Inhibitors of arachidonate-regulated calcium channel signaling suppress triggered activity induced by the late sodium current

Disturbances in myocyte calcium homeostasis are hypothesized to be one cause for cardiac arrhythmia. The full development of this hypothesis requires (i) the identification of all sources of arrhythmogenic calcium and (ii) an understanding of the mechanism(s) through which calcium initiates arrhythmia. To these ends we superfused rat left atria with the late sodium current activator type II Anemonia sulcata toxin (ATXII). This toxin prolonged atrial action potentials, induced early afterdepolarization, and provoked triggered activity. The calmodulin-dependent protein kinase II (CaMKII) inhibitor KN-93 (N-[2-[[[3-(4-chlorophenyl)-2-propenyl]methylamino]methyl]phenyl]-N-(2-hydroxyethyl)-4-methoxybenzenesulphon-amide) suppressed ATXII triggered activity but its inactive congener KN-92 (2-[N-(4-methoxy benzenesulfonyl)]amino-N-(4-chlorocinnamyl)-N-methylbenzylamine) did not. Neither drug affected normal atrial contractility. Calcium entry via L-type channels or calcium leakage from sarcoplasmic reticulum stores are not critical for this type of ectopy as neither verapamil ((RS)-2-(3,4-dimethoxyphenyl)-5-{[2-(3,4-dimethoxyphenyl)ethyl]-(methyl)amino}-2-prop-2-ylpentanenitrile) nor ryanodine affected ATXII triggered activity. By contrast, inhibitors of the voltage independent arachidonate-regulated calcium (ARC) channel and the store-operated calcium channel specifically suppressed ATXII triggered activity without normalizing action potentials or affecting atrial contractility. Inhibitors of cytosolic calcium-dependent phospholipase A2 also suppressed triggered activity suggesting that this lipase, which generates free arachidonate, plays a key role in ATXII ectopy. Thus, increased left atrial late sodium current appears to activate atrial Orai-linked ARC and store operated calcium channels, and these voltage-independent channels may be unexpected sources for the arrhythmogenic calcium that underlies triggered activity.

Evidence that 2-aminoethoxydiphenyl borate provokes fibrillation in perfused rat hearts via voltage-independent calcium channels

We tested whether 2-aminoethoxydiphenyl borate (2-APB) induces arrhythmia in perfused rat hearts and whether this arrhythmia might result from the activation of voltage-independent calcium channels. Rat hearts were Langendorff perfused and beat under sinus rhythm. An isovolumic balloon inserted into the left ventricle was used to record mechanical function while bipolar electrograms were recorded from electrodes sutured to the base and the apex of hearts. Western and immunofluorescence analyses were performed on rat left ventricular protein extracts and left ventricular frozen sections, respectively. Rat ventricular myocytes express Orai 1 and Orai 3, and ventricle also contains the Orai regulator Stim1. Rat hearts (n=5) perfused with Krebs-Henseleit (KH) alone maintained sinus rhythm at 4.8 ± 0.1 Hz and stable mechanical function. By contrast, perfusing hearts (n=5) with (KH+22 μM 2-APB) provoked a period of tachycardic ectopy at rates of up to 10.8 ± 0.2 Hz. As perfusion with (KH+22 μM 2-APB) continued, the rate of spontaneous ventricular depolarization increased to 21.8 ± 1.2 Hz and became disorganized. Heart mechanical function collapsed as developed pressure decreased from 87 ± 8.8 to 3.5 ± 1.9 mm Hg. Flow rate did not change between normal (16.6 ± 0.9 ml/min) and fibrillating (17.4 ± 0.8 ml/min) hearts. The addition of 20 μM 1-[2-(4-methoxyphenyl)-2-[3-(4-methoxyphenyl) propoxy]ethyl-1H-imidazole (SKF-96365) to (KH+22 μM 2-APB) perfusates (n=4) restored sinus rhythm and heart mechanical output. These data indicate that activating myocardial voltage-independent calcium channels, possibly the Orais, may be a novel cause of ventricular arrhythmia.

Pharmacological evidence for Orai channel activation as a source of cardiac abnormal automaticity

Calcium transport through plasma membrane voltage-independent calcium channels is vital for signaling events in non-excitable and excitable cells. Following up on our earlier work, we tested the hypothesis that this type of calcium transport can disrupt myocardial electromechanical stability. Our Western and immunofluorescence analyses show that left atrial and ventricular myocytes express the Orai1 and the Orai3 calcium channels. Adding the Orai activator 2-aminoethoxydiphenyl borate (2-APB) to the superfusate of rat left atria causes these non-automatic muscles to contract spontaneously and persistently at rates of up to 10 Hz, and to produce normal action potentials from normal resting potentials, all in the absence of external stimulation. 2-APB likewise induces such automatic activity in superfused rat left ventricular papillary muscles, and the EC(50)s at which 2-APB induces this activity in both muscles are similar to the concentrations which activate Orais. Importantly, the voltage-independent calcium channel inhibitor 1-[2-(4-methoxyphenyl)-2-[3-(4-methoxyphenyl) propoxy]ethyl-1H-imidazole (SKF-96365) suppresses this automaticity with an IC(50) of 11 ± 0.6 μM in left atria and 6 ± 1.6 μM in papillary muscles. 1-(5-Iodonaphthalene-1-sulfonyl)-hexahydro-1,4-diazepine (ML-7), a second voltage-independent calcium channel inhibitor, and two calmodulin inhibitors also prevent 2-APB automaticity while two calmodulin-dependent protein kinase II inhibitors do not. Thus an activator of the Orai calcium channels provokes a novel type of high frequency automaticity in non-automatic heart muscle.

A pharmacological model for calcium overload-induced tachycardia in isolated rat left atria

Few experimental models produce spontaneous tachycardia in normal left atria to allow the study of the cellular mechanisms underlying this contributor to atrial fibrillation. We reported 2-aminoethoxydiphenyl borate (2-APB) that provokes sporadic spontaneous mechanical activity and calcium leak in isolated rat left atria. Since sarcoplasmic reticulum calcium leak in the presence of high calcium load may trigger tachyarrhythmias, we tested how conditions that increase calcium load affect 2-APB-induced ectopic activity. Exposing superfused rat left atria to (i) 30 nM isoproterenol, (ii) 3 microM forskolin, (iii) 300 nM (-)BayK 8644 ((4S)-1,4-Dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluormethyl)phenyl]-3-pyridinecarboxylic acid methyl ester), (iv) 300 nM FPL-64176 (2,5-Dimethyl-4-[2-(phenylmethyl)benzoyl]-1H-pyrrole-3-carboxylic acid methyl ester) or (v) 120 microM ouabain increases their force of contraction, evidence of calcium loading, but does not produce ectopic activity. Spontaneous mechanical activity occurs in left atria superfused with 20 microM 2-APB at 47+/-6 contractions/min in the absence of pacing. Any of these five agents increase rates of 2-APB-induced spontaneous mechanical activity to >200 contractions/min in the absence of pacing. Washing tachycardic left atria with superfusate lacking 2-APB restores normal function, demonstrating the reversibility of these effects. Decreasing superfusate sodium reverses this tachycardia and two hyperpolarization-activated current (I(f)) inhibitors blunt this ectopic activity. Thus conditions that increase atrial calcium load increase the frequency of spontaneous mechanical activity. Decreasing extracellular sodium and I(f) inhibitors suppress this spontaneous tachycardia suggesting forward-mode sodium-calcium exchange and I(f)-like activities underlie this activity. This model may help define cell pathways that trigger atrial tachyarrhythmias.

