Functional interaction between local anaesthetics and calcium antagonists in guineapig myocardium: 1. Cardiodepressant effects in isolated organs

Both local anaesthetics and calcium antagonists depress cardiac function. Therefore, we have studied the interaction of these compounds in the isolated myocardium of guineapigs. The negative inotropic effect of various local anaesthetics was investigated in left atria in the absence or presence of nitrendipine 10(-7) mol litre-1 (n = 7-8 in each group). In addition, the effect of bupivacaine was studied in the presence of several calcium antagonists. The factor by which the negative inotropic potency (EC50) of local anaesthetics was enhanced significantly in the presence of nitrendipine varied from mean 1.2 (SD 0.2) (benzocaine) to 3.0 (0.6) (bupivacaine). The EC50 of bupivacaine was lowered by all calcium antagonists. The potentiation factor varied from 1.4 (0.4) (verapamil) to 3.3 (0.6) (nifedipine). The effects of benzocaine (n = 12) and bupivacaine (n = 11) on the working heart were assessed either alone or after pretreatment with nifedipine 10(-8) mol litre-1. The effects of benzocaine on contractility remained unaltered in the presence of nifedipine, whereas the negative inotropic effect of bupivacaine increased significantly (for example, a 20% reduction in left ventricular maximum dP/dt occurred with bupivacaine 1.75 (0.16) x 10(-6) mol litre-1 alone compared with 1.1 (0.22) x 10(-6) mol litre-1 when combined with nifedipine). We conclude that the effects of some local anaesthetics, for example bupivacaine, on cardiac contractility are enhanced in the presence of calcium antagonists. The relevance of this interaction in patients remains to be determined.

Functional interaction between local anaesthetics and calcium antagonists in guineapig myocardium: 2. Electrophysiological studies with bupivacaine and nifedipine

The negative inotropic effect of local anaesthetics is potentiated by several calcium antagonists in guineapig myocardium [1]. Therefore, we studied which effects on cardiac ionic currents could be responsible for this interaction. Concentration-response curves for bupivacaine were studied in isolated guineapig atria and papillary muscles (slow action potentials). Effects on action potentials were assessed in the absence (n = 7 atria, n = 8 papillary muscles) or presence of nifedipine (8 x 10(-8) mol litre-1 in n = 8 atria, 10(-8) mol litre-1 in n = 8 papillary muscles). The effect on the Ca2+ current was assessed directly using the patch-clamp technique in guineapig ventricular myocytes. Bupivacaine reduced contractile force and upstroke velocity of atrial action potentials. Only the negative inotropic effect was potentiated in the presence of nifedipine. Force and upstroke velocity of slow action potentials were diminished by bupivacaine. Both variables were affected at significantly smaller concentrations of bupivacaine when given in combination with nifedipine. The Ca2+ current was reduced significantly by bupivacaine 5 x 10(-5) mol litre-1 (mean -18 (SD 7)%, n = 9). Its effect was accentuated in the presence of nifedipine 10(-9) mol litre-1 (-47 (4)%, n = 7). Bupivacaine 3 x 10(-4) mol litre-1 given alone exerted a comparable effect (-53 (4)%, n = 4). Variables indicative of Ca2+ channel function (contractile force, upstroke of slow but not normal action potentials, Ca2+ inward current) revealed potentiation of the effects of bupivacaine by nifedipine.

Pharmacological characterization of positive inotropic derivatives of 4-amino-7-methyl-1,8-naphthyridine-3-carboxylic acid

The positive inotropic effect of a series of 4-amino-7-methyl-1,8-naphthyridine-3-carboxylic acid derivatives was compared with the effects of known inotropic agents (ouabain, dihydroouabain, isoproterenol, adrenaline, histamine and isobutylmethylxanthine) in guinea-pig atrial and ventricular myocardial preparations. With respect to their functional effects, the 1,8-naphthyridine compounds are clearly different from drugs acting on the cAMP system, whereas several similarities with cardiac glycoside effects were found. Their ability to inhibit [3H]ouabain binding in guinea-pig cardiac membranes correlates well with their effects on myocardial contractile force. However, the latter effect was exerted by tenfold lower concentrations. The dissimilarities found between the 1,8-naphthyridines and digitalis may be due to a different type of interaction with the binding site on the (Na(+) + K+)-ATPase.

