Estimation of minimum alveolar concentration (MAC) for halothane, enflurane and isoflurane in spontaneously breathing guinea pigs

MAC for halothane, enflurane and isoflurane was determined in guinea pigs (Cavia porcellus) exposed to constant anesthetic concentrations (2.5 hours each) in a flow-through glass chamber. The following values were obtained (N = 8 for each anesthetic): 1.01 +/- 0.03 vol% for halothane, 2.17 +/- 0.04 vol% for enflurane, and 1.15 +/- 0.05 vol% for isoflurane. In guinea pigs, MAC for halothane and enflurane are similar to those reported for other rodents, while MAC for isoflurane is lower. The data indicate that guinea pigs possibly are more susceptible to isoflurane's anesthetic actions than other rodents.

Cardiovascular and lethal effects of cocaine in anesthetized dogs and guinea-pigs

Cardiovascular and lethal effects of i.v. cocaine infusion were studied in anesthetized mongrel dogs and Hartley guinea-pigs. Dogs were anesthetized with enflurane (2.25 vol%) or urethane (1500-1700 mg/kg i.v.) and guinea-pigs with urethane (1000 mg/kg i.p.). The rate of i.v. cocaine infusion was 0.91 mg/kg/min for dogs and 1.9 mg/kg/min for guinea-pigs. In spontaneously breathing dogs and guinea-pigs, death occurred by respiratory arrest, while in artificially ventilated dogs, the terminal stage of cocaine intoxication was peripheral vascular failure. Disturbances in cardiac rhythm (AV-dissociation, ectopic pacemaking activity or irregular rate) were not observed unless mean arterial blood pressure fell to levels inadequate for coronary perfusion. All animals showed an increase in the atrio-ventricular conduction interval (PR-interval). Cardiac arrest (fibrillation or standstill) was never the primary cause of death. Sympathomimetic actions of cocaine, as monitored by heart rate (HR) and mean arterial blood pressure (MAP), were not observed in dogs, while guinea-pigs exhibited only a consistent rise in MAP which was not accompanied by other signs of sympathetic activity. Changes in body temperature were not observed in either species. It is concluded that in anesthetized animals, the predominant cardiovascular and lethal effects of cocaine are the result of its local anesthetic (membrane-stabilizing) action, and that the contribution of its sympathomimetic effects, due to inhibition of neuronal uptake, is masked by its local anesthetic properties.

Effects of buffer magnesium on positive inotropic agents in guinea pig cardiac muscle

Experiments examined effects of extracellular Mg2+ concentration (Mgo2+) on dose-dependent actions of strophanthidin, norepinephrine, Bay K-8644 and extracellular Ca2+ (Cao2+) in electrically stimulated atrial and ventricular muscle isolated from guinea pig heart. Mgo2+ itself elicited a concentration-dependent negative inotropic effect. Elevation of Mgo2+ between 0.6 and 12 mM increased the concentration of strophanthidin necessary to produce its toxic effects without affecting the maximum developed tension prior to toxicity. Similarly, Mgo2+ did not alter the maximum contractile force elicited by cumulative addition of norepinephrine, Bay K-8644 or Cao2+, but increased their ED50 values. These data suggest that interactions between Mgo2+ and the four positive inotropic agents were not mediated by effects on receptor binding or Na+,K+-ATPase, but rather by alterations at one or more steps involved in excitation-contraction coupling.

Aging: effects on minimum alveolar concentration (MAC) for halothane in Fischer-344 rats

It is well-established that the anesthetic requirement (MAC) of volatile agents such as halothane is reduced in elderly patients. The current project was designed to determine whether a similar alteration in anesthetic requirement occurs in Fischer-344 (F-344) rats, an animal model often utilized in physiology and pharmacology to examine aging-related changes. Animals were exposed to increasing or decreasing levels of halothane. After equilibration at each concentration, the response to tail-clamping was used for MAC testing. MAC was reduced approximately 17% in aged (25 months) versus young adult (5 months) animals. From these data, it is concluded that the F-344 rat may be an adequate model for examination of age-dependent alterations in the actions of volatile anesthetics.

