The intrinsic control of myocardial contraction--ionic factors

Dynamics of interstitial calcium in rat myocardial ischemia reperfusion injury in vivo

Intracellular calcium overload is a key factor for myocardial ischemia reperfusion injury (IR). However, there was no report for interstitial calcium concentration dynamics. We investigated the interstitial calcium dynamics in rat myocardial IR model in vivo. A microdialysis system was involved, and the time delay of the system and recovery time was introduced and tested with a fluids switching method. Twelve SD rats were divided into IR or control group. Myocardial IR was induced by ligating (20 min) then releasing (60 min) the suture underlying left anterior descending branch. Mycrodialyisis probe was implanted into the left ventricular myocardium perfusion area for occlusion. Dialysate samples were collected every 10 min. Dialysate calcium concentration was detected with an atomic absorption spectrophotometer. Recovery time for the microdialysis system was 20 min, and recovery rate was 16%. Dialysate calcium concentration showed no changes during ischemia, descended immediately after reperfusion, reached the lowest level (67% of baseline value) 20 min after reperfusion, then escalated slowly. Recovery time was an important parameter for mycrodialysis technique, and it should not be neglected and needed to be tested. Our data suggest that interstitial calcium concentration in rats with myocardial IR in vivo kept steady in ischemia, descended rapidly at the initial reperfusion, then rebounded slowly. In conclusion, we introduced the concept of recovery time for microdialysis and provided a simple testing method.

Force-frequency relationship in intact mammalian ventricular myocardium: physiological and pathophysiological relevance

The force-frequency relationship (FFR) is an important intrinsic regulatory mechanism of cardiac contractility. The FFR in most mammalian ventricular myocardium is positive; that is, an increase in contractile force in association with an increase in the amplitude of Ca(2+) transients is induced by elevation of the stimulation frequency, which reflects the cardiac contractile reserve. The relationship is different depending on the range of frequency and species of animal. In some species, including rat and mouse, a 'primary-phase' negative FFR is induced over the low-frequency range up to approximately 0.5-1 Hz (rat) and 1-2 Hz (mouse). Even in these species, the FFR over the frequency range close to the physiological heart rate is positive and qualitatively similar to that in larger mammalian species, although the positive FFR is less prominent. The integrated dynamic balance of the intracellular Ca(2+) concentration ([Ca(2+)](i)) is the primary cellular mechanism responsible for the FFR and is determined by sarcoplasmic reticulum (SR) Ca(2+) load and Ca(2+) flux through the sarcolemma via L-type Ca(2+) channels and the Na(+)-Ca(2+) exchanger. Intracellular Na(+) concentration is also an important factor in [Ca(2+)](i) regulation. In isolated rabbit papillary muscle, over a lower frequency range (<0.5 Hz), an increase in duration rather than amplitude of Ca(2+) transients appears to be responsible for the increase in contractile force, while over an intermediate frequency range (0.5-2.0 Hz), the amplitude of Ca(2+) transients correlates well with the increase in contractile force. Over a higher frequency range (>2.5 Hz), the contractile force is dissociated from the amplitude of Ca(2+) transients probably due to complex cellular mechanisms, including oxygen limitation in the central fibers of isolated muscle preparations, while the amplitude of Ca(2+) transients increases further with increasing frequency ('secondary-phase' negative FFR). Calmodulin (CaM) may contribute to a positive FFR and the frequency-dependent acceleration of relaxation, although the role of calmodulin has not yet been established unequivocally. In failing ventricular myocardium, the positive FFR disappears or is inverted and becomes negative. The activation and overexpression of cardiac sarcoplasmic reticulum Ca(2+) ATPase (SERCA2a) is able to reverse these abnormalities. Frequency-dependent alterations of systolic and diastolic force in association with those of Ca(2+) transients and diastolic [Ca(2+)](i) levels are excellent indicators for analysis of cardiac excitation-contraction coupling, and for evaluating the severity of cardiac contractile dysfunction, cardiac reserve capacity and the effectiveness of therapeutic agents in congestive heart failure.