Pattern of interaction between dihydropyridine calcium antagonists and cationic amphiphilic drugs in isolated guinea-pig left atria

The pharmacodynamic interaction between dihydropyridine calcium antagonists and several cationic amphiphilic agents with respect to their negative inotropic action has been studied in isolated guinea-pig left atria. (+/-)-Bepridil (CAS 64706-54-3), (+/-)-cis-diltiazem (CAS 33286-22-5), falipamil (CAS 77862-92-1), (+/-)-gallopamil (CAS 16662-47-2), lidocaine (CAS 6108-05-0), procainamide (CAS 614-39-1), or quinidine (CAS 50-54-4) were added at fixed concentrations. Afterwards, concentration-response curves for nifedipine (CAS 21829-25-4), the S-enantiomer niguldipine.HCl and its R-enantiomer dexniguldipine.HCl (CAS 113165-32-5), (+/-)-nimodipine (CAS 66085-59-4) (+/-)-nitrendipine (CAS 39562-70-4), or (+/-)-isradipine (CAS 75695-93-1) were obtained. In most cases, a left-ward shift in the concentration-response curve of the dihydropyridine was observed. The extent of this shift varied between dihydropyridines, and it depended on the catamphiphilic compound present. A high correlation was found between the different dihydropyridines, when comparing the extent of displacement of their concentration-response curves by the various catamphiphilic test compounds. Although quantitative differences exist between the different dihydropyridines, they are affected in potency by other compounds with a rather similar pattern. Accordingly, some general predictions about the interactions of a given dihydropyridine might be possible using limited datasets.

Mechanisms of beta-adrenergic stimulation of cardiac Ca2+ channels revealed by discrete-time Markov analysis of slow gating

Individual cardiac Ca2+ channels cycle slowly between a mode of gating in which the channel is available to open, and one in which the channel remains silent. The regulation of this multisecond cycling process by isoproterenol was investigated by single-channel recording and the development of a discrete-time Markov model that describes the slow switching among modes in terms of (de) phosphorylation reactions. The results provide evidence that isoproterenol increases Ca2+ channel activity by a reciprocal regulatory mechanism: not only is the phosphorylation rate of the channel increased, but also the dephosphorylation rate decreased. The discrete-time Markov formalism should prove useful as a general tool for understanding the mode switching demonstrated by a number of ionic channels.

Evidence for physiological functions of protein phosphatases in the heart: evaluation with okadaic acid

Okadaic acid exerts a positive inotropic effect in cardiac preparations. We studied whether the positive inotropic effect of okadaic acid in cardiac preparations could be due to phosphatase inhibition and whether this inhibition affects the phosphorylation of cardiac proteins. In papillary muscles from guinea pigs, 30 microM okadaic acid increased force of contraction to 175% of predrug value. In isolated guinea pig ventricular cardiomyocytes, okadaic acid augmented single Ca(2+)-channel currents by enhancing channel availability. In homogenates from ventricles, 1 microM okadaic acid completely inhibited phosphorylase a phosphatase activity. In isolated 32P-labeled ventricular cardiomyocytes, 30 microM okadaic acid increased phosphorylation of phospholamban (PLB) and troponin inhibitor (TnI) to 325 and 284% of control, respectively. Furthermore, 30 microM okadaic acid increased phosphorylation of a hitherto unknown 23-kDa protein to 352% of control. It is concluded that the effects of okadaic acid could be mediated by increasing the phosphorylation state of several proteins including PLB, a 23-kDa protein, and TnI.