Effects of morphine pretreatment on cocaine cardiotoxicity in anesthetized guinea-pigs

Cardiovascular and lethal effects of intravenous cocaine infusion were compared in guinea-pigs pretreated chronically with morphine or saline. Alzet minipumps filled with either morphine solution (30 micrograms/kg/hr for 6 days) or isotonic saline were implanted subcutaneously in animals 6 days before cocaine infusion. On the morning of the 7th day, animals were anesthetized with urethane (1000 mg/kg i.p.) and an i.v. infusion of cocaine was begun at a rate of 1.9 mg/kg/min. All animals were spontaneously breathing room air throughout the experiments. Death occurred by respiratory arrest in all animals; however, there was a significant decrease in the time to respiratory arrest in the morphine-pretreated guinea-pigs. All animals showed a gradual increase in the atrio-ventricular conduction interval (PR-interval) and in mean arterial pressure up until the time that respiration ceased. There were no disturbances in cardiac rhythm before respiratory arrest. This suggests that the predominant cardiovascular and lethal effects of cocaine in urethane-anesthetized guinea-pigs are the result of its local anesthetic (membrane stabilizing) action, and that morphine potentiates this effect of cocaine, but does not alter the pattern of cocaine toxicity.

Aging: ouabain-sensitive 86Rb+ uptake rate and responsiveness to digoxin in rat left atrial muscle

Previous work in anesthetized rats has demonstrated that the sensitivity to cardiotoxic actions of cardiotonic steroids is increased in senescence, and studies in crude homogenates and partially purified membrane preparations have suggested that this altered responsiveness is related to an aging-associated reduction in the sarcolemmal content of Na,K-adenosine triphosphatase. This decrease in Na,K-adenosine triphosphatase could enhance the sensitivity to digitalis-like compounds by reducing the reserve capacity of the Na+-pump and thus the extent of digitalis-induced pump inhibition required before the onset of toxicity. Current experiments examined dose-dependent actions of digoxin in atrial muscle isolated from 3-, 12- and 24- to 25-month-old rats and determined if alterations in responsiveness correlated with changes in ouabain-sensitive 86Rb+ uptake rate, an estimate of Na+-pump activity. Atrial preparations from aged rats were more sensitive to the cardiotoxic actions of digoxin; however, the inotropic efficacy before the onset of toxicity was not affected by age. Both 1) the maximum attainable ouabain-sensitive 86Rb+ uptake rate and 2) the difference between maximum uptake rate and that monitored in preparations stimulated at 4.0 Hz decreased progressively with age. These results indicate that atrial muscle from aged rats is more sensitive to direct toxic effects of digoxin and suggest that this lower tolerance is mediated, at least in part, by a reduction in Na+-pump reserve capacity.

Interaction of BAY K-8644 with effects of digoxin in the dog heart-lung preparation

Concentration-dependent effects of BAY K-8644, a dihydropyridine calcium channel agonist, on heart rate (HR), left atrial pressure (LAP) and maximal rate of left intraventricular pressure rise (+dP/dtmax) were compared with those of isoproterenol and digoxin in dog heart-lung preparations during exposure to 0.8 vol% halothane. All three agents reduced LAP to nearly equal levels at maximally effective concentrations. Maximal increases in +dP/dtmax induced by BAY k-8644 and digoxin were of similar magnitude (35 and 31%, respectively), and both agents increased HR slightly. Isoproterenol elevated both +dP/dtmax and HR more than twice as much as the other agents. At a concentration which produced only small changes by itself, BAY K-8644 markedly increased the effects of digoxin on LAP and +dP/dtmax. Furthermore, the same concentration of BAY K-8644 significantly reduced toxic effects of digoxin (arrhythmogenesis, elevation of LAP, reduction in systemic output). These data indicate that in the isolated blood-perfused heart digoxin's positive inotropic action may be enhanced by an appropriate concentration of BAY K-8644, while adverse effects are diminished.

