Remineralization Potential of a Novel Biomimetic Material (Self-assembling Peptide P11-4) on Early Enamel Caries: An In Vitro Study

AIM

To assess the remineralizing potential of self-assembling peptide P_11-4 and compare it to the remineralizing potential of fluoride varnish using DIAGNOdent^TM, as well as the amount of mineral gain after application of fluoride varnish and self-assembling peptide P_11-4.

MATERIALS AND METHODS

This study included 20 premolars extracted during orthodontic therapy with all surfaces intact and free of hypoplastic regions, white spot lesions (WSL) and dental caries. The teeth sample for Curodont Repair^TM (self-assembling P_11-4) and Bifluorid 10® (fluoride varnish) was equally divided. On each tooth surface, a 2 × 2 mm window was created. The samples were immersed in a demineralizing solution for 96 hours before being subjected to DIAGNOdent^TM pen reading, ICDAS-II scoring, and scanning electron microscopy-energy-dispersive X-ray (SEM-EDX) analysis on one half of the sample. The remineralizing agents were applied to the second half of the sample according to the manufacturer's instructions and placed in artificial saliva for 21 days, with the artificial salvia being replaced every 24 hours. After 21 days, the second half of the sample was subjected to DIAGNOdent^TM pen reading, ICDAS-II score, and SEM-EDX analysis.

RESULTS

Following remineralization, the DIAGNOdent^TM pen and ICDAS-II score values differed statistically between the two groups, with the Bifluorid 10® group reporting higher mean values (p > 0.05) using t-test analysis. Energy-dispersive X-ray analysis using the t-test revealed a statistically significant result for remineralization (p < 0.05), with Curodont^TM Repair group (55.150.84) reporting better mean values than Bifluorid 10® for phosphorus and calcium, but Bifluorid 10® reporting a higher result in remineralization (p < 0.05) than Curodont^TM Repair for fluoride.

CONCLUSION

Curodont^TM Repair showed better remineralizing potential compared with Bifluorid 10® varnish. In terms of the mineral gain, Curodont^TM Repair showed better results for calcium and phosphorus post-remineralization. Whereas Bifluorid 10® showed a higher gain in terms of fluoride. Self-assembling peptide P_11-4 can be used as an alternative to fluoride varnish for remineralizing WSL.

CLINICAL SIGNIFICANCE

Self-assembling polypeptide P_11-4 is a novel remineralizing agent for initial enamel lesions, which is the least-invasive method of enamel remineralization.

Isoindolines: a new series of potent and selective endothelin-A receptor antagonists

1,3-Disubstituted isoindolines have been discovered as a new class of potent functional ET(A) selective receptor antagonists through pharmacophore analysis of existing nonpeptide endothelin antagonists. The structure-activity relationships for both the trans and the cis series of isoindolines are discussed.

Differential inhibition of the prejunctional actions of angiotensin II in rat atria by valsartan, irbesartan, eprosartan, and losartan

The effects of valsartan and other nonpeptide angiotensin II type 1 (AT(1)) receptor blockers on the prejunctional actions of angiotensin II were investigated in the isolated left atria of rat. Norepinephrine stores in rat atria were loaded with [(3)H]norepinephrine, and neuronal norepinephrine release was deduced from the radioactivity efflux. Angiotensin II (10(-9) to 10(-6) M) produced concentration-dependent enhancement of the electrical stimulation-induced efflux of [(3)H]norepinephrine from the preparation. Pretreatment of tissues with valsartan, irbesartan, eprosartan, or losartan (10(-8) to 10(-6) M) produced concentration-dependent inhibitions of the stimulation-induced efflux of radioactivity observed in the presence of angiotensin II (10(-7) M). The AT(1) receptor blockers did not decrease the "basal stimulation-induced overflow of radioactivity but rather selectively inhibited the angiotensin II-mediated augmentation of the response. Regression analyses of the inhibition of the angiotensin II-mediated response by valsartan, irbesartan, eprosartan, and losartan revealed corresponding log IC(50) values (log M, with 95% confidence intervals) of -7.78 (-8.19, -7.51), -7.65 (-8.02, -7.40), -7.12 (-7. 37, -6.86), and -6.75 (-7.00, -6.40), indicating that the IC(50) values for valsartan and irbesartan are significantly lower than those for eprosartan and losartan. Thus, valsartan is a potent inhibitor of the prejunctional facilitatory effect of angiotensin II on the release of norepinephrine from peripheral sympathetic nerves. This implies that the therapeutic domain of valsartan may be extended to include pathophysiological conditions such as congestive heart failure wherein prejunctional angiotensin II receptors apparently play a significant role. Whether the high potency of valsartan translates into a significant clinical advantage relative to the other agents tested remains to be ascertained.

