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To analyse the kinetic changes of uptake, washout and retention of Tc-sestamibi in order to evaluate the protective effects and possible mechanism of ischaemic preconditioning and adenosine preconditioning on myocardium injured by ischaemia/reperfusion.Isolated ischaemia/reperfusion rabbit heart models, as established by Langendorff, were used. Eighteen rabbit hearts perfused in Krebs-Henseleit (KH) buffer were randomly assigned to three groups: ischaemia/reperfusion (I/R, n=6), adenosine preconditioning (AD, n=6), and ischaemic preconditioning (IPC, n=6). Tc-sestamibi (55.5 MBq) in KH was perfused for 40 min and washed out for 40 min. The kinetic changes of Tc-sestamibi within myocardial tissue was monitored during the uptake and washout phases. Cardiac haemodynamic parameters, creatine kinase and lactate dehydrogenase leakage in coronary effluent, and myocardial infarct size were measured to assess myocardial injuries in rabbit hearts.In the early phases of uptake, there were no significantly different uptake rates of Tc-sestamibi between AD (before 20 min), IPC (before 15 min) and I/R myocardium (all P0.05). Uptake rates of Tc-sestamibi in myocardium of the three groups all tended to increase, with the uptake time increasing. In the late phases of uptake, AD and IPC were significantly higher than I/R (all P0.05). In the washout phases, the retention fractions of Tc-sestamibi in myocardium of the three groups all showed a descending tendency with washout time increasing. The retention fractions in AD and IPC were all higher than I/R (all P0.05). There were no statistical differences in uptake rates and retention fractions of Tc-sestamibi between AD and IPC (all P0.05). Cardiac haemodynamic parameters, creatine kinase and lactate dehydrogenase leakage, and myocardial infarct size demonstrated there is lighter injury in AD and IPC myocardium than in I/R (all P0.05). The retention of Tc-sestamibi and myocardial infarction weight were significantly negatively correlated (r=-0.8384, P0.001).Adenosine preconditioning has similar myocardial protective effects on ischaemia/reperfusion myocardium as does ischaemic preconditioning. Tc-sestamibi may be a sensitive and reliable measure for evaluating the importance and mechanism of ischaemic preconditioning and adenosine preconditioning.

The adenosine A(1)/A(2) receptor agonist AMP 579 has been reported to protect the heart against infarction even when administered after the onset of ischemia. The present study explored both the timing and the duration of treatment required to limit infarct size in in situ rabbit hearts subjected to 30 min of regional ischemia and 3 hours of reperfusion. In groups 1 and 2, AMP 579 was infused from 10 min before reperfusion and continued for either 30 or 40 min. In group 3, AMP 579 was begun at the onset of reperfusion and continued for 70 min. In group 4, AMP 579 was also infused for 70 min but begun 10 min after reperfusion. In untreated hearts 36.4 +/- 3.1% of the risk zone infarcted. Protection was observed only in hearts having a 70-minute infusion of AMP 579 starting at reperfusion (13.0 +/- 1.9%, P < 0.05). Therefore, AMP 579 must be present at the moment of reperfusion and have a continued presence of more than 30 min thereafter to protect. Importantly, because AMP 579 can protect when administered up to the time of reperfusion, it likely prevents a reperfusion injury, and, therefore, has impressive clinical potential.

Background —Atrial fibrillation (AF) is frequently associated with atrial dilatation caused by pressure or volume overload. Stretch-activated channels (SACs) have been found in myocardial cells and may promote AF in dilated atria. To prove this hypothesis, we investigated the effect of the SAC blocker gadolinium (Gd 3+ ) on AF propensity in the isolated rabbit heart during atrial stretch. Methods and Results —In 16 isolated Langendorff-perfused rabbit hearts, the interatrial septum was perforated to equalize biatrial pressures. Caval and pulmonary veins were occluded. Intra-atrial pressure (IAP) was increased in steps of 2 to 3 cm H 2 O by increasing the pulmonary outflow fluid column. Vulnerability to AF was evaluated by 15-second burst pacing at each IAP level. At baseline, IAP needed to be raised to 8.8±0.2 cm H 2 O (mean±SEM) to induce AF. A dose-dependent decrease in AF vulnerability was observed after Gd 3+ 12.5, 25, and 50 μmol/L was added. AF threshold increased to 19.0±0.5 cm H 2 O with Gd 3+ 50 μmol/L ( P 2 O at baseline but never during Gd 3+ . Atrial effective refractory period shortened progressively from 78±3 ms at 0.5 cm H 2 O to 52±3 ms at 20 cm H 2 O ( P 3+ 50 μmol/L had no significant effect on effective refractory period. Conclusions —Acute atrial stretch significantly enhances the vulnerability to AF. Gd 3+ reduces the stretch-induced vulnerability to AF in a dose-dependent manner. Block of SAC might represent a novel antiarrhythmic approach to AF under conditions of elevated atrial pressure or volume.

