Simulated ischaemia and reperfusion in isolated guinea pig ventricular myocytes.

OBJECTIVE: The objectives were (1) to develop a cellular model of simulated ischaemia and reperfusion in isolated ventricular myocytes; (2) to determine effects of simulated ischaemia and reperfusion on calcium current (ICa), transient inward current (ITI) and contraction; and (3) to determine whether pharmacological agents which alter intracellular sodium and calcium loading affect signs of calcium overload in reperfusion in this model. METHODS: Electrical activity was recorded with conventional and voltage clamp techniques. Cell shortening was measured with a video edge detector. Myocytes were equilibrated in Tyrode solution, exposed to simulated ischaemia (hypoxia, acidosis, lactate, hyperkalaemia, glucose-free) for 20 min, and reperfused with Tyrode solution. RESULTS: Ischaemia depolarised myocytes [-89(SEM 1) to -67(4) mV, p < 0.05], abbreviated action potential duration [APD90, 257(14) to 188(12) ms, p < 0.05], and abolished contractions. Contractions elicited by voltage clamp steps also were abolished in ischaemia; however, ICa decreased by only 51% [-0.98(0.08) to -0.50(0.06) nA, p < 0.05]. Signs of calcium overload, including aftercontractions, oscillatory afterpotentials, and ITI, occurred in 69% of myocytes in reperfusion. Upon reperfusion, both APD90 and ICa recovered slowly; however, contractions returned quickly and temporarily exceeded control. Amiloride during ischaemia and reperfusion lowered incidence of ITI in reperfusion, whereas nifedipine and lignocaine had no effect on ITI. CONCLUSIONS: This model of ischaemia and reperfusion in ventricular myocytes shows many features of multicellular preparations, such as membrane depolarisation and action potential duration shortening during ischaemia, and appearance of oscillatory afterpotentials upon reperfusion. Inhibition of contraction during ischaemia and recovery of contraction in reperfusion are independent of changes in APD90 or ICa. Induction of aftercontractions, oscillatory afterpotentials, and ITI in reperfusion is associated with reduced peak ICa. Amiloride most probably decreased signs of calcium overload in early reperfusion by inhibiting sodium loading via Na+/H+ exchange. Additionally, amiloride may inhibit ITI directly by blocking Na+/Ca2+ exchange.

Acridine orange fluorescence cytochemistry for detecting lymphocyte immunoreactivity.

Acridine orange staining reveals changes within 3 hours of in vitro stimulation of normal rat lymphocytes with mitogens, and of immune rat lymphocytes with the sensitizing antigen. An increased number of red fluorescent cytoplasmic organelles, presumably lysosomes are seen by fluorescence microscopy. Fluorimetry of the supernatants from stained cell suspensions suggests an overall decreased cell uptake of the dye. The microscopy and fluorimetry detected early events in the reaction of lymphocytes from tumour-bearing rats with the target tumour cells. It would appear that the changes in intracellular behaviour of the dye and in overall cell uptake after immune stimulation are a reflection of dissociated variations in internal and external cell membrane permeability, and may provide simple general means for recognizing cellular immune reactions.

Role of prostaglandins in the arrhythmogenic effects of ouabain on isolated guinea pig hearts.