Anilides of (R)-trifluoro-2-hydroxy-2-methylpropionic acid as inhibitors of pyruvate dehydrogenase kinase

The optimization of a series of anilide derivatives of (R)-3,3, 3-trifluoro-2-hydroxy-2-methylpropionic acid as inhibitors of pyruvate dehydrogenase kinase (PDHK) is described that started from N-phenyl-3,3,3-trifluoro-2-hydroxy-2-methylpropanamide 1 (IC(50) = 35 +/- 1.4 microM). It was found that small electron-withdrawing groups on the ortho position of the anilide, i.e., chloro, acetyl, or bromo, increased potency 20-40-fold. The oral bioavailability of the compounds in this series is optimal (as measured by AUC) when the anilide is substituted at the 4-position with an electron-withdrawing group (i.e., carboxyl, carboxyamide, and sulfoxyamide). N-(2-Chloro-4-isobutylsulfamoylphenyl)-(R)-3,3, 3-trifluoro-2-hydroxy-2-methylpropionamide (10a) inhibits PDHK in the primary enzymatic assay with an IC(50) of 13 +/- 1.5 nM, enhances the oxidation of [(14)C]lactate into (14)CO(2) in human fibroblasts, lowers blood lactate levels significantly 2.5 and 5 h after oral doses as low as 30 micromol/kg, and increases the ex vivo activity of PDH in muscle, kidney, liver, and heart tissues. However, in contrast to sodium dichloroacetate (DCA), these PDHK inhibitors did not lower blood glucose levels. Nevertheless, they are effective at increasing the utilization and disposal of lactate and could be of utility to ameliorate conditions of inappropriate blood lactate elevation.

Secondary amides of (R)-3,3,3-trifluoro-2-hydroxy-2-methylpropionic acid as inhibitors of pyruvate dehydrogenase kinase

N'-methyl-N-(4-tert-butyl-1,2,5,6-tetrahydropyridine)thiourea, SDZ048-619 (1), is a modest inhibitor (IC(50) = 180 microM) of pyruvate dehydrogenase kinase (PDHK). In an optimization of the N-methylcarbothioamide moiety of 1, it was discovered that amides with a small acyl group, in particular appropriately substituted amides of (R)-3,3,3-trifluoro-2-hydroxy-2-methylpropionic acid, are inhibitors of PDHK. Utilizing this acyl moiety, herein is reported the rationale leading to the optimization of a series of acylated piperazine derivatives. Methyl substitution of the piperazine at the 2- and 5-positions (with S and R absolute stereochemistry) markedly increased the potency of the lead compound (>1,000-fold). Oral bioavailability of the compounds in this series is good and is optimal (as measured by AUC) when the 4-position of the piperazine is substituted with an electron-poor benzoyl moiety. (+)-1-N-[2,5-(S, R)-Dimethyl-4-N-(4-cyanobenzoyl)piperazine]-(R)-3,3, 3-trifluoro-2-hydroxy-2-methylpropanamide (14e) inhibits PDHK in the primary enzymatic assay with an IC(50) of 16 +/- 2 nM, enhances the oxidation of [(14)C]lactate into (14)CO(2) in human fibroblasts with an EC(50) of 57 +/- 13 nM, diminishes lactate significantly 2.5 h post-oral-dose at doses as low as 1 micromol/kg, and increases the ex vivo activity of PDH in muscle, liver, and fat tissues in normal Sprague-Dawley rats. These PDHK inhibitors, however, do not lower glucose in diabetic animal models.