2-APB Induces Instability in Rat Left Atrial Mechanical Activity

Atrial contractile abnormalities are common clinical disorders but few pharmacological models can reliably produce such abnormalities in isolated atrial muscle. Since sarcoplasmic reticulum (SR) calcium leak may underlie these contractile irregularities, we investigated whether 2-aminoethoxydiphenyl borate (2-APB), a calcium leak-inducer, affects mechanical function in isolated, superfused rat left atria. Exposing left atria paced at 3 Hz to >10 microM 2-APB produced sporadic mechanical events that occurred in the absence of pacing stimulus. Prolonging atrial diastole in the presence of 2-APB produced spontaneous mechanical activity (SMA) defined as numerous mechanical events occurring in the absence of pacing stimulus. SMA depends on atrial sodium and chloride gradients as decreasing superfusate concentration of either ion suppressed SMA. Increasing superfusate potassium to produce an EK of approximately -74mV reversed SMA, revealing possible membrane potential sensitivity. Mechanical function decreased with time in left atria treated with 2-APB and low sodium or the anion transport inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) compared with atria exposed to low sodium or DIDS alone, suggesting 2-APB may decrease left atrial SR activator calcium. Thus, 2-APB produces instability in regular left atrial mechanical activity that may require forward-mode sodium-calcium exchange and chloride channel activities. This data identify a new model for studying atrial contractile abnormalities.

Activation of leptin expression by an inhibitor of carnitine palmitoyltransferase-1

Leptin may regulate peripheral fatty acid oxidation and invoke a feedback mechanism that affects leptin expression in adipocytes. The objective of this study, therefore, was to determine whether inhibiting systemic fatty acid oxidation at the level of carnitine palmitoyltransferase-1 (CPT1) affects leptin expression. To accomplish this objective, fed or overnight fasted rats were treated with 2-tetradecylglycidic acid (TDGA), a specific, irreversible CPT1 inhibitor, and acute changes in rat epididymal leptin expression and serum leptin content were measured using Northern, RT-PCR, and radioimmunoassay analyses. Overnight fasting decreased both epididymal leptin mRNA content and serum leptin. Treating overnight fasted rats with TDGA increased both their epididymal leptin mRNA and their serum leptin significantly in a time- and concentration-dependent manner. TDGA affected neither epididymal leptin mRNA nor serum leptin in fed rats where systemic fatty acid oxidation is low. These results support the conclusion that CPT1-linked fatty acid oxidation is a key modulator of leptin expression in fasting rats.

Occupation of the prostaglandin E2-type 1 receptor increases rat atrial contractility via a Y-27632-sensitive pathway

This study investigated whether rat left atria (LA) contain the prostaglandin E2 type 1 receptor (EP1) and whether EP1 occupation induces positive inotropic responses in superfused LA. Western analysis demonstrated that LA contain EP1 and the EP1 splice variant. Exposing isolated, superfused LA to 17-phenyl trinor PGE2, an EP1 agonist, increased isometric contractile force and its corresponding dF/dTs to approximately 70% of the isoproterenol maximum with an EC50 of approximately 80 nM. In contrast, agonists for EP2, EP3, and EP4 caused little change in LA function. While the EP1 antagonists SC-51089 and SC-19220 blocked 17-phenyl trinor PGE2-induced inotropy, neither prazosin, nadolol, atropine nor EI-283, a pan-specific protein kinase C inhibitor, affected 17-phenyl trinor PGE2-induced inotropy. However, Y-27632 and HA-1077, inhibitors of rho A-activated protein kinases, prevented and reversed the increase in LA contractility that occurred in the presence of 17-phenyl trinor PGE2. Thus, atria contain EP1 and EP1 occupation increases LA contractility via a pathway sensitive to inhibitors of rho A-activated protein kinases.

Peroxynitrite irreversibly decreases diastolic and systolic function in cardiac muscle

Much of the damaging action of nitric oxide in heart may be due to its diffusion-limited reaction with superoxide to form peroxynitrite. Direct infusion of peroxynitrite into isolated perfused hearts fails to model the effects of in situ formation because the bulk of peroxynitrite decomposes before reaching the myocytes. To examine the direct effects of peroxynitrite on the contractile apparatus of the heart, we exposed intact and skinned rat papillary muscles to a steady state concentration of 4-microM peroxynitrite for 5 min, followed by a 30-min recovery period to monitor irreversible effects. In intact muscles developed force fell immediately to 26% of initial force, recovering to 43% by 30 min. Resting tension increased by 600% immediately, and was still elevated 500% by 30 min. Nitrotyrosine immunochemistry showed that peroxynitrite can induce tyrosine nitration at low concentrations and is capable of penetrating 200-380 microm into the papillary muscle after a 5-min infusion. Decomposed peroxynitrite had no effect on either intact or skinned muscle developed force or resting tension. Our results show that peroxynitrite directly damages both developed force and resting tension of isolated heart muscle, which can be extrapolated to systolic and diastolic injury in intact hearts.

2-Tetradecylglycidic acid, an inhibitor of carnitine palmitoyltransferase-1, induces myocardial hypertrophy via the AT1 receptor

Activation of the antiogensin II, type 1 (AT1) receptor mediates the myocardial response to numerous hypertrophic stimuli. This study tested the hypothesis that 2-tetradecylglycidic acid (TDGA), an oxirane carboxylate inhibitor of mitochondrial carnitine plamitoyltransferase-1, induces myocardial hypertrophy via the AT1 receptor system. Male Sprague-Dawley rats treated with 10 mg TDGA/kg/day for 7 days had a heart wet weight:body weight ratio of 3. 58+/-0.16 mg/g compared with a ratio of 2.79+/-0.07 for rats treated with vehicle (P<0.05). The plasma level of antiogensin II was 117. 75+/-17.39 pg/ml in rats treated with 10 mg TDGA/kg/day compared with 54.0+/-11.38 pg/ml for rats treated with vehicle (P<0.05). The plasma level of angiotensin I in these two groups of rats was not different statistically. Rats treated with TDGA and given drinking water containing 1 mg losartan/ml had a heart wet weight:body weight ratio of 2.84+/-0.05 mg/g. This value was not statistically different from the value measured in rats given drinking water containing 1 mg losartan/ml and treated with vehicle alone. No significant difference in the heart wet weight:dry weight ratio occurred among these groups of rats. Finally, treating rats with TDGA or giving rats drinking water that contained 1 mg losartan/ml altered neither their heart rate nor their mean arterial blood pressure when compared with untreated rats. This data, therefore, suggests that oxirane carboxylates induce myocardial hypertrophy by activating the AT1 receptor independent of changes in systemic hemodynamics.

MDL-28170, a membrane-permeant calpain inhibitor, attenuates stunning and PKC epsilon proteolysis in reperfused ferret hearts

OBJECTIVES

This paper tests the hypothesis that calpains are activated in the ischemic (I)/reperfused (R) heart and contribute to myocardial stunning.

METHODS

Isolated ferret hearts were Langendorff perfused isovolumically, and subjected to 20 min of global I followed by 30 min of R in the presence or absence of 0.2 microM MDL-28170, a membrane-permeant calpain inhibitor. Right trabeculae then were isolated from these hearts, skinned chemically, and pCa(2+)-force curves obtained. Samples of left ventricle were extracted subjected to SDS-PAGE, and Western analyzed for PKC epsilon and PKM epsilon.

RESULTS

Perfused ferret hearts exhibit a 43% decline in left ventricular developed pressure during R. Pre-treatment of hearts with MDL-28170 prior to I significantly improves function during R. Trabecular myofilaments from normal hearts have a KD for Ca2+ of 6.27 +/- 0.06; I/R decreased the KD to 6.09 +/- 0.04; trabeculae from I/R hearts pre-treated with MDL-28170 have a KD of 6.28 +/- 0.04. Western analysis shows ferret hearts to contain a single approximately equal to 96 kDa species of PKC epsilon. I/R hearts contain the native PKC epsilon and a approximately equal to 25 kDa smaller species of PKC epsilon which corresponds to PKM epsilon, the calpain proteolyzed form of PKC epsilon. Pre-treatment of I/R hearts with MDL-28170 markedly diminishes PKM epsilon in reperfused hearts.

CONCLUSIONS

Mechanical stunning during R is sensitive to MDL-28170. Depressed mechanical function is reflected in a hyposensitization of trabecular myofilaments to Ca2+. Western analysis shows that PKM epsilon is present in R hearts.

