Short-term lisinopril treatment in old rats worsens impairment of angiotensin-induced reflex bradycardia.

To determine how short-term treatment with an angiotensin-converting enzyme (ACE) inhibitor affects drug-induced reflex bradycardia at different ages in conscious rats, we compared the magnitude of drug-induced reflex bradycardia before and after injecting bolus intravenous doses of lisinopril, 1 mg/100 g, in male Sprague-Dawley rats aged 4 (young) or 19 (old) months. Anesthetic artifacts were avoided by recording all drug-induced cardiovascular responses from femoral arterial cannulas implanted 1 week earlier. For eliciting reflex bradycardia, blood pressure was increased by graded intravenous infusion of angiotensin or phenylephrine. Impairment of reflex bradycardia in old rats occurred only during pressor responses to angiotensin but not when blood pressure was equally increased with phenylephrine. Subsequent administration of lisinopril affected neither pressor and reflex bradycardic responses to phenylephrine nor pressor responses to angiotensin. However, contrary to the baroreflex enhancement described previously by others, the reflex bradycardia induced by angiotensin was reduced by lisinopril treatment but only in old and not in young rats. Thus our results indicate that whereas angiotensin-induced reflex bradycardia was already impaired in old rats before lisinopril was given, it was reduced further after short-term lisinopril treatment.

Conscious obese rats have impaired reflex bradycardia and enhanced norepinephrine sensitivity.

Male Sprague-Dawley rats fed a condensed milk diet were classified as either "obesity susceptible" (OS) or "obesity resistant" (OR) based on body weight increases attained after 12 wk. Overall caloric intake in OS rats was higher than in chow-fed controls, and OS rats were heavier than chow-fed controls or OR rats. There were no significant differences in blood glucose, serum insulin, ventricular weight, basal blood pressure, or heart rate. Pressor responses recorded after combined blockade with atropine and propranolol to eliminate reflex effects were identical for vasopressin, but those to norepinephrine were larger in OS than in OR rats, whereas those to angiotensin were larger in OS than in control rats. When baroreflex sensitivity was assessed using intravenously infused sodium nitroprusside or phenylephrine to alter systemic arterial pressure, differences in reflex tachycardia were equivocal, but reflex bradycardia was clearly inhibited in OS rats. These results show that, although basal blood pressure was unaffected in OS rats, their impaired reflex bradycardia along with enhanced pressor responsiveness to norepinephrine could predispose them to subsequent development of hypertension.

Comparative toxicity of four chlorinated dibenzo-p-dioxins (CDDs) and their mixture. IV. Determination of liver concentrations.

Groups of male Sprague-Dawley rats were administered orally the following chlorinated dibenzo-p-dioxins (CDDs) in corn oil/acetone (95/5; v/v): 30-60 micrograms/kg 2,3,7,8-tetrachlorodibenzo-dioxin (tetra-CDD), 160-270 micrograms/kg1,2,3,7,8-pentachlorodibenzo-p-dioxins (penta-CDD), 630-1249 micrograms/kg 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin (hexa-CDD) and 5000-8000 micrograms/kg 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (hepta-CDD) or a mixture of the four homologues such that each was present in the mixture at one quarter of its dose as a single compound. Animals were killed at 2 and 8 days after dosing. Livers were immediately removed, and aliquots frozen in liquid nitrogen. Storage occurred at -80 degrees C until further use. About 0.2 g of each lyophilized rat liver was extracted, the extract purified by column chromatography and analyzed by GC/MS for CDD content. Results obtained suggest that the absorption of CDDs after oral administration decreases in the order of tetra-CDD > or = penta-CDD > hexa-CDD > hepta-CDD, indicating that the dose was an incomplete surrogate of exposure in parts I-III of this publication series (Stahl et al. 1992; Weber et al. 1992a,b). Moreover, data also support the notion that the pharmacokinetics of CDD mixtures at high doses are somewhat different from those expected based on single compound exposures. Our findings suggest that the intrinsic relative potency in terms of toxic equivalents (TEQ) of the higher chlorinated homologues is slightly greater (about a factor of 2) than suggested by Stahl et al.(ABSTRACT TRUNCATED AT 250 WORDS)

A pharmacodynamically responsive model of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) transfer between liver and fat at low and high doses.