Triterpene and diterpene inhibitors of pyruvate dehydrogenase kinase (PDK)

Several oximes of triterpenes with a 17-beta hydroxyl and abietane derivatives are inhibitors of pyruvate dehydrogenase kinase (PDK) activity. The oxime 12 and dehydroabietyl amine 2 exhibit a blood glucose lowering effect in the diabetic ob/ob mouse after a single oral dose of 100 micromol/kg. However, the mechanism of the blood glucose lowering effect is likely unrelated to PDK inhibition.

Characterization of CGS 31447, a potent and nonpeptidic endothelin-converting enzyme inhibitor

Optimization of an aminophosphonic acid series of compounds for inhibition of endothelin-converting enzyme (ECE) has led to the discovery of CGS 31447. This compound reversibly inhibited the activity of recombinant human ECE-1 with an IC50 value of 21 nM. The effect of CGS 31447 was not due to nonspecific chelation of the zinc ion at the catalytic center of ECE-1 by the phosphonic acid of the inhibitor. Determination of kinetic parameters of ECE-1 in the presence of 5-15 nM CGS 31447 revealed the competitive nature of the compound; a K1 of 7 nM was obtained. CGS 31447 infused at concentrations of 0.01, 0.1, and 1.0 microM inhibited the mean increase in big ET-1-induced pressor responses in isolated and perfused rat kidneys by 7, 39, and 68%, respectively, compared with the controls. These results demonstrate that CGS 31447 is a potent, reversible, and competitive inhibitor of ECE-1.

A novel model of conduit coronary constriction reveals local actions of endothelin-1 and prostaglandin F2alpha

We evaluated the effects of endothelin-1 (ET-1) and prostaglandin F2alpha (PGF2alpha) selectively on conduit coronary artery diameter using a novel approach in which the local concentration of vasoactive agent was controlled and maintained in vivo. ET-1 and PGF2alpha were applied topically (100 microl every 3 min) to the external surface of the left circumflex coronary artery (LCx) in anesthetized dogs or to the bathing medium of isolated canine LCx rings in parallel in vitro experiments. The dose-dependent constrictions obtained in vivo and in vitro were similar with each agent. Single, approximately maximally effective concentrations of PGF2alpha evoked an initial rapid contraction followed by a slow and sustained larger contraction in both preparations. In contrast, single concentrations of ET-1 elicited a rapid constriction that partially recovered (50-80%) in the ensuing 1.5-2 h despite continuous exposure to ET-1. After the ET-1 constriction reversed, PGF2alpha could still elicit a contraction, indicating a homologous endothelin receptor desensitization. Both agents maximally decreased conduit artery cross-sectional area in vivo by approximately 40% without significantly changing LCx resistance. Thus this in situ technique revealed effects of ET-1 and PGF2alpha on a localized segment of coronary artery that were not discernible with either intravenous or intracoronary administration.