Background The aims of the Langendorff-perfused rabbit heart study were to evaluate the arrhythmogenic consequences of myocardial contusion and to determine the mechanism of arrhythmia. Methods Six hearts were in the control group, and 24 hearts (intact heart protocol) were submitted to one of four different contusion kinetic energies (75, 100, 150, or 200 millijoules [mJ]; n = 6). Occurrence of arrhythmia, of an electrically silent area (i.e., area with no electrical activity), and of line of fixed conduction block were reported before and for 1 h after contusion. In 16 hearts (frozen hearts) submitted to cryoprocedure and contusion impact of 100 or 200 mJ, ventricular conduction velocities, anisotropic ratio, wavelengths, ventricular effective refractory period, and its dispersion were measured before and for 1 h after contusion. Using high-resolution mapping, arrhythmias were recorded and analyzed. Results The intact heart study showed that the number and seriousness of contusion-induced arrhythmias increased with increasing contusion kinetic energy, as did the number of electrically silent areas (five of six ventricular fibrillations and five of six electrically silent areas at 200 mJ). In the frozen heart study, immediately after contusion ventricular effective refractory periods were shortened and dispersed, and wavelengths were also shortened. The arrhythmia analysis showed that all ventricular tachycardias but one were based on reentry developed around an electrically silent area or a line of fixed conduction block. Conclusions Myocardial contusion has direct arrhythmogenic effects, and the seriousness of arrhythmia increases with the level of contusion kinetic energy. The mechanism of arrhythmia was mainly based on reentrant circuit around a fixed obstacle.

Background Single-site or multisite atrial pacing may reduce the incidence of atrial fibrillation in humans. The therapeutic mechanisms may include synchronization of atrial repolarization (repolarization “memory”) and/or decreased dispersion of atrial repolarization. These responses have not been well documented in intact atria. Methods and Results Monophasic action potential recordings were made from six atrial epicardial sites in 39 isolated perfused rabbit heart preparations during 3 hours of continuous right atrial, left atrial, or biatrial pacing. Action potential recordings obtained at times 0, 45, 90, 135, and 180 minutes were computer analyzed for activation time (AT) and 90% action potential duration (APD) at each site. No consistent relationship could be demonstrated between APD and AT at any time during atrial pacing (all P >.05). On average, left atrial APDs were longer than right atrial APDs by up to 6.3 ms at all times, regardless of the site of pacing ( P ≤.05). At all times, dispersion of atrial repolarization was minimized by left atrial pacing compared with right atrial pacing (21.6±9.1 versus 32.4±15.1 ms, respectively, at time 0; P P >.05). Conclusions No relationship can be demonstrated between atrial AT and APD in the isolated rabbit heart preparation. This differs from ventricular repolarization “memory,” which is demonstrable under the same conditions. Left atrial APD is, on average, longer than right atrial APD, suggesting spatial heterogeneity in repolarization. Dispersion of atrial repolarization is minimized by left atrial pacing in this preparation with no further advantage to biatrial pacing.

