Ca2+ localization and sensitivity in vascular smooth muscle

Extracellular Na+ dependency of free cytosolic Ca2+ regulation in aortic vascular smooth muscle cells

This study examined contribution of Na(+)-dependent processes to the regulation of free cytosolic calcium (Ca2+i) in cultured vascular smooth muscle cells (VSMC) using fura-2. Removal of Na+ from superfusate (replacement with choline) resulted in an increment of Ca2+i that was greatly augmented by pretreatment with ouabain. Under both conditions, Ca2+i increase was followed by partial recovery to a new steady state that was still significantly higher than that seen before removal of external Na+ (Na+o). In ouabain-pretreated cells lowering of Na+o caused progressive increases in Ca2+i. Addition of NiCl2, a Na(+)-Ca2+ exchange inhibitor, completely blocked the increase in Ca2+i produced by removal of Na+o, indicating that the Na(+)-Ca2+ antiporter was responsible for observed Ca2+i changes. Ca2+i increase produced by reduction of Na+o was also seen after depletion of inositol trisphosphate-sensitive Ca2+ stores with repeated pulses of angiotensin II or after blockade of sarcoplasmatic reticulum Ca2+ release with TMB-8 but was not observed in the absence of external Ca2+. These observations indicate that the source of Ca2+i increase in response to changes in the transmembrane Na+ gradient is largely external, and potentiation of the Ca2+i surge by ouabain suggests Ca2+ influx via the Na(+)-Ca2+ exchanger operating in the reverse mode. The relative contribution of a Na(+)-dependent and -independent component of Ca2+i recovery was investigated by superfusing cells with ionomycin in a Na(+)-free medium and later adding Na+ to the medium. This Ca2+ ionophore increased Ca2+i to a peak, and this was followed by a rapid but partial recovery to a new steady state. Readdition of varying amounts of Na+ to the superfusate, in the continued presence of ionomycin, resulted in concentration-related decline in Ca2+i, thereby uncovering a substantial contribution of a Na(+)-dependent mechanism of Ca2+i regulation. Decline of Ca2+i produced by readdition of Na+ was blocked by addition of NiCl2 to the superfusate. Our findings thereby provide evidence for Ca2+i regulation in VSMC via a Na(+)-dependent mechanism, consistent with a Na(+)-Ca2+ exchanger, which acts as a Ca2+ efflux mechanism when Ca2+i is elevated. Na(+)-Ca2+ exchanger acts as a Ca2+ influx mechanism when intracellular Na+ is elevated by prior exposure to ouabain.

Clodronate inhibits contraction and prevents the action of L-type calcium channel antagonists in vascular smooth muscle

Clodronate (dichloromethylenebisphosphonate) decreased vasoconstriction of the isolated perfused rat tail artery mediated by norepinephrine and by Ca2+ in a K(+)-depolarizing solution. The norepinephrine contractile response was divided into two components by sequential manipulation of the composition of the perfusion fluid, where the first component is due to the release of Ca2+ from intracellular stores and the second to the influx of Ca2+ from extracellular fluid. Clodronate (20 microM) decreased only the first component of the response at a norepinephrine concentration of 50 nM, and both components of the response at a higher norepinephrine concentration (100 nM). The L-type Ca2+ channel blocking drugs, nicardipine (10 nM) and verapamil (1 microM), reduced the second component of the norepinephrine-mediated vasoconstriction, but in the presence of clodronate (20 microM) this blocking action was prevented. These results were confirmed by examining the interaction between clodronate and nicardipine on norepinephrine and K(+)-mediated lanthanum (La(3+)-resistant unidirectional 45Ca uptake. Nicardipine (1-10 nM) decreased the norepinephrine (100 nM) and K(+)-induced (60 mM) La(3+)-resistant unidirection 45Ca uptake in a concentration-dependent manner, but in the presence of clodronate (20 microM) this concentration-dependent response was abolished. Thus, clodronate not only reduced agonist-induced Ca2+ release from intracellular stores and Ca2+ influx through L-type Ca2+ channels but also prevented L-type Ca2+ channel antagonists from exerting their effect. These results indicate clodronate has two sites of action during vascular smooth muscle contraction: the first on intracellular mobilization of Ca2+ and the second on L-type Ca2+ channels.