In vitro and in vivo evaluation of an endothelin inhibitor reveals novel K+ channel opening activity

A low molecular weight endothelin (ET-1) inhibitor (Ex. 127, European Patent Application 404 525 A2, Takeda Chemical Ind., 1991), CGS 26061, was synthesized and evaluated to determine its mechanism of action. CGS 26061 (10 microM) failed to inhibit binding of [125I]ET-1 in porcine thoracic aorta and was without effect on ET-1-induced [3H]inositol phosphate accumulation in A7r5 cells. However, CGS 26061 relaxed porcine coronary arterial rings precontracted with ET-1. In addition, contractions to PGF2 alpha and low K+ (20 mM) but not high K+ were attenuated, suggesting that CGS 26061 (1, 10 microM) is a potassium channel opener. Patch-clamp experiments confirmed the K+ channel activity (0.1-10 microM). The originally re ported inhibition of ET-1-induced pressor responses by Ex. 127 (CGS 26061) was not replicated in the anesthetized dog or conscious rat nor was it shown to be antihypertensive in SHR. These data have identified CGS 26061 as a novel K+ channel opener with a unique cardiovascular profile.

The unusual binding properties of the endothelin receptor antagonist CGS 27830 distinguishes receptor/agonist interactions

CGS 27830 [meso-1,4-dihydro-5-methoxycarbonyl-2,6-dimethyl-4-(3- nitrophenyl)-3-pyridine carboxylic acid anhydride] is a nonpeptidic, insurmountable, endothelin (ET) receptor antagonist with approximately 10- to 20-fold selectivity for ETA receptors. CGS 27830 exhibits unusual binding properties which depend on the receptor and ligand: standard saturation binding experiments (coincubation of membranes with ligand in the absence or presence of antagonist) suggest that CGS 27830 is a competitive inhibitor of [125I]IRL 1620 binding to ETB receptors in rat cerebellar membranes (i.e., there was a change of apparent Kd with no change of maximum binding), but a noncompetitive inhibitor of [125I]IRL 1620 binding to ETB receptors in rat lung membranes (i.e., significant loss of total binding was observed). Although the antagonist appears to be a noncompetitive inhibitor of [125I]IRL 1620 binding to ETB receptors in rat lung membranes, CGS 27830 appears to be a competitive inhibitor of [125I]ET-1 binding to the same receptors as well as to ETA receptors in A7r5 cell membranes. Thus, CGS 27830 can distinguish [125I]IRL 1620 binding to ETB receptors in rat cerebellar and lung membranes, but not ET-1 binding to ETB receptors in these tissues. These unusual binding properties demonstrate that rat lung and cerebellum ETB receptors interact differently with IRL 1620 or ET-1.

Inhibition by nickel of endothelin-1-induced tension and associated 45Ca movements in rabbit aorta

Contractions induced by 10 nM endothelin-1 (ET) in the rabbit aortic media intimal layer were inhibited by prior exposure to 100 microM Ni++ (33.1%) or to a Ca(++)-free buffer (80.2%) but were unaffected by pretreatment with 0.1 microM nifedipine. Contractions elicited by phenylephrine (1 nM-100 microM) or K+ (10-50 mM) were not inhibited by 100 microM Ni++ but those induced by ET in tissues submaximally precontracted with 20 mM K+ were selectively antagonized by the divalent cation. The mechanism for the inhibitory action of Ni++ was ascertained by an examination of the effects of the cation on ET-induced alterations in the cellular distribution and mobilization of Ca++. Efflux of 45Ca from the muscle into a solution without added Ca++ was not altered by ET. Total or cellular 45Ca uptake (uptake after exposure to La and low temperature), at either low- or high-affinity sites in resting muscles was also not affected by the peptide. However, low-affinity cellular 45Ca retention in muscles depolarized with high K+ levels (160 mM) was significantly enhanced (45.1%) by ET. Ni++ did not alter 45Ca retention in control and K(+)-treated muscles but it blocked the additional incremental 45Ca uptake associated with ET (in the presence of high K+). Thus, Ni++ produced a selective blockade of an ET-activated Ca++ influx pathway, distinct from the dihydropyridine-sensitive L-type Ca++ channels, in rabbit aortic smooth muscle. This action by Ni++ apparently inhibits subsequent contractile responses of the muscle to ET.

