Suitability of urethane anesthesia for physiopharmacological investigations in various systems. Part 2: Cardiovascular system

Reversibility of the haemodynamic effects of anabolic steroids in rats

The haemodynamic effects of 6 weeks nandrolone decanoate treatment (total dose 30 mg.kg-1) (SG I, n = 12) and their reversibility were studied in anaesthetised, open-chest male rats exposed to 5 min isoproterenol (2.5 micrograms.kg-1) and CaCl2 (25.0 mg.kg-1) loads. In SG I, the heart weight and its ratio to body weight were greater than in the untreated rats (CG I, n = 13) (p less than 0.05 and p less than 0.01 respectively). The initial heart rate and the inotropic and chronotropic responses to isoproterenol were lower in SG I than in CG I (p less than 0.05 in all cases). Peripheral resistance decreased during both infusions in SG I but remained unaltered in CG I (p less than 0.05). 6 weeks after finishing anabolic steroid treatment (SG II, n = 11), in the CaCl2-test the ejection fraction (p less than 0.05) and stroke index were smaller than in control rats of the same age (CG II, n = 12). Mean aortic pressure was lower in SG II than in CG II. In the CaCl2-test peripheral resistance was initially higher, but decreased during the infusion in SG II while it increased in CG II (p less than 0.05 in both cases). In conclusion, anabolic steroid treatment reversibly reduces the left ventricular response to isoproterenol. It decreases peripheral vascular tone during inotropic loads. Six weeks after the cessation of treatment, the pumping efficiency of the heart is reduced.

Experimental diabetes in the rat: alterations in the vesical function

Vesical function has been studied in vivo in alloxan-diabetic rats. Rhythmic contractions evoked as a micturition reflex by a controlled distension of the bladder were recorded in urethane-anesthetized animals. These rhythmic contractions are essentially induced by the parasympathetic and inhibited by the sympathetic outflow. The study has been performed on male Sprague-Dawley rats. They were treated at the age of two months with vehicle (control group) or alloxan (100 mg/kg s.c.). Such treatment caused hyperglycemia (greater than 250 mg/dl) and glycosuria in most of the animals. In control animals, analyzed between 5 and 8 months of age, the contractions evoked by vesical distension appeared at a threshold volume between 0.5 and 1 ml, and they had a regular rate. In a first series of experiments, diabetic animals 3 months after treatment showed the evoked response at an average threshold volume of about 1.7 ml. while in animals with 6 months of experimental diabetes the average threshold volume was about 3.2 ml. The rate of contractions in diabetic animals became more and more irregular as the duration of diabetes increased. These marked functional alterations were associated with the evidence of bladder enlargement. In a second series of experiments, bovine brain gangliosides (10 mg/kg i.p./day) or saline were given to diabetic rats between the 30th and the 90th day after alloxan. Ganglioside administration significantly reduced bladder enlargement and the threshold volume for micturition reflex and improved the rate of bladder contractions. It is suggested that alterations of the autonomic nervous system can be expressed as functional vesical dysfunction in experimental diabetes.

Cardiotoxicity testing of organic solvents by coronary artery ligation in closed-chest rats

The effect in coronary artery occlusion was investigated in closed-chest anesthetized (urethane, pentobarbital, ether) and conscious rats. The parameters measured were: arrhythmia development, heart rate changes and mortality. Lowest mortality rate and less severe arrhythmias were observed in the urethane group. In urethane-anesthetized rats single oral administration of carbon disulphide (8.3 mmol/kg), chloroform (5.7 mmol/kg), trichloroethylene (16.7 mmol/kg) and ethanol (174 mmol/kg), respectively, 1 h prior to coronary occlusion reduced survival rate accompanied by marked influences on arrhythmia development. The coronary occlusion method in closed-chest rats under urethane-anesthesia appears to be a valuable tool for investigating cardiotoxic effects of chemicals.

