Conscious Animal Model Research Articles

Fosinopril, a Phosphinic Acid Inhibitor of Angiotensin I Converting Enzyme: In Vitro and Preclinical In Vivo Pharmacology

Fosinopril is the first member of a new chemical class of angiotensin I (AI) converting enzyme (ACE) inhibitors, the phosphinic acids. In vitro, SQ 27,519, the active moiety of the prodrug fosinopril, was a more potent inhibitor of purified rabbit lung ACE- (IC50 = 11 vs. 23 nM) and bradykinin-induced contractions of guinea pig ileum than captopril. In vivo, SQ 27,519 was equipotent to captopril as an inhibitor of an AI pressor response after intravenous (i.v.) administration to conscious rats and monkeys but appeared to be less potent in conscious dogs. After oral administration, fosinopril again was equipotent to captopril as an inhibitor of an AI pressor response in rats and monkeys and slightly less potent in dogs. However, both SQ 27,519 (i.v. studies) and fosinopril (oral studies) had a longer effect than captopril in all three species. When fosinopril was administered orally for 5 days, its effects on an AI pressor response were the same on days 1 and 5, suggesting lack of tolerance to the compound. The ACE inhibitory effect of captopril, but not fosinopril, was prolonged in conscious rats with glycerol-induced acute renal failure, suggesting that fosinopril is excreted by an extrarenal route. Finally, fosinopril had no effect on the pressor or chronotropic effects of norepinephrine (NE) or 1,1-dimethyl-4-phenylpiperinium (DMPP) or electrical stimulation of the sympathetic ganglia of pithed rats. Fosinopril attenuated the pressor, but not the chronotropic effects of tyramine. We conclude that fosinopril is a potent and long-lasting inhibitor of ACE in conscious animal models that does not impair adrenergic function or reflexes.(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions between circulating peptides and the central nervous system in hemodynamic regulation

Enkephalins and endothelins are endogenous peptides, which, at least at pharmacologic doses, produce complex hemodynamic responses after intravenous administration. The enkephalins, when injected into conscious animal models and humans, increase blood pressure, heart rate and minute ventilation. This response occurs by activation of specific opiate receptors located outside the blood-brain barrier; the actual mechanism involves an increase in adrenergic autonomic nervous system tone and a decrease in cholinergic tone. These opiate receptors may activate afferent fibers, perhaps nicotinic cholinoceptors; in many ways their properties are suggestive of chemoreceptors. Furthermore, enkephalin responses appear to be modulated by gamma-aminobutyric acid complexes, in that the reversal of the excitatory hemodynamic responses seen in the conscious state to vasodepressor responses after barbiturate anesthesia may result from alteration of the state of activation of the gamma-aminobutyric acid complex. The enkephalin receptors are localized to the vertebral artery vascular distribution; the specific site may be the area postrema, a blood-brain barrier-deficient circum-ventricular organ demonstrated to modulate heart rate and blood pressure and to represent a target site for circulating angiotensin II. Endothelin increases heart rate and blood pressure when infused slowly into conscious or anesthetized dogs, although barbiturates do blunt the increase in heart rate. The mechanism appears to involve modification of autonomic tone, but also some element of direct vasoconstrictor activity. Interestingly, rapid bolus doses of endothelin produce only vasodepressor responses, suggesting that the rate and concentration at which circulating endothelin reaches afferent receptors or vasoconstrictor sites on vascular smooth muscle may determine the net hemodynamic response observed. At higher doses, both enkephalins and endothelins may produce emesis, an action often mediated by the area postrema. These observations suggest mechanisms by which circulating peptides may communicate with vasomotor control centers within the central nervous system and potentially modulate cardiovascular responses.

Distribution of blood flow in the muscles of conscious animals during exercise

This study reviews the recent research on the distribution of blood flow within and among skeletal muscles in conscious animals during locomotory exercise. A brief description of the technique that has been used in these studies, the radiolabeled microsphere method, is presented, including a short discussion of the assumptions that must be met in using the methodology. Distinct patterns of flow distribution occur in the muscles during exercise that are related to the fiber-type composition of the muscles or muscle parts (and, presumably, the patterns of fiber recruitment) and the intensity of the exercise. The mechanisms that regulate the differential elevations in blood flow within and among the muscles at the initiation of exercise have not been elucidated. However, studies on the conscious animal model indicate that the control cannot be totally explained by local metabolic factors, sympathetic influences or mechanical effects.

Long-term hypotensive effect of beta-agonist in conscious dogs.