Estimates of beat to beat handling of activator calcium using measurements of [Ca2+]i in aequorin loaded ferret cardiac muscle

OBJECTIVE

The aims were (1) to measure simultaneously and on a beat to beat basis intracellular calcium concentration ([Ca2+]i) transients and force transients in isolated ferret cardiac trabeculae; (2) to obtain and compare independent estimates of the recirculating fraction of Ca2+ using the [Ca2+]i data and the force data (recirculating fraction is the fraction of activator Ca2+ taken up by the sarcoplasmic reticulum in each beat and, in the steady state twitch, the fraction of activator Ca2+ released by the sarcoplasmic reticulum); and (3) to estimate the amount of Ca2+ that returns to the sarcoplasmic reticulum and the amount that, during steady state contractions, enters the cytosol, presumably from the extracellular compartment, with each beat.

METHODS

Eight trabeculae were mounted in the myograph. The servo-controlled muscle length was 98% of the length at which developed force was maximal. A modified technique was used for chemical loading of aequorin, and a new method for computer controlled low level photon counting, storage, calibration, and analysis. [Ca2+]i transients and force transients were simultaneously recorded during potentiated beats, together with their respective decays toward control steady state [Ca2+]i transients and force transients. A modified test of postextrasystolic potentiation achieved with a brief train of rapid pacing followed by a pause was used to evoke the potentiated beats.

RESULTS

At 2.0 mM extracellular Ca2+ ([Ca2+]o), resting [Ca2+]i was 283(SD 77) nM. The resting tension was 1.6(0.3) g.mm-2. The steady state [Ca2+]i transient and the peak potentiated [Ca2+]i transient averaged 992(165) and 1290(154) nM respectively. The corresponding tensions were 4.0(1.9) and 8.7(3.1) g.mm-2 respectively. The recirculating fraction of Ca2+ calculated from the dissipation of the potentiated [Ca2+]i transient averaged 45(4)%. This recirculating fraction was indistinguishable from the one calculated with another method from the decay of the force potentiation.

CONCLUSIONS

This is the first study to estimate the recirculating fraction of activator Ca2+ using measurements of [Ca2+]i. The results indicate that over a wide range of [Ca2+]i and tensions the Ca(2+)-force relationship is well approximated by a straight line. At 2.0 mM [Ca2+]o it appears that some 450 nM of Ca2+ recirculates and that a similar amount per steady state beat enters the cytosol, probably from the extracellular compartment.

Beat-to-beat measurements of [Ca2+]i and force in ferret cardiac muscle after chemical loading of aequorin

This communication reports the development of a modified procedure for chemical loading of aequorin in small multicellular cardiac preparations, with special emphasis directed toward the implementation of a new method for computer-controlled low-photon counting and digital processing and analysis of the data to obtain intracellular Ca2+ concentration ([Ca2+]i). In eight ferret right ventricular trabeculae, we measured the mechanical performance and found that, at 1.25 mM extracellular Ca2+ concentration ([Ca2+]o), resting tension, developed tension, and time to peak tension were unchanged by the loading procedure. Estimated resting and peak systolic [Ca2+]i were 299 +/- 65 and 766 +/- 131 nM, respectively. Thirty minutes after raising the [Ca2+]o to 5 mM, there was a robust increase in mechanical performance, with peak systolic [Ca2+]i averaging 1,218 +/- 222 nM. The diastolic [Ca2+]i remained unchanged. In four other trabeculae, exposure to a low-Na(+)-containing superfusate demonstrated a remarkable beat-to-beat correspondence of increases in diastolic [Ca2+]i and resting tensions. The same beat-to-beat concordance was also observed between the rapidly changing amplitudes of peak [Ca2+]i and developed tension. In additional experiments, simultaneous recordings of [Ca2+]i and force transients were obtained during rapid pace pause maneuvers. These studies showed distinct and quantifiable fluctuations of [Ca2+]i in a 1:1 relation to the mechanical record to a frequency of at approximately 300 beats/min. These results demonstrate that beat-to-beat measurements of [Ca2+]i and tension transients can be obtained with good resolution in multicellular cardiac preparations.

A stable model of left ventricular dysfunction in an intact animal assessed with high fidelity pressure and cinemagnetic resonance imaging

OBJECTIVE

Numerous models of acute and chronic left ventricular dysfunction have been used over the years. However, few can produce a rapid onset of global systolic and diastolic dysfunction that is stable and potentially reversible. The aim of this study was to develop such a model.

METHODS

A model of left ventricular dysfunction was produced in six intact dogs using 1% halothane anaesthesia and pharmacological autonomic blockade with atropine (0.1 mg.kg-1) and propranolol (2 mg.kg-1). Left ventricular function was assessed by combined high fidelity pressure and cinemagnetic resonance imaging (cine-MR) during increases in afterload using infusions of angiotensin.

RESULTS

Left ventricular systolic dysfunction was characterised by a diminished resting ejection fraction of 45(SD 4)% and a depressed +dP/dtmax of 1537(100) mm Hg.s-1. Diastolic dysfunction was manifested by an increased left ventricular end diastolic pressure of 16(2) mm Hg, a decreased -dP/dtmax of -1705(369) mm Hg.s-1, and a prolonged time constant of left ventricular relaxation of 42(9) ms. As left ventricular systolic pressure steadily rose with angiotensin infusion from 87(7) to 124(13) to 152(10) mm Hg (p < 0.001), left ventricular ejection fraction decreased markedly from 45(4) to 35(4) to 27(4)% (p < 0.001). Left ventricular +dP/dtmax did not change [1537(100) to 1500(110) to 1498(84) mm Hg.s-1] in spite of a significant increase in left ventricular end diastolic pressure from 16(2) to 21(5) to 29(7) mm Hg (p < 0.001) and left ventricular end diastolic volume from 59(12) to 71(14) to 78(17) ml (p < 0.001). Individual slopes of the end systolic pressure-volume relationship were also low, ranging between 2.1 and 4.4 mm Hg.s-1 (r = 0.99 to 1.00), typical of impaired contractility.

CONCLUSIONS

Halothane anaesthesia in dogs pretreated with large amounts of propranolol and appropriate muscarinic cholinergic blockade produces a moderate decrease in baseline systolic and diastolic function in our intact dog model. However, left ventricular systolic function showed limited contractile reserve when challenged by physiological increases in systemic arterial pressure. Impaired systolic and diastolic function may, at least in part, be related to diminished activator calcium produced by halothane in addition to the well known negative inotropic action of beta adrenergic blockade.

Quantitative effects of sympathetic and vagal nerve stimulations on sinus and AV junctional rhythms

Responses of the sinus node and atrioventricular (AV) junctional pacemakers to autonomic denervation and to individual stimulations of the right and left stellate and both vagi were studied in 33 anesthetized dogs. Autonomic denervation depressed sinus node automaticity by only 18% from control, whilst AV junctional automaticity was reduced by 48.5% from control. Sympathetic and parasympathetic stimulation frequency-response curves (0.25, 0.5, 1, 2, 4, 8, 16 and 32 Hz) were obtained. In the sinus node the chronotropic responses to sympathetic stimulations reflect a bilaterally asymmetrical innervation with a right sided preponderance. In contrast, sinus slowing in response to either right or left vagal stimulations were indistinguishable when lower frequencies of stimulation were used. At 4 Hz and higher frequencies there is a right vagal preponderance. The AV junctional chronotropic responses suggest that this major subsidiary pacemaker receives a bilaterally symmetrical autonomic innervation. The chronotropic responses to individual nerve stimulations expressed as percent changes in sinus rate and AV junctional rate from their respective controls after autonomic denervation show that the AV junction is far more responsive than the sinus node to both sympathetic and parasympathetic stimulations. To allow for more meaningful comparisons the data were normalized using the respective maximum increase and maximum decrease of sinus node and AV junctional rates to left and right sympathetic and parasympathetic stimulations as the 100% reference. These normalized curves show that 50% of the maximal chronotropic responses were always achieved at a lower stimulus frequency in the AV junction than in the sinus node; shift of the AV junctional response curves to the left of the sinus node response curves by a 0.2 (sympathetic) and 0.3 (parasympathetic) log units was observed. These studies further showed that sympathetic activity in the AV junction is an absolute prerequisite to maintain regular AV junctional rhythms especially during the bradycardic episodes evoked in the study of vagal stimulus frequency-response curves.