Conversion of proendothelin-1 into endothelin-1 by aspartylproteases

The ability of cathepsin D, chymosin, pepsin and renin to produce endothelin-1 (ET-1) from proendothelin-1 (proET-1) was compared. No significant conversion was observed when proET-1 was incubated with up to 1 U of renin for 15 min at 37 degrees C. Cathepsin D generated, as well as degraded, ET-1 rapidly. Net production of ET-1 reached a maximum when 0.003 U of cathepsin D was used, and about 16% of the initial proET-1 was detected as ET-1 by HPLC. Pepsin up to 1 U converted proET-1 into ET-1 dose-dependently with a maximum of 71% conversion. A further increase of the amount of pepsin in the reaction mixture produced nonspecific cleavage of ET-1. Less than 10% of ET-1 remained in the presence of 15 U of pepsin. Chymosin also generated ET-1 dose-dependently, and a complete conversion was obtained at 1 U of enzyme. Greater than 1 U of chymosin only slightly degraded ET-1; at least 80% of ET-1 was still present when 15 U of chymosin was included in the assay. Other properties associated with the conversion of proET-1 into ET-1 by chymosin were investigated. Similar to authentic ET-1, the product of chymosin treatment caused contraction of isolated rabbit aortic rings, and pre-incubation of chymosin with pepstatin A abolished this contractile response.(ABSTRACT TRUNCATED AT 250 WORDS)

Functionally distinct endothelin B receptors in vascular endothelium and smooth muscle

IRL 1620 (Suc-[Glu9,Ala11,15]-ET-1 (8-21)) (0.1 nM - 1 microM), a novel ETB-selective endothelin (ET) agonist, produced endothelium-dependent relaxations in precontracted rabbit mesenteric artery (2 nM, EC50) and endothelium-independent contractions in porcine coronary artery (18 nM, EC50). ET-3 (0.1 nM-10 nM) produced qualitatively similar responses in the two tissues. The maximal contractions induced by IRL 1620 or ET-3 were substantially smaller (< 20%) than that produced by ET-1. BQ-123 (1 microM), an ETA receptor antagonist, inhibited responses to ET-1 without affecting IRL 1620- or ET-3-induced responses in either tissue. Thus functionally distinct ETB receptors mediating vasodilator and vasoconstrictor effects are located on the vascular endothelium and smooth muscle, respectively. The overall effect of ETB receptor activation on vascular tone is tissue-specific and presumably reflects differing receptor distribution at the two sites.

Identification of a novel aspartyl endothelin converting enzyme in porcine aortic endothelial cells

An enzyme which catalyzes the conversion of proendothelin-1 to the potent vasoconstrictor peptide endothelin-1 has been identified in the detergent extract of primary porcine aortic endothelial cell membranes. Partial purification was accomplished by anion exchange and Con A affinity chromatography. The enzyme was active at pH 4 and was inhibited by 100 nM peptstatin A. Hydrolysis products of proendothelin-1 were characterized by bioassay, RIA, HPLC and molecular mass analysis. Comparisons to cathepsin D and renin demonstrated that the endothelin converting enzyme activity from the porcine aortic endothelial cells was unrelated to the known enzymes. These results suggest that the processing of proendothelin-1 by endothelial cells involves a novel pepstatin-sensitive aspartyl protease.

Identification and partial purification of a thiol endothelin-converting enzyme from porcine aortic endothelial cells

Endothelin is a potent peptide vasoconstrictor. The final step in the processing of endothelin has been postulated to be the cleavage of the Trp21-Val22 peptide bond in proendothelin by a putative endothelin-converting enzyme. A soluble extract of primary porcine aortic endothelial cells was found to contain an enzyme activity that converted proendothelin-1 (proET-1) to an endothelin-1 (ET-1)-like peptide as determined by the rabbit aortic ring contraction assay. This enzyme was partially purified by DE52 ion-exchange chromatography. Incubation of proET-1 with the partially purified enzyme generated a product which had a retention time on HPLC identical to that of authentic ET-1. Further analysis of the product showed that it caused contraction of rabbit aortic rings, had a molecular weight identical to ET-1 as measured by fast atom bombardment mass spectrometry, and competed for [125I]ET-1 binding in an RIA using specific antibodies which recognize the carboxy terminal tryptophan of ET-1. The enzyme activity could be inhibited by thiol protease inhibitors such as Z-phe-pheCHN2 and p-hydroxymercuribenzoate, but not by serine- or metalloprotease inhibitors. The optimal pH for the enzymatic activity was between 7.0 and 7.5, and no activity was detected at pH 4.0. These results demonstrate that this thiol protease is a potential endothelin-converting enzyme.