Na(+)-Ca2+ exchange function underlying contraction frequency inotropy in the cat myocardium

In most mammalian species, an increase in stimulation frequency (ISF) produces an increase in contractility (treppe phenomenon), which results from larger Ca2+ transients at higher frequencies, due to an increase in sarcoplasmic reticulum Ca2+ load and release. The present study attempts to elucidate the contribution of the Na(+)-Ca2+ exchanger (NCX) to this phenomenon. Isolated cat ventricular myocytes, loaded with [Ca2+]i- and [Na+]i-sensitive probes, were used to determine whether the contribution of the NCX to the positive inotropic effect of ISF is due to an increase in Ca2+ influx (reverse mode) and/or a decrease in Ca2+ efflux (forward mode) via the NCX, due to frequency-induced [Na+]i elevation, or whether it was due to the reduced time for the NCX to extrude Ca2+. The results showed that the positive intropic effect produced by ISF was temporally dissociated from the increase in [Na+]i and was not modified by KB-R7943 (1 or 5 microM), a specific blocker of the reverse mode of the NCX. Whereas the ISF from 10 to 30 beats min(-1) (bpm) did not affect the forward mode of the NCX (assessed by the time to half-relaxation of the caffeine-induced Ca2+ transient), the ISF to 50 bpm produced a significant reduction of the activity of the forward mode of the NCX, which occurred in association with an increase in [Na+]i (from 4.33+/-0.40 to 7.25+/-0.50 mM). However, both changes became significant well after the maximal positive inotropic effect had been reached. In contrast, the positive inotropic effect produced by ISF from 10 to 50 bpm was associated with an increase in diastolic [Ca2+]i, which occurred in spite of a significant increase in the relaxation rate and at a time at which no increases in [Na+]i were detected. The contribution of the NCX to stimulus frequency inotropy would therefore depend on a decrease in NCX-mediated Ca2+ efflux due to the reduced diastolic interval between beats and not on [Na+]i-dependent mechanisms.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

Interaction of the Na+-K+ pump and Na+-Ca2+ exchange via [Na+]i in a restricted space of guinea-pig ventricular cells

1. The whole-cell Na+-K+ pump current (INa-K) and Na+-Ca2+ exchange current (INa-Ca) were recorded in guinea-pig ventricular myocytes to study the interaction between the two Na+ transport mechanisms. 2. INa-K was isolated as an external K+-induced current, and INa-Ca as an external Ca2+- induced or Ni2+-sensitive current. The experimental protocol used for one ion carrier did not affect the other. 3. The amplitude of INa-K decreased to 54 +/- 17 % of the initial peak during continuous application of K+ with 20 mM Na+ in the pipette. The outward INa-Ca, which was intermittently activated by brief applications of Ca2+, decreased during activation of INa-K, and recovered after cessation of INa-K activation. These findings revealed a dynamic interaction between INa-K and INa-Ca via a depletion of Na+ under the sarcolemma. 4. To estimate changes in Na+ concentration ([Na+]i) under the sarcolemma, the reversal potential (Vrev) of INa-Ca was measured. Unexpectedly, Vrev hardly changed during activation of INa-K. However, when INa-Ca was blocked by Ni2+ at the same time that INa-K was activated, Vrev changed markedly, maximally by +100 mV, immediately after the removal of Ni2+ and K+. 5. Subsarcolemmal [Na+]i was calculated from the Vrev of INa-Ca on the assumption that the subsarcolemmal Ca2+ concentration ([Ca2+]i) was fixed with EGTA, and mean [Na+]i was calculated from both the time integral of INa-K and the cell volume. The subsarcolemmal [Na+]i was about seven times greater than the mean [Na+]i. 6. The interaction between the Na+-K+ pump and Na+-Ca2+ exchange was well simulated by a diffusion model, in which Na+ diffusion was restricted to one-seventh (14 %) of the total cell volume.