The purpose of this study was to investigate the arterial pressure response to long-term administration of beta-agonists in the chronically instrumented conscious animal model. Chronically instrumented dogs were given intravenous infusions of ritodrine (2 micrograms.kg-1.min-1) for a period of 2 wk. Several cardiovascular and renal parameters were monitored before, during, and after the ritodrine infusion, and renal function curves were constructed. After the 1st wk of infusion, a new steady state was reestablished, and this was characterized by hypotension, reduced plasma protein concentration, elevated cardiac output, expanded extracellular fluid space, and near normal levels of activity of renin-angiotensin-aldosterone systems. The renal function curve during ritodrine infusion shifted to the left with no change in slope. We propose the following: 1) the persistence of hypotension is most probably related to the resetting of the arterial pressure-kidney blood volume servocontrol mechanisms, and 2) the persistent elevation of cardiac output and reduction in peripheral resistance are most probably related to increased oxygen and nutrient demand during beta-agonist infusions.

Conscious guinea-pig aerosol model for evaluation of peptide leukotriene antagonists

A new conscious animal model for evaluating leukotriene antagonists is described. The model consists of monitoring the change in the respiratory pattern induced by aerosol administration of various airway constrictors in six guinea pigs secured in a plexiglass chamber by a neck yoke. The animals are pretreated with indomethacin (10 mg/kg, i.p.) and propranolol (5 mg/kg, i.p.) 30 min prior to the challenge. After a 30-min stabilization period, the animals are challenged by various agonists delivered via a Monaghan ultrasonic nebulizer at a flow rate of 2.0 L/min for 5 min. The end point is defined as the onset of slow, labored abdominal breathing (dyspnea) measured in seconds. Peptide leukotrienes (LTs) (30 nM-60 μM) produced concentration-related decreases in time to dyspnea with a rank order of potency of LTD 4 > LTC 4 > LTE 4. LTD 4 was 1,000-fold more potent than histamine or carbachol. Pretreatment of the animals with either FPL55712 or LY171883 delayed the time to reach dyspnea induced by LTD 4. In contrast, pyrilamine, cyproheptadine, and phenoxybenzamine failed to alter LTD 4-induced dyspnea. The results indicate that this model is useful in assessing the efficacy of LT receptor antagonists in vivo.

Thoracic epidural anaesthesia in conscious and anaesthetized rats. Effects on central haemodynamics compared to cardiac beta adrenoceptor and ganglionic blockade

A simple technique for cannulation of the thoracic epidural space in rats was described. 40-50 microliter of epidural bupivacaine 5 mg/ml induced a distribution of sensory analgesia from lower cervical to lower thoracic segments. With this model, effects of thoracic epidural anaesthesia (TEA) on mean arterial pressure (MAP), cardiac output (CO), systemic vascular resistance (SVR), stroke volume (SV), heart rate (HR), central venous pressure (CVP), left ventricular end-diastolic pressure (LVEDP) and maximal increase of pressure in the left ventricle (max dp/dt) were studied in six groups of animals: 1) In conscious animals (n = 10) MAP, CO, SV and HR decreased significantly by 12%, 25%, 10% and 16%, respectively, while SVR increased significantly by 20% during TEA; 2) In chloralose-anaesthetized animals (n = 7) the reduction in CO during TEA was less pronounced and there were no significant changes in SV or SVR; 3) In conscious animals (n = 6) LVEDP, CVP and max dp/dt decreased significantly during TEA; 4) Hexamethonium, when administered to pharmacologically vagotomized conscious animals during TEA (n = 8), induced a significant decrease in SVR (23%) but no change in HR; 5) Changes in haemodynamics after cardiac adrenoceptor blockade with metoprolol, in conscious animals (n = 12), did not differ significantly from those seen during TEA, except for an unchanged SV after metoprolol; 6) 50 microliters of bupivacaine (5 mg/ml) when given i.v. to conscious animals (n = 8) did not affect CO, SV, HR or TPR significantly, while MAP increased slightly but significantly. Thus, in this conscious animal model, TEA almost completely and rather selectively blocked probably mediated by a reflex activation of unblocked sympathetic efferents.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracoronary alpha 2-adrenergic receptor blockade attenuates ischemia in conscious dogs during exercise.

Studies on the role of alpha-adrenergic constrictor tone in the coronary vascular bed during ischemia were performed in dogs running on a treadmill. The animals were instrumented with a left ventricular pressure transducer, and regional systolic wall thickening (%WTh) was assessed by sonomicrometry in the anterior and posterior walls of the left ventricle. An intracoronary catheter was implanted chronically in the circumflex coronary artery, and a hydraulic cuff was placed proximally around the artery. After beta-adrenergic blockade with propranolol (0.8 mg/kg i.v.), acute stenosis of the coronary artery was performed during running in five dogs to induce severe regional myocardial dysfunction in the posterior wall. Intracoronary infusion of the selective alpha 2-adrenergic blocking agent idazoxan (80 micrograms/kg) improved %WTh in the ischemic region from 5.1 +/- 1.6 to 10.8 +/- 2.8% (p less than 0.05), without any significant effect on the anterior wall. Blood flow to the subendocardium of the posterior wall (radioactive microspheres) increased from 0.17 +/- 0.05 to 0.45 +/- 0.30 (ml/min)/g (p less than 0.05). It is concluded that in exercising dogs subjected to beta-adrenergic blockade, significant postjunctional alpha 2-adrenergic receptor-mediated coronary vasoconstriction exists, even during severe ischemia. Regional alpha 2-adrenergic receptor blockade can reduce regional ischemia and improve contractile function by attenuating exercise-induced sympathetic vasoconstriction in this conscious animal model.