Enantioselectivity of asocainol studied at different conditions: a novel approach to check the feasibility of molecular models of antiarrhythmic drug action

In terms of the "guarded receptor" hypothesis, changes in potency of Na+ channel blocking drugs reflect alterations in drug access to and/or egress from a compartment facing a binding site with constant affinity. Potency is therefore assumed to be determined by changes in drug diffusion, its mobility in the electric field, protonation etc. Hence, the potencies of enantiomers, i.e. compounds with identical physicochemical properties, should be influenced in a parallel manner by the condition. To test this prediction, actions of the enantiomers of the stereoselective antiarrhythmic drug asocainol were compared at various membrane potentials and stimulus frequencies. Several experimental models indicative of Na+ channel block were used: the elevation of the rectangular pulse stimulation threshold (RPT) and the suppression of alternating-current induced arrhythmia (ACT) were studied in guinea-pig atria. The reduction of the upstroke velocity of action potentials was measured in guinea-pig papillary muscles. The inhibition of whole-cell Na+ currents was investigated in isolated guinea-pig ventricular myocytes. In all these assays, (+)-asocainol was more potent than the (-)-enantiomer. Lowering the membrane potential and/or increasing the stimulus frequency enhanced the effects of both enantiomers. However, over a certain range of conditions, the potency of (+)-asocainol was more markedly affected than that of (-)-asocainol, indicating that the eudismic ratio between potencies of the two drugs is not constant. Accordingly, these findings are inconsistent with the guarded receptor hypothesis.

Frequency- and potential-dependency of the negative inotropic action of various dihydropyridine and non-dihydropyridine calcium antagonists

Transmembrane voltage and beat frequency are important determinants of the action of several organic calcium antagonists. This is well-known for the cationic amphiphilic calcium antagonists. We intended to assess the functional impact of these phenomena in cardiac muscle with special regard to dihydropyridines. Therefore, concentration-response curves were constructed in isolated guinea-pig left atria for the negative inotropic effect of various compounds. The dihydropyridines nifedipine, racemic nitrendipine, nisoldipine, and felodipine, and the enantiomers of isradipine were investigated at different stimulation frequencies (1 Hz, 2.5 Hz, 4.5 Hz), and at different extracellular K+ concentrations (2.7 mM, 5.4 mM, 10.8 mM). These drugs were compared with the cationic amphiphilic compounds gallopamil, verapamil and diltiazem. The potency of some dihydropyridines, particularly nitrendipine, could be modulated to a remarkable extent, covering several orders of magnitude. The potential-dependency of the drugs depended on stimulus frequency and ranged from less than a half to two orders of magnitude. At 2.5 Hz, the rank order of extent of potential-dependency was gallopamil greater than nitrendipine greater than diltiazem greater than verapamil = (+)-isradipine greater than (-)-isradipine greater than or equal to nisoldipine greater than or equal to felodipine = nifedipine. Based on data obtained from binding studies in intact atria and from patch-clamp measurements of calcium current blockade, a mathematical model was used which describes the observed potency changes.(ABSTRACT TRUNCATED AT 250 WORDS)

Marked dependence of the cardiac effects of gallopamil on the extracellular K(+)-concentration

1. In guinea-pig Langendorff hearts, the negative inotropic effect of the calcium antagonist gallopamil is shifted by 15-fold to the left, when the extracellular K(+)-concentration is raised from 2.7 to 8.1 mM. 2. In papillary muscles, the ability of gallopamil to shorten the action potential (AP) markedly depends on K+: 100-fold lower gallopamil concentrations were required at 10.8 mM, compared to 2.7 mM. 3. In isolated myocytes, a change in the holding potential from -90 to -70 mV displaces the gallopamil dose-response curve to block Ca2+ currents leftward by only 6-fold. 4. Tetraethylammonium (TEA, 10 mM) mimics the mitigating effect of low K+ on the gallopamil-induced AP-shortening. Hence, the K(+)-dependence of gallopamil may be comprised of modulation of Ca(2+)-channel and K(+)-channel blocking effects.

