Leukotriene D4: cardiovascular and sympathetic effects in spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats

Leukotriene D4 (1--20 micrograms/kg i.a.) administered to conscious spontaneously hypertensive rats (SHR) and WKY rats caused acute elevation of blood pressure in both groups, but only in SHR a prolonged hypotensive period followed the hypertensive event. SHR rats had tachycardia during the hypertensive phase and relative bradycardia during the hypotensive phase which was more pronounced and prolonged than in WKY rats. In SHR rats only, plasma epinephrine and norepinephrine were elevated (6- and 3-fold, respectively) at the peak of the hypertensive period. Pretreatment of SHR rats with indomethacin (5 mg/kg) potentiated the LTD4-induced pressor response and shortened the hypotensive-bradycardic effect of LTD4. This same biphasic, dose-dependent response to LTD4 (1--20 micrograms/kg i.v.) was present in pithed SHR rats. Therefore, a direct action of LTD4 on vascular smooth muscle and heart is suggested. In all WKY rats and some SHR rats, a bradycardic effect of LTD4 resulted from sinus bradycardia, whereas in pithed SHR rats impaired conduction varying from transient second degree atrioventricular block to complete heart block was observed. Electrocardiographic signs of ischemia were seen only in LTD4-injected, pithed SHR rats. These results suggest fundamental differences between SHR and WKY rats in regard to their sensitivity to lipoxygenase products.     

Characterization of the alpha(2)-adrenoceptor subtype, which functions as alpha(2)-autoreceptor in human neocortex

The pharmacological properties of the alpha(2)-adrenergic receptors regulating the release of norepinephrine were investigated in human neocortex. Slices were preincubated with [(3)H]norepinephrine, superfused under blockade of transmitter reuptake, and stimulated electrically. First, the autoinhibitory circuit of [(3)H]norepinephrine release was analyzed quantitatively by estimation of the K(d) of norepinephrine at the alpha(2)-autoreceptor (10(-7.99) M), the concentration of the endogenous transmitter causing this autoinhibition at a stimulation frequency of 3 Hz (10(-7.61) M), and the maximum inhibition obtainable through the autoreceptor (83%). Second, antagonist pK(b) values of nine antagonists were determined by using their pEC(50) values (negative logarithms of antagonist concentrations that increased the electrically evoked overflow of tritium by 50%) against the release-inhibiting effect of the endogenous transmitter. When compared with binding or functional data from the literature, the pK(b) values correlated best with the antagonist affinities at alpha(2A) binding sites. In contrast, the correlations with alpha(2B), alpha(2C), and alpha(2D) sites were not as good. It is concluded that in human neocortex prejunctional autoreceptors are alpha(2A).     

Platelet activating factor (PAF) and tumor necrosis factor-alpha (TNF alpha) interactions in endotoxemic shock: studies with BN 50739, a novel PAF antagonist

BN 50739, a new PAF receptor antagonist, was tested in vitro and in vivo for its capacity to block PAF, endotoxin and recombinant human tumor necrosis factor-alpha (rTNF)-mediated effects. In vitro, BN 50739 blocked PAF-induced platelet aggregation by 60 to 100% at 0.2-1 x 10(-7) M (P less than .002), respectively. In the conscious rat, pretreatment (30 min) with BN 50739 (n = 5-13) dose-dependently attenuated PAF-induced hypotension (-5 +/- 5 vs. - 43 +/- 2 mm Hg, P less than .01) and shortened the recovery time of mean arterial pressure (22 +/- 13 vs. 325 +/- 46 sec, P less than .01). BN 50739 (10 mg/kg i.p., n = 5-11) prevented endotoxin (14.4 mg/kg) induced-hemoconcentration (54 +/- 1 vs. 46 +/- 1%, P less than .01) and reduced 24-hr mortality (100 vs. 60%, P less than .05). Only partial protection was conveyed by BN 50739 against the hypotensive response to endotoxin (115 +/- 3 vs. 91 +/- 4 mm Hg, P less than .03). Also, BN 50739 attenuated the lipopolysaccharide-induced elevation of plasma thromboxane B2 (21.2 +/- 0.8 vs. 46.7 +/- 11.8 pg/100 microliters, P less than .01) and tumor necrosis factor-alpha (7523 +/- 3983 vs. 26,430 +/- 3541 U/ml, P less than .05), whereas leukopenia and thrombocytopenia remained unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)     

Respiratory and locomotor stimulation by low doses of dermorphin, a mu1 receptor-mediated effect

The selective opioid mu receptor agonist dermorphin increased the locomotor activity of rats dose dependently at 10 to 100 pmol/kg i.c.v. Respiratory rate, relative tidal volume and respiratory minute volume also increased unrelated to changes in locomotor activity. Higher doses, on the other hand, produced catalepsy and respiratory depression. Pretreatment of the rats with the mu1-selective antagonist naloxonazine (10 mg/kg i.v.) blocked the stimulant locomotor and respiratory effects of low doses of dermorphin (10-100 pmol/kg), but potentiated the respiratory depressant effect of a high dose (10 nmol/kg) of dermorphin. The selective benzodiazepine antagonist flumazenil (5 mg/kg), which has been shown previously to antagonize catalepsy and respiratory depression produced by relatively high doses of dermorphin, did not antagonize the respiratory or locomotor stimulant effect of dermorphin. The data suggest that mu1-opioid receptors are responsible for the low dose stimulant effects of dermorphin on locomotor activity and respiration whereas mu2 receptors mediate the respiratory depressant effect of dermorphin.     