Vascular smooth muscle contractile elements. Cellular regulation

For many years the simple view was held that contractile force in smooth muscle was proportional to cytosolic Ca2+ concentrations ([Ca2+]i). With the discovery that phosphorylation of myosin light chain by Ca2+/calmodulin-dependent myosin light chain kinase initiated contraction, regulation of the contractile elements developed more complex properties. Molecular and biochemical investigations have identified important domains of myosin light chain kinase: light chain binding sites, catalytic core, pseudosubstrate prototope, and calmodulin-binding domain. New protein phosphatase inhibitors such as okadaic acid and calyculin A should help in the identification of the physiologically important phosphatase and potential modes of regulation. The proposal of an attached, dephosphorylated myosin cross bridge (latch bridge) that can maintain force has evoked considerable controversy about the detailed functions of the myosin phosphorylation system. The latch bridge has been defined by a model based on physiological properties but has not been identified biochemically. Thin-filament proteins have been proposed as secondary sites of regulation of contractile elements, but additional studies are needed to establish physiological roles. Changes in the Ca2+ sensitivity of smooth muscle contractile elements with different modes of cellular stimulation may be related to inactivation of myosin light chain kinase or activation of protein phosphatase activities. Thus, contractile elements in smooth muscle cells are not dependent solely on [Ca2+]i but use additional regulatory mechanisms. The immediate challenge is to define their relative importance and to describe molecular-biochemical properties that provide insights into proposed physiological functions.

Inhibitory effect of 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8) in vascular smooth muscle

The inhibitory effects of 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8) on vascular smooth muscle contraction and cytosolic Ca2+ level ([Ca2+]i) were examined using isolated rabbit aorta loaded with a fluorescent Ca2+ indicator, fura-2. TMB-8 (100 microM) decreased the high K(+)-induced increase in muscle tension, and [Ca2+]i and 45Ca2+ influx to their respective resting levels. TMB-8 (100 microM) almost completely inhibited the increase in [Ca2+]i and 45Ca2+ influx due to norepinephrine although muscle tension was only partially decreased. A higher concentration of TMB-8 (300 microM) inhibited the remaining portion of the contraction without additional decrease in [Ca2+]i. The inhibitory effect of TMB-8 on high K(+)-induced contraction, but not on the norepinephrine-induced contraction, was antagonized by the increase in external Ca2+ concentrations or by the Ca2+ channel activators, CGP 28,392 and by Bay K8644. In Ca(2+)-free solution, norepinephrine-induced transient increases in [Ca2+]i and muscle tension and 100 microM TMB-8 inhibited these changes. The caffeine-induced transient increases in [Ca2+]i and muscle tension were also inhibited by TMB-8 at concentrations higher than those needed to inhibit the norepinephrine-induced transient changes. In permeabilized smooth muscle, TMB-8 (300 microM) did not inhibit the Ca(2+)-induced contraction. These results suggest that TMB-8 inhibits vascular smooth muscle contractility by inhibiting Ca2+ influx, Ca2+ release and Ca2+ sensitization of contractile elements.

Intracellular calcium levels in canine basilar artery smooth muscle following experimental subarachnoid hemorrhage: an electron microscopic cytochemical study

Changes in intracellular calcium levels in canine basilar arterial smooth muscle were semiquantitatively measured by an electron microscopic cytochemical technique using a combined oxalate-pyroantimonate method. Measurements made after subarachnoid hemorrhage were compared with those made after contraction induced by prostaglandin F2 alpha. Fifteen minutes after topical application of the drug, when the basilar artery was constricted by 20%, 15% of smooth muscle cells contained a large amount of intracellular calcium. One hour later, the diameter of the basilar artery and intracellular calcium precipitation returned almost to control levels. Fifteen minutes after the first intracisternal injection of autologous blood, when acute vasospasm was angiographically evident, 13% of smooth muscle cells contained a large amount of calcium. After 1 h, when acute vasospasm had already abated, the number of smooth muscle cells containing a large amount of calcium markedly increased to 37% and some smooth muscle cells showed early degenerative findings such as intracytoplasmic vacuoles including calcium accumulation. After 48 h, when delayed vasospasm had already started, the calcium deposits and early degenerative changes had decreased significantly. After 49 h and 4 days (1 h and 48 h after the second injection of blood), the change in the amount of calcium was the same as at 1 h and 48 h after the first injection, respectively, but degeneration of smooth muscle cells increased. Therefore, acute vasospasm after subarachnoid hemorrhage may be caused by an initial elevation of intracellular calcium levels, as is the case with drug-induced contraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Oxytocin contracts rat uterine smooth muscle in Ca2(+)-free medium without any phosphorylation of myosin light chain

Contraction of rat uterine smooth muscle related to phosphorylation state of myosin light chain under various conditions was investigated. In the Ca2(+)-containing medium, both high K+ and oxytocin induced marked contraction of the muscle accompanied by pronounced phosphorylation of myosin light chain. In the Ca2(+)-free medium, although both vanadate and oxytocin induced slight contraction, phosphorylation of myosin light chain was only evident for vanadate but not for oxytocin. It was suggested that another mechanism distinct from myosin light chain phosphorylation might be involved in Ca2(+)-independent contraction of uterine smooth muscle elicited by oxytocin.

