A23187 increases calcium permeability of store sites more than of surface membranes in the rabbit mesenteric artery

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

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

Effects of guanosine nucleotides on skinned smooth muscle tissue of the rabbit mesenteric artery

1. Effects of guanosine triphosphate (GTP) and guanosine 5'-o-(3-thio)triphosphate (GTP gamma S) on mechanical properties of skinned smooth muscle tissues of the rabbit mesenteric artery were investigated. 2. In skinned muscle tissues prepared by saponin, GTP (above 100 microM) and GTP gamma S (above 1 microM) enhanced the Ca2+-induced contraction (0.3 microM-Ca2+ buffered with 2 mM-EGTA) in the presence of 1 microM-ionomycin, a depletor of stored Ca2+. The concentration-response (pCa-tension) relationship observed in the presence of 10 microM-GTP gamma S shifted to the left with no change in the maximum response evoked by 10 microM-Ca2+. The action of GTP was reversible but that of GTP gamma S was not. 3. The enhancement of the Ca2+-induced contraction by GTP gamma S occurred with increases in the phosphorylation of myosin light chain and in the shortening velocity as measured with the slack test. 4. GTP gamma S had no effect on the Ca2+-independent contraction of skinned muscle tissues evoked by MgATP in Ca2+-free solution (4 mM-EGTA), following treatment with rigor solution containing adenosine 5'-o-(3-thio)triphosphate (ATP gamma S). 5. The present results indicate that GTP and GTP gamma S enhance the Ca2+-induced contraction in skinned muscle tissues due to increase in the Ca2+ sensitivity of contractile proteins. These enhancing actions of guanosine nucleotides on contractile proteins are discussed in comparison to those of protein kinase C.

Ryanodine inhibits the Ca-dependent K current after depletion of Ca stored in smooth muscle cells of the rabbit ileal longitudinal muscle

1. Effects of ryanodine on the membrane currents were investigated on dispersed smooth muscle cells of rabbit ileal longitudinal layer using voltage and patch clamp procedures. 2. With voltage clamp, membrane depolarization to 0 mV from the holding potential of -60 mV produced an inward Ca current (ICa) which was followed by transient and sustained outward currents (ITO and ISO, respectively). Prolonged depolarization of the membrane produced spontaneous oscillations of the outward current (oscillatory outward current; IOO) on ISO. 3. Ryanodine (30 microM) modified neither the basal membrane current recorded at the holding potential (-60 mV) nor ISO. Ryanodine inhibited both ITO and IOO in a concentration-dependent manner (IC50 = 5.5 and 4.5 microM, respectively, measured 12 min after application of ryanodine). These values were much higher than that observed in skeletal muscle for Ca release. 4. The time course of the ryanodine-induced inhibition of IOO was slow and the inhibition was irreversible. Caffeine (3 mM) enhanced the amplitudes of ITO and IOO in the presence of Ca, and only transiently enhanced IOO in the absence of Ca. However, following application of 10 microM ryanodine, 3 mM caffeine did not increase IOO. 5. Ryanodine (3-30 microM) slightly enhanced the amplitude of ICa evoked by depolarization pulses at potentials more negative than O mV but not that induced by larger depolarizations (positive potentials). 6. With patch clamp procedure, single Ca-dependent K channel currents were recorded in cell free and cell attached configurations. Application of 30 microM ryanodine transiently enhanced the Ca-dependent K current without any detectable changes in the amplitude of the single channel current recorded in the cell attached condition. In the inside-out membrane patch, when the intracellular membrane side was superfused with 1 microM Ca buffered with 10 mM EGTA, bath application of 10 microM ryanodine had no effect on the Ca-dependent K current. 7. It was concluded that both ITO and IOO are generated by Ca released from intracellular stores, mainly sarcoplasmic reticulum. Ryanodine appears to open irreversibly the Ca channel in the store and to inhibit the Ca-dependent K channel due to depletion of the stored Ca.