We have constructed a pharmacokinetic model for TCDD in the rat, emphasizing its transfer between plasma, lipid, and cytoplasmic compartments of the liver, white adipose tissue (WAT), and brown adipose tissue (BAT). Volumes of the lipid pools are controlled by a submodel of triglyceride (TG) metabolism and transport that responds via a receptor for TCDD in WAT cytoplasm with a Kd of about 2 nM. This submodel, and one for cytochrome P450IA2 induction, allowed us to simulate binding of TCDD to the induced P450IA2 binding sites at low doses (1 ng/kg to 10 micrograms/kg) independently of the decreased feed intake and hyperlipidemia associated with higher doses (20 to 120 micrograms/kg). In low-dose simulations, the induction of cytochrome P450IA2 binding sites for TCDD dominated the redistribution of TCDD between WAT and liver. When simulations were performed using a partitioning model with constant gastrointestinal flows, increased WAT lipolysis driven by reduced feed intake (from 10 to 120 micrograms/kg) is predicted to result in a decreased WAT volume and a sharp drop in the mass of WAT-associated TCDD, while initially increasing the levels of TCDD in liver. However, the observed concentration of TCDD in WAT increased in rats treated with a high dose (72 micrograms/kg) of TCDD. The rise in tissue concentrations could not be explained without incorporating decreased intestinal flows into the gastrointestinal absorption process, which increased the resorption of TCDD. TCDD concentrations in tissue increased only when the relative tissue volumes decreased more rapidly than the whole-body TCDD elimination rate.

Strain differences in baroreflex inhibition by centrally infused enalapril in old rats.

To determine whether inhibition of the brain renin-angiotensin system would affect baroreflexes similarly in old rats of different strains, we compared 24-month-old male Fischer 344 and Sprague-Dawley rats. Baroreflex sensitivity was tested while the rats were awake by recording reflex heart rate responses elicited as blood pressure was elevated with phenylephrine or lowered with sodium nitroprusside. Sprague-Dawley rats had higher blood pressures and lower heart rates initially. Chronic infusion of enalapril, a converting enzyme inhibitor, into a lateral cerebral ventricle (ICV) for two weeks lowered blood pressure in Sprague-Dawley but not in Fischer 344 rats. Furthermore, reflex bradycardia was unaffected in either rat strain, but reflex tachycardia was selectively suppressed in Fischer 344 rats. Thus, although time controls were not done to rule out spontaneous changes during the 14-day infusion period, these results suggest that central cardiovascular regulation does not change similarly with age in these two rat strains. As removal of the brain renin-angiotensin system lowered blood pressure in one strain and inhibited reflex tachycardia in the other, the divergence could mean that the brain renin-angiotensin system acts differently to keep blood pressure elevated in Sprague-Dawley rats and modulate reflex tachycardia in Fischer 344 rats.

Is a serotonergic mechanism involved in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced appetite suppression in the Sprague-Dawley rat?

