Ca-free Medium Research Articles

Strontium, barium, and manganese metabolism in isolated presynaptic nerve terminals.

To gain insight into the mechanisms by which the divalent cations Sr, Ba, and Mn affect neurotransmitter release from presynaptic nerve terminals, we examined the sequestration of these cations, in comparison to Ca, by mitochondrial and nonmitochondrial [presumably smooth endoplasmic reticulum (SER)] organelles and the extrusion of these cations from isolated nerve terminals. Sequestration was studied in synaptosomes made leaky to small ions by treatment with saponin; efflux was examined in intact synaptosomes that were preloaded with the divalent cations by incubation in depolarizing (K rich) media. The selectivity sequence for ATP-dependent mitochondrial uptake that we observed was Mn much greater than Ca greater than Sr much greater than Ba, whereas that for the SER was Ca greater than or equal to Mn greater than Sr much greater than Ba. When synaptosomes that were preloaded with divalent cations were incubated in Na- and Ca-free media, there was little efflux of Ca, Ba, Sr, or Mn. When the incubation was carried out in media containing Na without Ca, there was substantial stimulation of Ca and Sr efflux, but only slight stimulation of Ba or Mn efflux. In Na-free media, the addition of 1 mM Ca promoted the efflux of all four divalent cations, probably via Ca-divalent cation exchange. In summary, the sequestration and extrusion data suggest that, with equal loads, Mn will be buffered to the greatest extent, whereas Ba will be least well buffered. These results may help to explain why Mn has a very long-lasting effect on transmitter release, while the effect of Sr is much briefer.

Lack of involvement of Ca 2+ conductance change in ATP-induced depolarization of the guinea-pig vas deferens

The double sucrose-gap method was used to examine the electrial responses of the guinea-pig vas deferens to ATP and their possible dependence on external Ca 2+. Normally ATP induced a depolarization and an increase in membrane conductance and both effects were concentration-dependent. The reversal potential of the 10 −4 M ATP-induced depolarization was 27.1 mV positive to the resting membrane potential of the tissue. This value was quite similar to that previously obtained for the 3 × 10 −5 M ATP-induced depolarization. The smooth muscle membrane was depolarized by 5.9 mV in a Ca-free medium, in which ATP also caused a depolarization, associated with an increase in membrane conductance. The reversal potential of the depolarization induced by ATP (10 −4 M) in the Ca-free medium was 26.5 mV positive to the resting membrane potential. The results suggest that, in this tissue, ATP induces membrane depolarization with little effect on Ca 2+ conductance.

Membrane Ca2+ interactions and contraction in denervated rat soleus muscle.

Under voltage clamp conditions contractile responses and ionic currents of single fibres isolated from rat soleus, denervated for more than 20 days, were recorded in Na-free TEA containing solutions. The relationship between membrane potential and contraction has been analysed under various conditions. The addition of trivalent cations (La3+, Gd3+) resulted in a dose dependent reduction of the contractile response and similar effects were produced by polymyxin B (0.05-0.5 mM). By contrast in the presence of phospholipase D (1-5 U/ml) contractions were significantly increased for all values of depolarization. The time course of the change of tension amplitude after the application of Ca-free medium, was dependent on the amplitude, the duration and the frequency of the depolarization. Upon depolarization glycerol-treated fibres generated contractile responses which were similar to those recorded in normal muscle and were also dependent on [Ca]o. It is proposed that in denervated soleus muscle the negatively charged phospholipids at the outside of the membrane were involved in the depolarization-contraction coupling by means of their Ca binding properties. The quantity of Ca binding sites would be dependent on [Ca]o and membrane potential and their binding properties modified during and/or following variation in membrane potential.