Ryanodine reduces the amount of calcium in intracellular stores of smooth-muscle cells of the rabbit ear artery

We have investigated the mechanism of action of ryanodine on intact and skinned smooth-muscle cells of the rabbit ear artery. The amplitude of the phasic response induced by low noradrenaline (NA) concentrations (less than 30 nM) was inhibited by 10 microM ryanodine, while that elicited by high NA concentrations (greater than 100 nM) was not affected. The phasic contractions induced by both low and high NA concentrations in Ca2+-free solution containing 2 mM EGTA were suppressed by 10 microM ryanodine. The rate of 45Ca efflux in Krebs solution was enhanced by 10 microM ryanodine, while the increased 45Ca efflux induced by 10 microM NA was inhibited by ryanodine. 10 microM ryanodine did not affect the contractile proteins in saponin-treated smooth-muscle cells. The intracellular Ca2+ stores of these skinned cells could be filled by exposing these cells to a solution containing 0.6 microM Ca2+. After a wash in a Ca2+-free solution, a contraction due to a release of the accumulated Ca2+ could be induced by either 25 mM caffeine or 20 microM inositol 1,4,5-trisphosphate (InsP3) or 10 microM A23187. These contractions did not occur if 10 microM ryanodine was present during Ca2+ loading. The addition of ryanodine during the Ca2+-free wash did not affect the subsequent force development. These observations indicate that ryanodine, in the presence of Ca2+, depletes the intracellular Ca2+ stores, and that this depletion is responsible for the inhibition of the component of the NA-induced contraction which depends on the release of Ca2+ from intracellular stores.

Properties of intracellular calcium stores in pregnant rat myometrium

1. The properties of the Ca2+ stores in myometrium of 21-day pregnant rats were studied by recording the contractile responses of saponin-treated skinned muscles. 2. After accumulation of Ca2+ into the stores in the presence of 5 mM NaN3, inositol 1,4,5-trisphosphate (InsP3) at concentrations exceeding 3 microM produced a contraction. The amplitude of this contraction was maximal at about 20 microM. A second application of 20 microM InsP3 produced a smaller contraction than the first one. However after reloading the stores with Ca2+, 20 microM InsP3 produced a contraction of the same amplitude as the initial one. 3. After application of 20 microM InsP3, 1 microM A23187 still evoked a large contraction. If A23187 was applied first, the subsequent application of InsP3 or A23187 no longer induced a contraction, even after Ca2+ loading. 4. Guanosine triphosphate (GTP) or arachidonic acid, both 100 microM neither evoked a contraction nor enhanced the subsequent contraction elicited by 20 microM InsP3. 5. Caffeine 25 mM did not induce a contraction nor did it affect the contraction elicited by 20 microM InsP3. 6. The results indicate that in pregnant rat myometrium InsP3 releases Ca2+ from intracellular stores as has been proposed in vascular smooth muscles.

Effects of inositol phosphates on the membrane activity of smooth muscle cells of the rabbit portal vein

The effects of intracellular perfusion of inositol 1,4,5-trisphosphate (InsP3) or inositol 1,3,4,5-tetrakisphosphate (InsP4) on electrical responses of smooth muscle cell membranes of the rabbit portal vein were studied using the whole cell voltage clamp technique. Depolarisation to 0 mV from a holding potential of -60 mV, evoked inward Ca (Ica), transient outward (ISO), oscillatory outward (IOO) and sustained outward (ISO) currents. Generation of IOO was dependent on the [Ca]o, but it was also generated in 0 mM Ca solution for over 10 min. From amplitude histograms, IOO was divided into two components. Reduction in [Ca]o inhibited the appearance of but not the amplitudes of both IOO components. However, the larger component of IOO was more resistant to a reduction in [Ca]o than the smaller one. InsP3 (10 microM) increased the frequency of both IOO components to a greater extent than their amplitude, but the larger component was more sensitive to InsP3 than the smaller one. The increase in the occurrence of IOO induced by InsP3 did not occur following pretreatment with 3 mM caffeine or 1 nM A23187. In normal PSS, InsP3 was evoked by a depolarising pulse positive to -40 mV, whereas following perfusion with InsP3 (10 microM), IOO was evoked at -60 mV. In normal PSS, intracellular perfusion with 10 microM InsP4 changed neither the frequency nor the amplitude of IOO, and the amplitudes of ICa, ITO and ISO were also unchanged. However, in 10 mM Ca solution, 10 microM InsP4 generated IOO at a membrane potential of -60 mV.(ABSTRACT TRUNCATED AT 250 WORDS)

Mobilization of intracellular calcium by extracellular ATP and by calcium ionophores in the Ehrlich ascites-tumour cell