Increased Ca2+ signaling after alpha-adrenoceptor activation in vascular hypertrophy

In an effort to explain the increased sensitivity to agonists of hypertrophic vascular muscle, intracellular Ca2+ concentration ([Ca2+]i)-signaling mechanisms were studied in normal and hypertrophic rat aortas from normotensive and coarctation-hypertensive rats. Based on both fura 2 fluorescence and aequorin luminescence measurements, qualitatively different patterns of Ca2+ mobilization occur in normal and hypertrophic rat aortic muscle. Normal rat aortic muscle contracts to phenylephrine with little or no increase in [Ca2+]i, whereas the angiotensin II-induced contraction is accompanied by a marked [Ca2+]i transient. In contrast, hypertrophic rat aortic muscle shows a dramatic increase in Ca2+ signaling after phenylephrine stimulation. Moreover, both the amplitude of the angiotensin-induced [Ca2+]i transient and the contractile sensitivity to this agonist are decreased in the hypertrophic muscle. Our results strongly suggest that the amplitude of the [Ca2+]i transient after agonist stimulation determines the contractile sensitivity and that there is an altered coupling of the alpha-adrenoceptor in the hypertrophic vascular muscle.

12-Deoxyphorbol 13-isobutyrate contracts isolated rat thoracic arteries

The vasocontractile effect of 12-deoxyphorbol 13-isobutyrate (DPB), an activator of protein kinase C (PKC), was studied to clarify the mechanism of the vascontractile response elicited by activation of PKC. DPB induced both a sustained increase in intracellular Ca2+ levels and contraction in isolated rat thoracic artery. For a given increase in intracellular Ca2+ levels. DPB induced a greater contraction than high K+ or ionomycin. In Ca(2+)-free media, DPB induced concentration-dependent contraction with a slow rate of rise without causing detectable changes in intracellular Ca2+ levels. The DPB-induced contractions in Ca(2+)-free media were less inhibited by the inhibitors of calmodulin or myosin light chain kinase than ionomycin-induced contractions in normal media were. These results indicate that activation of PKC might increase the Ca2+ sensitivity of contractile elements due to mechanisms not associated with the Ca(2+)-calmodulin pathway.

The inhibitory action of okadaic acid on mechanical responses in guinea-pig vas deferens

Okadaic acid and nifedipine inhibited contractions induced by noradrenaline (NA), KCl and ATP in guinea-pig vas deferens. NA, KCl and ATP induced initial spike-like changes followed by a sustained increase in cytosolic Ca2+ levels ([Ca2+]cyt) and tension. Okadaic acid inhibited the sustained increments in [Ca2+]cyt and muscle tension due to NA and ATP more than the initial spike-like changes, whereas nifedipine more strongly inhibited the initial spike changes. Okadaic acid also inhibited the KCl-induced contraction with only a small inhibition of the stimulated [Ca2+]cyt. By contrast, nifedipine (10(-7) M) inhibited the increments in both [Ca2+]cyt and muscle tension due to KCl. Okadaic acid markedly inhibited the maximal contractile response to Bay K 8644 but nifedipine only shifted the response curve to the right without affecting the maximum responses. In a Ca2(+)-free medium containing EGTA and nifedipine, okadaic acid did not inhibit the residual phasic contractile response to NA (10(-4) M) but the contractile response to a subsequent addition of Ca2+ (1.2 mM) was suppressed. These results suggest that, in guinea-pig vas deferens, okadaic acid has an inhibitory effect on smooth muscle contraction but not on the intracellular Ca2+ mobilization. The inhibitory effect may be due to the inhibition of Ca2+ influx and the possible interference of contractile elements.

Myosin light chain kinase phosphorylation: regulation of the Ca2+ sensitivity of contractile elements

Purified myosin light chain kinase from smooth muscle is phosphorylated by cyclic AMP-dependent protein kinase, protein kinase C and the multifunctional calmodulin-dependent protein kinase II. Since phosphorylation in a specific site (site A) by any one of these kinases desensitizes myosin light chain kinase to activation by Ca2+/calmodulin, kinase phosphorylation could play an important role in regulating smooth muscle contractility. This possibility was investigated in 32P-labelled bovine tracheal smooth muscle. Treatment of tissues with carbachol, KCl, isoproterenol, or phorbol 12,13-dibutyrate increased the extent of kinase phosphorylation. Six primary phosphopeptides (A-F) of myosin light chain kinase were identified. Site A was phosphorylated to an appreciable extent only with carbachol or KCl, agents which contract tracheal smooth muscle. The extent of site A phosphorylation correlated to increases in the concentration of Ca2+/calmodulin required for activation. These results show that cyclic AMP-dependent protein kinase and protein kinase C do not affect smooth muscle contractility by phosphorylating site A in myosin light chain kinase. It is proposed that phosphorylation of myosin light chain kinase in site A, perhaps by calmodulin-dependent protein kinase II, may play a role in reported desensitization of contractile elements in smooth muscle to activation by Ca2+.