Inhibition of Antigen-Induced Contraction of Guinea Pig Isolated Tracheal Muscle with 2-n-butyl-3-dimethylamino-5,6-methylenedioxy indene (MDI-A), indane (MDI-B) and 8-(diethylamino)octyl-3,4,5-trimethoxy benzoate hydrochloride (TMB-8)

The effects of three intracellular calcium antagonists on antigen-induced contraction of sensitized guinea pig tracheal muscle were investigated. The agents employed were: 2-n-butyl-3-dimethylamino-5,6-methylenedioxy indene hydrochloride (MDI-A), 2-n-butyl-3-dimethylamino-5,6-methylenedioxy indane hydrochloride (MDI-B) and 8-(diethylamino)octyl-3,4,5-trimethoxy benzoate hydrochloride (TMB-8). Each of these agents caused a concentration-dependent inhibition of the antigen-induced contraction of sensitized tracheal smooth muscle. Moreover, in histamine and leukotriene D4 (LTD4) incited contractions of guinea pig tracheal muscle, they showed antagonistic action. The induced tracheal muscle contraction, achieved through the addition of compound 48/80 to a solution of ethylene glycol bis (beta-aminoethylether)-N,N,N1, N1-tetraacetic acid (EGTA) in a contained Ca-free medium, was completely inhibited by individual pretreatment with MDI-A, MDI-B and TMB-8. In the case of tracheal muscle contraction induced by CaCl2 in a high potassium, Ca-free medium, only TMB-8 inhibited contraction. Lastly, none of these three intracellular calcium antagonists affected the antigen-induced release of histamine and slow reacting substance of anaphylaxis (SRS-A). These results suggest that MDI-A and MDI-B inhibit the antigen-induced contraction of sensitized guinea pig tracheal muscle by interfering with the contractile responses caused by histamine and LTD4.

Ultrastructure of single smooth muscle cells contracted by carbachol and calcium ion.

The ultrastructural changes of single smooth muscle cells of guinea pig taenia coli during contraction evoked by Ca2+ (Ca) and carbachol (CCh) were investigated by transmission and scanning electron microscopy. Cells, prepared in the presence of a high concentration (140 mM) of KCl, without addition of Ca, exhibited smooth surfaced contour. The contraction of these cells were dose dependent upon Ca. With the increase in Ca concentration, the contraction was accompanied with formation of processes (0.15 micron thick and 1 micron long), membrane evaginations of various sizes and shapes and cores densely packed with filaments at the bottoms of all evaginations. Cells, prepared in the presence of 0.18 mM Ca and at a physiological concentration (2.7 mM) of KCl, had two populations: One had shallow invaginations of the cell membrane and the other had spinous processes. Cells of both types were responsive to CCh and the spinous processes disappeared during contraction but evagination did not appear. Differences in ultrastructural changes during contractions between single smooth muscle cells and intact tissues may be due to the fact that single cells are mechanically free from adjacent cells and connective tissues. It is also possible that membrane properties change during isolation, particularly when cells are isolated in Ca free medium.

Adenosine 5′-triphosphate release evoked by electrical nerve stimulation from the guinea-pig gallbladder

The endogenous release of adenosine 5'-triphosphate (ATP) from strips of guinea-pig gallbladder during transmural stimulation (TS) was measured with a firefly luciferine-luciferase reaction. TS (15V, 1 ms, 0.5–5 Hz, for 1 min) caused a rapid and marked increase of ATP release in a frequency-dependent manner. Both ATP release and contractions evoked by TS (15 V, 5 Hz, 1 ms) were completely abolished in Ca-free medium. BaCl 2 (3 × 10 −3 M), a direct muscle stimulant, produced almost the same degree of contractile tension as TS (15 V, 5 Hz, 1 ms) while the ATP release induced by BaCl 2 was significantly reduced to about 60 percent of that induced by TS. Atropine (10 −6 M) significantly reduced TS-evoked contraction without affecting ATP release. It was suggested, therefore, that some of the ATP release induced by TS was of neural origin. Theophylline (a P1-purinoreceptor antagonist) 10 −6 M, quinidine (a non-specific P2-purinoreceptor antagonist) 10 −6 M and apamin (a potassium channel blocking agent) 10 −8 M had no effects on TS-evoked contraction and ATP release, suggesting the absence of a presynaptic autoregulatory mechanism of ATP release in the guinea-pig gallbladder.

