisms exist for agonist-mediated enhancement of the Ca(2+)-force relationship. In alpha-toxin-permeabilized antrum, ACh (10 microM) with GTP (100 microM) or GTP gamma S (100 microM) increased the Ca(2+)-induced contraction at clamped levels of Ca2+. Phorbol 12,13-dibutyrate (PDBu, 10 microM) also increased the contractile force at a given level of Ca2+.(ABSTRACT TRUNCATED AT 400 WORDS)