We have studied the changes of the intracellular free calcium concentration ([Ca2+]i) effected by external ATP, which induces formation of inositol trisphosphate, and by the divalent cation ionophores ionomycin and A23187. Both, ATP (40 microM) and ionophores (1-80 mumol/l cells ionomycin; 20-400 mumol/l cells A23187), produced a transient rise of [Ca2+]i which reached its maximum within 15-30 s and declined near resting values (about 200 nM) within 1-3 min. When the [Ca2+]i peak surpassed 500 nM a transient cell shrinkage due to simultaneous activation of Ca2+-dependent K+ and Cl- channels was also observed. The cell response was similar in medium containing 1 mM Ca2+ and in Ca2+-free medium, suggesting that the Ca mobilized to the cytosol comes preferently from the intracellular stores. Treatment with low doses of ionophore (1 mumol/l cells for ionomycin; 20 mumol/l cells for A23187) depressed the response to a subsequent treatment, either with ionophore or with ATP. Treatment with ATP did also inhibit the subsequent response to ionophore, but in this case the inhibition was dependent on time, the stronger the shorter the interval between both treatments. This result suggests that the permeabilization of Ca stores by ATP is transient and that Ca can be taken up again by the intracellular stores. Refill was most efficient when Ca2+ was present in the incubation medium. Addition of either ATP or ionomycin (1-25 mumol/l cells) to cells incubated in medium containing 1 mM Ca2+ decreased drastically the total cell Ca content during the following 3 min of incubation. In the case of ATP the total cell levels of Ca returned to the initial values after 7-15 min, whereas in the case of the ionophore they remained decreased during the whole incubation period. These results indicate that Ca released from the intracellular stores by either ATP or ionophores is quickly extruded by active mechanisms located at the plasma membrane. They also suggest that, under the conditions studied here, with 1 mM Ca2+ outside, the Ca-mobilizing effect of ionophores is stronger in endomembranes than in the plasma membrane.

Effect of milrinone on contractility and 45Ca movements in the isolated rabbit aorta

The effects of milrinone on 45Ca movements and on contractile responses were studied in rabbit aortic smooth muscle. Milrinone, 10(-7)-5 x 10(-4) M, inhibited the responses induced by high K and noradrenaline (NA). These effects were observed when milrinone was added before or after the induced contractions and were similar in aortas with or without endothelium. The Ca-induced contractions of K-depolarized aortas as well as the contractions induced by caffeine in normal and in Ca-free solution were also inhibited dose dependently by milrinone. Milrinone reduced the 45Ca influx in resting or non-stimulated aortic rings as well as in aortic rings stimulated by NA. On the other hand, neither the 45Ca influx stimulated by high K or the 45Ca efflux stimulated by NA were altered by pretreatment with milrinone. It is concluded that milrinone inhibits vascular smooth muscle contractility by probably acting at multiple sites to decrease the availability of intracellular free Ca required for activation.

Effects of a phorbol ester on acetylcholine-induced Ca2+ mobilization and contraction in the porcine coronary artery

1. The effects of 12-O-tetradecanoylphorbol-13-acetate (TPA), an activator of protein kinase C, have been investigated on intact and chemically skinned muscle strips of the porcine coronary artery. 2. In the presence or absence of extracellular Ca2+, TPA (0.1-1 nM) slightly enhanced the amplitude of ACh (10 microM)-induced contractions but at 100 nM, inhibited the contractions by approximately 50%. 3. ACh (10 microM) reduced the amount of [32P]phosphatidylinositol 4,5-bisphosphate (PIP2) and increased the amount of [32P]phosphatidic acid (PA) in the presence or absence of Ca2+. TPA (over 1 nM) dose-dependently inhibited the hydrolysis of PIP2 induced by ACh. 4. ACh (over 0.1 microM) dose-dependently increased the intensity of fura-2 fluorescence in dispersed single-cell suspensions. TPA (over 1 nM) dose-dependently inhibited the increase of the Ca2+ transient evoked by ACh, but it did not modify the ionomycin-induced Ca2+ transient or the resting fluorescence. These inhibitory effects of TPA occurred over a similar dose range to that which inhibited ACh-induced PIP2 break-down. 5. When the relationship between ACh-induced contraction amplitude and Ca2+ transient was observed in the presence or absence of 10 nM-TPA, TPA greatly reduced the Ca2+ transient but did not modify the amplitude of contraction. 6. In saponin-treated skinned muscle strips, TPA (10 nM) or 1,2-diolein (50 micrograms/ml) with phosphatidylserine (PS; 50 micrograms/ml) increased the amplitude of contraction evoked by various concentrations of Ca2+ (0.1-1.0 microM) without any change in the maximum amplitude of the Ca2+-induced contraction. 7. TPA (10 nM) with PS (50 micrograms/ml) increased the amplitude of contraction evoked by 10 microM-inositol 1,4,5-trisphosphate in chemically skinned muscle strips. 8. It is concluded that TPA inhibits the ACh-induced [Ca2+]i increase by inhibiting the hydrolysis of PIP2, but that it enhances the Ca2+ sensitivity of the contractile proteins. These results indicate that ACh-induced contractions are controlled by negative feed-back regulation of PIP2 hydrolysis together with a positive feed-back regulation of the Ca2+ sensitivity of the contractile proteins. This may depend on the on-going level of protein kinase C activation.