Time-dependent changes in Ca2+ sensitivity during phasic contraction of canine antral smooth muscle

1. Relationships between cytosolic Ca2+ concentration ([Ca2+]cyt), myosin light chain (MLC) phosphorylation and muscle tension were examined in circular smooth muscle of canine gastric antrum. 2. Electrical slow waves induced a transient increase in [Ca2+]cyt and muscle tension. [Ca2+]cyt increased before the initiation of contraction and reached a maximum before the peak of the phasic contractions. Following the first Ca2+ transient, a second rise in [Ca2+]cyt was often observed. The second Ca2+ transient was of similar magnitude to the first, but only in some cases was this increase in [Ca2+]cyt associated with a second phase of contraction. Relaxation occurred more rapidly than the restoration of resting levels of [Ca2+]cyt. 3. Acetylcholine (ACh; 3 x 10(-7) M) increased the amplitude of Ca2+ transients, caused MLC phosphorylation and increased the force of contraction. The decay of contraction and MLC dephosphorylation preceded that of [Ca2+]cyt. 4. Increasing external K+ (to 25-40 mM) caused a sustained increase in [Ca2+]cyt, but little change in resting tension. This suggests that the Ca2+ sensitivity decreased as [Ca2+]cyt increased. Increasing K+ to 59.5 mM further increased the level of [Ca2+]cyt, induced MLC phosphorylation and caused a transient contraction. When normal levels of K+ were restored, the rates of MLC dephosphorylation and relaxation exceeded the rate of decay in [Ca2+]cyt. 5. Removal of external Ca2+ in depolarized muscles decreased [Ca2+]cyt below the resting level without affecting resting tension. Readmission of Ca2+ to depolarized muscles caused force to develop at [Ca2+]cyt levels below the original resting level, suggesting that Ca2+ sensitivity was increased when the resting level of [Ca2+]cyt was decreased. 6. The phosphatase inhibitor, calyculin-A (10(-6) M), induced tonic contraction and MLC phosphorylation without an increase in [Ca2+]cyt. During these contractures, electrical activity caused transient increases in [Ca2+]cyt and phasic contractions which were superimposed upon the Ca(2+)-independent contracture. In the presence of calyculin-A, relaxation occurred in two phases. The initial, rapid phase of relaxation was not significantly affected by calyculin-A, but the slow phase was significantly decreased. 7. These results suggest that the relationship between [Ca2+]cyt, MLC phosphorylation and contraction changes as a function of [Ca2+]cyt in canine antral muscles. This may be due to a Ca(2+)-and time-dependent phosphatase that regulates the level of myosin phosphorylation.

Attenuation of contraction of isolated canine coronary arteries by enflurane and halothane

Contraction of vascular smooth muscle such as that existing in coronary arteries is regulated in part by Ca++ entry into cells via Ca++ channels. Volatile anaesthetics are known to attenuate agonist-induced coronary artery constriction. The purpose of this experiment was to determine if 1.5 MAC concentrations of halothane or enflurane attenuated contractions evoked by activation of one type of Ca++ channel--the potential operator channel. In the current experiment, potential operator channels were activated by depolarizing isolated canine coronary artery rings with high concentration of K+, causing Ca++ entry and vessel contraction. Rings without endothelium were suspended for isometric force measurement in organ chambers containing aerated Krebs-Ringer solution. Maximum response to Ca++ in rings depolarized with K+ was 120 +/- 5 per cent in untreated versus 101 +/- 3 per cent in rings treated with enflurane (P less than 0.01). The maximum response was 123 +/- 6 per cent in untreated versus 111 +/- 5 per cent during halothane administration (P less than 0.05). In contrast, nifedipine 10(-9) M depressed maximum contractions from 114 +/- 5 per cent to 37 +/- 4 per cent (P less than 0.01) and nifedipine 10(-8) M depressed contractions to 30 +/- 4 per cent (P less than 0.01). In a further series of experiments, sustained contractions were depressed by continued administration of the anaesthetics, indicating no loss of anaesthetic effect with time. The results indicate that 1.5 MAC halothane and enflurane attenuate contractions of canine coronary arteries evoked by depolarization and Ca++ entry through potential operated channels. However, neither halothane nor enflurane exhibited the marked depressant effect exerted by nifedipine.

Effects of a novel smooth muscle relaxant, KT-362, on contraction and cytosolic Ca2+ level in the rat aorta

1. Inhibitory effects of a novel smooth muscle relaxant, KT-362 (5-[3-([2-(3,4-dimethoxyphenyl)-ethyl]amino)-1-oxopropyl]-2,3,4,5- tetrahydro-1,5-benzothiazepine fumarate), on contraction and the cytosolic Ca2+ level ([Ca2+]cyt) in isolated vascular smooth muscle of rat aorta were examined. 2. KT-362 inhibited the contractions induced by high K+ and noradrenaline. The inhibitory effect was antagonized by an increase in external Ca2+ concentration. A Ca2+ channel activator, Bay K 8644, did not change the effect of KT-362 on high K+-induced contraction. 3. [Ca2+]cyt, measured with fura-2-Ca2+ fluorescence, increased during the contractions induced by high K+ or noradrenaline. KT-362 decreased [Ca2+]cyt and muscle tension stimulated by high K+ or noradrenaline. By contrast, a Ca2+ channel blocker, verapamil, inhibited the noradrenaline-induced increase in [Ca2+]cyt with only partial inhibition of the noradrenaline-induced contraction and KT-362 inhibited the verapamil-insensitive portion of the contraction without changing [Ca2+]cyt. 4. In a Ca2(+)-free solution, noradrenaline and caffeine induced a transient contraction following a transient increase in [Ca2+]cyt. KT-362 inhibited the increments due to noradrenaline but not those induced by caffeine. 5. These results suggest that KT-362 inhibits vascular smooth muscle contraction by inhibiting Ca2+ channels, receptor-mediated Ca2+ mobilization, and receptor-mediated Ca2+ sensitization of contractile elements.