Background Several investigators have indirectly implicated oxygen free radicals in reperfusion injury following ischemia in the isolated rabbit heart. Methods A quantitative assessment of free-radical production during ischemia and reperfusion was made using electron paramagnetic resonance (EPR) spectroscopy. Serial frozen (77 degrees K) tissue biopsies of the left ventricular wall in isolated rabbit hearts were performed during perfusion, ischemia, and reperfusion. These were pulverized into a fine powder that was filled into EPR tubes. EPR spectra of the tissue were recorded at 77 degrees K. Results Three predominant signals were seen on EPR analysis: signal A, g = 2.028; signal B, g = 2.005, and signal C, g = 1.940. All three signals were present during the perfusion period and decreased in size during the period of ischemia. After reperfusion, all three signals doubled in size. Conclusion This study lends direct support to the theory that free radicals are generated during myocardial reperfusion and may result in reperfusion injury.

BACKGROUND Although the existence of myocardial mechanoelectrical feedback is well established, the mechanism of arrhythmia induction by ventricular dilatation or stretch remains insufficiently defined. In particular, controversy exists when comparing the arrhythmogenic potential of chronic versus acute myocardial stretch. Also, assessment of cellular electrophysiological effects of myocardial stretch has been incomplete. METHODS AND RESULTS To evaluate the electrophysiological and arrhythmogenic effects of slow versus rapid ventricular wall stretch, we developed an isolated Langendorff-perfused rabbit heart model in which left ventricular (LV) volume can be changed by a computer-controlled servopump. Cellular electrophysiological effects and premature ventricular excitations (PVEs) and their origin produced by the volume increases were assessed by a multiple-site monophasic action potential (MAP) recording system and by volume-conducted ECGs obtained by immersing the entire preparation in a saline-filled tank. Volume was increased either gradually with slow volume ramps (0.1 ml/sec) or suddenly by volume pulses of varying pulse waveforms (three different amplitudes and five different rise velocities) applied randomly 250-350 times to each of eight hearts. Gradual LV volume loading caused gradual decreases in MAP resting and action potential amplitude, whereas rapid, transient volume pulses caused transient depolarizations. Despite similar membrane potential effects of stretch, gradual volume increases rarely (11%) produced PVEs, even with large volume loads, whereas rapid volume pulses of moderate amplitudes regularly triggered PVEs (45-100% of interventions). Logistic regression analysis showed that the probability of PVE occurrence increased independently with both the amplitude and the velocity of the volume increase, with the greatest sensitivity to stretch velocity exhibited at low and intermediate pulse amplitudes. Faster volume pulse rise velocities triggered PVEs at a lower instantaneous pulse amplitude than lower rise velocities, further corroborating the dependence of stretch-activated arrhythmias on the velocity of stretch. In contrast, an increase in the basic ventricular volume had no effect on the probability of PVE occurrence during the volume pulses. The MAP recordings demonstrated spatial variability in the extent of local depolarizations and site of PVE origin; transient depolarizations occurred, and PVEs originated most often in the posterolateral region of the left ventricle. CONCLUSIONS Membrane depolarization is caused by both gradual and rapid ventricular stretch, but PVEs are more easily elicited by rapid stretch. Regions of greater myocardial compliance that experience greater relative stretch may act as "foci" for stretch-activated arrhythmias during dynamic ventricular loading. These whole-heart data corroborate the existence of stretch-activated membrane channels in ventricular myocardium and may help explain ventricular ectopy under conditions of differential ventricular loading, as in ventricular dyskinesia, or regional muscle traction, as in mitral valve prolapse syndrome.