The major cause of TCDD-induced death in rats is a progressive voluntary feed refusal which has been correlated with reduced gluconeogenesis. Since centrally administered TCDD does not cause death or decreased feed intake in rats, the ability of TCDD to suppress appetite via peripheral mechanisms acting on the central nervous system was examined in two experimental models. First, it was found that the feed intake of rats on scheduled feeding cycles was not decreased by blood transfused from rats with TCDD-induced appetite suppression (8 days after a lethal dose of TCDD, i.p.). In contrast, a similar transfusion from normal, satiated rats did reduce feed intake of recipient rats by approximately 40%, suggesting that TCDD-treated rats are not satiated but rather that they are not hungry. In the second study tryptophan (the amino acid precursor of the neurotransmitter serotonin) was measured in the plasma and tryptophan, serotonin, norepinephrine and dopamine in the hypothalamus as well as dopamine and its metabolites in the striatum 4, 8, and 16 days after TCDD dosage (125 micrograms/kg, i.p.). Progressive time-dependent increases in tryptophan levels in plasma and brain were paralleled by increases in brain serotonin and 5-hydroxyindoleacetic acid (the primary metabolite of serotonin) in TCDD-treated rats. No changes were observed regarding the other biogenic amines. It is suggested based on these data and on substantial evidence from the published literature that a serotonergic mechanism may be involved in TCDD-induced feed intake reduction.

Chromatographic studies on the binding, action and metabolism of (-)-deprenyl.

Serum binding, the effect on striatal dopamine release and the metabolism of (-)-deprenyl [N-methyl-N-propargyl(2-phenyl-1-methyl)ethylammonium chloride], TZ-650 [N-methyl-N-propargyl(2-phenyl)ethylammonium chloride] and J-508 [N-methyl-N-propargyl(indanyl)ammonium chloride] were investigated using various chromatographic methods. A strong interaction between (-)-deprenyl and macroglobulins was found. Deprenyl enhanced the dopamine release from striatal slices of the rat brain and also inhibited the dopamine-DOPAC conversion. Deprenyl analogues showed either smaller or no effect. Hydroxylation of (-)-deprenyl takes place in the para position, in addition to the usual oxidative N-dealkylations, which are known from various metabolic studies on N-substituted phenylalkylamines.

Selective pressor enhancement by monoamine oxidase inhibitors in conscious rats.

To compare the cardiovascular effects of chronic monoamine oxidase (MAO) A and B inhibition, rats were given s.c. injections of saline clorgyline or l-deprenyl daily for 3 weeks. Indwelling vascular catheters and a Doppler flow probe were implanted chronically to allow subsequent recording of femoral pressure, heart rate and iliac blood flow before and during the treatment while the rats were awake. On days 7 and 21, average femoral pressure were significantly higher in rats treated with either saline or l-deprenyl than in those treated with clorgyline. Pressor responses elicited by injecting graded doses of phenylephrine, angiotensin or tyramine i.v. were always accompanied by bradycardia and reduced iliac flow. Magnitude of all responses was unaltered in control rats treated with the saline vehicle. In rats treated with l-deprenyl responses to tyramine were enhanced slightly, but in those treated with clorgyline enhancement was more pronounced and included not only responses to tyramine but also those to phenylephrine and angiotensin. Because clorgyline inhibits MAO A whereas l-deprenyl inhibits MAO B our findings imply that enhanced pressor responsiveness depends on inhibition of MAO A rather than MAO B.

Studies on serum binding of some drugs.

Binding of three potential antidepressive compounds, EGYT-3615, EGYT-475 (trelibet) and EGYT-2760 to human serum was investigated by equilibrium dialysis method using radiolabelled drugs. All three compounds bound to serum proteins and the binding was sensitive to pH and salt concentration. The binding of trelibet and EGYT-3615 was not saturable at a wide range of drug concentration. The results show that EGYT-3615 has both specific and non-specific, while trelibet has only non-specific binding sites on serum proteins.

Measurement of tissue catecholamines of obese rats by liquid chromatography and electrochemical detection.

The content and turnover of norepinephrine was measured in the interscapular brown adipose tissue (BAT), white adipose tissue (WAT) and heart of lean and monosodium glutamate treated obese rats (GOR). An HPLC-ECD method was used for efficient separation and sensitive detection of catecholamines with an appropriate sample preparation, especially for lipid rich tissues. The lower basal norepinephrine level and the slower norepinephrine turnover in organs of the glutamate obese rats indicate a diminished activity of the sympathetic nervous system which may be a reason for a reduced thermogenic response and in this way contributes to the development of obesity.