We examined the hypothesis that endogenous prostaglandins participate in the arrhythmogenic influence of ouabain in guinea pig hearts. Addition of ouabain (10 ng/ml) resulted in a 5-fold increase in the release of 6-keto-prostaglandin F1 alpha in the coronary effluent. Ten of 13 hearts studied (77%) demonstrated arrhythmic activity with a mean time to the onset of arrhythmias of approximately 35 min. The nonsteroidal antiinflammatory drugs indomethacin and acetylsalicylic acid which significantly inhibited the efflux of 6-keto-prostaglandin F1 alpha also reduced the incidence of arrhythmias to 10 of 30 hearts studied. In those hearts in which arrhythmias occurred, the time to onset was significantly increased to approximately 50 and 55 min for acetylsalicylic acid and indomethacin, respectively. In contrast, exogenous prostaglandin F2 alpha (0.1 and 1 ng/ml) and prostacyclin (0.1 and 10 ng/ml) increased the incidence of arrhythmias to 100% (10 of 10 hearts studied) and decreased the time to onset to approximately 10 min. These prostaglandin pretreatments were also able to reverse the protective actions of both acetylsalicylic acid and indomethacin. Other concentrations (10 ng/ml prostaglandin F2 alpha and 1 ng/ml prostacyclin) had no influence either on the incidence of arrhythmias or their time to onset. Prostaglandin E2 (0.1 ng/ml) produced a modest but not significant decrease in the time to onset of arrhythmias although this concentration was significantly effective in reversing the nonsteroidal antiinflammatory drug effects. The inotropic, chronotropic and coronary constricting actions of ouabain were unaffected either by nonsteroidal antiinflammatory drug or prostaglandin pretreatment. These studies suggest that prostaglandins are involved, at least in part, in the arrhythmogenic actions of ouabain in the isolated guinea pig heart.

The use of the fluorescent antibody test to detect Escherichia coli with K88 and 987P pilus antigens.

Direct fluorescent antibody tests were used to detect Escherichia coli possessing K88 and 987P antigens. Identification of bacteria was accomplished on suspensions of organisms from clinical isolates, on frozen sections and impression smears from small intestine and on faecal smears. This assay makes possible the rapid identification of E. coli possessing K88 and 987P pilus antigens.

Detection of antibodies to Eperythrozoon ovis by the use of an enzyme-linked immunosorbent assay.

Specific antibody to Eperythrozoon ovis was detected by an enzyme-linked immunosorbent assay (ELISA) in the sera of infected sheep. In the presence of parasite antigen, positive control serum showed a reaction approximately eight times that of negative serum. When compared to an immunofluorescent antibody test (IFAT), the ELISA was eight times more sensitive. Positive control sera gave a titre of 1:3200 by IFAT and 1:25,600 by ELISA. Through the use of a reference titration curve ELISA could be used as a semi-quantitative system to determine antibody levels in test sera.

Antibody responses of sheep vaccinated with foot rot vaccines.

Enzyme-linked immunosorbent assay (ELISA) and crossed immunoelectrophoresis (IEP) were used to investigate antibody responses of sheep vaccinated with a double adjuvanted or single adjuvanted commercial foot rot vaccine. ELISA detected an antibody response of greater magnitude to the double adjuvant vaccine compared with the single adjuvant vaccine. Sera from sheep vaccinated with double adjuvant vaccine recognised at least six antigens of Bacteroides nodosus in crossed IEP while sera from the single adjuvant vaccinated sheep recognised one antigen. The use of non-denatured antigens of B nodosus in ELISA and crossed IEP enabled quantitative comparisons of antibody responses to the different foot rot vaccines to be made.

Diverse lamb genotypes. 3. Eating quality and the relationship between its objective measurement and sensory assessment.

The effect of genotype on eating quality was evaluated on m. Longissimus thoracis et lumborum (LTL) muscle of 60 lambs. The lambs were sired by a selection of Texel (T), Poll Dorset (PD), Border Leicester (BL) and Merino (M) rams, crossed with Border Leicester x Merino (BLM) and Merino (M) ewes giving six genotypes (TxBLM, PDxBLM, TxM, PDxM, BLxM and MxM). The relationships between sensory panel assessment of eating quality attributes and pH, cooking loss and shear force were also investigated. No significant differences were observed between genotypes for panel assessment of tenderness, juiciness, aroma liking, aroma strength, flavour liking, overall acceptability and rating. MxM lambs had a significantly (P<0.05) higher flavour strength than BLxM lambs. pH was a poor indicator of any eating quality attributes, except aroma strength (r=0.3, P<0.05). Warner Bratzler shear force value (WB) and tenderness showed a significant (P<0.001) negative correlation (-0.7). Tenderness, flavour and juiciness were the most important sensory attributes, explaining 86.5% of the total variation in overall acceptability.

Vaccination against lumpy jaw and measurement of antibody response in wallabies (Macropus eugenii).