Cellular Mg++ accumulation is altered by extracellular Na+ and directly affects agonist-induced mobilization of Ca++ in vascular smooth muscle

Effects of altered Na+, Ca++ and Mg++ concentrations on 45Ca and 28Mg distribution and binding as well as of changes in cellular Mg++ on mobilization of Ca++ by added norepinephrine (NE) were examined in the rabbit aortic media-intimal layer. Uptake of 45Ca at cellular high affinity sites was decreased by Mg++ much more than 28Mg uptake was altered by Ca++. Substitution of Na+ affects 45Ca uptake primarily at extracellular (La( )-accessible) binding sites. Muscles were pre-loaded with Mg++ by incubation in a low-Na+ solution (75% Na+ replaced isosmotically with sucrose) for 30 min followed by a 90 min exposure to a similar solution also containing 15 mM MgCl2. These tissues, upon examination in normal (154 mM) Na(+)-containing solution, indicated decreased retention of that cellular, high-affinity Ca++ fraction important for NE-induced contractile response. Accordingly, release of 45Ca from this site and associated tension responses to added NE were attenuated in these muscles. These results suggest that variations in extracellular Na+ concentration modulate binding and subsequent mobilization of activator Ca++ by agonists through alterations in cellular Mg++ content in vascular smooth muscle.

Effects of pH and anion substitution on magnesium accumulation in rabbit aortic smooth muscle

The effects of anion substitution, pH and extracellular Mg2+ concentration on 28Mg accumulation were examined in rabbit aortic smooth muscle. Accumulation of 28Mg (expressed as a 28Mg/Mg2+ ratio) was not changed when the concentration of added, nonradioactive MgCl2 was increased from 1.5 to 15.0 mM. The 28Mg efflux rate was increased by added MgCl2 (0.15, 0.5 or 1.5 mM) in a dose-related manner after a similar delay of 5-10 min. Addition of 1.5 mM MgCl2, MgSO4 or magnesium aspartate hydrochloride enhanced 28Mg efflux and inhibited accumulation of 28Mg to the same extent. An increase or decrease in extracellular pH correspondingly increased or decreased 28Mg accumulation. However, the 28Mg efflux rate was not altered when extracellular pH was decreased. Efflux of 28Mg was increased by added 1.5 mM MgCl2 at pH 7.4 but not at pH 5.8. Thus, the net uptake of Mg2+ appears to be proportional to the concentration of extracellular Mg2+ in rabbit aorta. Low external pH decreases 28Mg retention in rabbit aorta by inhibiting the uptake of 28Mg rather than by increasing 28Mg efflux. Effects of added Mg2+ on transmembrane movements of 28Mg are not altered by changes in the associated anion. Extracellular Mg2+ appears to enter the cell and exchange with an intracellularly located pool of 28Mg in the same manner, regardless of whether the accompanying anion is sulfate, chloride or monoaspartate hydrochloride.