Effect of stimulation frequency on intracellular Na+ activity in rat atrial muscle

Mean intracellular Na+ activity (aNai) was measured in rat left atrial muscle stimulated at increasing frequencies between 0 and 12 Hz. Low-pass filtered signals from conventional and ion-selective microelectrodes were used to determine aNai. Preparations were bathed in a low Ca2+ (0.1 mM) Krebs-Henseleit buffer containing 1.0 mM Mn2+ to abolish contractile motion and permit stable impalements. Under these conditions, aNai increased progressively with frequency from 5.8 +/- 1.5 mM at 0 Hz to a maximum of 12.7 +/- 2.1 mM, which was observed at 10, 11, or 12 Hz. Further increases in frequency exceeded the effective refractory period, and aNai tended to decrease. These data suggest that aNai can be approximately doubled in rat atrial muscle by increasing the depolarization rate from 0 to 10-12 Hz, a range that has been shown to elicit a two- to three-fold elevation in Na+-pump activity in similar preparations.

Myocardial recovery from the cardiac depressant effects of enflurane and halothane

Recovery from the cardiac depressant effects of enflurane and halothane was examined in the dog heart-lung preparation (HLP) and in right ventricular muscle isolated from guinea pig hearts. In the HLP. recovery was studied under two conditions: (1) After a two-hour exposure to anesthetic concentrations increasing from 0.36 to 1.2 MAC, and (2) after a one-hour exposure to a single concentration that raised the left atrial pressure (LAP) to 9 to 10 mmHg. Under either condition, +dP/dtmax. was significantly less depressed with enflurane and returned to preanesthetic control levels, while recovery with halothane remained significantly below control. Following the longer exposure. recovery of the LAP and left ventricular function curves (LVFC) was significantly less with halothane; however, this difference was not observed after the shorter exposure period. In electrically paced, isometrically contracting right ventricular strips exposed for one hour to 2.25 vol% enflurane (a concentration that reduced contractility by 45%), force development returned within 60 minutes to values above preanesthetic control values. After an identical depression for one hour with halothane (0.80 vol%), force development recovered to values less than those observed following enflurane. These data indicate that the recovery from anesthetic-induced negative inotropic effects in isolated cardiac preparations is better with enflurane than halothane.

Effects of theophylline on inotropic and arrhythmogenic actions of cardiotonic steroids in guinea pig cardiac muscle

This study was designed to examine effects of theophylline, a methylxanthine, on both the positive inotropic and toxic actions of cardiotonic steroids in cardiac muscle isolated from guinea pig heart. In electrically paced left atrial muscle, 0.3 mM theophylline reduced both the maximum developed tension observed in the presence of increasing concentrations of strophanthidin and the dose of this steroid that first elicited extrasystoles. Similarly, 0.3 mM theophylline decreased the time to onset of arrhythmias produced by 5 microM digoxin and the fractional occupancy of specific binding sites on Na,K-adenosine triphosphatase by digoxin at the onset of these dysrhythmic events. A higher level of theophylline (6.5 mM) severely diminished or prevented the positive inotropic and arrhythmogenic actions of cardiotonic steroids while promoting the contracture elicited by these digitalis-like compounds. In spite of the severe contracture observed in the presence of 6.5 mM theophylline plus 5 microM digoxin, the digoxin fractional occupancy was significantly less than that observed at the onset of digoxin-induced extrasystoles and contracture in the absence of theophylline. In radiolabeled ligand binding experiments, 6.5 mM theophylline reduced the affinity of specific binding sites for ouabain while having no effect on receptor density. These results, when considered in light of previous reports by other investigators, suggest that moderate concentrations of methylxanthines promote cardiotonic steroid-induced arrhythmias by increasing Ca++ influx and its uptake into sarcoplasmic reticulum. Higher levels seem to antagonize the arrhythmogenic actions by inhibition of sarcoplasmic reticular Ca++ uptake and by antagonism of receptor binding.