Potentiation of cardiodepressive action among calcium antagonists from different classes: evidence for a mechanism at the single calcium channel level

The ability of calcium antagonists and antiarrhythmic agents to potentiate the negative inotropic effects of calcium antagonists was investigated in guinea-pig left atria. The potency of nitrendipine was enhanced by several amphiphilic agents by one order of magnitude or more (by pretreatment with quinidine or bepridil). The effect of preincubation with bepridil was investigated for a larger number of dihydropyridines. Only some of them were potentiated like nitrendipine. There was no potentiation between any two members of the same chemical group, i.e. between two dihydropyridines or two catamphiphilic calcium antagonists. The interaction between bepridil and nitrendipine was studied in more detail. In atria, its extent was influenced by several conditions, such as the stimulus frequency, the incubation temperature, or the extracellular K+ concentration. In measurements of whole-cell calcium currents in guinea-pig myocytes, the interaction was found to take place in a quantitatively similar manner. At the single channel level, an enhancement of the effects could also be demonstrated. It appears here that both drugs interact by binding to the same channel molecule. We conclude that the interaction may be due to 1.: an amphiphilic drug (like bepridil) binding to the channel very transiently and thus briefly favouring the inactivated channel state, which means that 2.: the other drug (like nitrendipine) has a higher chance to be bound because of its high affinity towards inactivated channels. Alternative explanations are also discussed.

The positive inotropic drugs DPI 201-106, BDF 9148, and veratridine increase ouabain toxicity and [3H]ouabain binding in guinea pig heart

The interaction of three positive inotropic compounds, which are modulators of sodium channels, with the cardiac glycoside ouabain was investigated in isolated guinea pig atria. In the presence of DPI 201-106 (3 x 10(-7) M), of its new acetidine derivative BDF 9148 (10(-7) M), or of veratridine (10(-6) M), the threshold ouabain concentration to induce toxicity was lowered by a factor of 2. This effect can be explained by the observation that specific equilibrium [3H]ouabain binding in intact atria was elevated by these compounds in the appropriate concentrations. The binding results were analyzed by means of a previously established model of "positive cooperative ouabain binding to intact myocardium," which describes the relationship between cellular sodium homeostasis and ouabain binding. The extent to which the compounds increased ouabain binding was in good quantitative agreement with the observed shift in the threshold concentration of ouabain toxicity. The increase of specific [3H]ouabain binding is likely initiated by a gain in cytosolic sodium. It takes place at the cellular level only, since a direct enhancement of [3H]ouabain binding to isolated cardiac membranes was not found. In conclusion, at least under some conditions, new inotropic drugs acting as sodium channel modulators can increase the risk of digitalis toxicity.

Modulation of ouabain binding in beating ventricular myocardium from guinea-pigs: effects of lidocaine and monensin

To further elucidate the interdependence between digitalis sensitivity and the cellular Na+ load, the influence of two Na+ load modifying drugs, monensin and lidocaine, on the concentration-dependence of ouabain binding and ouabain effects was studied in beating ventricular strips from guinea-pig heart. Monensin (3 x 10(-6)M), a Na+ ionophore known to elevate Na+ influx, enhanced 3H-ouabain binding (by approximately 40%) as well as the ouabain effect at non-toxic ouabain concentrations, and it shifted the threshold for toxicity towards threefold lower ouabain concentrations. Lidocaine (2 x 10(-4)M), a Na+ channel blocker, lowered ouabain binding by about one third, and it extended the ouabain concentration range tolerated without toxicity by a factor of three. In the concentrations used, neither compound exerted any direct effect on ouabain binding studied in isolated cardiac membranes. The binding data obtained in the muscle strips were well fitted by a mathematical model which quantifies the dependence of ouabain binding on the underlying (Na+ +K+)-ATPase activity. These findings provide evidence for an indirect drug-induced modulation of ouabain binding via the interference with the cellular Na+ load.

Diastolic tension and contraction amplitude in calcium-loaded rat ventricular myocardium are differently affected by drugs

1. To obtain a measure of drug effects on myocardial function during diastole, the following experimental protocol was designed: rapid electrical stimulation (5 Hz) at high Ca02+ caused an elevated diastolic tension, which could be subjected to drug-induced alterations. 2. Antiarrhythmic drugs (quinidine, propafenone, procainamide, mexiletine) were able to lower diastolic force without appreciably decreasing contraction amplitude. Calcium antagonists (nifedipine, verapamil) lowered both parameters in parallel. 3. Veratridine and Bay K 8644 both enhanced diastolic tension, but only Bay K 8644 concomitantly elevated contraction amplitude. 4. These findings may be explained when taking into account differential actions of sodium- and calcium channel modulating drugs, respectively, on cellular Ca2+ movements. In quantitative terms, the non-linear dependence of myocardial force on Cai2+ also had to be considered.