Successful protection against lumpy jaw disease in a colony of captive wallabies (Macropus eugenii) was induced by vaccination with a commercial ovine footrot vaccine. No mortalities attributable to lumpy jaw were observed in 69 vaccinated animals while six of 42 unvaccinated control wallabies died of the disease. Vaccinated animals exhibited significant increases in antibody titres to Bacteroides nodosus after the first and second doses of vaccine. Titres were measured by an enzyme-linked immunosorbent assay.

Effects of transmembrane potential on oscillatory afterpotentials induced by acetylstrophanthidin in canine ventricular tissues.

Transmembrane activity was recorded from isolated canine false tendons or trabeculae by using standard differential microelectrode techniques. Exposure of the tissues to acetylstrophanthidin (AS; 0.5-2 X 10(-7) g/ml) induced oscillatory afterpotentials in false tendons but not in muscle. When the transmembrane potential of the isolated tissues was altered by externally applied current, progressive depolarization of false tendons caused the amplitude of oscillatory afterpotentials to increase to a maximum and then decrease. Hyperpolarization abolished oscillatory afterpotentials and unmasked activity attributable to the normal pacemaker mechanism. Any level of manifest oscillatory afterpotential-related toxicity, including phasic decreases in excitability, could be elicited by careful selection of membrane potential. The effects of the imposed membrane potentials were immediately reversed with the return of membrane potential to control levels. In muscle, exposed to AS but not exhibiting oscillatory afterpotentials, depolarization revealed oscillatory afterpotentials. In both false tendons and muscle, depolarization to membrane potentials of -50 mV or less before exposure to AS elicited depolarization-induced automaticity. Subsequent exposure of the tissues to AS abolished this activity. This study demonstrates an important role of membrane potential in digitalis toxicity.

The effects of tension on acetylstrophanthidin-induced transient depolarizations and aftercontractions in canine myocardial and Purkinje tissues.

Transmembrane potentials and contractile activity were recorded from isolated canine Purkinje and ventricular muscle preparations exposed to acetylstrophantidin (AS) and subjected to a resting tension equal to 80% of that required to elicit peak developed tension. AS induced transient depolarizations (TD's) accompanied by aftercontractions in Purkinje tissue. AS also induced aftercontractions in 11 muscle preparations, and in seven of these the mechanical events were associated with TD's. Aftercontractions and TD's in both and the coupling intervals were directly related to the preceding basic cycle length (BCL). The amplitudes of aftercontractions and TD's reached a maximum at a coupling interval of 600-700 msec. Tension increased the amplitude of TD's in Purkinje tissue and promoted the appearance of TD's in muscle. TD's in muscle occasionally reached threshold in the presence of tension. The results of this study suggest that stretch or increased resting tension may promote the types of cardiac arrhythmias that are causally related to digitalis-induced TD's. The results are compatible with the hypothesis that TD's are caused either by a transmembrane influx of calcium or by an internal release of calcium ions.

Digitalis arrhythmias: role of oscillatory afterpotentials.

Digitalis-induced OAP provide a mechanism of automaticity that may be responsible for many arrhythmias induced by cardiac glycosides. In response to digitalis, OAP occur in tissues of the specialized conducting systems of both ventricles and atria and, under the influence of tension, occasionally in ventricular myocardium. Digitalis, in toxic doses, suppresses "normal" pacemaker activity possibly in part by enhancing overdrive suppression. In contrast to "normal" pacemaker activity, OAP exhibit, both in magnitude and rate of depolarization, postpacing acceleration. This plus the coupled nature of OAP are important characteristics in the generation of complex arrhythmias by OAP. Conduction disturbances may also be related to OAP. At early stages of intoxication OAP may speed conduction of superimposed beats relative to earlier or later beats. More advanced stages of intoxication are associated with conduction block. The occurrence of digitalis-induced OAP is promoted by high concentrations of calcium, low concentrations of potassium, and moderate stretch. OAP can be suppressed by high concentrations of potassium, reduction of extracellular calcium, and exposure to antiarrhythmic agents including diphenylhydantoin, verapamil, and aprindine. The effectiveness of the latter two agents may be related to ability to block transmembrane calcium currents. Digitalis-induced OAP in atrial tissue can be abolished by acetylcholine. A transmembrane current possibly but not necessarily carried by calcium appears to underly the occurrence of OAP. This current demonstrates kinetic properties different from those of the slow inward current associated with the plateau of the cardiac action potential. Calcium is intimately involved in the mechanism causing OAP and may be responsible for aftercontractions observed in conjunction with OAP. Aftercontractions greatly affect contractility and may be responsible at least in part for some of the inotropic actions of digitalis. Thus the occurrence of OAP may be linked to the inotropic actions of digitalis. Digitalis-induced OAP provide a mechanism of automaticity with characteristics paralleling automatic behavior observed in intact animals intoxicated with digitalis. The relative importance of OAP in the genesis of clinically important arrhythmias awaits further investigation.