Alterations in 28Mg distribution and movements in rabbit aortic smooth muscle

The distribution and transmembrane fluxes of 28Mg were examined in the isolated media-intimal layer of rabbit aorta. Accumulation of 28Mg was slow and not complete after a 3-hr incubation. The major portion of the cellular Mg++ is not exchangeable. The 28Mg efflux rate was increased by 1.5 mM nonradioactive Mg++ after a time lag of 5 to 10 min; this increase was blocked reversibly by decreasing bathing solution temperature to 4 degrees C. A rapid and sustained increase in 28Mg efflux rate was elicited with added EDTA. Accumulation of 28Mg by rabbit aorta was increased more than 5-fold by substituting sucrose for NaCl in the bathing solution. Q10 values obtained for Mg++ accumulation in rabbit aorta incubated at different temperatures either in normal solution or low-Na+ solution ranged from 1.3 to 2.0. Uptake of 28Mg was inhibited substantially by 60 mM added K+, 1.5 or 15.0 mM La , 7 mM neomycin or 1 microgram/ml of antimycin A. These ions and drugs did not significantly increase 28Mg efflux when added during the slow component phase of the washout. Thus, the major portion of slowly accumulated Mg++ appears to be stored and exchanged intracellularly. The transmembrane movements of Mg++ depend upon simple diffusion, Mg++-Mg++ exchange and a transport process that is increased when Na+ is decreased. The EDTA-induced increase in 28Mg efflux rate may result from nonspecific membrane permeability increases, whereas ions and drugs decrease cellular Mg++ content by reducing uptake rather than increasing loss.

Specific actions of gallium on norepinephrine-induced tension and associated 45Ca movements in rabbit aortic smooth muscle

Gallium ion (Ga) dose-dependently (60-360 microM) inhibited contractions induced by norepinephrine (NE, 1 microM) in rabbit aortic (and media intimal) strips, but did not affect contractions elicited with high K+ (80 mM) solution. The initial phasic portion of the NE-induced response was either unaffected or only slightly (less than 10%) reduced, but the tonic portion of the response was inhibited completely by higher concentrations (greater than or equal to 300 microM) of Ga . In resting muscles, the equilibrated (90 min) 45Ca uptake was not altered by Ga (360 microM). Also, 45Ca efflux from either high- or low-affinity Ca++ binding sites was unaltered by Ga . The effects of Ga (360 microM) on 45Ca retained after a subsequent 60-min washout at 0.5 degrees C in an isosmotic (80.8 mM) La solution were also examined. High affinity La -resistant 45Ca released by NE (1 microM) was not altered by Ga . Under conditions favoring low affinity Ca++ uptake, 45Ca retention in control and K+-treated muscles was not changed by Ga , but the additional incremental 45Ca uptake associated with NE (in the presence of high K+) was blocked. Thus, Ga appears to have a selective inhibitory action on NE-associated 45Ca uptake without affecting either resting and high K+-induced 45Ca uptake or that 45Ca fraction released by NE. This action may result from a selective blockade by Ga of receptor-linked Ca++ channels in rabbit aortic smooth muscle.

Dissociation of actions of BRL 34915 in the rat portal vein

In rat portal vein, 0.5 and 5.0 microM BRL 34915 [(+/-)-6-cyano-3,4-dihydro-2,2-dimethyl-trans-4-(2-oxo-1-pyrrolidyl++ +)-2H- benzo[b]pyran-3-ol] abolished spontaneous rhythmic movements and norepinephrine (NE)-induced tension responses, respectively. Only the higher (5 microM) concentration of BRL 34915 increased 42K efflux and inhibited the NE-induced increase in 42K efflux. These results suggest that BRL 34915 inhibits spontaneous rhythmic movements and NE-induced tension responses by differing mechanisms of action and that only the block of the NE-induced tension response is related to K+ permeability or conductance changes.