BACKGROUND Polyethylene glycol, covalently linked to superoxide dismutase (PEG-SOD), has a long plasma half-life (greater than 30 hours) and has been proposed as an effective agent for reducing free radical-mediated injury ischemia and reperfusion. METHODS AND RESULTS Using an isolated rabbit heart perfused with arterial blood from a support rabbit, we have demonstrated that pretreatment with PEG-SOD (30,000 units/kg, intravenous bolus, 12-24 hours before 60 minutes of normothermic global ischemia), combined with addition of PEG-SOD to the blood perfusion circuit (30,000 units/kg to the support rabbit) and inclusion of PEG-SOD (150 micrograms/ml) in a cardioplegic solution, enhanced the postischemic recovery of left ventricular developed pressure (LVDP) from 51 +/- 6 to 74 +/- 9 mm Hg (p less than 0.05; n = 9 per group). In further studies we showed that, whereas maximum protection was obtained when PEG-SOD was given as a combined pretreatment and additive to both the cardioplegic and the reperfusate solutions (postischemic LVDP recovery increased from 44 +/- 4% in the control group to 70 +/- 3% in the PEG-SOD group), the administration of PEG-SOD during pretreatment plus cardioplegia or during reperfusion alone also resulted in a significant improvement in postischemic function (62 +/- 7% and 60 +/- 3%, respectively). However, the use of PEG-SOD as a cardioplegic additive alone failed to afford protection (47 +/- 4% recovery of LVDP). In dose-response studies (with 0, 3,000, 6,000, 12,000, 30,000, or 60,000 units/kg; n = 8 per group), maximum recovery of LVDP was obtained with the administration of 12,000 units/kg of PEG-SOD. Studies of the plasma activity of PEG-SOD confirmed its long half-life and showed that the treatment with PEG-SOD either 1 hour or 12-24 hours before the study resulted in similar levels of plasma activity. In an attempt to assess any involvement of blood-borne elements in the protection afforded by PEG-SOD, studies were also carried out in the crystalloid-perfused rabbit heart, and no protection was observed. Similarly, no protection was observed at any one of a variety of doses in the crystalloid-perfused rat heart. CONCLUSIONS PEG-SOD can afford protection in the blood-perfused rabbit heart; this protection is dose dependent and probably involves some action of PEG-SOD on blood-borne elements, possibly leukocytes.

Cardiac ischemia decreases the rate of oxidative phosphorylation (OXPHOS) and the contents of cardiolipin and cytochrome c (CYTc) in subsarcolemmal mitochondria (SSM) in the isolated rabbit heart. CYTc release first requires damage to the inner mitochondrial membrane to delocalize CYTc to the intermembrane space, followed by breach of the outer membrane. The decrease in cardiolipin content allows CYTc detachment from the inner membrane. It is still unclear how CYTc passes the outer membrane for release into cytosol. We propose that ischemia increases outer-membrane leakage by depletion of bcl-2 content, and that oxidants generated by the electron transport chain (ETC) during ischemia favor bcl-2 depletion. We used blockade of the proximal ETC at complex I during ischemia with amobarbital (AMO) to test the role of ETC during ischemia. Langendorff perfused rabbit hearts were treated with AMO (2.5 mM for 1 min) or vehicle immediately before 30 min global ischemia (37°C). Time controls were perfused for 45 min. SSM were isolated at the end of ischemia. CYTc content (reduced minus oxidized spectra), OXPHOS and bcl-2 (western blotting) were measured. Ischemia decreased OXPHOS with TMPD-ascorbate as substrate (electron donor to complex IV via CYTc) and the contents of CYTc and bcl-2. In contrast, AMO preserves OXPHOS, CYTc and bcl-2. Thus, blockade of electron transport preserves bcl-2 content during ischemia with enhanced CYTc retention by SSM. The ETC contributes to mitochondrial damage during ischemia, depleting cardiolipin in the inner membrane and bcl-2 in the outer membrane favoring the two steps required for release of CYTc from mitochondria during ischemia and reperfusion.

Bupivacaine, but not lidocaine, may cause severe cardiac dysrrhythmias in case of accidental intravascular injection. In an attempt to discriminate between a pharmacokinetic and a pharmacodynamic (or both) origin to these differences, we used an isolated rabbit heart model with constant coronary inflow to compare the myocardial uptake and disposition kinetics of lidocaine and bupivacaine. Drug concentration in the outflow perfusate was assayed and surface electrocardiogram was recorded. Drug uptake and disposition kinetics were modeled with a two-compartment open model. An Emax model was used to describe the increase in QRS duration in relation with drug concentration in the central compartment. Lidocaine and bupivacaine exhibited similar myocardial pharmacokinetics (i.e., a rapid decrease in the outflow concentration upon drug administration discontinuation). Bupivacaine-induced maximum increase in QRS duration (Emax) was 15 times superior to lidocaine Emax. The steady-state perfusate concentration producing half Emax was the same for both drugs. We conclude that bupivacaine-induced QRS widening decreases almost at the same rate as does lidocaine-induced QRS widening when drug administration is terminated. Therefore, the different cardiac effects of lidocaine and bupivacaine are not due to differences in myocardial uptake and disposition kinetics.