Relationship between acetylstrophanthidin-induced aftercontractions and the strength of contraction of canine ventricular myocardium.

Contractile activity was recorded from isolated canine ventricular muscle exposed to acetylstrophanthidin (AS), 0.5 to 2 X 10(-7) g/ml. Development of the positive inotropic effect of AS was accompanied by the appearance of aftercontractions (AC) coupled to the driven responses. The amplitudes of AC increased with the number of preceding beats. Test beats occurring during the ascending limb of AC were potentiated and the potentiation increased with the amplitude of the AC. Beats falling during the descending limb decreased in strength as the amplitude of the AC increased. As eliminated the frequency dependence of restitution. However, staircase phenomena persisted and were then found to be dependent on the phase relationship of each beat to underlying AC. These findings suggest an important role for AC in the inotropic actions of digitalis and in determining strengh-interval relationships of ventricular muscle treated with digitalis.

Mechanisms of action of beta-adrenergic agents on the arrhythmogenic transient inward current in rabbit Purkinje fibers.

Catecholamines increase the amplitudes of oscillatory afterpotentials (OAP) and peak magnitude of the transient inward current (Iti) responsible for OAP. The objectives of this study were to determine whether beta-adrenoceptor stimulation can induce Iti, and to determine the mechanism by which beta-adrenoceptor stimulation increases the magnitude of Iti. Experiments were performed using standard two electrode voltage--clamp techniques in isolated rabbit Purkinje fibers. Holding potential was either -50 or -80 mV. The Iti was elicited by repolarizing steps, following 1.5 or 3 s activating steps to potentials near 0 mV. Isoproterenol (ISO) failed to induce the Iti at concentrations from 10(-8) to 10(-6)M. However ISO (10(-7)M) significantly increased peak magnitude of spontaneously occurring Iti (P less than 0.05), or Iti induced by acetylstrophanthidin (AS) (P less than 0.05). ISO also shifted the minimum activation voltage 10 mV more negative (P less than 0.05). The current-voltage relationship demonstrated that ISO significantly increased the range of potentials over which Iti greater than or equal to 5 nA occurred, but did not significantly shift the voltage at which maximum peak current was observed. Effects of ISO on Iti were blocked by 10(-7)M propranolol or atenolol. Mn2+ (2 mM) or verapamil (2 microM) blocked the slow inward current (Isi) more than 80% before substantially decreasing peak Iti. Either agent blocked stimulation of Isi but not Iti by ISO at 10(-7)M. In contrast, quinacrine (20 microM), an inhibitor of Na(+)-Ca2+ exchange, abolished stimulation of Iti by ISO while having no significant effect on Isi. Our results indicate that beta-adrenoceptor stimulation cannot induce Iti in rabbit Purkinje fibers, but can enhance the Iti induced by other means, by stimulating Na(+)-Ca2+ exchange.

Effects of lidocaine on reperfusion arrhythmias and electrophysiological properties in an isolated ventricular muscle model of ischemia and reperfusion.