Influence of ionic modification on electrical activity of Purkinje fibers obtained from dogs with 1-day-old myocardial infarction

A stable sustained rhythmic activity (SRA) occurred in 62 of 192 specimens isolated from the infarcted subendocardium of 48 dogs 24 h after a left descending coronary artery occlusion. Changes in [Na+]o and/or [Ca2+]o of the superfusate allowed us to distinguish two types of responses, suggestive of two different mechanisms for SRA. In type 1 responses at constant [Na+]o, the rate of SRA decreased when [Ca2+]o was increased and increased when [Ca2+]o was decreased. When [Ca2+]o was held constant, the rate of SRA was directly related to [Na+]o. In type 2 responses, at constant [Na+]o, the rate of SRA was directly proportional to [Ca2+]o. In contrast, at constant [Ca2+]o, SRA was inversely proportional to [Na+]o. When a constant [Ca2+]o/[Na+]o3 ratio was maintained, the type 2 response became indistinguishable from the type 1 response. The combination of lower temperature (36 degrees C) and high initial [Ca2+]o (2.7 mM) favored the type 2 response (28 of 32 preparations). In contrast, 25 of 30 preparations studied at 39 degrees C and 1.35 mM [Ca2+]o showed the type 1 response. These results suggest that SRA in the 24-h infarct model can be due to both abnormal automaticity (type 1) or triggered activity (type 2) and that changes in temperature and ionic milieu will largely determine which of the two mechanisms is responsible for SRA.

Excitation-contraction coupling model to estimate the recirculating fraction of activator calcium in intact cardiac muscle

Potentiated contractions were evoked with rapid pace pause maneuver in 14 length-clamped ferret papillary muscles paced 12 times/min at 25 degrees C. At 1.25 mM [Ca2+]o the average steady-state force was 2.94 +/- 1.08 g/mm2 and the potentiated contraction averaged 10.96 +/- 1.61 g/mm2. At 5.0 mM [Ca2+]o the steady-state force increased to 6.18 +/- 1.23 g/mm2 and the potentiated contraction averaged 12.08 +/- 1.15 g/mm2. Under the conditions of these experiments the potentiated contraction obtained at 5.0 mM [Ca2+]o is equal to the maximum twitch tension (Fmax) these muscles can generate. We have previously shown that Fmax is an equivalent of maximal calcium activated force. Since there is a beat to beat nearly exponential decay of the evoked potentiation, the fraction (= fraction x) of the potentiation that is not dissipated with each beat is nearly constant. Using an excitation-contraction coupling model we have previously found that x reflects a measure of the recirculating fraction of activator calcium. Because the tension-calcium relationship is better characterized by a sigmoidal curve, we have now incorporated the Hill equation in the model. To account for the inverse relationship between [Ca2+]i and the magnitude of the slow inward current, a term for negative feedback (h) was also included. We have determined the quantity (x-h) because x and h could not be determined separately. The quantity (x-h) was denoted as x'. The average values of x' at 1.25 and 5.0 mM [Ca2+]o were significantly different (p less than 0.0001), approximately 20% at the lower [Ca2+]o and about 50% at the higher [Ca2+]o.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of ryanodine on contractile performance of intact length-clamped papillary muscle

Extent, time course, and underlying mechanisms of the negative inotropic effect of ryanodine were examined in 22 length-clamped ferret right ventricular papillary muscles paced 12/min at 25 degrees C. After 60 minutes of exposure to 5 microM ryanodine a new steady state was attained with developed forces averaging 10-15% of maximum twitch force. Ryanodine does not pharmacologically excise the sarcoplasmic reticulum (SR) in this preparation. Ryanodine does not appreciably inhibit the ability of the SR to take up Ca2+ as evidenced by the potentiated beats obtained after a short pause that are nearly as large after ryanodine as before. On comparing equipotent beats before and after ryanodine, we found that ryanodine actually increases the rate at which Ca2+ is released during the twitch if the SR Ca2+ stores are equal or similar. The evidence for this conclusion is a larger maximum rate of tension rise and briefer time to peak tension after ryanodine. Since ryanodine increases the time that SR Ca2+ release channels are open and decreases their conductivity, it must follow that the former effect predominates over the latter in our experiments. Ryanodine increases the leakiness of the SR during diastole probably by inhibiting closure of SR Ca2+ release channels. The evidence for this conclusion is as follows: the early peak of the restitution curves after ryanodine, the brevity of the time required for a rested state contraction after ryanodine, and the small amplitude of the steady-state contraction at a rate of 12/min. The SR leaks even in the absence of ryanodine, but if external Ca2+ is so high that Ca2+ loss from the cell is slowed or a Ca2+ leak into the cell through the sarcolemma cancels the SR leak, then the effects of the SR leak are minimized. The evidence for this conclusion is the time required for rested-state contraction to occur or the slope of the descending limb of restitution curve; however, in presence of ryanodine even high external Ca2+ cannot prevent rapid depletion of SR Ca2+ stores. Even though we have presented evidence for a mechanism whereby ryanodine increases the number of open SR Ca2+ release channels in both systole and diastole, we do not mean to imply that most of them stay open in diastole; the SR would leak too fast to accumulate any Ca2+ for the potentiated beat. Thus, probably most channels close after being open a certain length of time, even in the presence of ryanodine.

Maximal twitch tension in intact length-clamped ferret papillary muscles evoked by modified postextrasystolic potentiation

A modified test of postextrasystolic potentiation achieved with a brief episode of rapid pacing followed by a 6-second pause (RPP maneuver) was used to evoke maximal force in isolated intact ferret right ventricular papillary muscles. Maximal RPP tensions were examined under length-clamped conditions and compared with the steady-state forces obtained when further increases in [Ca2+]o, did not further increase force and to the tensions recorded at the point of saturation of force when similarly length-clamped muscles were subjected to caffeine-induced tetanization. The results show that the calculated maximal twitch tension achieved with RPP is comparable to the 25-35 g/mm2 observed in intact single skeletal muscle fibers. The study also shows that the beat-to-beat decay of the potentiated contraction is exponential. While the amount of the constant fractional beat-to-beat decay is a function of [Ca2+]o, it is not influenced by length. During the decay of potentiation, the ratio of the potentiation of any beat divided by that of the previous beat is a constant, called (X). With certain assumptions, it is shown that (X) is a measure of the fraction of activator calcium taken up by the sarcoplasmic reticulum in each beat and, in the steady state, the fraction of activator calcium that comes from the sarcoplasmic reticulum. The (X) amounted to 33%, 50%, and 65% when [Ca2+]o was 1.25, 2.50, and 5.0 mM, respectively. Thus, at 1.25 mM [Ca2+]o, some two thirds of the total calcium required to activate the myofilaments comes from the extracellular compartment during excitation and only one third is contributed via release from the sarcoplasmic reticulum. In the region of optimal myofilament overlap, RPP force-length curves are remarkably shallow and almost indistinguishable from the sarcomere length-tension relation observed in skinned single cardiac cells. Tetanus plateau tensions are significantly smaller than RPP forces at any length, and the slope of the tetanus force-length curves is greater than that obtained with RPP. Thus, and by exclusion, we also suggest that caffeine may exert significant downstream inhibitory effects.