Relaxation response of isolated canine veins to agents that act on the adenylate cyclase-cyclic AMP system: further investigation

The present study was undertaken to clarify whether there is a deficiency in the relaxation mediated by the adenylate cyclase-cyclic AMP (cAMP) system in the portal vein as compared to the saphenous vein. Longitudinal strips of the portal vein and helical preparations of the saphenous vein were used. The relaxation response to various agents was examined under conditions such that the venous preparations were previously contracted by methoxamine in equipotent concentrations (EC80), i.e., 10(-6) M for portal vein and 10(-5) M for saphenous vein. The saphenous vein relaxed fully in response to isoproterenol but the portal vein relaxed only to 29% of the maximum relaxation induced by papaverine 10(-4) M. However, dibutyryl cAMP and 8-bromo-cAMP, membrane permeable derivatives of cAMP, 3-isobutyl-1-methylxanthine and papaverine, phosphodiesterase inhibitors, and forskolin, a direct stimulator of adenylate cyclase, relaxed portal and saphenous veins similarly though with quantitative differences. The results suggest that there is no profound deficiency in the adenylate cyclase-cAMP system but there may be a deficiency in the coupling between surface beta-adrenoceptors and adenylate cyclase or there may be a low density of beta-adrenoceptors in the portal vein.

Guinea-pig megakaryocytes can respond to external ADP by activating Ca2(+)-dependent potassium conductance

1. The responses of megakaryocytes to adenosine diphosphate (ADP) were studied using whole-cell patch electrodes and a Ca2(+)-sensitive fluorescent dye, Fura-2. Megakaryocytes (diameter, 17-42 microns) were mechanically dissociated from the bone marrow of adult guinea-pigs and ADP (1-10 microM) was pressure-applied to megakaryocytes under recording. 2. In megakaryocytes immersed in standard saline, ADP evoked an obvious outward current at a membrane potential of -63 mV. The current was identified as a K(+)-carried current, since the reversal potential depended distinctly on the external K+ concentration, but it showed no changes after removal of external Na+. The amplitude of evoked K+ currents showed considerable intercell variation, which is presumably due to differences of current density in the membrane. 3. During application of ADP, the evoked K+ current was not sustained but slowly decayed to become negligible within 10-20 s, suggesting the appearance of desensitization. The response of the megakaryocyte to ADP recovered slowly and returned to an original level after 4-5 min of continuous washing. 4. When the intracellular free Ca2+ concentration ([Ca2+]i) was measured using the Ca2(+)-sensitive fluorescent dye, Fura-2, application of 10 microM-ADP induced an increase of [Ca2+]i by about 5-fold, which was followed by a gradual decay to the original level within 30-50 s. Roles of internal Ca2+ for activating the K+ current were confirmed by observing (1) enhancement of evoked currents by the use of internal saline containing no Ca2+ chelators and (2) generation of prolonged K+ current by application of a Ca2+ ionophore, A23187, to the megakaryocyte. 5. In a fraction of the megakaryocytes, spontaneous hyperpolarization of the resting membrane potential was observed. The hyperpolarization seemed to result from the activation of K+ channels in the membrane, which was caused by spontaneous release of Ca2+ from the internal storage site. 6. It was concluded that megakaryocytes of the guinea-pig can respond to external ADP by increasing [Ca2+]i and consequently by activating Ca2(+)-dependent K+ channels in the membrane.

Mechanisms of pinacidil-induced vasodilatation

The mechanism of the vasodilator effect of pinacidil was examined. Pinacidil (0.1-100 microM) inhibited the increases in cytosolic Ca2+ ([Ca2+]i) and muscle tension due to norepinephrine in rat aorta. In contrast, a Ca2+ channel blocker, verapamil, inhibited the norepinephrine-stimulated [Ca2+]i more strongly than the contraction. Higher concentrations of pinacidil (3-100 microM) inhibited the verapamil-insensitive portion of the contraction and [Ca2+]i. An inhibitor of ATP-sensitive K+ channels, glibenclamide, antagonized the inhibitory effect of low concentrations (less than or equal to 10 microM) of pinacidol. Pinacidil did not change the contraction induced by Ca2+ in vascular smooth muscle permeabilized with Staphylococcus aureus alpha-toxin. Norepinephrine (in the presence of GTP), 12-deoxyphorbol 13-isobutyrate (in the absence of GTP), and treatment with GTP gamma S potentiated the contraction of permeabilized smooth muscle induced by the addition of Ca2+. Pinacidil (100 microM) inhibited the potentiation due to GTP gamma S or norepinephrine but not to phorbol ester. These results suggest that pinacidil has dual effects on vascular smooth muscle contraction. At lower concentrations (greater than 0.1 microM), it decreases [Ca2+]i, possibly by activating ATP-sensitive K+ channels. At higher concentrations (greater than 3 microM), it may additionally inhibit the receptor-mediated, GTP-binding protein-coupled phosphatidyl inositol turnover.

