The main structural feature of direct thrombin inhibitor LK-732 responsible for the appropriate interaction at the thrombin active site is a strong basic group. A possibility that a strong basic ...group of LK-732 might contribute to the mast cell degranulation effect and consequent reduction of tracheal air flow (TAF) and fall of mean arterial blood pressure (MAP) in rats was investigated in the present study. At doses up to 5 mg/kg (i.v.), LK-732 did not cause significant changes of TAF and MAP. At 7 mg/kg (i.v.), a sudden reduction of TAF and a fall of MAP was observed within 5 min after LK-732 administration (75% mortality, p = 0.007). A less basic direct thrombin inhibitor LK-658 (21 mg/kg, i.v.) did not significantly disturb TAF and MAP. A reduction of TAF and a fall of MAP caused by LK-732 (7 mg/kg, i.v.) was almost completely abolished in rats with degranulated mast cells (0% mortality, p = 0.008). LK-732 concentration-dependently degranulated rat peritoneal mast cells in vitro (pEC(50) = 1.92 +/- 0.05 muM). A structure-activity relationship (SAR) study revealed that the terminal basic groups attached to the aromatic ring are responsible for the mast cell degranulation effect. A good correlation was observed between mast cell degranulation and pK(b) of analogues of LK-732 (R(2) = 0.49), but not between mast cell degranulation and thrombin K(i) (R(2) = 0.23). LK-732-induced reduction of TAF, the fall of MAP and high mortality originate from LK-732-induced mast cell degranulation. As judged by the SAR study, this effect could be overcome by reducing the basicity of LK-732.
The antithrombotic potential of new direct thrombin inhibitors built on the azaphenylalanine scaffold (LK-732, LK-639 and LK-731) and their amidoxime prodrugs (LK-658, LK-633 and LK-730) was studied ...in comparison to argatroban and nadroparin in two rat models of venous thrombosis, induced either by complete stasis combined with hypercoagulability (model 1) or by partial stasis combined with vessel injury (model 2). In initial experiments LK-732 was established as the most promising antithrombotic of the LK inhibitors and as such was further tested. In model 1, intravenous bolus administration of LK-732 produced a dose-dependent inhibition of thrombus formation with an ID50 value of 1.3 mg/kg. This ID50 value was approximately four times higher than the ID50 value of argatroban (0.3 mg/kg; p=0.011). However, in model 2, LK-732 and argatroban decreased thrombus weight by 50% at similar ID50 values (3.8 mg/kg vs 3.0 mg/kg, respectively; p=0.726). The ex vivo anticoagulant effect of LK-732 was substantially weaker compared to argatroban at doses that produced comparable antithrombotic effects. After subcutaneous administration, in vivo thrombus weight reduction of LK inhibitors (10 mg/kg) ranged between 22 to 48%. However, their oral antithrombotic effect at a dose of 30 mg/kg was rather low. LK amidoxime prodrugs failed to produce a substantial antithrombotic effect after subcutaneous (10 mg/kg) as well as after oral administration (30 mg/kg). In conclusion, thrombin inhibitors built on the azaphenylalanine scaffold represent a new group of intravenously effective antithrombotics. However, optimisation of the oral antithrombotic effect of amidoxime prodrug LK-658 of the lead inhibitor LK-732 is required for justifying further development of these inhibitors.