Differentiation between negative inotropic drugs by means of potentiated post-rest contractions in guinea-pig heart

1. It was tested using isolated guinea-pig atria whether negative inotropic drugs could be differentiated by measuring force under both equilibrium conditions and after a rest period leading to potentiated beats. 2. A drug believed to interfere with sarcoplasmic reticular function, ryanodine, preferentially suppressed potentiated beats, whereas the calcium antagonists gallopamil and felodipine selectively depressed force under continuous stimulation. 3. Gentamicin and doxorubicin also exerted characteristic effects on the two force parameters, the former resembling calcium antagonists, the latter ryanodine. 4. The method used is suitable for a descriptive classification of negative inotropic drugs, possibly according to their cellular mode of action.

Beta-adrenergic stimulation of calcium channels occurs by potentiation of high-activity gating modes

cAMP-dependent phosphorylation clearly increases current through cardiac L-type Ca channels, but the molecular manifestation of this effect remains controversial. Previous work implicates either an increase in the number of functional channels or graded changes in the gating of individual channels. We now find that single cardiac Ca channels display three patterns of activity ("modes") and that isoproterenol or 8-bromoadenosine 3',5'-cyclic monophosphate redistributes the relative proportions of modes such that the two most active (mode 1, bursts of brief openings; mode 2, very long-lasting openings) are favored (P less than 0.05; n = 7). Conversely, a pattern of sparse brief openings (mode 0a) is selectively inhibited (P less than 0.01). Despite differences in the relative frequencies of the various modes before and during drug exposure, the gating within each mode is not detectably changed. We conclude that potentiation of highly active modes of Ca channel gating underlies the enhancement of calcium influx by beta-adrenergic stimulation.

The binding of (+)-isradipine by guinea-pig intact atria

1. The accumulation of [3H]-(+)-isradipine (PN 200-110) was measured in quiescent guinea-pig left atria with normal (K+ 2.7 mM) or lowered (K+ 40 mM) membrane potential. 2. Under control conditions (2.7 mM K+) a high affinity binding of (+)-isradipine could not be detected. If, however, the atria were partially depolarized to about -30 mV by 40 mM K+, high affinity binding became evident displaying a dissociation constant of 4.2 x 10(-11) M and a capacity of 9.7 nmol kg-1 wet wt. 3. The depolarization-induced binding was reversible upon repolarization of the atria although isradipine was still present in the medium. This indicates that the high affinity binding sites disappear as soon as the cell membranes become polarized. 4. Isradipine belongs to the less hydrophobic dihydropyridines, but nevertheless the unsaturable binding led to an accumulation of about 84 fold. At a concentration of 2 x 10(-8) M (+)-isradipine, which reduces the contractile force by 50%, the cellular concentration will rise to more than 10(-6) M.

On the ocular distribution of cardiac glycosides in guinea pigs following acute administration

Tritiated ouabain, digitoxin, and digoxin were given IV to anesthetized guinea pigs. The tritium content of the ocular tissues (cornea, iris, lens, vitreous body, retina, choroid, sclera, and optic nerve) was measured after 1 and 3 h and compared with the amount of tritium found in the brain and some peripheral organs. High levels of digitoxin were found in the brain and retina. Digoxin was detected in large quantities in the retina, but only small levels were found in brain tissue. Ouabain was nearly absent in the retina and cerebrum, whereas in most other ocular and peripheral tissues the three cardiac glycosides were present in comparable quantities. The IP injection of tritiated digoxin resulted in lower absolute tissue levels, but a pattern of distribution similar to that observed in anesthetized guinea pigs was found in conscious animals.