TMB-8 can block twitches without blocking high K+ or caffeine induced contractures in frog's skeletal muscle

TMB-8 [8-(diethylamino)-octyl-3,4,5-trimethoxybenzoate] is known to inhibit calcium ion dependent processes in several tissues by stabilizing some intracellular stores of membrane-bound calcium. TMB-8 was used to study the excitation-contraction (E-C) coupling process in frog's skeletal muscle. TMB-8 (5 X 10(-5) - 10(-4) M) blocked electrically evoked twitches but not high K+ (123 mM)- or caffeine (2.36 mM)-induced contractures in isolated, curarized toe muscles. TMB-8 (10(-4) M) produced a small decrease (16%) in the action potential of frog's sartorius muscle fibres. However, reducing extracellular Na+ to 44.7 mM produced a similar reduction (17%) in action potential amplitude but did not suppress the twitch; i.e. it produced only a small increase (about 10%) in twitch amplitude. It is known that potassium contractures are produced by extracellular Ca++ ions which enter through calcium channels in the t-tubules and that caffeine produces contractures by sensitizing the sarcoplasmic reticulum to Ca++-induced Ca++ release. The present results suggest that TMB-8 blocks twitches by preventing the release of Ca++ ions bound to the intracellular surface of the t-tubular membrane which is often called the store of 'trigger-calcium' ions.

Mechanism for the opioid-induced twitch potentiations of frog's skeletal muscle

The twitch-potentiating effects of opioids in the frog's skeletal muscle which are naloxone resistant and nonstereospecific were further studied. The rapid kinetics of the onset and of the offset (following washout) of the opioid effect indicates that the site for this action is the surface membrane of the muscle fibre. On the other hand, the lack of any twitch-potentiating effect by naloxone methylbromide, a quaternary derivative of naloxone, suggests that opioids which potentiate the twitch must enter the lipid phase of the membrane to act. Intracellular microelectrode experiments revealed no relation between the opioid effects on membrane electrical events and twitch potentiation. Blocking slow calcium channels with D-600 did not modify the opioid-induced twitch potentiation. The twitch potentiation was antagonized by increasing the extracellular calcium concentration, [Ca2+]o, to 8.64 mM. The effects of closely spaced multiple electrical pulses revealed that the opioids decreased the summated response relative to predrug controls. The results suggest that opioids facilitate the process of excitation-contraction coupling in the frog's skeletal muscle by the release of an additional amount of "trigger calcium" following a single electrical stimulus, thereby generating a potentiated twitch.

Displacement of the surface membrane bound calcium of the skeletal muscle fibers of the frog: effects of lanthanum and opioids

Three opioids, viz. methadone, morphine and meperidine, were tested separately for their effects on the rate of 45Ca++ efflux from frog toe muscles. The experimental procedures were designed to provide reliable measurements of the displacement of 45Ca++ from surface binding sites using the thin and light toe muscle. La (2 mM) was used to test the ability of the experimental procedure to detect the displacement of calcium bound superficially to the muscle fiber membrane. La not only enhances the rate of efflux of 45Ca++ from surface membrane superficial stores but also inhibits the efflux of intracellular calcium. Of the opioids tested, only methadone (10(-4) or 10(-3) M) caused a slight but significant increase in the rate of 45Ca++ efflux from the surface bound (i.e., the La -displaceable) store of calcium of the muscle. The "calcium-displacement" effect of methadone was much weaker than that seen with La . Morphine (10(-4) or 10(-3) M) or meperidine (10(-4) or 10(-3) M) did not show any such effect on the bound 45Ca++ desaturation rate even under experimental conditions modified to enable the detection of the displacement of even small amounts of 45Ca++ from superficial binding sites on the muscles.

Naloxone-resistant effects of opioids on the twitch in frog skeletal muscle

Morphine and several other opioid agonists including the enkephalins caused a dual action on the twitch in the isolated, curarized, and electrically stimulated frog toe muscle; a potentiating action at low drug concentrations and a potentiation followed by an inhibitory action at higher concentrations. The twitch potentiation was found to be nonstereospecific and resistant to antagonism by naloxone. The inhibitory action too was naloxone-resistant and is probably due to a nonspecific local anesthetic effect of the opioids on the electric properties of the frog skeletal muscle fibre membrane.