Contractile properties of chemically skinned fibers from pregnant rat myometrium: existence of an internal Ca-store

Smooth muscle fibers isolated from pregnant rat myometrium were skinned by saponin treatment. Properties of the contractile system and the involvement of an intracellular source of activator calcium were studied. (1) In the presence of 4 mM total EGTA, skinned fibers contracted in a concentration-dependent manner to micromolar applications of calcium ions. Rat myometrium exhibited a high calcium sensitivity (80% of the maximum contraction was achieved in the presence of 10(-6) M Ca2+). (2) Several divalent cations induced concentration-dependent contractile responses in skinned uterine muscle. The rank order of potency was: Ca2+ greater than Mn2+ greater than Sr2+ greater than Ba2+. All these divalent cation-activated contractions were antagonized by trifluoperazine in a concentration-dependent manner. (3) Pretreatment of skinned fibers with cAMP (5 X 10(-5)-5 X 10(-4) M) depressed cation-activated contractions. This effect was dependent on the free cation concentration. (4) In the presence of a low EGTA concentration (0.1 mM) the pCa-tension curve was shifted to the right with a 2.5-fold increase in the Ca-concentration required to induce half-maximum contraction. (5) After Ca-loading (10(-6) M Ca2+ for 3 min in low EGTA-containing solution), total replacement of K+ ions by choline induced a small and tonic contraction. In these conditions, the ionophore A23287 (5 X 10(-8) M-5 X 10(-5) M) and inositol 1,4,5-triphosphate (InsP3; 2 X 10(-6) M-2 X 10(-5) M) also produced contractions of skinned uterine fibers even in the presence of NaN3 (5 mM) and of NaCN (5 mM).(ABSTRACT TRUNCATED AT 250 WORDS)

Positive inotropic and chronotropic effects of the calcium ionophore A23187 (calimycin) on guinea-pig atria

The inotropic and chronotropic effects of the calcium ionophore A23187 (calimycin = CA) in isolated, superfused, electrically driven, auxotonically contracting left and spontaneously beating right guinea-pig atria were examined at different Ca2+ concentrations. 10(-6) to 10(-5) mol/l CA shows significantly positive inotropic and chronotropic effects. The inotropic effect of CA can be significantly diminished by relatively high concentrations of Ca2+ channel antagonists and relatively low concentrations of ryanodine. The positive chronotropic effect of CA can be reduced slightly, but significantly, by beta-adrenoceptor antagonists and histamine H2-receptor antagonists. From this we infer that the positive inotropic effect of CA is mainly due to the release of Ca2+ from cardiac sarcoplasmic reticulum, whereas catecholamine and histamine release appear to contribute to the positive chronotropic effect of the ionophore.

Extracellular calcium and human isolated airway muscle: ionophore A23187 induced contraction

In order to investigate the role played by extracellular calcium mobilization in activating human airway contraction, we studied the effects of A23187, a calcium ionophore, in human isolated bronchial spiral strips. In this preparation, ionophore induced a concentration dependent contraction from 10(-7) M to 10(-5) M which resulted from a direct effect on smooth muscle cells and was not a consequence of mediator release. Ionophore-induced contraction was dependent upon an entry of extracellular calcium which did not occur through the verapamil sensitive voltage dependent channel. Maximal ionophore contraction was 97 +/- 11% (n = 5) of the maximal histamine contraction but only 46 +/- 11% (n = 5) of the maximal carbachol contraction. However, when extracellular calcium concentration was doubled to 5 mM before addition of ionophore, the significant difference in amplitude between carbachol and ionophore maximal contraction was abolished. At physiological calcium concentrations addition of carbachol or histamine to the plateau of the ionophore maximal contraction produced a significant increase in the tension. Verapamil blocked the increase in ionophore tension produced only by histamine. These results suggest that (i) calcium mobilization from the extracellular source alone can produce contraction comparable in magnitude to that induced by histamine or carbachol. (ii) Extracellular calcium mobilization through different pathways has a cumulative effect on human airway contraction.