Interpretation of [3H]ouabain binding in guinea-pig ventricular myocardium in relation to sodium pump activity

1. The attempt was made to analyse the complex [3H]ouabain binding curves obtained in intact cardiac ventricular preparations electrically stimulated at different frequencies. The result of this analysis was used to draw conclusions from the binding curves on the frequency dependence of sodium pump activity. 2. [3H]Ouabain binding to isolated, electrically stimulated (1.5 Hz) ventricular strips of guinea-pig hearts was investigated. The positive inotropic effects were studied in separate experiments. Specific [3H]ouabain binding barely reached an equilibrium within 3 h of incubation. A binding curve was constructed using the equilibrium values of specific [3H]ouabain binding obtained at different ouabain concentrations. This binding curve revealed a concentration-proportional component at positive inotropic concentrations and a saturating component at high, toxic concentrations. At very low, inotropically ineffective ouabain concentrations, however, binding values were higher than expected from a linear relationship between ouabain concentration and binding. 3. The peculiar shape of the binding curve could be largely accounted for by a mathematical model, which takes into consideration biochemical properties and physiological regulation of the sodium pump. The model predicts a concentration-proportional pattern of binding which takes place in the non-toxic ouabain concentration range. The slope of the concentration-proportional component of the binding curve should represent a measure of sodium pump activity. 4. Investigation of binding curves at various stimulation frequencies revealed that, as predicted by the model, the slope of the concentration-proportional component of the binding curves was increased and the maximum non-toxic equilibrium binding was decreased with increasing beat frequencies. 5. Quantitative evaluation of the binding curves led to the conclusion that sodium pump activity is a linear function of stimulation frequency in guinea-pig ventricular preparations, the activity in resting preparations amounting to about 15% of the maximum activity. Comparison of the present results with former studies on sodium pump function suggests that [3H]ouabain binding reflects steady-state sodium pump activity. If the complex pattern of binding curves is taken into consideration, [3H]ouabain binding measurements may serve as a means of studying sodium pump function.

[The acute physiology score as a stratification and prognostic criterion in patients in a surgical intensive care ward]

In order to compare different groups of intensive-care patients and therapeutic interventions a measuring system is needed reflecting the patient's pathophysiologic status with great accuracy. The APACHE-system designed by Knaus et al. (APS) meets the requirements when comparing groups of patients. This prospective study was undertaken to determine whether APS is useful to forecast survival and death of 764 individual patients of a surgical ICU. The results were compared to daily statements obtained from the head surgeon of the intensive-care department. Both the physician and APS gave true judgements in 95% of the cases. Overestimation of the operative procedures by the physician was revealed to be a major source of error in wrong statements. When a combination of APS and physician's judgement was used the forecasts were found true in 99% of the patients. The authors conclude that APS by itself is not able to give a sufficient prognosis in an individual ICU patient but is an excellent tool to assure or modify the physician's opinion.

Acrihellin, a cardioactive steroid escaping from the organ-bath

The concentration of acrihellin rapidly declines in oxygenated Tyrode-solution, because the compound escapes from the organ-bath being enriched in droplets sprayed from the surface of the bubbled solution. As checked by radiochromatography, acrihellin remains chemically unaltered during this process. Hellebrin and hellebrigenin persist in gassed Tyrode-solution, suggesting that the 3 beta-substituent dimethylacrylic acid endows acrihellin with amphiphilic properties, thus promoting its enrichment at gas-water interphases. Measurements of the inotropic effects in guinea pig left atria performed at concentrations of acrihellin kept constant yielded a dose-response curve, which closely resembles that of the conventional cardioactive steroid ouabain.