[The appearance and development of ventricular rhythm disorders in the 1st 24 hours after the start of experimental myocardial infarct in dogs]

The emergence of ventricular tachycardia (VT), its temporal progress and response to high-frequency stimulation were studied in 10 dogs subjected to two-step occlusion of the left descending coronary artery (LDA). Within two hours after the LDA ligation under atrioventricular block all animals developed an atrioventricular nodal rhythm of 37 +/- 9 pulses per minute that could be suppressed by high-frequency ventricular stimulation, i.e. the so-called overdrive suppression (OS) phenomenon was occurring. Three or four hours after LDA ligation, atrioventricular block brought out VT in 8 dogs. The onset of VT was always abrupt, its episodes being short-lived at first and growing progressively longer with time. Once VT was established, its rate increased gradually to reach the peak that exceed the base line VT rate by 21 +/- 9% 2 or 3 hours later. As the VT rate increased, the OS phenomenon grew less pronounced and disappeared altogether as the VT peak was reached. The results suggest that the abrupt emergence of VT 3 or 4 hours after the onset of myocardial infarction can be a result of ectopic pacemaker activity of partially depolarized fibres that in some cases may be due to the effects of a trigger mechanism.

The effects of negative chronotropic interventions on sinus node recovery time

The effects of multiple increases in sinus cycle length on sinus node recovery time (SNRT) were examined in 5 dogs. Pacing was performed from the left atrial appendage for 30 and 60 seconds using at least 4 different pacing cycle lengths selected between 230 and 620 msec. Each dog received propranolol (1 mg/kg, IV) prior to any measurements. The effects of increases in sinus cycle length on SNRT were first assessed during 2 levels (4 and 8 Hz) of continuous vagal stimulation. From a control cycle length of 439 +/- 28 msec (mean +/- SE), the vagal stimulations lengthened the sinus cycle lengths to 604 +/- 10 msec and 758 +/- 16 msec respectively. Sinus cycle length was then prolonged by combined muscarinic and beta-receptor blockade resulting in a sinus cycle length of 549 +/- 9 msec. Autonomic blockade plus verapamil (3-10 mg IV) resulted in sinus cycle lengths of 612 +/- 14 and 721 +/- 18 msec respectively, which were not significantly different from those obtained with vagal stimulation. Data relating SNRT to the sinus cycle length, pacing cycle length, duration of pacing and the negative chronotropic interventions used to achieve the changes in the sinus cycle length were analyzed via covariance analysis. The results demonstrate that the single most important determinant of SNRT is the sinus cycle length. Furthermore, equivalent increases in sinus cycle length whether obtained by vagal stimulation, autonomic blockade or intravenous verapamil results in SNRTs that are not significantly different. Therefore, in the sinus node, changes in the rate of pacemaker activity, regardless of how they are achieved, will largely determine the changes in SNRT.

Differential interaction of adrenergic and cholinergic effects on AV junctional automaticity and AV conduction

The effects of postsynaptic autonomic interactions on atrioventricular (AV) junctional automaticity and AV conduction were studied in six canine heart in situ using direct injections of norepinephrine (NE) and physostigmine (PSM) into the AV node artery. Injection of NE (0.05 microgram/ml, 2 ml) caused an AV junctional rhythm (AVJR) in every dog. After injection of PSM (10 micrograms/ml, 2 ml), the responses of AVJR to NE were virtually identical to those observed before cholinesterase inhibition (160 +/- 13 vs 162 +/- 12 bpm). In contrast, this moderate cholinesterase inhibition still had a readily demonstrable negative dromotropic effect. In any given dog, depressed AV conduction was characterized by one of two types (I and II) of retrograde atrial capture during AVJR. Before PSM in the AV junction, onset of atrial depolarization during AVJR preceded the onset of ventricular depolarization in both type I and type II responses. After PSM, atrial depolarization occurred later with respect to ventricular depolarization (i.e., during or mostly after ventricular activation) in type I, whereas in the type II responses atrial depolarizations began much earlier than before PSM, thus being completed long before the onset of ventricular activation. Because of such differential responsiveness of AV junctional automaticity and AV conduction and because of the two types of intranodal conduction observed after administration of PSM into the AV junction, we can postulate that under appropriate autonomic imbalance retrograde or antegrade AV block could readily develop in spite of preserved AV junctional automaticity.

Differential effects of sympathetic activity on AV junctional automaticity and AV conduction

Rapid ventricular response during episodes of supraventricular tachycardia are often followed, on abrupt cessation of the tachycardia, by prolonged pauses terminated by a sluggish and sometimes erratic escape of a supraventricular pacemaker. Such chronotropic-dromotropic paradoxes are readily reproduced in the animal laboratory following elimination of the sinus node and bilateral decentralization of the stellate ganglia and vagi. This study examined whether left stellate stimulation (0.5, 1, 2, 4, 8 and 16 Hz) or lack thereof differentially affected AV junctional automaticity and AV conduction. In the absence of any sympathetic neural activity (maximal sympathetic deficit), the AV junctional rate averaged a mere 22 +/- 2 percent of its peak performance, whereas under the same conditions, anterograde AV conduction averaged 73 +/- 5 percent and retrograde VA conduction 56 +/- 13 percent of their respective peak performances. On comparing the response curve (normalized responses) for AV junctional automaticity with that obtained for anterograde AV conduction the differences were significant at all frequencies between 0 and 4 Hz. Retrograde VA conduction (as assessed by the fastest ventricular pacing rate still conducted 1:1 to the atria) was always significantly less than anterograde AV conduction (as assessed by the fastest atrial pacing still conducted 1:1 to the ventricles). These results indicate that AV junctional automaticity is considerably more affected by sympathetic deficit than are either anterograde or retrograde AV conduction. In other words, AV junctional automaticity is far more dependent upon sympathetic input than AV conduction. While sympathetic influence is critical to the escape and maintenance of AV junctional automaticity both anterograde and retrograde AV conduction are remarkably resilient even under conditions of severe sympathetic deficit.

Atrioventricular junctional tachycardia during heart block

The response of the atrioventricular (AV) junction to brief intense adrenergic stimulation applied during episodes of second degree heart block achieved by acetylcholinesterase paralysis in the AV junction was examined in six dogs. Despite profound depression of AV conduction due to enhanced cholinergic activity, strong local adrenergic stimulation still readily elicited AV junctional tachycardia. Increase in cholinomimetic influences in the AV junction did not prolong transatrial or His bundle-ventricular conduction times. During AV junctional rhythm and retrograde atrial capture (n = 4), neither the sequence of retrograde atrial activation nor the atrial electrogram configurations were altered. In the two remaining dogs the AV junctional tachycardia was associated with AV dissociation. These findings suggest that the acetylcholine-induced depression of AV conduction is located in the AV node region exclusively. More important, however, is the demonstration that retrograde atrial activation originating from a pacemaker located in the AV node or immediate vicinity could actually precede the inscription of the H spike by a considerable amount of time, further suggesting that anterograde conduction from the pacemaker site to the bundle of His is far more depressed by acetylcholine than is the concomitant retrograde conduction from the pacemaker site to the atrium. Thus, inference of the origin of a subsidiary pacemaker from the P wave configuration or the relation of the A wave to the His bundle electrogram, or both, may lead to erroneous conclusions.

Cardiac autonomic efferent activity during baroreflex in puppies and adult dogs

Blood pressure and heart rate were recorded in 15 anesthetized puppies (6-10 wk, 1-6 kg) and 18 adult mongrel dogs (greater than 1 yr, 18-26 kg) before and during acute blood pressure changes achieved with nitroglycerin or phenylephrine (4 and 8 micrograms/kg iv). Overall heart rate responses to blood pressure changes in adults were significantly (P less than 0.05) greater than those in puppies. Following control baroreflex responses, two multifiber efferent preparations from the discrete thoracic cardiac nerves (sympathetic, n = 48; parasympathetic, n = 18) were simultaneously recorded and analyzed by microprocessor. Severing of the nerves significantly attenuated the heart rate responses to blood pressure changes in puppies only, suggesting less redundancy of the neural regulation of the sinus node in the puppy. The pressure-induced reflex changes in the sympathetic or parasympathetic efferent nerve activities were not significantly different between adult dogs and puppies. There were no significant differences in reflex activities in right-sided (n = 29) vs. left-sided (n = 19) sympathetic nerves in either puppies or adult dogs. Preganglionic sympathetic fibers in puppies (but not adult dogs) were more responsive to blood pressure changes than were postganglionic sympathetic fibers. Thus baroreceptor reflex control in the puppy is less developed than in the adult canine heart, and the maturational difference in neural regulation of the heart is at or beyond the efferent nerve terminals.