Evidence for medetomidine as a selective and potent agonist at alpha 2-adrenoreceptors

The activity on alpha-adrenoreceptors of medetomidine ((+/-)-4-(alpha,2,3-trimethylbenzyl)imidazole), an alpha-methyl derivative of detomidine, has been characterized in vivo and in vitro using detomidine, MPV 207, MPV 295, azepexole, clonidine and xylazine for reference purposes. Medetomidine (1-100 micrograms/kg i.v.) was a hypotensive and bradycardic compound in anaesthetized rats. Furthermore, it induced vasopressor (PD50 1.7 microgram/kg) and sympatho-inhibitory (ID50 1.6 microgram/kg) actions in pithed rats, the effects being antagonized by idazoxan (0.3 mg/kg i.v.) but not by prazosin (0.1 mg/kg i.v.). Medetomidine (30-300 micrograms/kg i.m.) had an alpha 2-adrenoreceptor mediated sedative effect on chicks. Medetomidine was, overall, more potent than detomidine, MPV 207, clonidine, xylazine, MPV 295 or azepexole in central (sedation in the chick) and peripheral (cardiac presynaptic in the pithed rat) actions on alpha 2-adrenoreceptors. Clonidine had, however, about an equal potency to medetomidine in the vascular smooth muscle of the pithed rat. Like detomidine and MPV 295, medetomidine had no agonistic activity in the rat aortic ring, but high concentrations antagonized methoxamine-induced contractions, giving a pA2 value of 5.68 for alpha 1-adrenoreceptor antagonism. The overall lipophilicity (log P') of medetomidine in the octanol/buffer (pH 7.4, 24-26 degrees C, HPLC technique) was 2.80. In summary, the experimental data suggest that medetomidine is a lipophilic compound with selective alpha 2-adrenoreceptor-stimulating properties and high potency. It may, therefore, prove to be a suitable pharmacologic tool for interventions in alpha 2-adrenoreceptor mediated effects in the autonomic nervous system.

Differences in cardiovascular responses to peripherally administered GABA as influenced by basal conditions and type of anaesthesia

The cardiovascular (blood pressure, heart rate, cardiac contractility) effects of i.v. gamma-aminobutyric acid (GABA) were investigated in guinea-pigs anaesthetized with barbitone or urethane. GABA (0.1-10 mg kg-1) produced a transient 'depressive' effect on cardiovascular parameters which in barbitone-anaesthetized animals was followed by a transient 'excitatory' effect. Resting cardiovascular parameters were higher in urethane-as compared to barbitone-anaesthetized animals. Picrotoxin pretreatment (2 mg kg-1, i.v.) barely affected the cardiovascular changes produced by GABA in barbitone-anaesthetized animals. In picrotoxin pretreated animals anaesthetized with urethane, GABA produced an initial depression of cardiovascular parameters followed by an excitatory phase. Hexamethonium (20 mg kg-1, i.v.) suppressed or reduced markedly the GABA-induced cardiovascular changes both in barbitone- or urethane- anaesthetized animals. Reserpine pretreatment lowered resting cardiovascular parameters. In these animals, regardless of type of anaesthesia, the effects of i.v. GABA were of the 'excitatory' type only. Reserpine pretreated animals anaesthetized with barbitone were selected for further experiments. Various GABAA receptor agonists (homotaurine, muscimol, THIP, 5-aminovaleric acid) mimicked the 'excitatory' effect of GABA in reserpine pretreated animals anesthetized with barbitone and prevented the effects of subsequent GABA administration. On the other hand (+/-)-baclofen, a selective GABAB receptor agonist, had a slight depressant effect and did not prevent the 'excitatory' cardiovascular effects of GABA. Neither bicuculline nor picrotoxin pretreatment prevented the 'excitatory' cardiovascular effect of i.v. GABA in reserpine pretreated, guinea-pigs anaesthetized with barbitone. In adrenalectomized guinea-pigs or in preparations receiving i.v. phentolamine plus propranolol, GABA produced only a small 'depressant' effect on cardiovascular parameters. These findings demonstrate that GABA exerts a neuromodulatory effect on cardiovascular function via peripheral actions which is influenced by: type of anaesthesia resting values of cardiovascular parameters degree of activity of the sympathetic nervous system and catecholamine release from the adrenal medulla.

Suitability of urethane anesthesia for physiopharmacological investigations. Part 3: Other systems and conclusions

The suitability of urethane anesthesia for physiopharmacological experiments in various systems is briefly reviewed. Urethane anesthesia appears to be suitable for various types of studies on respiratory function and on reflex activation of motility of the urinary bladder and some sections of the intestinal tract. However, urethane produces a variety of potentially disturbing side-effects at endocrine and renal level.