Calcium ions trigger the expansion in bistable myelin

Ultrastructural studies of nerve myelin emphasize its static aspects. However, current work indicates that the myelin multilayer is poised for a supramolecular transition from a compact to an expanded form. In teleosts, the membrane pair becomes 27 Å thicker. The trigger for this expansion is physiological levels of Ca. Varying proportions of the myelin can be trapped in the expanded state by transferring an excised specimen from Ca-containing to Ca-free medium in order to prevent expansion of the remaining compact myelin. Images of the myelin multilayer show that the expansion occurs in the aqueous spaces between lipid bilayers. Previously we demonstrated the reversal of expansion by removing Ca under special conditions. These results together lead us to suggest that myelin may expand locally in normal nerve function, while wholesale expansion may be an early step in myelin breakdown in vivo.

Excitation-contraction coupling in non-spiking smooth muscle in the gill ofAplysia

1. Excitation-contraction coupling (E-C Coupling) was investigated in the non-spiking longitudinal efferent vein muscle (LEVM) from theAplysia gill. The sucrose-gap method was used to study the E-C coupling in response to acetylcholine (ACh) and to elevated external potassium (Ko). 2. LEVMs from mature and oldAplysia were used in this study. The response to high Ko and ACh was not significantly different in the two groups. 3. Depolarization increased directly with the concentration of either Ko (50 to 500 mM) or ACh (1 μM to 1 mM). It preceded the development of tension, which increased with depolarization. These stimulants did not evoke action potentials. 4. In the presence of MnCl2 (10–20 mM) for 30 min, ACh-contracture was reduced to near zero; in contrast K-contracture was only reduced to 60% of the control level; and depolarization was unaffected. In a nominally Ca-free medium for 30 min, ACh-contracture was reduced to zero, whereas K-concentration persisted at 16% of the control level. Hence, ACh and high Ko apparently mobilize different Ca stores to initiate E-C coupling: In the non-spiking LEVM, ACh-contracture primarily involves influx of Ca, whereas K-contracture appears to rely on primarily cellular Ca. The results are contrasted to those in a spiking molluscan muscle. 5. We propose that mobilizing both stores of calcium to initiate E-C coupling insures synchronized contraction of LEVM fibers. The similarity of activation in vitro, by Ko and ACh, and in situ, by neuronal firing, is shown and discussed.

Influence of membrane potential on calcium efflux from giant axons of Loligo.

Experiments are described in which Ca efflux is monitored in axons under voltage clamp. As Ca efflux consists of more than one component, conditions were sought where one component predominates. Thus external Na-dependent Ca efflux can be studied in relative isolation either at pH 9.0 or in fully poisoned axons immersed in Ca-free media; external Ca-dependent Ca efflux can be studied in fully poisoned axons immersed in Na-free media and the Na-independent, energy requiring, pump is best examined in Na and Ca-free sea waters. Both in unpoisoned axons at pH 9.0 and fully poisoned axons at pH 7.8, the external Na-dependent Ca efflux is activated by hyperpolarization and inhibited by depolarization. Depolarizations achieved either electrically or by exposure to high K are roughly comparable and the inhibition brought about by high K can largely be removed by electrical hyperpolarization to the initial resting potential. In both Na sea waters and choline sea waters containing 100 mM-Na, Ca efflux is increased e-fold over approximately 50 mV. In choline sea water, external Ca-dependent Ca efflux from fully poisoned axons is unaffected by voltage over the range -80 to -30 mV. But addition of K or Li activates the flux and this activation is increased by hyperpolarization and decreased by depolarization, suggesting that the activating cation may also be transported into the axon. The Na-independent, energy-requiring, flux is inhibited by electrical hyperpolarization and stimulated by electrical depolarization. External K also stimulates the flux and part of this stimulation can be removed by electrical hyperpolarization. These data show that the energy-dependent pump is sensitive to membrane potential in the physiological range and suggest that it may be an electrogenic process. The finding that voltage affects the energy-dependent (uncoupled) pump and external Na-dependent fluxes in opposite directions may help explain why the total Ca efflux from intact axons responds to potential in a very variable manner.