Carvedilol, a new vasodilator and beta adrenoceptor antagonist, is an antioxidant and free radical scavenger.

The antioxidant effect of carvedilol, a new vasodilating, beta adrenoceptor blocker was studied and compared with five other beta blockers. Carvedilol rapidly inhibited Fe(++)-initiated lipid peroxidation, measured as thiobarbituric acid reactive substance (TBARS), in rat brain homogenate with an IC50 of 8.1 microM. Under the same conditions, the IC50 values of atenolol, pindolol propranolol, celiprolol and labetalol were over 1.0 mM. Carvedilol protected against Fe(++)-induced alpha-tocopherol depletion in rat brain homogenate with an IC50 of 17.6 microM; propranolol, celiprolol and labetalol, up to 200 microM, did not show any effect. Using dihydroxyfumarate/Fe(++)-ADP as a OH.radical generating system and 5,5-dimethyl pyrroline-N-oxide (DMPO) as a trapping agent, the characteristic DMPO-OH signals were monitored by electron paramagnetic resonance. Carvedilol dose-dependently decreased the intensity of the DMPO-OH signal, with an IC50 of 25 microM, whereas propranolol, at 500 microM, and U74500A, a 21-aminosteroid, at 100 microM, had no effect. The antioxidant effect of carvedilol mainly resides in the carbazole moiety, and the substitution of a hydroxyl group at certain positions on the phenyl ring of either carbazole or the ortho-substituted phenoxylethylamine part of carvedilol resulted in an increase in antioxidant activity. Furthermore, the protective effect of carvedilol analogs against OH.-mediated neuronal death positively correlated to their antioxidant effect. We conclude that carvedilol is a far more potent antioxidant than other commonly used beta blockers. The apparent mechanism of carvedilol's inhibition of lipid peroxidation is mainly via scavenging free radicals. This novel property of carvedilol may contribute to the known cardioprotective activity of this compound.     

Prostacyclin Reversal of Lethal Endotoxemia in Dogs

Severe endotoxemia, a condition where microembolization and intravascular coagulation are thought to play important roles, was treated experimentally with prostacyclin (PGI₂). In a study of 24 dogs, 8 control animals injected with 1.75 mg·kg⁻¹ of endotoxin died within 24 h. Six animals given intravenous aspirin 100 mg/kg, 30 min after endotoxin died. 9 of 10 dogs infused with 100 ng PGI₂·kg⁻¹·min⁻¹ for 3 h, given 30 min after the injection of endotoxin survived 24 h (P < 0.025). Injection of endotoxin resulted in a: (a) maximal 62% fall in mean arterial pressure (P < 0.001); (b) transient doubling of mean pulmonary arterial pressure (P < 0.001); (c) initial 70% drop in cardiac index (P < 0.001); (d) decline in blood platelets from 213,700 to 13,700/mm³ (P < 0.001), and leukocytes from 7,719 to < 750/mm³ (P < 0.001); (e) depressed urine output (P < 0.001); (f) 34% decrease in blood fibrinogen (P < 0.01) and an increase in fibrin degradation products > 50 μg/ml (P < 0.001); (g) fivefold increase in circulating cathepsin D titer (P < 0.005) and (h) increase in blood norepinephrine (P < 0.005), dopamine (P < 0.005), and epinephrine (P < 0.001). Aspirin treatment led to an increase in mean arterial pressure (P < 0.001) and mean pulmonary arterial pressure (P < 0.005), but cardiac index, urine flow, platelets, leukocytes, fibrin degradation products, and cathepsin D levels remained similar to untreated controls. After infusion of PGI₂ there was a: (a) prompt increase of cardiac index to base-line levels; (b) late increase in mean arterial pressure (P < 0.005) after the discontinuation of PGI₂ treatment (c) restoration of urine output; (d) increase in circulating platelets to levels still below base line but above untreated control animals (P < 0.05); (e) no effect on circulating leukocyte levels; (f) fall in fibrin degradation products to 11.2 μg/ml (P < 0.05); (g) decline in cathepsin D levels to values 60% lower than the untreated controls (P < 0.025); and (h) reduction in plasma norepinephrine levels to base line at 4 h (P < 0.005). Although the mode of PGI₂ action is not clear, it is effective in the treatment of experimental endotoxemia.     