Transmembrane electrical activity was recorded from endo- and epicardium of isolated segments of guinea pig right ventricles with standard microelectrode techniques. An ECG was also recorded by two electrodes placed at opposite ends of the tissue bath. Regular stimulation was delivered to the endocardium. Tissues were exposed to simulated ischemia for 15 min and then were reperfused with "normal" Tyrode's solution. Rapid sustained or nonsustained ventricular tachycardia, bigeminy or trigeminy with characteristics of transmural reentry occurred in early reperfusion in 14 of 20 hearts (70%). Arrhythmias were accompanied by prolongation of transmural conduction time and abbreviation of endocardial effective refractory period. With lidocaine, at 1, 5, 10 and 50 microM, reperfusion arrhythmias occurred in 53.3, 22.2, 20.8 and 14.3% of hearts, respectively. The decreased incidence of arrhythmias was statistically significant for 5 to 50 microM lidocaine (P less than .01). The antiarrhythmic effect did not correlate with changes in transmural conduction time, endocardial effective refractory period, or endocardial excitability. However, antiarrhythmic concentrations of lidocaine selectively depressed epicardial excitability and significantly increased endo- to epicardial conduction block during late ischemic and early reperfusion periods. Epicardial inexcitability extended to late diastole and conduction block was not restricted to premature beats. Thus, in transmural reentry in which the epicardium is an essential component of the circuit, lidocaine may interrupt the circuit by selectively rendering this component inexcitable.

Contribution of Na(+)-Ca2+ exchange to stimulation of transient inward current by isoproterenol in rabbit cardiac Purkinje fibers.

Cellular mechanisms underlying beta-adrenergic stimulation of the arrhythmogenic transient inward current (TI) were investigated by using a two-microelectrode voltage-clamp technique in rabbit cardiac Purkinje fibers. TI induced by elevating [Ca2+]o to 30 mmol/L and substituting [Na+]o with N-methyl-D-glucamine (NMG) chloride had a distinct reversal potential (EREV) of -25 mV, suggesting that Na(+)-Ca2+ exchange was not the charge carrier for TI. In the absence of [Na+]o, isoproterenol (ISO, 0.01 to 5.0 mumol/L) had no effect on either inward or outward TI or on the current-voltage relation of TI. However, ISO (0.1 mumol/L) significantly increased both inward and outward TIs without affecting the EREV of TI, if [Na+]o was present. Pretreatment with propranolol (0.2 mumol/L) or atenolol (0.2 mumol/L) abolished the stimulatory effects of ISO. Addition of propranolol (0.2 to 0.5 mumol/L) after the effects of ISO had developed caused only partial reversal of TI stimulation. This indicates persistence of stimulatory effects downstream from the initial agonist-receptor interaction. Forskolin (1 mumol/L), a direct adenylate cyclase activator, also strongly increased both inward and outward TI in the presence of [Na+]o. These effects also were abolished when [Na+]o was substituted by NMG. Inward and outward TIs enhanced by either ISO or forskolin were reversed by two putative Na(+)-Ca2+ exchange blockers, dodecylamine (20 mumol/L) and quinacrine (20 mumol/L). These results suggest that beta-adrenergic stimulation of TI is mediated by the Na(+)-Ca2+ exchange; stimulation likely involves phosphorylation of the exchanger or some factor that modulates exchanger activity.

Contractions in guinea-pig ventricular myocytes triggered by a calcium-release mechanism separate from Na+ and L-currents.