The slow phase of the staircase in guinea-pig papillary muscle, influence of agents acting on transmembrane sodium flux.

Guinea-pig papillary muscles contracting at frequencies of 0.25 to 1 Hz after rest periods long enough to be followed by a rested-state contraction showed a biphasic staircase phenomenon. An initial phase was completed within about 6 beats (fast phase of the staircase). Then a slow phase followed reaching a steady state after about 4 min only. The effect on this slow phase exerted by drugs known to influence transmembrane Na-flux was investigated. Dihydroouabain in concentrations (1.5-5 X 10(-5) mol/l), causing no increase in the rested-state contraction considerably augmented the slow phase of the staircase thereby prolonging the time from the onset to the steady state by severalfold. The fast phase, however, remained unchanged as far as the steepening slow phase did not influence it. The rest decline of force of contraction, measured by repeated interruption of stimulation, was considerably prolonged by dihydroouabain (to about sixfold the control value by 5 X 10(-5) mol/l). However, dihydroouabain did not influence the time course by which the force of contraction decreased after lowering the [Ca]0 from 3.2 to 0.8 mmol/l. Stimulation of the muscles in Ca-free medium produced a transient increase in force of contraction as visualized by test contractions after addition of Ca. This positive inotropic aftereffect which depended on the frequency of stimulation in the Ca free solution was augmented severalfold by 1.5 to 3 X 10(-5) mol/l dihydroouabain.(ABSTRACT TRUNCATED AT 250 WORDS)

Excitatory effect of Clostridium perfringens alpha toxin on the rat isolated aorta.

Clostridium perfringens alpha toxin caused contraction of the isolated aorta of the rat in a dose-dependent manner. The contractile action caused by the toxin was inhibited or abolished by calcium antagonists such as nifedipine, verapamil and cinnarizine, or a Ca-free medium, but was not affected by phentolamine, chlorpheniramine, atropine, tetrodotoxin or a low Na medium. The toxin stimulated Ca uptake into the aorta in a dose-dependent manner. 8-N,N'-diethylaminooctyl-3,4,5-trimethoxybenzoate (TMB-8) blocked significantly both the toxin- and noradrenaline (NA)-induced contractions. Trifluoperazine (TFP) and N-(6-aminohexyl)-5-chloro-1-naphtharene sulphonamide (W-7) did not affect the contractile activity of the toxin but blocked the NA-induced contraction. The toxin also stimulated the 32P phosphate labelling of phosphatidylinositol (PI) and phosphatidic acid (PA) in the preparation. These results indicate that the toxin-induced contraction, which is different from that induced by NA, is the result of a direct action of the toxin on the aorta and is due to an increased Ca2+ permeability across the smooth muscle membrane. It is suggested that the contractile response to the toxin is associated with activation of phospholipid metabolism and enhanced entry of Ca into the aorta.

Effects of verapamil on the response of the guinea‐pig tracheal muscle to carbachol