Our study elucidates some mechanisms of contractions or relaxations of isolated porcine left anterior descending coronary artery (LAD) induced by two peptides from the honeybee venom, melittin and ...apamin. Contractions or relaxations were measured on relaxed or precontracted arteries, respectively. Melittin at lower concentrations (0.1–10 μg/ml) induced transient relaxation, and contraction at higher concentrations (≥7 μg/ml). The removing of the endothelium diminished the melittin-induced relaxation but did not affect the maximal contraction. The inhibition of prostaglandin and nitric oxide (NO) synthesis (by indomethacin and by N-omega-Nitro-
l-arginine, respectively) and the use of K
+ channel inhibitors (apamin and charybdotoxin) showed that melittin evoked relaxation via an endothelium-dependent mechanism (NO production), and by activation of charybdotoxin-sensitive K
+ channels of smooth muscle. Apamin alone did not affect contraction or relaxation, but the inhibition of NO and prostanoid production revealed the involvement of apamin-sensitive K
+ channels of smooth muscle in melittin-induced relaxation. Our data show that melittin and apamin could affect contractility of porcine LAD at concentrations similar to those encountered in multiple honeybee stings in humans. Melittin could directly affect contractility of porcine LAD, whereas apamin acts as a modulator of the relaxant response to melittin.
The deleterious intracellular Ca
2+ overload in the ischemic–reperfusion injury of the heart can be even more expressed in subjects with acute renal failure in whom maintenance of intracellular Ca
2+ ...has already been disturbed in normoxia. To study the influence of acute renal failure in ischemic–reperfusion injury on the heart, we used isolated Langendorff's hearts of guinea pigs with gentamicin-induced acute renal failure. We examined arrhythmias, heart contractility and myocardial cell damage during reperfusion. Two specific Ca
2+ channel antagonists, mibefradil (0.1 and 1 μM) and verapamil (0.1 μM), were used to test the possible involvement of T-type and L-type Ca
2+ channels in these processes. We exposed hearts to 50 min of zero-flow global ischemia and 60 min of reperfusion. During reperfusion, unrecoverable ventricular fibrillation appeared more often in hearts of animals with acute renal failure than in control hearts (80% vs. 0%, respectively). Mibefradil, but not verapamil, applied either pre- or post-ischemically, terminated ventricular fibrillation in all hearts of animals with acute renal failure. Mibefradil (0.1 μM only) improved contractility in hearts of animals with acute renal failure during reperfusion by 30%. During reperfusion, lactate dehydrogenase (LDH) release rate increased less in hearts of guinea pigs with acute renal failure than in control hearts and only verapamil decreased it additionally. Thus, our results suggest a more important role of T- than of L-type Ca
2+ channels in ischemic–reperfusion injury in isolated guinea pig hearts with acute renal failure.
During ischaemia and reperfusion increased cytosolic Ca2+ is one of the important causes for ischaemic-reperfusion myocardial injury. In the present study we compared effects of preferentially L-type ...Ca2+ antagonists nitrendipine (NT) and lacidipine (LP), and of mibefradil (MB) a Ca2+ antagonist with higher affinity to T- than to L-type channels on myocardial function during reperfusion. Coronary flow (CF), heart rate (HR), left ventricular pressure (LVP), lactate dehydrogenase (LDH) release rate and ECG were registered during 40 min of reperfusion following 30 min of global zero flow ischaemia in Langendorff's isolated rat hearts. Either NT (100 nmol/L) or LP (10 nmol/L) or MB (100 nmol/L) was added to Krebs-Henseleit solution 10 min before ischaemia till the end of experiments. All three drugs influenced CF, HR and LVP. All of them decreased LDH release rate (P < 0.05, in microkat/g x min) when compared with control hearts (53.2 +/- 5.1): MB (19.4 +/- 4.3) > LP (30.7 +/- 6.6) > NT (43.3 +/- 2.8). NT reduced the duration of continuous arrhythmias at the beginning of reperfusion (to 59.1 +/- 6.1% of ischaemic controls) as well as the number of single arrhythmic events arising during the whole period of reperfusion (to 26.1 +/- 6.0% of ischaemic controls). MB diminished only single arrhythmic events during reperfusion to 39.1 +/- 17.3% of ischaemic controls. LP did not affect the onset of arrhythmias. Results of our experiments indicate a relatively greater importance of T-type than of L-type Ca2+ channels in the arising of postischaemic myocardial damage.