Activation of propylbenzilylcholine mustard-sensitive muscarinic cholinoceptors more effectively utilizes cytosolic Ca2+ for contraction in guinea-pig intestinal smooth muscle

A 50-min treatment of longitudinal smooth muscle of guinea-pig ileum with propylbenzilylcholine mustard (PrBCM, 3 x 10(-6) M) irreversibly inhibited the responses elicited by carbachol. However, a 90-min treatment with PrBCM had no further significant inhibitory effect on the responses to carbachol, suggesting that there are two subtypes of muscarinic cholinoceptors, PrBCM-sensitive and PrBCM-insensitive receptors. Carbachol caused a rapid increase in cytosolic Ca2+ concentrations ([Ca2+]i), which was followed by a rapid increase in muscle tension in both untreated and PrBCM-treated preparations. There was a positive correlation between [Ca2+]i (R340/380) and tension developed in response to carbachol. A regression line for the two responses was obtained in each preparation. The slope of the line obtained with untreated preparations was steeper than that obtained with PrBCM-treated preparations. These data suggest that, upon activation, PrBCM-sensitive receptors use cytosolic Ca2+ more effectively than PrBCM-insensitive receptors.

Okadaic acid uncouples myosin light chain phosphorylation and tension in smooth muscle

Tracheal smooth muscle precontracted with carbachol relaxes upon the addition of 3 microM okadaic acid. Although cytosolic Ca2+ concentrations decrease, myosin light chain remains highly phosphorylated (50%). In smooth muscle treated with carbachol alone or carbachol plus okadaic acid 32P is incorporated into a single peptide on myosin light chain which corresponds to the site phosphorylated by myosin light chain kinase. Treatment with okadaic acid alone does not result in myosin light chain phosphorylation or tension development. These results suggest that a cellular mechanism other than myosin light chain phosphorylation can regulate contractile tension.

Cytoplasmic calcium and the relaxation of canine coronary arterial smooth muscle produced by cromakalim, pinacidil and nicorandil

1. In order to investigate the vasodilator mechanisms of the K+ channel openers, cromakalim, pinacidil and nicorandil, we measured changes in cytoplasmic Ca2+ concentration [( Ca2+]i) simultaneously with force by a microfluorimetric method using fura-2, a calcium indicator, in canine coronary arterial smooth muscle cells. 2. The three K+ channel openers all produced a concentration-dependent reduction of [Ca2+]i in 5 and 30 mM KCl physiological salt solution (PSS) but failed to affect [Ca2+]i in 45 and 90 mM KCl-PSS. 3. Cromakalim only partly inhibited (-45%) the 30 mM KCl-induced contractures, whereas pinacidil and nicorandil nearly abolished contractions produced by 45 mM, 90 mM and 30 mM KCl-PSS. 4. Tetrabutylammonium (TBA), a nonselective K+ channel blocker, or glibenclamide, a supposed adenosine 5'-triphosphate (ATP)-sensitive K+ channel blocker, abolished the reduction of [Ca2+]i caused by the three K+ channel openers and the relaxant effect of cromakalim, whereas they only slightly attenuated the relaxant effects of pinacidil and nicorandil. 5. The increase in [Ca2+]i produced by 45 or 90 mM KCl-PSS in the presence of pinacidil or nicorandil was abolished by 10(-5) M verapamil, indicating that the increase in [Ca2+]i was caused by the influx of extracellular Ca2+ and that pinacidil and nicorandil did not affect the voltage-dependent Ca2+ channel directly. 6. The [Ca2+]i-force relationship in the presence of cromakalim was not distinguishable from that of control. 7. The [Ca2+]i-force curve was shifted to the right by pinacidil and nicorandil. 8. These results show that cromakalim is a more specific K+ channel opener than pinacidil and nicorandil, and that vasodilatation produced by cromakalim in this study is predominantly a result of a reduction of [Ca2+]i due to the closure of voltage-dependent Ca21 channels by hyperpolarization. In contrast, additional mechanisms are involved in the vasodilator actions of pinacidil and nicorandil. One of these is related to a reduction in the sensitivity of contractile proteins to Ca2 . The latter mechanism of nicorandil is akin to that of nitroglycerin. K+ channels opened by these K+ channel openers may be ATP-sensitive ones which are blocked by glibenclamide.