Binding of (-) deprenyl to serum proteins.

14C-Deprenyl binding to serum proteins has been investigated using equilibrium dialysis and gel chromatography. Experiments in the equilibrium dialysis cells were performed both for binding of deprenyl to the serum proteins and the dissociation of the previously serum bound deprenyl. Comparative investigations were made with substance E, this reference compound showed reversible binding contrary to deprenyl that remained partially irreversibly bound to the serum proteins even in such conditions when substance E became totally released.

Effects of vinblastine on malondialdehyde formation, serotonin release and aggregation in human platelets.

Effects of the microtubular agent vinblastine on human platelet malondialdehyde formation, [14C]serotonin release and aggregation were studied in suspensions of [14C]serotonin-labelled platelets. Vinblastine caused dose-dependent inhibition of malondialdehyde formation and aggregation in platelet suspensions stimulated with thrombin, ADP or palmitaldehyde acetal phosphatidic acid (PGAP). Malondialdehyde formation, aggregation and [14C]serotonin release caused by threshold doses of thrombin were reduced but not abolished by 100 muM vinblastine; 30-100 muM vinblastine abolished ADP- and PGAP-induced malondialdehyde formation and [14C]serotonin released and transformed ADP- and PGAP-induced irreversible aggregation to a diminished reversible response. Arachidonate conversion to malondialdehyde catalysed by human platelet microsomes was inhibited by vinblastine and the cyclooxygenase inhibitors indomethacin and aspirin, but not by salicylate. Vinblastine inhibited the microsome-catalysed formation of malondialdehyde from prostaglandin H2. It is concluded that vinblastine inhibits the thromboxane pathway of arachidonate metabolism in stimulated platelets, consequently inhibiting release and aggregation, and that this effect of vinblastine may be, at least in part, independent of its antimicrotubular actions.

Opiate agonists inhibit noradrenaline release via opiate B-receptors and N-cyclopropylmethylnorazidomorphine (CAM) blocks electrically evoked contractions via histamine in the mouse vas deferens.

N-cyclopropylmethylnorazidomorphine (CAM) reduced slightly the stimulation evoked 3H-noradrenaline outflow from the mouse vas deferens and this effect was not antagonized by naloxone. Azidomorphine, however, inhibited potently the release of noradrenaline and the effect was completely antagonized by CAM and naloxone. The field stimulation induced release of acetylcholine remained unchanged in the presence of CAM. Atropine leaving the release of noradrenaline unchanged caused 50-60% inhibition of responses to electrical stimulation. Histamine and 48/80 inhibited the twitch responses to electrical stimulation and this effect was antagonized by H1-receptor antagonists (mepyramine, chloropyramine phenindamine, thenalidine, cyproheptadine), but not by cimetidine. H1-receptor antagonists increased the release of noradrenaline and facilitated neuromuscular transmission; cimetidine was ineffective. H1-blocking antihistamines completely inhibited the blockade of the electrically evoked contractions caused by CAM, whereas cimetidine left them unaltered. The effects of morphine and azidomorphine were only moderately antagonized by H1-blocking antihistamines and the effect of methionine enkephalin remained unchanged. In the presence of atropine and H1-blocking antihistamines CAM antagonized the effect of the opiate agonists competitively. It was concluded that in the mouse vas deferens opiate agonists inhibit noradrenaline release via opiate B-receptors. CAM inhibits the twitch responses in this test via histamine and H1-receptors were found to be involved in this mechanism.

A new model for the study of opiate B-receptors: isolated splenic strip of the cat.

Noradrenergic neurotransmission in the electrically stimulated isolated splenic strip of the cat was found to be modulated via opiate receptor. Pharmacodynamic parameters of morphine, methionine-enkephalin, azidomorphine, naloxone and N-cyclopropylmethyl-norazidomorphine (CAM) were determined. As CAM proved to be more potent than naloxone in this organ, the cat splenic strip is a useful model for the in vitro testing of opiate B-receptors.