1. Unloaded cell shortening and membrane currents were examined in isolated guinea-pig ventricular myocytes at 37 degrees C using video edge detection and single-electrode voltage clamp. 2. Inward Na+ currents were eliminated by lidocaine, tetrodotoxin, replacement of extracellular Na+ with choline chloride or sucrose, or by voltage inactivation of Na+ channels. In the absence of Na+ current, the threshold for contraction was approximately -50 or -55 mV. 3. Verapamil (5 microM) and nifedipine (2 microM) failed to inhibit contractions at negative membrane potentials when positive conditioning pulses were used to maintain intracellular Ca2+ stores via Na(+)-Ca2+ exchange. In contrast, 200 microM Ni2+ inhibited these contractions. 4. Contractions were abolished when the extracellular solution was nominally Ca2+ free. However, contractions were restored by as little as 50 microM extracellular Ca2+. 5. Ryanodine (30 nM) completely abolished contractions initiated by depolarizing steps from -65 to -40 mV, but had minimal effects on contractions initiated by depolarizing steps from -40 to +5 mV. Subtraction of contraction-voltage relations determined in the presence of ryanodine from control relations revealed a ryanodine-sensitive component of contraction. This component activated at -55 mV and reached a plateau near -25 mV. 6. The amplitudes of contractions initiated by depolarizing steps from -40 mV were directly proportional to the magnitude of Ca2+ current (ICa). In contrast, contractions initiated by steps from either -55 or -65 mV were not proportional to ICa. These contractions appeared at potentials negative to the threshold for L-type Ca2+ current, increased to a plateau at more positive potentials and did not decrease at potentials at which ICa decreased. 7. Subtraction of the contraction-voltage relationship determined from a membrane potential of -40 mV from that at -55 mV revealed a component of contraction with a negative activation threshold whose amplitude was not proportional to inward current. The shape of this relationship was virtually identical to that of the ryanodine-sensitive component of contraction. 8. This study identifies a component of contraction associated with Ca2+ release from sarcoplasmic reticulum (SR) which can be separated from other mechanisms of contraction on the basis of membrane potential. Our observations suggest that this voltage-dependent release mechanism is a true trigger mechanism which activates a portion of cardiac contraction which is attributable to SR Ca2+ release.

Pro- and antiarrhythmic effects of d-sotalol and dl-sotalol in an isolated tissue model of ischemia and reperfusion.

Pro- and antiarrhythmic effects of dl-sotalol and d-sotalol were compared with their electrophysiological actions in an isolated tissue model of simulated ischemia and reperfusion. Microelectrode recordings were made from endo- and epicardium of isolated guinea pig right ventricular free walls. An electrocardiogram also was recorded by two electrodes at opposite ends of the tissue bath. Regular stimulation was delivered to the endocardium. Tissues were exposed to simulated ischemia for 15 min and then were reperfused with normal Tyrode's solution. Arrhythmias with characteristics of transmural reentry occurred in 33% of hearts in ischemia and 73% of hearts in early reperfusion. Arrhythmias were accompanied by prolongation of transmural conduction time (CT) and abbreviation of endocardial action potential duration (APD) and effective refractory period. Both dl-sotalol and d-sotalol (100 microM) significantly (P < .05) prolonged endocardial APD, effective refractory period and epicardial APD under preischemic conditions; however, these effects were lost during simulated ischemia and early reperfusion. dl-Sotalol abolished arrhythmias in ischemia and reduced the incidence of reperfusion arrhythmias to 30%. This agent attenuated prolongation of transmural CT by ischemia and decreased the incidence of conduction block. In contrast, d-sotalol (100 microM) increased arrhythmias in ischemia to 80% and did not change the incidence of reperfusion arrhythmias (60%). The proarrhythmic effects of d-sotalol were accompanied by prolongation of transmural CT, increased incidence of conduction block and decreased epicardial excitability. Thus, in this model of global ischemia and reperfusion, antiarrhythmic effects were observed with dl-sotalol but not with the predominantly type III isomer, d-sotalol.

Automaticity and entrance block induced by focal depolarization of mammalian ventricular tissues.

Isolated canine interventricular septa were studied with standard microelectrode techniques. Focal automaticity was induced by applying depolarizing current through an extracellular pipet in contact with the right bundle branch (RBB) of the ventricular specialized conducting system. Automaticity appeared with depolarization to transmembrane potentials of -50 mV or less. The spontaneous activity was neither depressed nor accelerated when overdrive suppression was attempted. Activity originating within the focus propagated into fully polarized surrounding tissue. However, entrance block, phasically related to the spontaneous cycle length, was an intrinsic property of these foci. Early premature beats initiated outside the focus failed to enter the focus, but the resulting electrotonus delayed the next automatic beat. Late premature beats captured and thereby accelerated the focus. Thus, the automatic foci could be extrafocal activity. Consequently, continous pacing at various rates precipitated complex rhythms with fixed coupling. Similar foci with exit conduction, entrance block, and electrotonic modulation also were demonstrated in focally depolarized papillary muscles in feline septal preparations. The unique properties of focally depolarized areas in which spontaneous activity is generated at low membrane potentials provide a mechanism capable of generating a wide array of arrhythmias.