Influences of changes in calcium concentration and verapamil on the cardiac depressant effect of ethanol in cat papillary muscle

1. In isolated cat heart papillary muscle electrically driven at a constant rate the depressant effects of increasing concentrations of ethanol on peak tension developed (PTD) was studied in Ringer-Locke solution with different calcium concentrations and with the addition of verapamil. 2. Ethanol induced a concentration dependent decrease in PTD that was significantly greater for each concentration of ethanol in hypocalcic medium (1.1 mM) than in normocalcic medium (2.2 mM). 3. In normocalcic (2.2 mM) medium, verapamil (5.1 x 10(-4) mM) plus ethanol (48.6 and 97.2 mM) produced a decrease in PTD to values significantly greater than those obtained by the addition of ethanol and verapamil alone. Therefore a potentiation of the effects of ethanol by verapamil was observed when both drugs act simultaneously. 4. In hypercalcic medium (4.4 mM), verapamil plus ethanol (48.6 and 97.2 mM) produced a slight decrease in PTD that was significantly less than that observed in normocalcic and hypocalcic mediums. 5. In hypocalcic medium (1.1 mM) verapamil plus ethanol (48.6 and 97.2 mM) produced a decrease in PTD that was of the same relative magnitude (%) as that observed in normocalcic medium. However no potentiation of the combined effects of verapamil plus ethanol was observed in hypocalcic medium.

Influence of the force-frequency relation on left ventricular function during exercise in conscious dogs

BACKGROUND

The magnitude of the force-frequency effect on myocardial contractility in the conscious animal has been studied at rest, but it has not been assessed during exercise.

METHODS AND RESULTS

The influence of heart rate (HR) changes were evaluated during treadmill exercise in eight preinstrumented, conscious dogs in which high-fidelity left ventricular (LV) pressure, LV volume (by sonomicrometry), and aortic pressure were measured. Under resting conditions, end-systolic pressure-volume relations were obtained using inferior vena caval occlusion. Dogs were run on a treadmill, and the intrinsic exercise HR was reduced by infusion of a specific bradycardic drug (UL-FS 49 0.5 mg/kg) during continuing exercise while HR was maintained at 240 beats per minute by atrial pacing. At 6 minutes of running at a fixed, paced HR when a stable drug effect had been achieved, no effects of UL-FS 49 on measures of LV contractility were detected compared with exercise before drug administration. HR was then reduced stepwise from 240 to 210, 180, or 150 beats per minute in a random manner, returning to 240 beats per minute between steps. Progressive reductions in measures of myocardial contractility occurred as the HR was slowed, and reduction of rate from 240 to 150 beats per minute reduced the LV maximum positive dP/dt by 31% and (dP/dt)DP40 by 21% despite increases in LV end-diastolic pressure. The entire end-systolic pressure-volume could not be determined during exercise, but beat-averaged end-systolic pressure-volume points during exercise were progressively shifted to the right and downward by slowing the exercise HR. Thus, a pronounced negative inotropic influence of slowing the heart was observed during exercise, and the rate of ventricular relaxation (tau) was also significantly prolonged.

CONCLUSIONS

These findings indicate that force-frequency effects on the inotropic state of the intact LV are markedly enhanced by exercise.

Role of aiNa in positive force-frequency staircase in guinea pig papillary muscle

In the ventricular papillary muscle of guinea pig heart, membrane potential, intracellular sodium activity (aiNa), and twitch force were measured simultaneously and continuously for many hours at stimulation rates of 0, 0.5, 1, 2, 3, 4, 5, and 6 Hz to investigate the relation of aiNa to twitch force and membrane potential both in the steady state and during the changes in these variables. After an increase in stimulation rate, both aiNa and twitch force increased progressively, reaching steady-state levels. The relation between twitch force and aiNa in the steady state was generally sigmoidal over the range of 0.5-5 Hz and steep in the 1- to 4-Hz range. After either increase or decrease in stimulation rate, the time course of change in aiNa was exponential and similar to that of change in twitch force. Moreover, the force-aiNa relation observed after increase in stimulation rate from 0.5 to 3 Hz resembled that observed after decrease in the rate from 3 to 0.5 Hz, indicating an absence of hysteresis in the relation. The results suggest that an increase in aiNa is an important factor involved in the force staircase. As stimulation rate was increased from 0.5 to higher rates (5 or 6 Hz) and then decreased back to 0.5 Hz, a hysteresis phenomenon was observed in the relation between twitch force and aiNa. This suggests that some secondary factor may alter the relation between twitch force and aiNa. As stimulation rate increased and aiNa rose, the steady-state diastolic membrane potential hyperpolarized. This result is consistent with the view that an increase in aiNa enhances the electrogenic Na+-K+ pump and hyperpolarizes the cell membrane.