Serial production of controlled periods of temporary heart block used to unmask and assess latent ventricular automaticity during experimental acute myocardial ischemia

This study examined the onset, time course of development and response to overdrive stimulation of ventricular tachycardia in 10 dogs that underwent a Harris two-stage ligation of the left anterior descending coronary artery. Transient (12 +/- 3 minutes) complete atrioventricular (AV) block was produced 2, 3, 4, 5, 8, 12, 16, 20 and 24 hours after onset of infarction through selective injection of physostigmine salicylate into the AV node artery. Seven of the 10 dogs had early transient arrhythmic episodes that occurred within 20 to 40 minutes after coronary occlusion but none of the dogs had any spontaneous ventricular tachycardia in the ensuing 2 hours. Two hours after left anterior descending coronary artery ligation, complete AV block unmasked in every dog a slow (37 +/- 9 beats/min) AV junctional rhythm readily suppressed by overdrive. Three hours after coronary ligation, AV block revealed a monomorphic ventricular tachycardia (106 +/- 10 beats/min) in 3 of the 10 dogs. Four and five hours after coronary ligation, five and eight dogs, respectively, had ventricular tachycardia during AV block and in three the tachycardia was polymorphic. The two remaining dogs did not develop ventricular tachycardia during the 24 hours of observation. Ventricular tachycardia always began abruptly, first with brief and then longer bursts. Soon after onset the rate of tachycardia began to increase to reach a plateau 2 to 3 hours later at frequencies 21 +/- 9% greater than the initial tachycardia rate. Concomitant with this increase in rate there was a steady decline of overdrive suppressibility and during the plateau phase there was little or no overdrive suppression.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential modulation of autonomic activity by ethmozin and ethacizin (analog of ethmozin) on the canine sinus node and atrioventricular junction

The chronotropic and dromotropic actions of ethmozin and its diethylamine analog ethacizin were studied in the presence and absence of combined muscarinic, beta- and alpha-adrenoreceptor blockade in the intact canine heart in situ (n = 38). Injections of ethacizin, 5, 10 and 25 micrograms/ml, into the sinus node artery caused an immediate and significant (p less than 0.001) sinus bradycardia of 2, 6 and 11%, respectively. Injection of 25 and 50 micrograms/ml of ethacizin into the atrioventricular (AV) node artery significantly (p less than 0.001) prolonged AV conduction time with occasional second degree heart block. Conduction delay was located exclusively during the AH interval of the His bundle electrogram. Autonomic blockade did not alter the negative chronotropic or negative dromotropic effects of ethacizin. Ethacizin, 25 micrograms/ml, injected into the sinus node artery immediately reduced the sinus node response to vagal stimulations by 30% and the effect of acetylcholine, 0.1 micrograms/ml, injected into the sinus node artery by 50%. Ethacizin, 25 micrograms/ml, injected into the AV node artery immediately reduced the duration of complete AV block elicited by vagal stimulation or intranodal acetylcholine, 0.5 micrograms/ml, by 90%. Ethacizin caused a minor reduction in sinus node response to right stellate stimulations without, however, altering the sinus node response to intranodal norepinephrine. Ethmozin injections of up to 50 micrograms/ml into the sinus and AV node arteries had no chronotropic or dromotropic effects. Ethmozin had a minor and variable vagolytic action but significantly (p less than 0.05) reduced the sinus node response to sympathetic nerve stimulation. Hence, ethacizin, in contrast to ethmozin, has a direct depressing action on both the sinus node and the AV junction.(ABSTRACT TRUNCATED AT 250 WORDS)

Ventricular echo beats during ventricular pacing or junctional bradycardia: studies in the normal canine heart

In 15 adult dogs ventricular echoes were elicited during sinus rhythm by incremental ventricular pacing and during atrioventricular (AV) junctional rhythm by depressing simultaneously AV junctional automaticity and retrograde AV nodal conduction. Concomitant slowing of AV junctional automaticity and conduction was achieved by selective intranodal administration of verapamil. In three dogs incremental pacing from either ventricle failed to retrogradely activate the atria, and in each case the site of block was found to be in the AV node. In two dogs with retrograde atrial capture there was little or no rate-dependency of retrograde ventriculoatrial (VA) conduction. During incremental ventricular pacing a single ventricular echo beat was observed in 10 of the 12 dogs that had atrial capture, and the atrium appears to be an essential link in the production of each ventricular echo. Ventricular echo occurred when the time allotted for retrograde VA conduction amounted to 70 +/- 4% of the duration of the ventricular pacing cycle length. During AV junctional rhythm, a single ventricular echo was elicited in half of the dogs and in each of those cases intranodal verapamil produced a profound depression of retrograde VA conduction. These experiments suggest that retrograde AV nodal longitudinal dissociation occurs in the slow current-dependent cells of the AV node.

Differential inotropic actions of ethmozine and ethacizin (diethylamine analog of ethmozine)

The direct inotropic actions of ethmozine and of its diethylamine analog, ethacizin, were studied in the presence of muscarinic and beta-adrenoreceptor blockade in 12 ferret right ventricular papillary muscles. In each muscle ethmozine caused a small but consistent and significant (p less than 0.05) increase in contractile performance, whereas ethacizin significantly (p less than 0.05) diminished contractility. Although both phenothiazines are fast channel blockers, it appears that the net positive inotropic action of ethmozine is due to its stimulatory effect on the slow inward current and that the negative inotropic action of ethacizin is largely due to its recently demonstrated decreases of the slow inward current.

Differential sympathetic-parasympathetic interactions in sinus node and AV junction

This study examined the effects and interactions evoked by various combinations of parasympathetic (P) and sympathetic (S) stimulations on sinus node automaticity and atrioventricular (AV) conduction (both anterograde and retrograde) in 11 two-mo old anesthetized puppies. The respective effects of these autonomic maneuvers were assessed by covariant analysis using a linear regression model of the form Y = alpha 0 + alpha 1 X base line + alpha 2 X sequence + alpha 3 X S + alpha 4 X S2 + alpha 5 X P + alpha 6 X P2 + alpha 7 X (P X S).... The effects of parasympathetic and sympathetic stimulation, of the quadratic term S2, and of the interaction term (P X S) were highly significant (P less than 0.0001) on both sinus node automaticity and AV conduction (anterograde and retrograde). In contrast, the effect of the quadratic term P2 was significant on sinus node automaticity only. Sequence of stimulation whether (SP) or (PS) had no significant effect on either chronotropic or dromotropic properties. Furthermore, whereas augmented antagonism with parasympathetic predominance over sympathetic activity was readily demonstrable in the control of sinus rate, the reverse, i.e., augmented antagonism with sympathetic preponderance over parasympathetic activity, was found to apply to the control of AV conduction (both anterograde and retrograde). Hence, the nature and extent of parasympathetic-sympathetic interactions vary considerably, depending on which cardiac structure and/or function is under scrutiny. It appears that AV conduction is especially sensitive to modulation of sympathoadrenergic activity, whereas sinus node automaticity is particularly responsive to cholinomimetic influences.