Different aspects of the intrinsic regulation of rate-induced variations of functional refractory period of atrioventricular node (FRPN) were studied in isolated rabbit heart preparations. First, the hypothesis that these variations originate from the net interaction between facilitation and fatigue was tested. For a constant fast rate, selective effects of faciliation and of steady-stage fatigue were independently shown to shorten and prolong, respectively, FRPN while their combined effects were shown to result in intermediate changes corresponding to the sum of their individual effects. Second, selective and combined effects on FRPN were shown to start for rates corresponding to the upper half of the 1:1 nodal conduction range and to reach their maximums at the fastest rate tested. Third, the time-courses of fatigue-induced prolongations in nodal conduction time and FRPN were shown to be closely linked. Facilitation effects on conduction time and FRPN were confirmed, as previously shown for in situ dog hearts, to be linked, but time-independent. Fourth, FRPN was shown not to correspond to particular limits in its subintervals, but to be, nevertheless, related to nodal refractoriness. Fifth, it was demonstrated that, in conditions of combined facilitation and transient fatigue such as those prevailing in current endocavitary investigations of nodal function, FRPN could be shortened, left unchanged or prolonged by a constant fast rate depending on its duration. In conclusion, the present study demonstrates the dual origin of rate-induced FRPN variations, their rate and time dependence, their relation to changes in nodal refractoriness, and their involvements in various nodal responses.

To determine the effect of streptokinase on the ischemic myocardium independent of its effects on the occluding thrombus, the isolated rabbit heart, perfused with Krebs-Henseleit solution, was subjected to a 45-min period of ischemia--83% reduction in myocardial (perfusion) flow--plus anoxia (95% N2 and 5% CO2), followed by restoration of perfusion and reoxygenation. Streptokinase, 75 or 150 IU/min, was infused starting 15 min before reperfusion and continuing for 30 min after reperfusion. Compared with the control group, streptokinase was associated during reperfusion with a significant dose-dependent greater restoration or smaller depression of ventricular function, dP/dt, and developed pressure. To determine if streptokinase effects were mediated during the ischemic or reperfusion phase, the high streptokinase dose was administered in either the last 30 min of the ischemia or the first 30 min of reperfusion. The improvement in recovery of left ventricular function was primarily in the group having streptokinase administered only during the ischemic period. Thus, streptokinase affects the ischemic myocardium so that there is an acceleration in the recovery of ventricular function or a reduction of the impairment in ventricular function during myocardial reperfusion.

Thirteen rabbit atrioventricular nodes were studied both morphologically and electrophysiologically. Two of these nodes were subsequently reconstructed in three-dimensional fashion. From the morphological standpoint, it was shown that the atrioventricular node was divided into a smaller enclosed portion and a larger open portion by a fibrous collar derived from the central fibrous body. Three distinct nodal cell types were identified within these nodal segments. The open node was mostly occupied by transitional cells which merged proximally with the atrial myocardium. As they entered the enclosed node, some of these transitional cells merged into a knot of midnodal cells. Others passed circumferentially round this knot and together with the midnodal cells, joined with a tract of lower nodal cells. The latter cells were continuous with the His bundle anteriorly, but they also extended into the open node posteriorly. It was possible to correlate the activation sequence of the node accurately with the disposition of these cells. Using both reconstructions and techniques to mark cells from which action potentials had been recorded, it was shown that the transitional cells correlated with the AN zone of the node. Cells with N potentials were located in the environs of the midnodal cell knot. The anterior portion of the lower nodal cells correlated with the NH nodal zone. The posterior extension of the lower nodal cells and the overlay fibers of the anterior transitional cells both functioned as dead-end pathways. Histologically distinct tracts were not identified within the internodal atrial myocardium.