The effects of verapamil on the contraction of the guinea-pig tracheal smooth muscle induced by calcium (Ca2+) or barium (Ba2+) were investigated in three different conditions: (a) in excess K solution, (b) in the presence of carbachol, and (c) in excess K solution containing carbachol. In order to clarify the contractions, the effects of removal and readdition of the divalent cations were also investigated. In Ca2+-loaded tissues, application of carbachol in Ca-free medium produced a transient contraction, the magnitude of which decreased the longer the duration of exposure to Ca2+-free solution. In Ca2+-depleted, Ba2+-loaded tissues, application of carbachol in a Ba2+- and Ca2+-free medium produced a transient contraction the magnitude of which decreased the longer the duration of exposure to the Ba2+- and Ca2+-free solution. After exposure to Ca2+-free solution for 40 min, the sensitivity of the tissue to Ca2+ was greater in the presence of 30 microM carbachol (ED50 = 0.06 mM) than in the presence of 40 mM K+ (ED50 = 0.3 mM). The Ca2+-sensitivity in the presence of 30 microM carbachol plus K+ (40 mM) was not different from that in the presence of 30 microM carbachol alone. In Ca2+-free solution, the sensitivity of the tissue to Ba2+ in the presence of 40 mM K+ (ED50 = 1.4 mM) was not different from that observed in the presence of 30 microM carbachol (ED50 = 1.3 mM). 6 After exposure to Ca2+-free solution, verapamil produced a parallel rightward shift in the concentration-response curves to added Ca2+ and Ba2+ in the presence of either 40 mM K+, 30 microM carbachol or 40mM K+ plus 30 microM carbachol. 7 The pA2 values of verapamil against Ca2+ responses in the presence of 40 mM K+, 30 microM carbachol and 40 mM K+ plus 30 microM carbachol were 7.0, 6.5 and 6.5, respectively. The pA2 values of verapamil against Ba2+ responses under these conditions were 7.1, 7.0 and 7.1, respectively. 8 It is concluded that the sustained contraction produced by carbachol requires the influx of Ca2+ and that Ba2+ can substitute for Ca2+ in this process. Furthermore, the ionic channels which admit Ca2+ may be modified by carbachol to different degrees depending on the presence of Ca2+ or Ba2+. Such changes alter the affinity of the channel to verapamil.

Comparison of the Effects of the Vasoconstrictive Dihydropyridines BAY K 8644 and CGP 28392 on Isolated Rat Aorta

The effect of BAY K 8644 was compared to that of CGP 28392 in isolated rat thoracic aorta. In low concentration both dihydropyridines had a direct concentration dependent contractile effect that could be eliminated in Ca-free medium and by the Ca-antagonist nifedipine. Readdition of Ca to the Ca-free medium restored the contractile response to both agents. In high concentrations (greater than or equal to 10(-5) M), both agents depressed active tone. In physiological salt solution containing 15.8 mM, potassium BAY K 8644 was far more potent in contracting rat aorta than CGP 28392. The responses to BAY K 8644 and CGP 28392 were not affected by alpha- and beta-adrenoceptorblockade. Both compounds shifted the noradrenaline- and potassium-concentration response curve to the left and increased the maximal response. Ultraviolet radiation (UVR) had a reversible and marked effect on BAY K 8644 and no effect on CGP 28392 contractions. The UVR-mediated relaxation of BAY K 8644-induced contractions could partly be eliminated by raising extracellular Ca. The results showed that differences according to potency and mode of action exist between BAY K 8644 and CGP 28392 in rat aorta. To explain the effect of UVR on BAY K 8644-induced contractions, a modification of a Ca channel model is proposed that implies the existence of "photosensitive proteins" located to "Ca antagonist" binding sites. According to this hypothesis, UVR may induce a reversible binding of BAY K 8644 to "Ca antagonist receptors."

Changes in intracellular Ca2+ distribution of rat peritoneal mast cells before and after histamine release

Rat peritoneal mast cells were stained with quin 2, a fluorescent Ca2+ chelator. By means of a fluorescence microscope and real time image processer, it was revealed that the fluorescence derived from the Ca-quin 2 complex was weak in the area occupied by the nucleus and distributed unevenly in the cytoplasm of the resting cells so as to encompass the individual granules. When compound 48/80 or substance P was added in a Ca-free medium, the fluorescence intensity of quin 2 increased markedly all over the cell, suggesting that a large amount of Ca2+ was released from intracellular Ca stores. The increase in the fluorescence intensity produced by compound 48/80 or substance P in a Ca-free medium was inhibited by pretreatment with certain drugs eliciting an increase of c-AMP levels, such as dibutyryl c-AMP and theophylline, or by some anti-allergic drugs providing a membrane stabilizing action.