Reversal of intracellular toxicity of the trichothecene mycotoxin T-2 with monoclonal antibody

The trichothecene mycotoxin T-2 is a potent inhibitor of intracellular protein synthesis. We have previously shown that a mouse immunoglobulin G1 monoclonal antibody (15H6) specific for T-2 toxin can neutralize the in vitro protein synthesis inhibitory effect of the toxin in human B-lymphoblastoid cultures, and protect rats from lethal toxemia. We now report that these monoclonal antibodies can induce the net efflux of [3H]-T-2 toxin from poisoned human B-lymphoblastoid cells in vitro, and restore protein synthesis. Administration of the monoclonal antibodies (250 mg/kg) 30 min before infusion of a lethal dose (1 mg/kg) of T-2 toxin causes the sequestration of the toxin in the plasma compartment. When administered 35 min after T-2 toxin, a time when the bulk of toxin is in the tissues, the monoclonal antibodies facilitate the migration of toxin back into the plasma compartment. These data demonstrate that monoclonal antibodies can be of therapeutic value against an intracellular toxin.     

Mechanisms of central hemodynamic and sympathetic regulation by mu opioid receptors: effects of dermorphin in the conscious rat

The effects of i.c.v. administered dermorphin, a highly selective mu-opioid agonist, on cardiac function and renal, mesenteric and hindquarter blood flow were studied in conscious rats. Core temperature, blood gases, arterial plasma levels of norepinephrine, epinephrine, dopamine, 3,4-dihydroxyphenylalanine and dihydroxyphenylacetic acid (DOPAC) also were examined. Cardiac output was measured using a thermodilution technique and regional blood flows using directional pulsed Doppler velocimetry. Dermorphin, at doses of 0.1-100 nmol/kg, increased blood pressure and hindquarter blood flow, renal and mesenteric resistance, and core temperature. Higher doses (1-5 mumol/kg) caused respiratory depression, acidosis, and shock despite profound sympatho-adrenomedullary stimulation. Circulating levels of catecholamines were significantly increased at the dermorphin doses of 0.1-100 nmol/kg. At the 100 nmol/kg dose, plasma levels of epinephrine, norepinephrine, the dopamine metabolite dihydroxyphenylacetic acid and the catecholamine precursor 3,4-dihydroxyphenylalanine were increased by 2-15-fold. The data indicate that mu opioid receptor stimulation exerts potent effects on cardiorespiratory functions, activates the sympathoadrenomedullary system and produces a pattern of blood flow changes consistent with the stress-induced "defense" response (skeletal muscle vasodilation and splanchnic vasoconstriction). Excessive mu opioid receptor stimulation leads to shock due to respiratory and hemodynamic collapse.     

Inhibition of norepinephrine and acetylcholine release from human neocortex by omega-conotoxin GVIA

Superfused slices of human neocortex, prepared from surgically removed tissue (to gain access to subcortical tumors) and prelabeled selectively with [3H]norepinephrine (NE) or [3H]choline, were stimulated electrically to evoke tritium overflow. This tritium overflow was abolished by the sodium channel blocker tetrodotoxin and by withdrawal of extracellular Ca++. Thus, the action potential-induced, exocytotic tritium overflow supports the assumption of a quasiphysiological release of NE from noradrenergic and of acetylcholine (ACh) from cholinergic nerve terminals, respectively. In addition, the modulation of NE release by adrenoceptor ligands displayed the appropriate pharmacology of alpha-2 autoreceptors; ACh release was modulated by muscarinic ligands. Both NE and ACh release decreased with the age of the patients. The effects of drugs on NE and ACh release were not age-related. The peptide modulator of the N-type voltage sensitive Ca++ channel, omega-conotoxin GVIA, inhibited NE release with an IC50 of about 14 nM and ACh release with an IC50 of about 3 nM, whereas L-type modulators were ineffective. The binding of [125I]omega-conotoxin GVIA to human neocortical membranes was of high affinity (KD = 1.3 pM) to one site (nH = 0.97) of substantial density (maximum binding = 878 fmol/mg of protein); the binding of the L-type modulator [3H]isradipine to these membranes was also of high affinity (KD = 89 pM) to one site (nH = 1.03) of lesser density (maximum binding = 429 fmol/mg of protein). In conclusion, Ca++ entry through N-type Ca++ channels, rather then L-type Ca++ channels, predominates in subserving NE and ACh release from noradrenergic and cholinergic nerve terminals, respectively, of human neocortex.     

Central autonomic effects of dermorphin in conscious rats

Central cardiovascular and respiratory effects of dermorphin were studied in conscious rats. Intracerebroventricular administration (0.1 or 1 nM) of dermorphin increased the systolic and diastolic blood pressure whereas a high dose (50 nM) decreased blood pressure. Dermorphin at 1 nM increased respiratory rate but caused hypoxia, acidosis and hypercapnia; at 50 nM, respiratory rate was suppressed. Both of these doses produced catalepsy. Stimulation of the sympathoadrenomedullary axis at the pressor period elicited by 1 nM dermorphin was evident by high circulating levels of epinephrine and norepinephrine. N-methyl-atropine reversed the severe bradycardia induced by dermorphin and completely prevented this phenomenon if given as pretreatment. Neither blood pressure nor respiratory depression was altered by the muscarinic blocker. Naloxone (1 mg/kg) reversed the respiratory, cardiovascular and sympathetic effects of dermorphin as well as the catalepsy. These data show that dermorphin has central autonomic effects which are naloxone reversible and mediated by both sympathetic and parasympathetic pathways.