Intracellular microelectrodes and small unipolar leads applied to endocardial surface of the right atrium in vitro were used to study the complex extracellular wave patterns recorded from the neighborhood of the cardiac pacemaker during spontaneous activity. Sinus activity propagated slowly toward the site of atrial invasion on the venous border of the crista terminalis. Atrial activation was marked by a primary negative wave that appeared 20-40 msec after pacemaker firing. Two sources of complex multiphasic waves were found. First, potentials from transitional sinus tissue propagated toward the atrium and caused low-voltage waves that preceded and slurred the onset of the atrial initial negativity. Second, bundles and layers of the crista terminalis muscle were excited asynchronously around the invasion region, as if cross-connections were infrequent. Waves originating from this source occurred after the firing of the invasion site. No extracellular wave could be associated with the firing of the true pacemaker cells. The sinoatrial ring bundle (SARB) yielded a discrete biphasic deflection along most of its way toward the coronary sinus. This potential appeared most frequently after that of the adjoining cristal muscle, raising questions about the functional role of the SARB as an internodal preferential pathway.

Controversy persists over the relative tolerance of the immature myocardium to global ischemia. Thus, we evaluated the physiologic effects of 30, 60, and 180 minutes of global ischemia in an isolated, isovolumic rabbit heart model, at 3 different ages: newborns (less than 1 week of age) (n = 36), juveniles (4 to 6 weeks old) (n = 36), and adults (5 to 7 months old) (n = 36). Following 30 and 60 minutes of ischemia, respectively, adults recovered 87 +/- 4% (mean +/- SEM) and 90 +/- 7% of baseline systolic function, and juveniles recovered 91 +/- 10% and 85 +/- 8%. In contrast, newborns recovered only 27 +/- 6% and 28 +/- 4% of baseline systolic function (p less than 0.05 compared to adults and juveniles). During ischemia, newborn hearts became stiff more rapidly, reaching 361 +/- 46% of baseline stiffness by 60 minutes, whereas adults and juveniles were at 122 +/- 33% and 92 +/- 18% of baseline stiffness (p less than 0.05 newborns compared to adults and juveniles). With reperfusion after 60 minutes of ischemia, the work efficiency of the newborn heart deteriorated to 39 +/- 7% of baseline, compared with 95 +/- 7% and 91 +/- 7% of baseline efficiency in the adult and juvenile hearts (p less than 0.05, newborns compared to adults and juveniles). The ratio of tissue wet-to-dry weights were similar in all age groups after ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)

An accelerating ventricular tachycardia ending in fibrillation can be initiated in isolated hearts by procaine injections after the administration of epinephrine, but not in untreated hearts. The run of tachycardia consists of a supraventricular or idioventricular impulse followed by an accelerating train of premature systoles coupled to it. Fibrillation results when the train reaches a frequency of 10 to 12 per second. In hearts with their bundles of His cut, epinephrine injections alone cause only a ventricular tachycardia of constant frequency. Therefore, the initiation of ventricular arrhythmias by procaine must entail the production of A-V blockade and some process which causes the tachycardia to accelerate.

Ethanol in saline solution (15%, v/v) was infused into anesthetized rabbits at a rate of 0.494 ml/min for the first 12 minutes and then at 0.247 ml/min for 108 minutes. Three hours after the infusion, heart triglyceride and lipoprotein lipase were assayed. Oxidation and esterification of fatty acids (palmitate -14 C as an indicator) were assessed by using either tissue homogenates or perfused hearts taken from the rabbits. Oxidation-reduction states of the perfused hearts were examined by measuring the tissue levels of dehydrogenase-linked substrates. The infusion of ethanol resulted in 180% increase in heart triglyceride content, but the infusion of norepinephrine (3 µg/kg/min) did not change the content. No change in plasma free fatty acids and triglyceride or heart lipoprotein lipase activity was detected. Addition of ethanol had little effect on the distribution of palmitate -14 C in the lipids of tissue slices and homogenates. On the other hand, prior infusion of ethanol resulted in depression of 14 CO 2 production (70 and 50%) and enhanced fatty acid esterification into triglyceride (270 and 170%) both in homogenates and perfused hearts. Mitochondrial and cytoplasmic redox states were shifted to more oxidized states by ethanol infusion. It is postulated from these results that an accumulation of triglyceride in the rabbit heart in response to ethanol administration is a result of decreased fatty acid oxidation rather than of increased triglyceride uptake or increased fatty acid synthesis.