Mechanisms of flunarizine-induced vasodilation in the rabbit mesenteric artery

The vasodilating effects of flunarizine on smooth muscle strips of rabbit mesenteric artery have been investigated and compared with those of nifedipine. Flunarizine (30-300 nM) dose-dependently inhibited Ca2+-induced contractions in Ca2+-free solution containing 100 mM K+. Double reciprocal analysis showed that this inhibition was either competitive at low concentrations (30-100 nM; nifedipine-like) or noncompetitive at high concentrations (0.3-1 microM). The latter seemed to be partly related to an inhibition of contractile proteins as estimated from Ca2+-induced contractions in saponin-treated chemically skinned muscle strips. In contrast to the actions of nifedipine, flunarizine inhibited norepinephrine (NE)-induced contractions more than those induced by high K+, and at 0.3 microM, this agent totally blocked NE-induced contraction. Flunarizine also inhibited NE-induced contraction in Ca2+-free solution containing 2 mM EGTA. In Ca2+-free solution, NE rapidly hydrolyzed phosphatidylinositol 4,5-bisphosphate (PI-P2) and produced phosphatidic acid (PA). Flunarizine (30 and 300 nM), but not nifedipine (100 nM), inhibited NE-induced hydrolysis of PI-P2 and production of PA. However, flunarizine (100 nM) did not modify the contraction induced by 10 microM inositol 1,4,5-trisphosphate in chemically skinned muscle strips. It is concluded that flunarizine inhibits both voltage-dependent (nifedipine-like) and receptor-operated Ca2+ influx induced by NE and also inhibits NE-induced Ca2+ release from intracellular stores due to inhibition of the hydrolysis of PI-P2.

Regulation of ion permeability in frog brain venules. Significance of calcium, cyclic nucleotides and protein kinase C

1. The effect on ionic permeability of frog brain endothelium of various second messengers was studied by a technique based on continuous recording of the electrical resistance of the venular endothelium in vivo. 2. Augmentation of the cytosolic Ca2+ concentration in endothelial cells induced with the ionophores ETH 1001 and A23187 increased ion permeability significantly as reflected in the reduced electrical resistance. 3. The electrical resistance fell reversibly within 1-2 s after administration of Ca2+-activating agents. The maximal effect was a reduction to about 0.70 times the pre-experimental resistance value. The resistance decrease was similar to that induced by several inflammatory mediators (Olesen & Crone, 1986). 4. Administration of the following agents did not change the electrical wall resistance: 8-bromo-cyclic AMP, dibutyryl-cyclic AMP, forskolin, 8-bromo-cyclic GMP, dibutyryl-cyclic GMP, sodium nitroprusside, phorbol myristate acetate (a protein kinase C stimulator). Changes in cytosolic Mg2+ did not affect permeability. 5. Ca2+ may be an important cytosolic signal in the endothelial cell, acting as an intracellular mediator for several permeability-augmenting substances.

Cellular calcium regulates outward currents in rabbit intestinal smooth muscle cell

The nature of transient and oscillatory outward currents (ITO and IOO) in fragmented smooth muscle cells (smooth muscle ball, SMB) from the longitudinal muscle layer of the rabbit ileum, was studied using a single electrode voltage clamp technique. With a high K+ solution containing 0.3 mM ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid (EGTA) in the pipette and physiological salt solution (PSS) in the bath, the Ca inward current was followed by a large transient outward current (ITO) and spontaneous oscillations of the outward current (IOO) on the sustained outward current (ISO) were elicited by a depolarizing pulse, positive to -30 mV (holding potential of -60 mV). When the internal fluid of the SMB was replaced with Cs+-tetraethylammonium+ (TEA+) solution, or when the concentration of EGTA in the pipette was increased to 4 mM, using the intracellular perfusion technique, both ITO and IOO were abolished. In Mn2+ solution both currents were also inhibited. Bath application of TEA+, procaine or A23187 completely blocked both ITO and IOO. Caffeine (0.3-1 mM) enhanced the amplitude of ITO and generations of IOO, and concentrations of caffeine over 3 mM transiently enhanced, but finally suppressed both these currents. These results suggest that the generation of ITO is closely related to the Ca2+ influx, whereas the generation of IOO may be initiated by an increment in the intracellular concentration of Ca2+, possibly released from store sites.