On the cooperativity of ouabain-binding to intact myocardium

A theoretical concept is presented which proposes that binding of ouabain to intact myocardium should be positive cooperative. It is based on the assumption that the myocardial Na/K-ATPases expose the ouabain-binding site only at a particular conformation adopted during a turnover cycle. The turnover rate and thus the ouabain-binding properties are regulated by the cytosolic Na-ion-concentration Nai. Any occupation of cellular Na/K-ATPases should affect the ouabain-binding properties of the unoccupied Na/K-ATPases, because their turnover rate is increased via an elevated Nai. A computer model which takes into account the interrelationships of the Na/K-ATPases both with Nai and with the ouabain-concentration predicts that ouabain-binding should proceed in a concentration-proportional fashion as long as the Na-load can be counterbalanced by non-occupied Na/K-ATPase molecules. The concentration-proportional binding reflects a positive cooperativity. Experimental results reveal that (3H)ouabain-binding to Na/K-ATPase of electrically stimulated guinea-pig left atria was in fact concentration-proportional under certain experimental conditions. The biological significance of the proposed concept remains to be elucidated.

Sodium load and high affinity ouabain binding in rat and guinea-pig cardiac tissue

An estimation of the actual Na/K-ATPase transport activity in intact cardiac cells was made by measuring the binding of [3H]-ouabain to rat and guinea-pig ventricular strips. At the low [3H]-ouabain concentration of 1 nM equilibrium binding was hardly obtained after an incubation time of five hours. Different procedures known to alter the sodium load of the cardiac preparations influenced [3H]-ouabain binding: the sodium ionophore monensin enhanced [3H]-ouabain binding, the local anaesthetic dibucaine and a reduction of external sodium ion concentration diminished [3H]-ouabain binding; [3H]-ouabain binding was similarly affected by these procedures in the rat and guinea-pig. Since [3H]-ouabain binding occurred predominantly at the high-affinity binding sites of rat myocardium under the applied experimental conditions, it was concluded that these binding sites represent Na/K-ATPase molecules involved in sodium ion transport.

Action of ouabain on rat heart: comparison with its effect on guinea-pig heart

The inotropic dose-response curve of ouabain in rat cardiac ventricular strips exceeded a concentration range of two decades (1 X 10(-7) M to 3 X 10(-5) M) displaying an intermediate plateau phase. In guinea-pig ventricular strips the inotropic ouabain concentrations spanned only one decade (1 X 10(-7) M-1 X 10(-6) M). Ouabain-intoxication in guinea-pig ventricular strips occurring at 3 X 10(-6) M consisted of arrhythmia and contracture, while in rat ventricular strips at the toxic concentration of 1 X 10(-4) M only a progressive increase in diastolic tension was observed. By means of atomic absorption spectroscopy the ouabain-induced loss of cellular potassium and gain of sodium in rat ventricular strips was detected only at concentrations of ouabain higher than 10(-4) M. Ouabain reduced the activity of Na/K-ATPase prepared from rat and guinea-pig cardiac ventricles to half of its maximum at 6.5 X 10(-5) M in rat and 1.0 X 10(-6) M in guinea-pig, rat heart Na/K-ATPase thus being about 60 fold less sensitive towards ouabain. Specific [3H]-ouabain binding to membrane suspensions prepared from rat and guinea-pig ventricles was characterized by a similar affinity in rat (KD = 4 X 10(-8) M) and guinea-pig (KD = 13 X 10(-8) M). The number of ouabain binding sites in rat membranes was only about 10% of the number found in guinea-pig membranes. In rat the presence of additional ouabain-binding with low affinity and high capacity seemed possible, but could not be verified for methodological reasons. In the light of the biochemical results and binding data, the wider range of ouabain concentration exerting a positive inotropic effect in the rat may be attributed to the existence in the latter of two populations of receptors with different affinities for ouabain and different capacities. In contrast, in the guinea-pig, there is a single population. Nevertheless it is probable that all the receptors in both species are part of the Na/K-ATPase complex and mediate a positive inotropic effect after ouabain-binding in an identical manner.

The use of biofeedback techniques in occupational therapy for persons with chronic pain

The use of biofeedback in occupational therapy to aid the person with chronic pain in the resumption of his daily functional activities is discussed. The chronic pain syndrome and how it disrupts performance of activities is examined, as well as occupational therapy strategies for assessment and treatment using biofeedback, and indications for evaluating treatment outcomes. The authors assume readers have basic familiarity with biofeedback theory, equipment and its operation.