Ventricular fibrillation has been studied by driving the ventricles of the isolated rabbit heart electrically and observing whether fibrillation persisted after stimulation was stopped. The hearts were perfused by solutions of different ionic composition and the proportion of hearts in which persistent fibrillation was seen was determined for each solution. The proportion was controlled from 0 to 100 per cent according to the amount of K + , hearts fibrillating spontaneously in .25 N K + . A similar study was made by varying Ca ++ . Fibrillation was arrested by ATP and prolonged by dinitrophenol. Fibrillating hearts lost more K + than when they were not fibrillating. Fibrillation appeared to depend on disturbances of the metabolic processes concerned with ion movements.

The sympathetic innervation of the rabbit heart, as a function of age, was studied by measuring the cardiac concentration of catecholamines and observing the anatomic distribution of sympathetic nerves by the monoamine fluorescense technique. The cardiac concentration of norepinephrine in late gestation was quite low; the levels rose progressively after birth to reach adult levels by about three weeks of age. Similar small amounts of epinephrine were found in the hearts at all ages. Substantially less change in adrenal catecholamines accompanied advancing age. At all ages a close correlation was noted between the norepinephrine levels and the histochemical demonstration of sympathetic innervation. Intensely fluorescent, terminal varicosities were observed within large preterminal nerve trunks only in the youngest animals, suggesting that the sympathetic nerves move into, rather than form within, the heart. Chromaffin cells were observed in the hearts at all ages.

Isolated rabbit hearts were perfused with a suspension of red cells which assured adequate oxygenation. Ouabain added to the perfusate (2.8 x 10 -7 M final concentration) promptly increased the force of contraction and oxygen consumption of the hearts. These events appeared to occur simultaneously. The extraction ratio of palmitate-1- 14 C added to the perfusate (0.31) was not altered by ouabain. However, ouabain increased the oxidation of the extracted palmitate from an average of 34% to an average of 71% at the time intervals studied.

Supraventricular tachycardia in an isolated rabbit heart preparation was repeatedly initiated and terminated by carefully timed atrial premature beats. Transmembrane action potentials of AV nodal cells were recorded simultaneously by a "brush electrode" consisting of 10 microelectrodes. Surface electrograms of atrium and His bundle were also recorded. The moments of activation of 54 different AV nodal cells, both during regular driving of the atrium and during tachycardia were ascertained. Premature atrial beats introduced during tachycardia would either "reset" the tachycardia or terminate it. The sequence of activation of the AV nodal cells when initiating tachycardia, during tachycardia itself, and when premature beats were interpolated during tachycardia warrant the conclusion that a circus movement in the AV node, based on functional longitudinal dissociation of the upper AV node, was the underlying mechanism of the arrhythmia.

The antagonistic effect of morphine on the positive inotropic response of ouabain was studied in 96 isolated rabbit hearts, using a modified Langendorff preparation. Decreased calcium (Ca) in the perfusate resulted in increased depression of peak left ventricular dP/dt by morphine (10(-4) gm/ml), and the number of beats required to reach the depression was proportionally reduced with the reduction of calcium chloride from 2.16 mM to 0.54 mM. Likewise, the positive inotropic effect of ouabain (10(-6) gm/ml) was proportionally reduced by the same reduction of calcium chloride concentration in the perfusate, but the number of beats required to reach the cardiotonic effect was proportionally increased with Ca deficiency. The opposing effects of ouabain and morphine were mutually antagonistic regardless of the order of drug administration. When the hearts had been previously exposed to morphine, ouabain stimulation was no longer beat-dependent. Following prior exposure to ouabain, the beat-dependency of morphine depression on the heart also was partially antagonized. These results suggest that the level of maximal dP/dt during morphine depression or ouabain stimulation is directly related to the extracellular concentration of Ca. The number of beats required to attain the depression or stimulation is probably a reflection of the rate at which equilibrium is established between the mobile fraction of extracellular and intracellular Ca, since transmembrane movement of Ca is believed to occur during each beat. A possible common site of morphine and ouabain effect may be the cellular membrane, where ionic flux occurs.