Multiunit electrical activity in conscious rats during an immune response

Mechanisms by which the central nervous system may be influenced during the course of an immune response probably exist but remain obscure. In an attempt to determine any neurophysiological changes during such responses, we have employed a conscious animal model bearing chronically implanted recording electrodes in the preoptic area/anterior hypothalamus ( PO AH ) and hypothalamic paraventricular nucleus (PVN). Rats were sensitized to sheep red blood cells (SRBC) injected intraperitoneally. Basal PO AH multiunit activity (MUA) increased significantly to a maximum 5 days after SRBC injection and correlated with the initial appearance of anti-SRBC serum antibodies. Significant decreases in PO AH MUA were recorded on Days 3 and 8 following the sensitization. PVN MUA decreased significantly for the first 3 days following immunization and then returned to a basal rate before increasing on Day 6. On the ninth and tenth days following the SRBC injection, both PO AH and PVN MUA levels had returned to those recorded before immunization. A further group of animals was examined for PO AH MUA changes during induction of a secondary response to SRBC. Firing rates increased significantly between Days 4 and 9 following the injection, the maximum increase being on Day 6. The profile of this response was different from that recorded during the first response, with no decreases recorded. The results are discussed in terms of neuroimmunomodulatory mechanisms such as those influencing neuroendocrine secretory function.

Telemetry of electrophysiologic variables from conscious dogs: System design, validation, and serial studies

The necessity for conscious animal models in the study of cardiac physiology has been established for hemodynamics. The characterization of the electrophysiologic properties of the heart has not been performed in a serial fashion in a conscious animal model. Implanted telemetry sensing devices for recording atrial, ventricular, and His electrograms and stimulation systems for both atrium and ventricle allowed serial evaluation of eight dogs for up to 4 months. There were significant fluctuations in some electrophysiologic variables with time, particularly heart rate, sinus node recovery time, and pacing rate for induction of Wenckebach block. This variability did not appear to deviate with time after implantation, and no significant differences in the basal electrophysiologic state were found between the period early after surgery ( 0 to 2 1 2 weeks ) and later (3 to 8 weeks).

Ventricular refractory periods in relation to rate and test-site VT intervals in anesthetized and conscious dogs: a canine model for conscious state measurements.

The functional relationship of ventricular effective refractory period (ERP) with basic cycle length (BCL) of stimulated ventricular depolarization and VT intervals of test-site unipolar ventricular electrograms were studied in five conscious and eight anesthetized (sodium pentobarbital, 30 mg/kg) open-chested dogs. The range of BCLs studied was 300 to 1000 ms, achieved through ventricular stimulation following chemically-induced complete AV block. In the conscious animal model developed for this study, two-to-four ventricular electrode leads were exteriorized and the ventricular rate was maintained by an implanted programmable VVI pacemaker. In all animals studied, the BCL-ERP relation was closely represented by the empirical equation ERP = A-B.Exp(-k.BCL), and the VT-ERP relation over the same range of BCLs was linear: ERP = C + D.VT. The correlation coefficients were in the range of 0.991 to 0.999. The mean values of the parameters in the above equations determined by the appropriate non-linear or linear regression analysis showed significant differences between the two groups of animals studied. In three conscious animals a strong linear correlation between the test-site VT intervals and simultaneously measured QT intervals measured from the lead II surface electrogram was demonstrated (r = 0.993 to 0.998). For a fixed site of stimulation, the morphology of ventricular depolarization complexes as well as the corresponding T-waves remained essentially unaltered with BCL for both myocardial and surface electrograms. The possible applications of developed canine model and the results of the present study include: (1) the study of the rate-dependent effects of cardioactive drugs on ventricular electrophysiology and, (2) the improved design of electronic refractory periods of rate programmable pacemakers.