Effects of the diethylamino analog of ethmozin (ethacizin) upon sympathetic and parasympathetic efferent activity to the canine heart

This study examined the effects of the diethylamino analog of ethmozin (ethacizin) (1 mg/kg i.v.) on the spontaneous and reflexly elicited efferent activity in thoracic cardiac sympathetic and parasympathetic nerves. Nitroglycerin and phenylephrine (4 and 8 micrograms/kg i.v.) were administered to 15 anesthetized mongrel dogs while monitoring blood pressure and heart rate. In each dog two cardiac nerves were isolated and efferent neurograms were recorded simultaneously and analyzed by microprocessor. Ethacizin significantly attenuated the spontaneous sympathetic efferent activity in both left and right, preganglionic (n = 8) and postganglionic (n = 14) sympathetic nerves to the heart. In contrast, reflex changes in sympathetic activity elicited by baroreceptor challenges were not affected by ethacizin. Also, ethacizin did not significantly affect either spontaneous or baroreceptor reflex-induced parasympathetic efferent activities in eight preganglionic nerves. Thus, this new phenothiazine derivative may exert part of its antiarrhythmic action through a reduction of the spontaneous sympathetic tonic discharges to the heart. The fact that ethacizin reduced neither the reflex-induced changes in sympathetic or parasympathetic activities nor influenced the tonic vagal discharges further suggests that the compound is not likely to interfere with reflexly mediated cardiovascular adaptive changes.

Differences in the determinants of overdrive suppression between sinus rhythm and slow atrioventricular junctional rhythm

Sinus node recovery time was compared to the recovery time of a slow atrioventricular junctional rhythm in each of the same seven pentobarbital anesthetized dogs. Recovery time and the first five cardiac cycles were examined after pacing atria and ventricles for 20, 40, and 60 seconds at four or more pacing cycle lengths. Data relating recovery times and return to control conditions to prepacing cycle length, pacing cycle length, duration of pacing, site of pacing, and origin of rhythms were analyzed by covariance analysis. From the analyses, the relative contribution of the determinants are: the prepacing cycle length 73%, the site of pacing 3.5%, the pacing cycle length 2%, and the interaction of the site of pacing and pacing cycle length 1% for sinus node recovery time; and for slow atrioventricular junctional rhythm recovery time, the duration of pacing 40%, the interactions between the duration of pacing and the pacing cycle length 27%, and the prepacing cycle length 9%. A modified exponential decay model predicted 8 beats for return to prepacing conditions during sinus rhythm and 66-100 beats during atrioventricular junctional rhythm. We conclude that the single most important determinant of sinus node recovery time is the prepacing cycle length. Pacing cycle length and site of pacing have a significant but small influence on sinus node recovery time and duration of pacing, beyond 20 seconds, has no significant influence. In contrast, duration of pacing is the most important determinant of slow atrioventricular junctional recovery time. Another major determinant of slow atrioventricular junctional recovery time is the interactions between pacing cycle length and duration of pacing. Prepacing cycle length has a minor influence, and site of pacing has no influence, on slow atrioventricular junctional recovery time.

Negative chronotropic and parasympatholytic effects of alinidine on canine sinus node and AV junction

The direct effects of alinidine (N-allyl-clonidine) on the sinus node and atrioventricular (AV) junction were studied in 18 anesthetized dogs. Stimulus frequency-response curves to right stellate ganglion and right cervical vagus stimulations as well as responses to norepinephrine or acetylcholine were determined before and after selective perfusion of alinidine into the sinus node artery. Alinidine (1 microgram/ml) had no effect on spontaneous sinus rate [148 +/- 5 (SE) beats/min]. However, alinidine concentrations of 5, 10, and 25 micrograms/ml produced significant (P less than 0.05) sinus slowing to 138, 127, and 121 beats/min, respectively. Recovery to control rate was dose dependent and took from 4 to 33 min. Sinus rate increases with right stellate stimulations were not affected by alinidine. However, sinus rate decreases with right vagal stimulations were significantly (P less than 0.01) attenuated by alinidine. The negative chronotropic effects of acetylcholine were not influenced by alinidine. Alinidine (1-100 micrograms/ml into AV node artery) had no effect on the A-H interval of the His bundle electrogram. However, alinidine (10 and 25 micrograms/ml) diminished the AV block produced by stimulation of the left vagus in electrically paced hearts but not the negative dromotropic actions of directly administered acetylcholine. Thus alinidine has direct negative chronotropic effects, no effect on sinus node responses to sympathetic stimulation, ability to diminish sinus node and AV junctional responses to vagal stimulations without interference at the cholinergic muscarinic receptor, and 4) no effect on AV nodal conduction.

Parasympathetic and sympathetic efferent traffic during a cardiogenic hypertensive chemoreflex

Activation of a cardiac chemoreceptor with serotonin elicits a reflex which includes changes in heart rate, contractile force, regional blood flow and hypertension. In six anaesthetised dogs we simultaneously recorded parasympathetic and sympathetic efferent traffic elicited during this cardiogenic reflex. The parasympathetic fibres were confirmed by reciprocal frequency changes with changes in blood pressure. The sympathetic fibre activity (anterior ansa subclavia) was attenuated or eliminated by ganglionic blockade or by clonidine. Whereas the phasic sympathetic multifibre discharge was only followed by a quiet period, the parasympathetic multifibre discharge was both preceded and followed by a quiet period. The sympathetic discharge preceded the parasympathetic discharge by 683 +/- 170 ms. These autonomic efferent discharges were not elicited by administration of serotonin into the carotid artery, but were abolished by pretreatment with the serotonin antagonist, cyproheptadine. Cyproheptadine blockade could be overcome by increasing the serotonin concentration tenfold. This remarkable neural asynchrony has important implications concerning the electrical stability of the heart.

Indirect stimulatory action of the calcium channel blocker AQA-39

AQA-39 is a new bradycardia-inducing drug chemically related to verapamil that reduces potassium conductance and blocks calcium channels. In canine ventricular trabeculae studied at 25 degrees C and at a pacing rate of 12 stimuli per minute. AQA-39, in concentrations of 1 and 2 X 10(-5) M, had a significant positive (and only positive) inotropic effect. Propranolol significantly diminished this positive inotropic action of AQA-39. After catecholamine depletion with reserpine, AQA-39 still elicited a significant increase in contractility but the magnitude of the increment in contractile performance was considerably less than the one observed when normal muscles were exposed to AQA-39. AQA-39 had no significant inotropic action on reserpinized and atropinized muscles. From these results we can conclude that AQA-39 is not a beta adrenoceptor agonist although adrenergic influences via neuronal norepinephrine release are mediating part of the inotropic effect of the drug. Furthermore, at this low rate of stimulation and at the concentrations used, AQA-39 has no direct inotropic action of its own but part of its indirect stimulatory action is mediated through an antimuscarinic effect.

No electrical instability after intracoronary streptokinase administered into sinus node or AV node of dogs

Intracoronary streptokinase (SK) is being used to treat acute myocardial infarction. Complications including atrioventricular (AV) block and both supraventricular and ventricular arrhythmias have been described. Selective cannulation of the nutrient arteries of the sinus node and AV node in the pentobarbital-anesthetized dog provides a model to study the specific pharmacologic effects of drugs administered directly to these structures. Selective perfusion of 2 ml normal saline into the sinus node artery of five dogs resulted in a transient 38.6% decrease in heart rate (148 +/- 9.6 to 91 +/- 12.7 bpm). This control response is typical of the method. Administration of 100, 500, 1000, 2000, and 4000 IU of SK/ml in these same dogs resulted in similarly brief bradycardias of 41.2%, 40.2%, 39.4%, 41.2%, and 40.3% (p = NS) below the control rate, respectively. The tachycardia induced by 0.0125 and 0.025 micrograms/ml of norepinephrine (24.2% and 31.4% sinus acceleration) was unaffected by the simultaneous administration of 500, 1000, and 2000 IU SK/ml. AV conduction was unchanged by selective perfusion of 1000, 2000, and 4000 IU SK/ml into the AV node artery of seven dogs, respective AV intervals being 111, 114, and 116 msec, with the control interval being 109 msec. Thus after selective perfusion of either the sinus node or AV node with SK in clinically used concentrations, we found no significant chronotropic or dromotropic effect in the dog.