Effects of procaine on pharmaco-mechanical coupling mechanisms activated by acetylcholine in smooth muscle cells of porcine coronary artery

The action of procaine on pharmaco-mechanical coupling activated by application of acetylcholine (ACh) was investigated using collagenase-treated dispersed intact and skinned smooth muscle cells and intact muscle tissues of the porcine coronary artery. ACh reduced stored 45Ca2+, and this action was prevented by procaine in intact dispersed cells. The maximum reduction in the level of stored 45Ca induced by caffeine (25 mM) or inositol 1,4,5-trisphosphate (InsP3; 3 microM) was also prevented by procaine in the skinned muscle cells in the presence or absence of ATP. However, inhibitions of the latter required higher concentrations of procaine than the former. Release by 10 microM ACh of Ca2+ from its store site in the presence or absence of extracellular Ca2+ was also inhibited by procaine and was detected using the quin2 fluorescence method. In these smooth muscle tissues, ACh (above 10 nM) reduced the amount of phosphatidylinositol 4,5-bisphosphate (PI-P2) and dose dependently increased the amount of phosphatidic acid. Procaine inhibited the hydrolysis of PI-P2 activated by ACh, thus reducing the amount of InsP3 and the release of Ca2+ from the store site. It is concluded that procaine has multiple actions on the porcine coronary artery, and one of the actions related with pharmacomechanical coupling appears through inhibition of hydrolysis of PI-P2 induced by ACh.

Mechanisms of the Ba2+-induced contraction in smooth muscle cells of the rabbit mesenteric artery

The mechanism of the Ba2+-induced contraction was investigated using intact and saponin-treated skinned smooth muscle (skinned muscle) strips of the rabbit mesenteric artery. After depletion of Ca2+ stored in the caffeine-sensitive site, greater than 0.65 mM Ba2+ evoked contraction in muscle strips depolarized with 128 mM K+ in Ca2+-free solution in a dose-dependent fashion, and the ED50 values for Ca2+ and Ba2+ were 0.5 mM and 1.2 mM in intact muscle strips, respectively. Nisoldipine (10 nM) blocked the contraction evoked by high K+ or 10 microM norepinephrine (NE) in the presence of 2.6 mM Ba2+, but did not block the contraction evoked in the presence of 2.6 mM Ca2+. These results may indicate that Ba2+ permeates the voltage-dependent Ca2+ channel. In skinned muscle strips, the ED50 values for Ca2+ and Ba2+ were 0.34 and 90 microM, respectively, as estimated from the pCa- and pBa-tension relationships. Calmodulin enhanced and trifluoperazine inhibited the Ba2+- and Ca2+-induced contractions. After the application of Ba2+ or Ca2+ with ATP gamma S in rigor solution, myosin light chain (MLC) was irreversibly thiophosphorylated, as estimated from the Ba2+- or Ca2+-independent contraction. Furthermore, both divalent cations phosphorylated MLC, as measured using two-dimensional gel electrophoresis, to the extent expected from the amplitudes of the contraction evoked by these cations. Thus, Ba2+ is capable of activating the contractile proteins as Ca2+ does. The amount of Ca2+ or Ba2+ stored in cells was estimated from the caffeine response evoked in Ca2+-free solution in intact and skinned muscle strips. After the application of 0.3 microM Ca2+ or 0.1 mM Ba2+ for 60 s to skinned muscle strips after the depletion of Ca2+ stored in cells, caffeine produced a contraction only upon pretreatment with Ca2+ but not with Ba2+. When Ba2+ was applied successively just after the application of Ca2+, the subsequently evoked caffeine-induced contraction was much smaller than that evoked by pretreatment with Ca2+ alone. The above results indicate that Ba2+ permeates the voltage-dependent Ca2+ channel but may not permeate the receptor-operated Ca2+ channel, it releases Ca2+ from store sites but is not accumulated into the store site, and it directly activates the contractile proteins via formation of a Ba2+-calmodulin complex.