Myocardial responsiveness to isoproterenol and calcium: a comparison of SD and F344 rats

1. Inotropic effects of isoproterenol and extracellular Ca2+ were compared in left atrial muscle isolated from F344 and SD rats. Preparations from the F344 strain were more sensitive to the actions of both agents. 2. The chronotropic action of isoproterenol was not different in right atria isolated from the two strains. 3. This suggests that the strain-related difference in responsiveness to the inotropic effect of isoproterenol is not caused by heterogeneity in the beta-adrenoceptor/adenylate cyclase system but rather by variations in excitation-contraction coupling.

Comparison of cardiac effects of enflurane, isoflurane, and halothane in the dog heart-lung preparation

Minimum alveolar concentration (MAC) was determined in intact dogs (N = 10 for each anesthetic) to be 2.12 +/- 0.04 vol% for enflurane (ENF), 1.28 +/- 0.04 vol% for isoflurane (ISO), and 0.94 +/- 0.03 vol% for halothane (HAL). Then, the direct cardiac effects of these three anesthetics were studied at 0.36, 0.6, 1.0, and 1.2 MAC in the dog heart-lung preparation (HLP): an in situ whole heart preparation devoid of major extracardiac influences and reflex control. All three agents produced concentration-dependent decreases in heart rate (HR) that became significantly different from control at 0.6 MAC. HAL and ISO reduced +dP/dtmax by the same degree at all MAC levels, becoming statistically significant at 0.6 MAC, while a significant reduction in +dP/dtmax for ENF occurred first at 1.0 MAC. Marked increases in left atrial pressure (LAP) were observed at 1.0 MAC for all anesthetics and the first significant depression of systemic output (SO) occurred at 1.0 MAC. Each agent produced significant shifts of the left ventricular function curves (LVFC) to the right with each consecutive MAC fraction. Marked reductions in the slope of the LVFC were first observed at 1.0 MAC, and this change in slope was more pronounced with ENF. At 1.2 MAC, ENF seemed to produce a more severe cardiodepression than HAL or ISO, as suggested by a greater incidence of cardiac failure; however, this was not statistically significant. In general, the data suggest that at MAC fractions up to 0.6, ENF is less cardiodepressant than ISO or HAL, but that ENF has a tendency to be more depressant than HAL or ISO at concentrations higher than 1.0 MAC.

Effects of chronic delta-9-THC treatment on cardiac beta-adrenoceptors in rats

This study was designed to determine if chronic treatment with delta-9-tetrahydrocannabinol (THC) alters cardiac beta-adrenoceptors in the rat. Following daily oral administration of 10 or 20 mg/kg THC or an equivalent volume of control solvent for 90 days, rats were sacrificed, and sarcolemmal membranes were prepared from ventricular myocardium. Beta-adrenoceptor density and binding affinity estimated with (-)[3H]dihydroalprenolol; a beta-adrenergic antagonist, were not significantly affected by treatment with THC when compared to vehicle controls. These results suggest that the tolerance to cardiovascular effects of THC which develops during chronic exposure in the rat is not associated with alterations in cardiac beta-adrenoceptors as monitored by radiolabeled antagonist binding.

Influence of BAY K-8644 on positive inotropic agents in guinea pig atrial muscle

This study examined the influence of BAY K-8644, a dihydropyridine Ca2+ agonist, on the positive inotropic effects of strophanthidin, isoproterenol, methoxamine and extracellular Ca2+ (Ca2+0) in atrial muscle isolated from guinea pig heart. BAY K-8644 enhanced both the maximum developed tension observed in the presence of strophanthidin and the sensitivity to its toxic effects. The maximum contractile force observed in the presence of methoxamine was also elevated by BAY K-8644 pretreatment; however, the ED50 value for methoxamine was not affected. The maximum contractile force elicited by BAY K-8644 alone or by strophanthidin or methoxamine in combination with BAY K-8644 was approximately the same as that produced by isoproterenol alone. The Ca2+ agonist did not alter the maximum developed tension elicited by increasing concentrations of isoproterenol or Ca2+0; however, it reduce both the ED50 for Ca2+0 and the concentration of isoproterenol necessary to produce maximum contractility. These results suggest that combinations of BAY K-8644 and cardiac glycosides can elevate contractile force to a level greater than that produced by cardiac glycosides alone.