Studies of changes in breathing and blood pressure accompanying a cardiogenic hypertensive chemoreflex compared in conscious and anesthetized dogs

Serotonin activates a cardiogenic hypertensive chemoreflex characterized by sudden hypertension. This study in 16 awake and 12 anesthetized dogs was conducted to examine the influence of changes in breathing or heart rate as they may influence the reflex hypertension. In all 28 dogs, either anesthetized or awake, there is a variable but brief initial reflex bradycardia and in all dogs there is transient ventilatory stimulation, but the response in every dog was characterized by hypertension within at most 10 seconds after administering serotonin. Treatment with atropine regularly eliminated the reflex bradycardia and caused the hypertension to appear soon (average of 6 seconds to maximal level). Atropine did not alter the reflex stimulation of breathing. Since atropine eliminated any transient bradycardia (and associated hypertension) but did not prevent reflex changes in breathing, we do not believe that breathing alteration plays any significant hemodynamic role in this reflex.

Acute effects of amiodarone upon the canine sinus node and atrioventricular junctional region

Amiodarone was selectively perfused into the sinus node artery and atrioventricular node artery of 51 dogs. Amiodarone had an immediate negative chronotropic and dromotropic effect. Threshold concentration was 2.5 micrograms/ml. 25 and 50 micrograms/ml of amiodarone injected into the sinus node artery slowed the heart by 25.6 +/- 3.1 and 33.7 +/- 2.6 beats/min (mean +/- 1 SEM), respectively. Amiodarone 25 and 50 micrograms/ml injected into the AV node artery during AV junctional rhythm slowed the AV junctional pacemaker by 12.2 +/- 1.8 and 17.4 +/- 1.7 beats/min, respectively. Injections of amiodarone into the AV node artery during sinus rhythm regularly increased AV conduction time sometimes causing 2 degrees AV block at the highest concentration used. Impaired conduction was exclusively measured at the level of the A-H interval in the His electrogram. Neither atropine nor propranolol prevented the negative chronotropic effects of amiodarone. Amiodarone had no significant effect on sinus node response to either stellate stimulation or intranodal administration of norepinephrine. The negative chronotropic action of amiodarone was significantly enhanced when amiodarone was administered in a perfusate containing low (0.6 mM) instead of normal calcium. Taken collectively these observations indicate that amiodarone has immediate depressant electrophysiologic effects on both the sinus node and the AV junction and that these early effects might involve the blockade of the slow channel.

Differential effect of verapamil isomers on sinus node and AV junctional region

The l- and d-isomers of verapamil were selectively perfused into the sinus node artery and atrioventricular (AV) node artery of 48 dogs. Injection of l-verapamil into the sinus node artery during sinus rhythm and into the AV node artery during AV junctional rhythm depresses both sinus rhythm and AV junctional rhythm significantly more than does the d-isomer. l-Verapamil is three to four times more powerful than d-verapamil. Injection of the isomers into the AV node artery during sinus rhythm rapidly impairs AV conduction. Increments in conduction time are measured exclusively at the level of the A-H interval of the His bundle electrogram, and l-verapamil is six times more powerful than d-verapamil. Neither d- nor l-verapamil in concentrations that exert a profound negative chronotropic effect or cause AV block, has any significant effect on transatrial or His bundle conduction. Thus these concentrations of d-verapamil have little or no significant effect on the fast sodium channel, but both verapamil isomers affect the slow channel. The main difference in action between l- and d-verapamil appears to be only quantitative in nature. The sinus node is significantly more sensitive to the negative chronotropic action of verapamil than is the AV junctional pacemaker, and this differential responsiveness appears to be related to the different intrinsic rates of the two pacemakers. During sinus rhythm (either in the presence or absence of atropine) sinus node automaticity is less affected than AV conduction when verapamil is given parenterally. We propose that this greater negative dromotropic effect of verapamil is also in part due to a rate-dependent process, since during sinus rhythm AV junctional cells have to be depolarized at frequencies significantly higher than their intrinsic rates.

Post-excitatory depression in thoracic sympathetic efferent neural traffic during a cardiogenic hypertensive chemoreflex

Serotonin injected in the left atrium activates a cardiogenic hypertensive chemoreflex in dogs. To elucidate patterns of the neural traffic, records were obtained from thoracic sympathetic efferent nerves (either the anterior ansa of the left stellate ganglion or the T4 input to the left stellate) in 8 anesthetized dogs with chest open. Serotonin (200 micrograms, left atrium) caused a massive sympathetic discharge during the hypertension and bracardia characteristic of the chemoreflex. Following the initial sympathetic discharge, there was a consistent post-excitatory depression of neural traffic, to a level significantly less than control discharge (two-tailed p less than .05). This post-excitatory depression began 11 +/- 5.4 (S.D.) seconds after injection of serotonin and 6.6 +/- 5.3 seconds after the peak neural discharge. It lasted 140 +/- 94 seconds, being maximal initially with gradual recovery. Complete block of the hypertension by the combined administration of phentolamine, propranolol, and nitroglycerin failed to abolish the efferent neural events, including post-excitatory depression, in all but one dog. We conclude that post-excitatory depression in thoracic sympathetic efferent neural traffic cannot be mediated exclusively through the secondary engagement of a baroreceptor mechanism and that it most likely is an integral part of the cardiogenic hypertensive chemoreflex.

The direct negative chronotropic action of prazosin on the canine sinus node

The direct and indirect chronotropic effects of prazosin were studied in 22 anesthetized dogs using the technique of selective perfusion of the sinus node. Concentrations of 1, 10, 50 and 100 micrograms/ml of prazosin produced cumulative, dose-related reductions in mean sinus rat of 4 +/- 2; 10 +/- 6; 15 +/- 9 and 27 +/- 15 beats/min, respectively (P less than .0001). Intranodal prazosin did not significantly alter the positive chronotropic sinus node response to norepinephrine (0.05 and 0.1 micrograms/ml) administered via the same route. Sinus rate increases during stimulus frequency response curves after right stellate stimulation were not influenced by prior administration of prazosin. Sinus bradycardia elicited by submaximal vagal stimulation also was not affected by intranodal prazosin.l In six reserpinized dogs, methoxamine (10 and 25 micrograms/ml) injected into the sinus node artery caused a sinus rate increase of 10 4/- 2 and 14 +/- 3 beats/min (P less than .001). Intranodal prazosin (50 micrograms/ml) prevented this effect. Taken together, these results indicate that prazosin exerts a direct negative chronotropic action on the canine sinus node and that this effect is at least in part mediated through interference with alpha-1 receptors located in or near the sinus node. Prazosin has little or no effect on the presynaptic alpha-2 receptors nor does it interfere with the cholinergic mechanism in the sinus node.

Effects of selectively produced changes in cardiac rhythm or conduction upon flow in the superior vena cava

The effects of changes in cardiac rhythm or conduction on the rate of volume flow in the superior vena cava (SVC) were studied in 25 dogs anesthetized with sodium pentobarbital. Slowing of sinus rate from 145 +/- 16 to 114 +/- 12 beats/min was achieved by selective perfusion of Ringer's solution into the sinus node artery. During sinus slowing there was a continuous change in the systolic-diastolic flow ratio measured in the SVC. At control sinus rate (net), total forward flow as 6.3 ml/beat with a systolic volume of 6.5 ml and a diastolic volume of -0.2 ml. At the slowest bradycardia, total forward flow had increased by 25%. Since there was no significant change in the systolic volume, the entire 25% increase in forward flow occurred during ventricular diastole. During atrioventricular (AV) block (second-degree or complete), achieved by selective perfusion of eserine into the AV node artery, unimpaired acceleration of flow was readily demonstrable in the SVC even in the absence of ventricular contraction. Thus systolic ventricular suction (vis a fronte) has little or no effect on right atrial filling. During high-degree AV block, the normal atrial contraction readily opened the tricuspid valve, and when both atrium and ventricle were filled to or near maximum capacity, atrial contraction was consistently vigorous enough to propel blood into the pulmonary artery. Given appropriate hemodynamic circumstances, these results confirm the remarkable effectiveness of right atrial contractile performance.