Carp (Cyprinus carpio) heart has a high sensitivity to the positive inotropic effect of strophanthidin despite negative force-frequency relationships

1. The relationship between response of the heart to increased stimulation frequency and digitalis sensitivity was examined comparing the positive inotropic effect of strophanthidin and [3H]ouabain binding to sarcolemmal Na+, K+-activated adenosine triphosphatase (Na+, K+-ATPase) in carp heart, which showed a negative force-frequency relationship, and in guinea-pig heart, which has a positive relationship. 2. In ventricular muscle preparations isolated from carp heart, strophanthidin increased developed tension with a half-maximal effect observed at 0.31 microM, indicating a relatively high digitalis sensitivity of this preparation. 3. The positive inotropic effect was not altered by concentrations of propranolol sufficient to block beta-adrenergic receptors. 4. Specific binding of [3H]ouabain to homogenates obtained from ventricular muscle of carp heart showed a single class of binding sites with a Kd value of 26 nM. 5. Potency of strophanthidin to produce the positive inotropic effect and affinity of the binding sites for [3H]ouabain were both higher in carp heart compared to those in guinea-pig heart. 6. These results demonstrate a clear dissociation between the force-frequency relationship and the sensitivity of heart muscle to the positive inotropic effect of cardiotonic steroids. 7. The latter is primarily determined by affinity of sarcolemmal Na+, K+-ATPase for the cardiotonic steroids.

The mechanism of action of maitotoxin in relation to Ca2+ movements in guinea-pig and rat cardiac muscles

Maitotoxin (MTX), the most potent marine toxin known, produced a dose-dependent positive inotropic effect on guinea-pig isolated left atria and rat ventricle strips at concentrations of 0.1 ng to 4 ng ml-1. MTX (4 ng ml-1) also exhibited a positive chronotropic effect on guinea-pig right atria. The MTX-induced inotropic effect was almost abolished by Co2+ or verapamil, but was little affected by propranolol, reserpine or tetrodotoxin. The tissue Ca content of guinea-pig left atria was increased by MTX (2-30 ng ml-1) in a dose-dependent manner, and this increase was markedly inhibited by Co2+ or verapamil. Furthermore, on the rat isolated cardiac myocytes MTX (0.01-10 ng ml-1) caused an arrhythmogenic effect which was followed by their transformation into irreversibly rounded cells. The effects of MTX on the isolated cells were inhibited by verapamil or Ca2+-free solution. These results suggest that the excitatory effects of MTX on heart muscle are caused by a direct action on the cardiac muscle membrane mainly due to an increase in Ca2+ permeability.

The effects of membrane potential, extracellular potassium, and tetrodotoxin on the intracellular sodium ion activity of sheep cardiac muscle

The intracellular sodium ion activity was measured using liquid ion-exchange microelectrodes with rapid response times in sheep Purkinje fibers and ventricular muscle under voltage control. The mean sodium ion activity in quiescent Purkinje fibers was 8.5 mM at a holding potential of -80 mV. With maintained hyperpolarizing (-110 mV) or depolarizing (-40 and 0 mV) voltage steps, sodium ion activity increased or decreased, respectively. At 0 mV, the mean steady state value for the sodium ion activity was 3.8 mM. Following a voltage step to 0 mV, or back to -80 mV, the time course of the sodium ion activity change could be fitted by single exponentials, with similar half-times. Increasing the extracellular potassium ion concentration from 5.4 to 15 mM did not alter the steady state value of the sodium ion activity at clamped voltages of -80 or 0 mV, which suggests that the external potassium ion activating site of the Na-K pump was saturated. With the extracellular potassium concentration 0 mM (holding potential -80 mV), the sodium ion activity increased. When maintained depolarizing steps to 0 mV were applied, the sodium ion activity decreased by up to 20 mM. This large fall in sodium ion activity is assumed to represent partial reactivation of the Na-K pump due to potassium ion accumulation in clefts. We also studied the stimulation-dependent change in sodium ion activity. Trains of action potentials or short duration depolarizing voltage clamp steps caused a frequency dependent rise in sodium ion activity. The magnitude of the rise of sodium ion activity was not altered by lengthening the duration of each voltage clamp step, but was inhibited by tetrodotoxin or by holding the membrane potential at -50 mV between depolarizing steps. These results show that sodium ion activity is a complex function of membrane voltage, depolarization frequency, and time. The rise in sodium ion activity with stimulation appears to depend on sodium ion entry regulated by the sodium channel, and may be important in the modulation of intracellular calcium and tension through the Na+-Ca++ exchange mechanism.