Effects of norepinephrine on BAY K-8644-induced contraction in porcine coronary artery

BAY K-8644 is a dihydropyridine calcium agonist which produces contraction in several isolated vascular preparations--including porcine coronary artery. This study examined the ability of norepinephrine to relax porcine coronary artery previously contracted with BAY K-8644 and compared this to effects on preparations contracted with histamine. Norepinephrine relaxed BAY K-8644 and histamine-treated preparations to approximately base line developed tension. The ED50 value for norepinephrine was the same in the presence of increasing concentrations of either BAY K-8644 or histamine.

Effects of ryanodine on inotropic and arrhythmogenic actions of cardiotonic steroids

In addition to its role in myocardial excitation-contraction coupling, the sarcoplasmic reticulum may be involved in arrhythmogenic actions of cardiotonic steroids. This study in isolated cardiac muscle demonstrates that ryanodine can prevent the arrhythmias and reduce the positive inotropic effect produced by cardiotonic steroids without affecting their specific binding to sites on Na,K-ATPase. These data suggest that the antagonism is mediated by ryanodine-induced alterations in Ca2+ release from the sarcoplasmic reticulum.

Can maximum ouabain-sensitive 86Rb+ uptake rate be obtained by increasing Na+ influx?

Increasing stimulation frequency and monensin were utilized in attempts to enhance Na+ influx and thereby elicit the maximum rate of ouabain-sensitive 86Rb+ uptake in rat atrial muscle. Increasing stimulation frequency between 0 and 9 Hz enhanced the rate of ouabain-sensitive 86Rb+ uptake, whereas the uptake rate appeared to decrease at higher frequencies. Monensin (1.0-5.0 microM) increased the rate of uptake in atrial preparations stimulated at 6 Hz. A higher concentration of monensin produced a significant decline in uptake rate. The maximum rate of ouabain-sensitive 86Rb+ uptake produced by monensin in preparations stimulated at 6 Hz was not significantly different from the maximum elicited by stimulation frequency alone. The results suggest that either method of increasing Na+ influx may be used to maximize the rate of ouabain-sensitive 86Rb+ uptake and thereby estimate the Na+ pump capacity of rat atrial muscle; however, it is possible that the inhibitory actions which were apparent at very high levels of monensin and stimulation frequency may mask the true maximum value.

The positive chronotropic effects of Bay K-8644 and calcium as influenced by temperature

The positive chronotropic effects of the dihydropyridine calcium agonist BAY K-8644 and calcium were studied in spontaneously beating right guinea-pig atria. Within the temperature range from 22 to 37 degrees C, the following observations were made: (1) The basal pacemaker frequency increased linearly with the ambient temperature; (2) the increase in pacemaker rate produced by a maximally effective concentration of BAY K-8644 was of the same magnitude as that produced by calcium; (3) the increases in pacemaker rate produced by BAY K-8644 and by calcium were directly proportional to the increase in temperature above 22 degrees C; and (4) at 37 degrees C, the apparent ED50 for the positive chronotropic effect of calcium was 2.3 X 10(-3) M and of BAY K-8644 was 0.7 X 10(-7) M. At temperatures below 22 degrees C, irregular pacemaking activity or complete arrest of pacemaking activity frequently occurred. The data are in strong support of the assumption that BAY K-8644 elicits its positive chronotropic effect by increasing the availability of calcium ions.