Equinatoxin II (EqT II) is a basic polypeptide toxin from the sea anemone Actinia equina (L.). Its LD50 in mice is 33 g/kg. The cause of death after intravenous application has been attributed to the ...circulatory failure resulting in the cardiotoxic effects. In Langendorff's rat and guinea-pig heart preparations EqT II caused dose dependent decrease in the coronary flow (CF). Morphologic changes of different cell cultures incubated with EqT II are the result of Ca2+ entry through the newly formed discrete pores. Pores in the cell membranes are composed of the toxin oligomeres. In the present study we tried to evaluate the dependence of vasoconstrictor effects of EqT II on isolated rat hearts upon the Ca2+ concentration in the perfusion solution. EqT II did not affect the CF in the group without Ca2+. The strongest effect was observed in the group with 1.5 mM Ca2+ where the CF decreased to 7.7±7 %. The results of our experiments indicate that the effects of EqT II on CF depend on Ca2+ concentration in the extracellular solution.
Death after i.v. administration of equinatoxin II (EqT II) has been attributed to the circulatory failure resulting from cardiotoxic effects. The mechanism of action is unknown. The aim of the ...present work was to study the effects of the toxin on vascular tone in the isolated porcine coronary artery and on coronary flow in the isolated pig heart. EqT II caused concentration-dependent contractions of rings of the isolated epicardial porcine coronary artery with an EC50 value of 89+/-5 nM (n=5-6) and maximal effect of about 140% of the contraction induced by 20 nM KCl. On Langendorffs porcine heart preparation EqT II caused a dose-dependent decrease of coronary flow. At EqT II doses lower than 0.05 micromol/100 g of heart weight there were no measurable effects of the toxin. At dose 0.5 micromol/100 g the toxin decreased coronary flow to less than 9.8+/-2.5% of the control value. The constrictory effect of the toxin on isolated porcine coronary arteries was diminished by the L-type calcium channel antagonist nicardipine (NC). NC in 1 microM concentration almost completely abolished the effect of EqT II on coronary flow. Our results confirmed involvement of L-type calcium channels in the vasoconstrictory effects of EqT II on epicardial coronary arteries.
The qualitative and quantitative effects of the (+)-
S and (−)-
R enantiomers and of the racemic mixture of the Ca
2+ channel antagonist, nicardipine, were compared on the isolated porcine coronary ...artery with intact and removed endothelium. All three forms of nicardipine inhibited the contractions induced by KCl (5–90 mM) in both vessel preparations. The potency (IC
50) of the (+)-
S and (−)-
R enantiomers and of the racemic mixture was 6.6, 31.8 and 10.9 nM in the vessel with endothelium and 6.4, 41.9 and 9.8 nM in the vessel without endothelium. The parameters of the concentration–response curves for each form of nicardipine at a submaximal KCl (60 mM) concentration and the potency ratios between the two enantiomers ((+)-
S/(−)-
R) were not statistically significantly different (
P>0.05) in the two vessel preparations. In conclusion, qualitatively, all three forms of nicardipine showed only Ca
2+ channel antagonistic effects in both vessel preparations. Quantitatively, the inhibition of contraction was stereoselective, the (+)-
S enantiomer being the most potent, and was endothelium-independent.
The venom of the honeybee Apis mellifera induces cardiovascular dysfunction. As its effects on coronary arteries have not yet been described, we studied the effects of the whole honeybee venom ...(non-volatile part) in the isolated porcine left anterior descending coronary artery (LAD) and the influence of L-type Ca2+ channel blocker, lacidipine, upon the venom effects in LAD. The venom produced concentration dependent contractions (7-70 microg/ml) of the porcine LAD; maximal effect of the venom was approximately the same as the effect of 30 mM KCl. Lacidipine concentration dependently (0.1-10 microM) and significantly (P < or = 0.05) decreased the venom-induced vasoconstriction. The results indicate the involvement of L-type Ca2+ channels in coronary contraction, induced by bee venom.