Effects of prostaglandin I2 synthesized in the endothelium and in the smooth muscle on mechanical properties of the canine thoracic aorta

In circular-cut strips prepared from canine thoracic aorta, acetylcholine (ACh) and A23187 relaxed endothelium-intact tissues [E(+) preparations] pre-contracted with noradrenaline or excess concentrations of K. These relaxations were associated with marked increases in the amount of 6-keto PGF1 alpha. After removal of the endothelium [E(-) preparations] the relaxation ceased, and the amounts of 6-keto PGF1 alpha were markedly reduced. In E(+) preparations, application of indomethacin attenuated the increase in 6-keto PGF1 alpha induced by ACh or A23187 in the presence of noradrenaline or high K, but not the endothelium-dependent relaxations. In E(-) preparations, ACh (0.1-10 microM) neither increased the amount of 6-keto PGF1 alpha nor produced a contraction. In dispersed single endothelial cells, A23187 markedly increased but 118 mM K did not modify the amount of 6-keto PGF1 alpha. Both noradrenaline and high K increased the production of 6-keto PGF1 alpha in the E(-) preparations but to a lesser extent than that in the E(+) preparations. This action was attenuated by indomethacin. The amplitude of the noradrenaline- and K-induced contractions was enhanced with indomethacin pretreatment in both E(+) and E(-) tissues. PGI2-Na (10 nM), reduced the amplitude of noradrenaline-induced contractions, concentration dependently and to the same extent in both E(+) and E(-) preparations. These results indicate that synthesis of PGI2 in the endothelium is not causally related to the endothelium dependent relaxation. PGI2 synthesized in the endothelium may not act directly on the muscle tissue, but PGI2 synthesized in the smooth muscle tissue may produce an inhibition of contraction.

Inorganic phosphate regulates the contraction-relaxation cycle in skinned muscles of the rabbit mesenteric artery

The effects were investigated of inorganic phosphate (Pi) on the Ca2+-dependent and Ca2+-independent contractions evoked in chemically skinned smooth muscles of the rabbit mesenteric artery. The relation between the concentration of Ca2+ and tension showed a sigmoidal curve in the range of pCa 7-5.5. Pi (over 1 mM) inhibited the Ca2+-induced contraction, shifted the pCa-tension curve to the right and increased the Hill number from 2 to 3. Calmodulin did not change the Hill number and attenuated the inhibitory action of Pi as estimated from the shift of the curve, but this agent did not modify the increased Hill number in the presence of Pi. Pi consistently inhibited the Ca2+-independent contractions provoked by application of trypsin-treated myosin light chain kinase, of MgATP following adenosine-5'-o-(3-thiotriphosphate) (ATP gamma S) and Ca2+, and of a solution containing high Mg2+. These inhibitory actions of Pi were inversely proportional to the amplitude of the contraction. When Pi was applied simultaneously with ATP gamma S and Ca2+, there was no change in the amplitude of Ca2+-independent contractions provoked by the application of MgATP. The amplitude of the rigor contraction evoked by ATP-free solution was less than 7% of that of the 10 microM-Ca2+-induced contraction. When ATP was removed from the solution during the Ca2+ contraction, the rigor contraction was also generated. Pi did not inhibit either type of contraction. With a decrease in the concentration of Ca2+ from 10 microM to below 1 nM, the tissue relaxed at a slower rate than the rate of rise of the Ca2+-induced contraction. The slow relaxation was not modified by a change in the concentration of EGTA or addition of 1 microM-calmodulin. Pi reduced, and high Mg2+ prolonged the time required for the relaxation. This action of Pi was not prevented in the presence of calmodulin or of high Mg2+. The rates of rise and fall of the Ca2+-induced contraction depended on the concentration of MgATP, and Pi consistently inhibited the Ca2+-induced contraction in the presence of any given concentration of MgATP. We conclude that Pi may accelerate the detachment of cross-bridges between the contractile proteins. Thus, the amplitude of Ca2+-induced contraction is slightly inhibited and the relaxation is markedly facilitated. However, the site of action of Pi may differ from that of MgATP.(ABSTRACT TRUNCATED AT 400 WORDS)

A phorbol ester has dual actions on the mechanical response in the rabbit mesenteric and porcine coronary arteries