Seasonal variations in drug response and staircase phenomena in atrial muscle from a hibernating rodent (Spermophilus richardsonii)

1 Electrically driven atrial muscle from a hibernating rodent (Spermophilus richardsonii) showed marked seasonal variations in inotropic responsiveness to both the cardiac glycoside actodigin and to raised extracellular calcium concentration. In contrast, glycoside toxicity was apparent and comparable in all tissues. 2 These variations were accompanied by changes in force-frequency ('staircase') characteristics of the tissues. 3 In contrast, no seasonal variation was observed in either positive inotropic responses to rubidium (a non-glycoside inhibitor of Na+ + K+ ATP-ase) or negative inotropic responses to methacholine. 4 In summer atria the calcium antagonist, verapamil (6 x 10(-7) M) abolished the positive staircase phenomenon but did not modify the positive inotropic actions of actodigin. 5 These data do not support the hypothesis that 'staircase' and glycoside-induced inotropism are mediated through a common mechanism.

Increase in intracellular sodium ion activity during stimulation in mammalian cardiac muscle

Changes in stimulation rate alter the electrical and mechanical characteristics of myocardial cells. We have investigated the possibility that intracellular sodium activity (aiNa) changes with stimulation and correlates with changes in contraction strength. Two kinds of liquid membrane Na+-selective microelectrodes were used to measure aiNa in guinea pig and sheep ventricular muscle and in sheep Purkinje strands. Stimulation produced a rate- and time-dependent elevation of aiNa. Small increases in aiNa were seen at stimulation rates as slow as 0.2 Hz, and faster rates of stimulation elevated aiNa by over 30%. The changes seen in Purkinje strands and ventricular muscle were similar. Following a period of stimulation, aiNa and Vm returned to their pre-stimulus levels with the same time courses. This is consistent with the suggestion that the post-stimulation hyperpolarization is the result of an increased rate of electrogenic Na+ extrusion. The effects of stimulation on aiNa and tension were compared with those of ouabain. The comparison suggests that rapid stimulation could produce increased contraction strength as the result of a substantial gain in intracellular calcium via a Na-Ca exchange mechanism, but that this is only one of several factors determining the force-frequency relationship.

Inotropic effects of digitoxin in isolated guinea-pig heart under conditions which alter contraction

In order to examine which calcium pool(s) might contribute to the therapeutic action of digitalis, the positive inotropic effect of digitoxin was quantified under conditions in which different calcium pools were either predominant or suppressed. Isolated left atrial muscle of guinea-pig heart was stimulated at 0.5 Hz at 30 degrees C. After a brief rest period, the first contraction (post-rest contraction) observed when electrical stimulation was resumed was markedly greater than the contraction observed under 0.5 Hz stimulation. Post-rest contraction was apparently dependent on the rest period and related to a ryanodine-sensitive, verapamil-insensitive calcium pool. Post-rest contraction was moderately enhanced by 0.2 microM digitoxin, either in the absence or presence of verapamil. A step-wise increase in the frequency of stimulation following the rest period caused a typical staircase phenomenon, which was markedly suppressed by verapamil but not by ryanodine. Digitoxin markedly augmented the staircase in the absence or presence of ryanodine. Paired-pulse stimulation markedly increased the developed tension, which was slightly reduced by verapamil but not by ryanodine. Digitoxin substantially increased the developed tension evoked by paired-pulse stimulation. In left atrial preparations of rat heart, an increase in stimulation frequency decreased the force of contraction; however, paired-pulse stimulation increased the force, indicating that the enhancement of developed tension by an increase in stimulation frequency and that by paired-pulse stimulation have different mechanisms. Thus, the positive inotropic action of digitoxin does not appear to be restricted to a specific calcium pool; however, the inotropic effect was greater under the conditions in which superficial calcium pool plays a predominant role.