Inositol 1,4,5-trisphosphate releases Ca2+from a nonmitochondrial store site in permeabilized rat cortical kidney cells

We have recently shown that inositol 1,4,5-trisphosphate (IP3) releases Ca2+ from the endoplasmic reticulum of pancreatic acinar cells and suggested that IP3 may function as a second messenger of hormonal receptors to mobilize Ca2+ from intracellular stores (Streb et al, 1983, Streb et al, 1984). In rat kidney cortical tubules and microdissected mouse proximal tubules, an increased turnover of polyphosphoinositide metabolism following hormonal stimulation with angiotensin II-amide and phenylephrine has been reported (Wirthensohn et al, 1984; Wirthensohn et al, 1985). This suggests that IP3, one of their hydrolysis products, increases during hormonal stimulation. We therefore investigated the effect of angiotensin II-amide and IP3 on intracellular Ca2 stores in saponin-treated cells and homogenate from rat kidney cortex. Saponin-treated isolated cortical kidney cells or homogenate was incubated in a high K+ buffer in the presence of MgATP and respiratory substrates. Ca2+ uptake was determined by measuring the free Ca2+ concentration of the surrounding medium with a Ca2+ specific macroelectrode. Addition of cells or homogenate to the incubation medium resulted in a decrease of the medium free Ca2+ concentration until a steady-state concentration of 5.7 +/- 0.2 X 10(-7) mole/l was obtained. In the presence of mitochondrial inhibitors Ca2+ uptake rate was reduced, whereas the steady-state concentration was unchanged. In contrast, in the presence of the CA2+-ATPase inhibitor vanadate mitochondrial uptake proceeded at the same rate as the control, but the steady-state concentration was higher (6.9 +/- 0.2 X 10(-7) mole/l).(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular site of “Ca reversal”: Inhibition of uterine smooth muscle contraction in Ca-free medium by a minute amount of Ca ion released from mitochondria by drugs

1. 1. Sodium azide (NaN 3) and dinitrophenol (DNP) at low concentrations caused “Ca reversal”, i.e. inhibition of oxytocin-induced tonic contraction of estrogen-dominated rat uterine smooth muscle in Ca-free solution. 2. 2. This inhibition was not due to depletion of ATP by NaN 3 or DNP. 3. 3. Higher concentrations of NaN 3 and DNP caused additional contraction. 4. 4. NaN 3 and DNP dose-dependently released Ca ion from mitochondria isolated from the estrogen-dominated rat uterine smooth muscle in vitro. 5. 5. Electron microscopic studies have shown that in estrogen-dominated rat uterine smooth muscle cells, cytoplasmic membranes proliferate, resulting in compartmentalization of the myofilament-sarcoplasmic system and its separation from the receptor-effector system in the surface folds of the plasma membranes, which also contain some mitochondria. 6. 6. It is proposed that low concentrations of NaN 3 and DNP release a small amount of Ca ion from these outer mitochondria and this Ca ion acts on the intracellular “site of Ca reversal” to induce reversal, i.e. inhibition. 7. 7. Higher concentrations of NaN 3 and DNP are proposed to release a large amount of Ca ion from the central mitochondria near myofilaments and so induce contraction. 8. 8. The “site of Ca reversal” was shown to be intracellular as our previous postulation.

The calcium receptor for Ca reversal is blocked by manganese in rat uterine smooth muscle

1. 1. The effects of Mn ion on Ca reversal, which is the name for the inhibitory action of a minute amount of Ca ion on contraction of rat uterine muscle in Ca-free medium (exo-Ca-free contraction), was tested. 2. 2. Mn ion inhibited Ca reversal by antagonizing the intracellular action of a minute amount of Ca ion in inhibiting exo-Ca-free contraction. 3. 3. As the concentration of Ca ion required for Ca reversal was very low and Mn ion antagonized its action, the action site of Ca ion for Ca reversal must be very specific and can be called “The Ca receptor for Ca reversal”.