Cytosolic free calcium of aorta in hypertensive rats. Chronic inhibition of angiotensin converting enzyme

Cytosolic free calcium concentration ([Ca2+]i) and muscle tension were simultaneously measured in aortic tissue isolated from spontaneously hypertensive rats (SHR), normotensive Wistar-Kyoto (WKY) rats, and SHR chronically treated with a novel angiotensin converting enzyme inhibitor, CS-622. In the presence of 2.5 mM Ca2+ in the bathing solution, aortic [Ca2+]i measured with fura-2 was higher in SHR than in WKY rats, and it was almost the same in CS-622-treated SHR and untreated WKY rats. Increase of external Ca2+ concentration from zero to 2.5 mM elicited a contraction in SHR aortas but not in aortas from both CS-622-treated SHR and untreated WKY rats. When the aortas were contracted by 60 mM K+, however, [Ca2+]i as well as developed tension was similar in the three groups. CGP-28392 (10(-6) M), a Ca2+ channel activator, induced a rhythmic activity superimposed on a gradual increase of [Ca2+]i and tension in SHR aortas but not in the aortas of CS-622-treated SHR or untreated WKY rats. Nicardipine (10(-7) M) decreased the resting [Ca2+]i and the resting tone in SHR aortas, but not in WKY rat aortas. These results suggest that SHR aortas have a higher myogenic tone due to increased [Ca2+]i than WKY rat aortas and that the increased [Ca2+]i is attributed to alterations of dihydropyridine-sensitive Ca2+ channels in SHR aortas. Further, the decrease of the vascular tone induced by long-term administration of the angiotensin converting enzyme inhibitor may be due to a reduction of increased [Ca2+]i in SHR.

Greater contraction is induced by pilocarpine than by carbachol at the same levels of cytosolic Ca2+ concentration in isolated longitudinal muscle of guinea pig ileum

Pilocarpine, which activates propylbenzilylcholine mustard (PrBCM)-sensitive cholinoceptors, and carbachol, which activates PrBCM-resistant ones, induced an increase in cytosolic Ca2+ concentration ([Ca2+]i) and tension development in a concentration dependent manner in isolated longitudinal muscle of guinea pig ileum. A positive correlation between [Ca2+]i and tension development due to both of the agonists was noted. The slope of regression line between two values for pilocarpine was steeper than that for carbachol, suggesting that pilocarpine induced carbachol. Thus an activation of PrBCM-sensitive cholinoceptors might enhance the Ca2(+)-sensitivity of the contractile elements.

Changes in cytosolic CA2+ and contraction induced by various stimulants and relaxants in canine tracheal smooth muscle

Effects of stimulants and relaxants on the cytosolic Ca2+ level [(Ca2+]cyt) and contraction were examined in isolated canine tracheal smooth muscle. High K+ and carbachol induced a sustained increase in [Ca2+]cyt and muscle tension. Cumulative addition of KCl induced a graded increase in [Ca2+]cyt and muscle tension. Cumulative addition of carbachol induced greater contraction than high K+ at a given [Ca2+]cyt 12-Deoxyphorbol 13-isobutyrate (DPB) (50 nmol/l) induced a small sustained contraction with little effect on [Ca2+]cyt. A higher concentration (1 mumol/l) of DPB induced a larger sustained contraction with a decrease in [Ca2+]cyt. DPB (50 nmol/l) potentiated the KCl-induced contraction without or with only a small additional increase in [Ca2+]cyt. By contrast, 1 mumol/l DPB potentiated the high-K(+)-induced contraction with a decrease in [Ca2+]cyt. Addition of 50 nmol/l or 1 mumol/l DPB in the presence of carbachol inhibited both [Ca2+]cyt and muscle tension. Verapamil, isoprenaline and forskolin did not change or slightly decreased [Ca2+]cyt and muscle tension in resting trachea. Verapamil inhibited the contraction and [Ca2+]cyt stimulated by high K+ and carbachol. Isoprenaline and forskolin inhibited the high-K(+)-induced contraction without changing [Ca2+]cyt, whereas these inhibitors inhibited carbachol-induced contraction with a relatively small decrease in [Ca2+]cyt. These results suggest that (a) sustained contractions induced by high K+ and carbachol are due to the sustained increase in [Ca2+]cyt, (b) carbachol increases the sensitivity of contractile elements to Ca2+, and (c) isoprenaline and forskolin inhibit the contraction by the decrease in [Ca2+]cyt and also by the decrease in the sensitivity of contractile elements to Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Dual effects of carbachol on cytosolic Ca2+ and contraction in intestinal smooth muscle