Effect of intracellular sodium on calcium uptake in isolated guinea-pig diaphragm and atria

(1) Effects of cellular sodium on the 45Ca uptake of isolated guinea-pig diaphragm and atria were studied. (2) Cellular sodium and calcium contents were higher in diaphragm compared to atria after incubating the tissues in normal Krebs-Henseleit solution. (3) Cellular sodium content in atria and diaphragm were reduced significantly by incubating the tissues in high potassium Krebs-Henseleit solution (K+ = 34.7 mM), while it was increased by incubating the tissues in the ice-cold low potassium and low calcium Krebs-Henseleit solution (K+ = 0.65 mM, Ca2+ = 0.2 mM). Cellular potassium content was changed inversely to the sodium content. (4) In atria, cellular content of calcium was not altered significantly by the above conditions. But in diaphragm, the cellular content of calcium was decreased slightly but significantly after incubation in the ice-cold low potassium and low calcium Krebs-Henseleit solution. (5) At normal cellular sodium levels, the 45Ca uptake of both tissues was similar. (6) The reduction of the cellular sodium content caused a significant decrease in the 45Ca uptake in both tissues. (7) When the cellular sodium content was increased in atrial preparations, a marked increase in the 45Ca uptake was observed. On the other hand, in diaphragm preparations, only a slight increase was observed, even when cellular sodium content was much higher than the normal level. (8) These results indicate that even when the intracellular sodium is increased by some physiological of pharmacological events, calcium influx through Na+/Ca2+ exchange mechanism is very slight and slow in diaphragm.

Inotropic effects and Na+,K+-ATPase inhibition of ouabain in isolated guinea-pig atria and diaphragm

Effects of ouabain on force of contraction were compared in electrically driven isolated tissue preparations of guinea-pig left atria and diaphragm. A distinct and steady positive inotropic effect of ouabain was observed in atrial preparations, whereas in diaphragm preparations, ouabain produced only a slight and transient positive inotropic effect, followed by the negative inotropic phase. The transient positive inotropic effect of ouabain was observed even in the absence of extracellular calcium, but was markedly dependent on the extracellular sodium concentration. In vitro [3H]ouabain binding studies revealed that the affinity of Na+,K+-ATPase for ouabain was about eight times higher and tissue concentration of the enzyme was significantly lower in diaphragm than in cardiac tissue. The Ki value for ouabain inhibition of the cardiac Na+,K+-ATPase was also approximately ten times higher than for the diaphragm enzyme. Ouabain-sensitive 86Rb uptake, an estimate of sodium pump activity, was inhibited by ouabain at a time when it produced its transient positive inotropic effect in diaphragm preparations. These results indicate that the lack of a distinct and steady positive inotropic effect of ouabain in diaphragm was due neither to the difference in the ouabain-Na+,K+-ATPase interaction between diaphragm and cardiac tissues nor the failure of sodium pump inhibition by ouabain in diaphragm.

Influence of arteriographic contrast media on the Na+/Ca++-ratio in blood

To investigate the basis of cardiodepressive side effects in coronary arteriography, the isolated effects of different contrast media on electrolyte levels in blood (and blood fractions) were quantified in vitro. Due to their sodium content and calcium chelation, ionic media cause an imbalance in the extracellar Na+/Ca++-ratio that determines myocardial contractility. Ca++-chelation exceeded that due to stabilizing additives indicating a binding by the contrast agents themselves. The amount of calcium bound varied with different media and was greatest with Renografin. It is suggested that the degree of Na+/Ca++-imbalance in blood explains the extent of acute cardiodepression.