Effects of BAY K-8644 on inotropic and arrhythmogenic actions of digoxin

Myocardial intracellular 'Ca2+ overload' may be involved in the direct arrhythmogenic actions of cardiotonic steroids. This proposal was examined by determining if the sensitivity of guinea-pig atrial muscle to digoxin-induced arrhythmias was affected by BAY K-8644, a 1,4-dihydropyridine derivative which promotes Ca2+ influx via slow channels. BAY K-8644 significantly reduced both the time required for a given concentration of digoxin to produce arrhythmias and the amount of digoxin bound to atrial muscle at the onset of arrhythmias. In addition, BAY K-8644 increased the maximum developed tension observed in the presence of digoxin before the onset of arrhythmias. Similar results were obtained with increasing concentrations of buffer Ca2+. In contrast, A23187, a Ca2+ ionophore, enhanced the sensitivity to digoxin-induced arrhythmias without affecting maximum developed tension. These results suggest that increases in intracellular Ca2+ enhance cardiac sensitivity to digoxin-induced arrhythmias and that the arrhythmogenic action may involve Ca2+ overload at a pool other than that which activates contractile proteins.

Stimulation frequency alters the inotropic response of atrial muscle to BAY K-8644

The inotropic and chronotropic effects of BAY K-8644 were examined in isolated guinea pig atria. The compound increased both rate and contractile force. Sensitivity to the inotropic effect was enhanced by increasing stimulation frequency between 0.5 and 3.3 Hz. The maximum developed tension elicited by the agent was reduced at 3.3 Hz. At BAY K-8644 concentrations up of to 3 X 10(-5) M, no dysrhythmic effects or other toxic signs such as an increase in resting tension were observed. These results are consistent with the suggestion that BAY K-8644 acts as a partial 'calcium agonist'.

Sanguinarine: a positive inotropic alkaloid which inhibits cardiac Na+,K+-ATPase

In isolated, isometrically contracting left guinea pig atria, sanguinarine, a benzophenanthridine alkaloid from the papaveracea Sanguinaria canadensis, produced a concentration-dependent positive inotropic effect. Between 2.3 x 10(-6) M and 6.5 x 10(-5) M, sanguinarine increased contractility by 108% which was comparable to the maximal inotropic effect of ouabain. Within the same concentration range, sanguinarine caused inhibition of Na+,K+-ATPase isolated from guinea pig myocardium. 100% inhibition of Na+,K+,ATPase activity occurred at 1 x 10(-4) M sanguinarine. The I50 for enzyme inhibition and the ED50 for the inotropic action of sanguinarine were the same (6-6.5 x 10(-6) M) indicating that both effects may be causally related.

Evidence for participation of catecholamines in cardiac action of ouabain: positive chronotropic effect

1 The shortening of cycle length (=positive chronotropic effect) by ouabain produced in isolated spontaneously beating atria of the guinea-pig was analyzed.2 The action of ouabain was dose-dependent; threshold response was seen at 1 x 10(-7) M, and maximal response occurred at 4 x 10(-7) M. The half-time of the ouabain effect was about 20 minutes.3 The positive chronotropic effect of ouabain was reduced to 40% by beta-adrenoceptor blockade (3.3 x 10(-9) M propranolol) or by reserpine-depletion of catecholamines. Incubation of reserpine-treated atria with noradrenaline partially restored the action of ouabain.4 The effect of ouabain was greatly dependent upon the calcium concentration. The optimal calcium level was 2.5 x 10(-3) M. Calcium and ouabain acted synergistically.5 Increasing calcium concentrations inhibited the positive chronotropic effect of noradrenaline in a manner similar to increasing ouabain concentrations.6 A hypothesis is proposed which explains the chronotropic effect of ouabain on the basis of two mechanisms: (1) increase of the catecholamine concentration affecting the pacemaker; (2) mobilization of calcium, i.e. increase of the biologically effective intracellular calcium level.