The effects of carbachol on muscle tension and cytosolic Ca2+ concentrations ([Ca2+]cyt), measured with fura-2, were examined in the guinea pig intestinal smooth muscle. Carbachol induced an initial transient increase followed by a sustained increase in [Ca2+]cyt and muscle tension. Higher concentrations of carbachol induced larger transient changes and smaller sustained changes. In the presence of carbachol, application of Ca2+ to a Ca2(+)-depleted muscle induced a contraction that was smaller in the presence of higher concentrations of carbachol. High concentrations of carbachol inhibited the high-K(+)-stimulated muscle tension and [Ca2+]cyt. Contractile and inhibitory effects of carbachol were inhibited by a muscarinic M2 antagonist. Increase in the external Ca2+ concentration or addition of BAY K 8644, a Ca2(+)-channel activator, antagonized the inhibitory effect. There was a linear correlation between log [Ca2+]cyt and muscle tension under the conditions employed in the present experiments (r = 0.949). These results suggest that lower concentrations of carbachol increase [Ca2+]cyt and induce contraction, whereas high concentrations of carbachol have an additional effect to decrease [Ca2+]cyt and inhibit contraction by a Ca2(+)-channel blocker-like action.

Actions of endothelin on isolated corpus cavernosum from rabbit and man

The effects of endothelin, a vasoconstrictor peptide produced by vascular endothelial cells, were investigated in isolated rabbit and human corpus cavernosum (CC). Preparations from both rabbit and man were potently contracted by endothelin in a concentration-dependent manner. The contractions developed slowly, could not be reversed despite frequent washings, and were only partly inhibited by the Ca2+ channel blocker nimodipine. Even in Ca2(+)-free medium containing the chelator EGTA a small contractile component persisted. In rabbit CC, the contractions in Ca2(+)-free medium were not affected by nimodipine, the Ca2(-)-channel agonist BAY K 86(44), or by depletion of intracellular Ca2(+) stores sensitive to noradrenaline (NA) and caffeine, but were almost abolished by the protein kinase C inhibitor H7. In both rabbit and man, carbachol and vasoactive intestinal polypeptide concentration-dependently relaxed preparations contracted by endothelin. The relaxations induced by carbachol were antagonized by atropine. Endothelin enhanced concentration-dependently the contractions induced by exogenously applied NA in rabbit CC. The enhancement was more pronounced at low concentrations of NA. This study shows that endothelin potently contracts isolated penile erectile tissue. The contraction seems to be mediated mainly by influx of Ca2+ through the cell membrane, which partly occurs through a pathway other than voltage-operated calcium channels. However, involvement of other mechanisms cannot be excluded. The results suggest that endothelin can play a role in penile erectile mechanisms.

Inhibitory effects of cadralazine and its metabolite, ISF-2405, on contractions and the level of cytosolic Ca2+ in vascular smooth muscle

The inhibitory effects of a hypotensive agent, cadralazine and its metabolite, ISF-2405, on the level of cytosolic Ca2+ ([Ca2+]cyt) and on contractions were examined in isolated vascular smooth muscle. Cadralazine slightly inhibited the transient norepinephrine-induced contraction in rabbit aorta and canine femoral, renal and mesenteric arteries and saphenous vein, and prostaglandin F2 alpha-induced contractions in canine basilar and coronary arteries. In contrast, ISF-2405 inhibited the contractions induced by prostaglandin F2 alpha in canine basilar and coronary arteries and those induced by norepinephrine in canine renal and femoral arteries and rabbit aorta. In aorta, ISF-2405 inhibited the increase in [Ca2+]cyt and muscle tension caused by norepinephrine. A Ca2+ channel blocker, verapamil, inhibited the norepinephrine-stimulated increase in [Ca2+]cyt more potently than it inhibited the increase in muscle tension, and ISF-2405 inhibited the verapamil-resistant part of the contraction. In Ca2(+)-free solution, norepinephrine induced transient increases in [Ca2+]cyt and muscle tension. ISF-2405 inhibited these changes. However, ISF-2405 did not inhibit the transient contraction induced by caffeine in the aorta. These results suggest that cadralazine is metabolized to ISF-2405 and inhibits vascular smooth muscle contraction by inhibiting receptor-mediated Ca2+ influx, Ca2+ release and Ca2+ sensitization of contractile elements.

Cyclic AMP and mechanisms of vasodilation

Cyclic AMP and the mechanism of vasodilation have been reviewed by first discussing the enzymes involved (adenyl cyclase, cyclic nucleotide phosphodiesterases, cyclic AMP-dependent protein kinase) and then agents that increase cAMP in smooth muscle. Two mechanisms of vasodilation are described: (i) effects on contractile proteins; (ii) effects on Ca2+ levels. Evidence for compartments of cAMP is also presented.

Ca2+-independent contraction of uterine smooth muscle

Rat uterine smooth muscle shows sustained contraction to oxytocin in Ca2+-free medium with EGTA, that is called "Ca-free contraction"(1). Participation of the rise in cytosolic free Ca2+ in this Ca-free contraction was tested. In Ca-free contraction, the cytosolic free Ca2+ level was not changed at all as measured with fura-2. Further, the chelation of cytosolic free Ca2+ with quin-2 did not at all affect Ca-free contraction. These results strongly suggest that Ca-free contraction is not triggered by Ca2+.