Mechanism of histamine secretion from rat mast cell by calcium reintroduction.

Rat mast cells treated with Ca-free medium were stimulated with Ca reintroduction and its secretory mechanism was investigated. The response was modulated by experimental manipulations which are supposed to alter the level of internal Na, suggesting the involvement of internal Na in this response.

Enhancement by Anemonia sulcata toxin II of spontaneous quantal transmitter release from mammalian motor nerve terminals

J. Molgo, M. Lemeignan and F. Tazieff-Depierre. Enhancement by Anemonia sulcata toxin II of spontaneous quantal transmitter release from mammalian motor nerve terminals. Toxicon 24, 441 – 450, 1986. — The action of Anemonia sulcata toxin II (ATX-II) on spontaneous quantal transmitter release from motor nerve terminals was investigated by recording miniature end-plate potentials (MEPPs) from isolated mouse phrenic nerve - hemidiaphragm nerve - muscle preparations. ATX-II (3.2 μM) when applied for 30 – 40 min to junctions bathed in a normal ionic medium enhanced about one hundred fold the rate of spontaneous MEPPs. Concomitantly, ATX-II depolarized the muscle fiber. The effect of the toxin on MEPP frequency was markedly reduced when junctions were exposed to Na-deficient solutions or pre-treated with dantrolene sodium (10 μM). ATX-II (0.24 – 3.2 μM) increased MEPP rate in junctions exposed to a Ca-free medium containing 2 mM EGTA and 2 mM Mg 2+ in a dose- and time-dependent manner. Tetrodotoxin (0.2 – 1 μM) prevented the effects of ATX-II on MEPP frequency and on the resting membrane potential of muscle fibers. Tetrodotoxin also antagonized the acceleration of MEPP induced by ATX-II. The experimental findings suggest that ATX-II acts to increase quantal transmitter output from motor nerve terminals by enhancing Na + influx through tetrodotoxin-sensitive presynaptic channels, since ATX-II action does not appear to depend upon entry of Ca 2+ from the extracellular medium. It is likely that ATX-II, by increasing intraterminal Na + concentration, may trigger calcium release from internal stores.

A Comparison of Effects of BAY K 8644 and CGP 28392, Dihydropyridines with Ca Agonistic Properties, on Spontaneous Mechanical Activity in Rat Portal Vein: Modification of Effect by Ultraviolet Radiation

The effect of CGP 28392 and BAY K 8644 was studied on isolated portal veins from male Wistar Kyoto rats. BAY K 8644 and CGP 28392 induced a concentration dependent increase in spontaneous mechanical activity in low but not in high concentrations. The portal vein was more sensitive to the effect of BAY K 8644 than to that of CGP 28392. In Ca free medium the increasing effect of BAY K 8644 and CGP 28392 on the mechanical activity was abolished and restored by readdition of calcium. Nifedipine abolished the augmenting effect of BAY K 8644 and CGP 28392 on mechanical activity. Ultraviolet radiation (UVR) depressed the effect of BAY K 8644 but had no effect on mechanical activity induced by CGP 28392. BAY K 8644 and CGP 28392 degradate slowly under influence of UVR; BAY K 8644 was far more resistant to the destroying effect of UVR than CGP 28392. The results indicate that both BAY K 8644 and CGP 28392 increase the spontaneous mechanical activity in rat portal vein by enhancing Ca influx. The effect of BAY K 8644 differs from that of CGP 28392 by being sensitive to UVR; this action seems not to be due to a direct UVR destroying effect of the substances, but rather to a BAY K 8644 induced sensibilisation of the tissue to UVR.