To clarify the role of protein kinase C in the mechanical response, the effects of 12-o-tetradecanoylphorbol-13-acetate (TPA), an activator of protein kinase C, were investigated on intact and skinned smooth muscle preparations of the rabbit mesenteric artery. TPA (0.1 microM) showed dual actions (initial enhancement followed by inhibition during long exposure) on the K+-induced contraction. The enhancement was marked in the presence of 39 mM-K+ but inhibition was the predominant effect in the presence of 128 mM-K+. Addition of 2.6 mM-Ca2+ to a Ca2+-free solution containing 2 mM-EGTA following application of A23187 (1 microM), produced contraction. TPA showed the same dual actions on this Ca2+-induced contraction. In chemically skinned muscles, TPA increased the amplitude of Ca2+-induced contractions evoked by low concentrations of Ca2+ (0.1-0.3 microM), but reduced those evoked by high concentrations of Ca2+ (1-10 microM). Both actions of TPA were facilitated in the presence of phosphatidylserine (PS). TPA with PS had no effect on the Ca2+-independent contraction evoked in relaxing solution containing 10 mM-EGTA and 4 mM-Mg ATP following application of adenosine-5-o-3-thiotriphosphate (ATP gamma S) and 0.3 microM-Ca2+. The amount of Ca2+ stored in cells estimated from the amplitude of the caffeine-induced contraction was not modified by application of TPA with PS in skinned or intact muscle tissues. The effects of TPA were investigated on the Ca2+ transient measured from the intensity of fluorescence of quin-2 in dispersed cell suspensions prepared from the porcine coronary artery. TPA had no effect on the Ca2+ transient in high K+ but enhanced the amplitude of the contraction. Amplitudes of the tonic response evoked by 39 mM-K+ in intact muscle tissues and the contraction induced by 0.3 microM-Ca2+ in skinned muscle were much the same. TPA with PS enhanced the amplitudes of both contractions to the same extent. From the above results, we concluded that TPA shows dual actions on the contractile machinery and may act on the regulatory systems of contractile proteins. Both excitatory and inhibitory actions of TPA depended on the concentration of Ca2+. However, the physiological action of protein kinase C as estimated from the action of TPA seems to be related to an excitatory action on the contractile machinery.

Inositol 1,4,5-trisphosphate activates pharmacomechanical coupling in smooth muscle of the rabbit mesenteric artery

To clarify the nature of the noradrenaline (NA)-induced contraction, the effects of NA on inositol phospholipid metabolism and the actions of inositol 1,4,5-trisphosphate (InsP3) on skinned muscle of the rabbit mesenteric artery were investigated. NA, in concentrations over 1 nM, reduced the amount of phosphatidylinositol 4,5-bisphosphate (PI-P2) and increased the amount of phosphatidic acid (PA). The maximum reduction in the amount of PI-P2 and the maximum increase in the amount of PA were observed in the presence of 1 microM-NA. With prolonged application of NA, the PI-P2 was gradually restored to near the control level, but with repeated applications of NA separated by rinses with Krebs solution, there was a consistent reduction of PI-P2. The NA-induced PI-P2 breakdown was inhibited by the alpha 1-adrenoceptor blocking agent, prazosin. After incubation of the tissue with radioactive inositol-containing solution, NA transiently increased the amount of radioactive InsP3 which was followed by increases in the amount of inositol 1,4-bisphosphate (InsP2) and inositol monophosphate (InsP). After accumulation of Ca by saponin-treated muscle cells of the dog mesenteric artery dispersed by collagenase, InsP3 released Ca stored in cells but InsP2 did not. A23187 (5 microM) but not InsP3 (up to 10 microM), depleted Ca accumulated in the presence of ATP. In saponin-treated skinned muscle tissues, InsP3 in concentrations over 0.3 microM, produced contraction following accumulation of Ca into the store site. InsP3 released Ca from the same source as caffeine. The release of Ca by InsP3 appeared in a concentration-dependent manner and this release also depended on the amount of Ca stored in cells (the median effective dose (ED50) was 3.0 microM in 0.1 microM-Ca and 1.0 microM in 0.3 microM-Ca). We concluded that NA activates alpha 1-adrenoceptors, thus hydrolysing PI-P2 and synthesizing InsP3. This product can release Ca stored in cells as estimated from the contraction in skinned muscle tissues, and also reduces the residual amount of Ca stored in skinned dispersed muscle cells. Contraction evoked by NA through pharmacomechanical coupling can be explained as a function of InsP3.