Biochemical and physiologic effects of thyroid hormone on cardiac performance

Recognition of the existence of ventricular myosin isozymes and their regulation by thyroid hormone may provide a biochemical basis for some of the effects of the hormone on myocardial performance. The isozymes, which are referred to as V 3, in order of decreasing electrophoretic mobility and ATPase activity, contain the same light chain components, but differ in their heavy chain composition. V 1 contains two A heavy chains, V 3 two B heavy chains, and V 2 one heavy chain of each type. Rabbit and several other mammalian species, including man, have a predominance of the low activity V 3 form. The relative proportions of these myosin forms seems consistent with a random association of the heavy chain types. Thyroid hormone administration leads to a predominance of the high activity V 1 form of the enzyme, indicating that A chain synthesis has been stimulated and B chain synthesis suppressed. By contrast, the V 1 form normally is predominant in rat ventricle; thyroid ablation procedures lead to expression of the V 3 form, indicating stimulation of B chain synthesis and suppression of A chain synthesis. Since the ATPase activity of myosin seems closely related to the intrinsic speed of cardiac contraction, a change in myosin isozyme composition from V 3 to the V 1 may explain the increase in velocity of shortening observed in papillary muscles from thyrotoxic cat and rabbit hearts. Similarly, the switch from V 1 to V 3 may account for the decrease in performance of hypothyroid rat cardiac muscle. However, in man and other species which normally have a predominance of the V 3 form, the reported changes in myocardial performance in hypothyroidism may be related to other factors, including alterations in electro-mechanical coupling, myxedematous infiltration and coronary atherosclerosis. Assessments of LV performance in thyrotoxic subjects and in conscious animal models have indicated that LV wall velocity is not usually greater than normal because of the increase in afterload. Although LV systolic and diastolic dimensions are often increased, ventricular filling pressures remain normal, and there is no evidence of sarcomere stretching or disruption. Acute B-adrenergic blockade has very little influence on any of these alterations in LV performance. Furthermore, animal experiments and computer simulation of the thyrotoxic circulatory system indicate that there are important changes in the peripheral vascular compartments as well as in the heart. These alterations, which include a decrease in peripheral arterial resistance and an increase in P ms, should be regarded as essential features of the high-output state associated with thyrotoxicosis. A change from the V 1 to the V 3 form also has been shown to occur in the rat ventricle during development, aging, chemically-induced diabetes, and in hemodynamic overload. In some cases, these changes seem to be associated with alterations in plasma thyroid hormone concentration. Also, in diabetes and pressure overload, the changes in myosin isozyme composition can be reversed by thyroxine treatment. Taken together, the experimental data suggest that thyroid hormone may play at least a permissive role in regulating ventricular myosin isozyme expression in many nonthyroidal cardiac conditions. The potential to alter myosin isozyme composition constitutes an important area for future research.

Detection of latent coronary stenosis in conscious dogs: regional functional and electrocardiographic responses to isoprenaline.

A conscious animal model was developed in which coronary stenosis could be produced while regional myocardial function and local surface electrocardiograms were measured. Responses to isoprenaline stress in the presence of mild (latent) coronary stenosis were then examined. In the absence of coronary stenosis, isoprenaline produced increases in regional function and no change in the surface VCG; at higher doses, increases in the endocardial ST segments occurred. After partial coronary stenosis, which produced no apparent regional dysfunction or electrocardiographic changes, isoprenaline infusion for 3 min (0.02 microgram . kg-1 . min-1) rapidly produced decreases in percentage wall thickening (average 17 +/- 4%, mean +/- SE, P < 0.01) and increases in the mean sum of VCG ST segments by 0.23 +/- 0.06 mV (P < 0.05). 1 min after stopping isoprenaline, most dogs showed further significant deterioration of both measures of ischaemia, but by 5 min there was no significant mean change from control. We conclude that in the presence of latent partial coronary stenosis, stress due to mild sympathomimetic stimulation alone can rapidly induce regional myocardial ischaemia. Deterioration of regional myocardial contractile function during such stress can provide as sensitive means of detecting latent coronary obstruction.

An effective conscious animal model of atrial fibrillation.

In 11 fully conscious dogs with chronically implanted spicardial electrodes, 50 Hz sine-wave electrical stimulation of the left atrium reliably and repeatedly produced an arrhythmia which was indistinguishable from atrial fibrillation in terms of its ECG appearance, the statistical properties of the ventricular response and responses to a number of pharmacological agents. In five of the 11 preparations this arrhythmia consistently persisted for periods in excess of 10 min following the cessation of stimulation, indicating an intrinsic cardiac basis for the arrhythmia and suggesting that it is true atrial fibrillation. A close correlation between the ventricular response to this arrhythmia and the degree of atrioventricular conductivity, together with the observation that dramatic changes can occur in ventricular responses without corresponding changes in atrial activity, suggest that the ventricular response is mainly a function of the atrio-ventricular conducting system. The experimental model described is useful for the study of both short and long term drug action no atrial fibrillation and for the evaluation of methods used therapeutically in attempts to terminate episodes of paroxysmal atrial fibrillation.