Effects of lithium and thallous ions on sodium pump activity in the guinea-pig heart and their relationship to the positive inotropic action

It has recently been demonstrated that both Tl+ and Li+ produce concentration- and time-dependent positive inotropic effects in guinea-pig atrial preparations although Tl+ inhibits and Li+ stimulates isolated Na+,K+-ATPase in vitro. In order to elucidate the mechanism of the positive inotropic actions of these cations, the effects of Tl+ and Li+ on sodium pump activity were studied. Active 86Rb uptake in guinea-pig ventricular slices, an estimate of sodium pump activity, was highly sensitive to the inhibitory effect of the cardiac glycosides. Preincubation of slices with Tl+ caused a dose- and time-dependent inhibition of active 86Rb uptake. Similar concentration- and time-dependent inhibition of active 86Rb uptake was observed when Na+ in a Krebs-Henseleit solution was partially replaced with Li+. Lithium, however, stimulated a partially purified Na+,K+-ATPase in vitro. During heart slice incubation, Tl+ and Li+ accumulated in a time-dependent manner. This accumulation was not readily reversible when slices were transferred into Tl+- or Li+-free solutions. It appears that the inhibition of sodium pump activity is related to the positive inotropic action of these cations.

Cardiovascular function in dogs with acute hypokalemia

The effects of acute hypokalemia on plasma electrolytes, cardiovascular function, osmolality, and hematocrit were investigated in anesthetized dogs for 2 hours. There was progressive increase in the total systemic vascular resistance, but no change in the osmolality and blood hematocrit. All of the hemodynamic parameters decreased except the preferred index of myocardial contractility, [dp/dt]/IIP, which increased during hypokalemia. The changes in this index of myocardial contractility were associated with changes in the plasma potassium but not the plasma sodium. The results suggest that hypokalemia-induced increases in myocardial contractility might be associated with an increased influx of Ca++ as a result of hypokalemia-induced inhibition of sarcolemmal Mg++ -dependent, Na+-K+-ATPase.

Isovolumic relaxation time in normal subjects and patients with cardiac disease: comparison of determinations made with echocardiographic techniques and apex cardiography

Isovolumic relaxation time (IVRT) was determined in 17 controls and 41 patients. Nine patients had ischemic heart disease (IHD), 7 mitral prolapse (MVPS), 13 hypertension (HPB), 7 pregnancy (P), and 5 cardiomyopathy (CM). Echocardiographic measurements of IVRT were made from the aortic second sound to the rapid opening of the mitral valve (A2D1). Determinations by apexcardiography were made from the aortic second sound to the 0 point (A2O). The IVRT was distinctly shorter when assessed by A2D1 than by conventional apexdardiography in conventional apexcardiography in controls (69.2 +/- 16.4 msec vs 118.7 +/- 16.5 msec) and in patients with cardiac disease. The IVRT in 9 older normal controls (mean age 47.7 years) was longer than in 8 younger ones (age 26.3 +/- 4.9 years). Patients with myocardial disease (IHD, HBP, and CM) had prolonged IVRTs when compared to normal subjects. Pregnant subjects had shortened intervals. IVRT may be a sensitive indicator of disturbances in myocardial contractility and may be shortened and enhanced contractility.

Membrane adenosinetriphosphatase: a digitalis receptor?

The enzyme Na+,K+-ATPase is a good model for receptor studies because of its known functional correlates. The binding of digitalis to the enzyme observed in vitro satisfied the criteria for receptor binding. Studies of the relationship between the digitalis binding and the drug action reveal an impressive correlation between these events but fail to provide proof of a causal relationship. Studies with other Na+,K+-ATPase inhibitors and agents that affect transmembrane Na+ movements (steps that would follow Na+,K+-ATPase inhibition) provide further supportive evidence that sodium pump inhibition and the resulting enhancement of intracellular Na+ transients cause the inotropic action of digitalis.

Diastolic intervals in cardiomyopathy and abnormal thyroid states: alterations in isovolumic relaxation time and rapid filling time of depressed and enhanced myocardial contractility

Diastolic and systolic time intervals were measured in 11 control subjects, 11 patients with cardiomyopathy, 7 hyperthyroid patients, and 5 hypothyroid patients. The isovolumic relaxation time (IVRT), rapid filling time (RFT), preejection period (PEP), left ventricular ejection time (LVET), and PEP/LVET ratio were found by simultaneously recording the ECG, phonocardiogram, external carotid pulse, and apexcardiogram. In cardiomyopathy the IVRT and RFT were prolonged (107.4 +/- 21.1 msec [P less than 0.01] and 111.0 +/- 10.0 [P less than 0.01] respectively) in comparison to the control subjects. (In the controls the IVRT was 85.7 +/- 18.4 msec and the RFT was 94.5 +/- 12.8 msec). In altered thyroid states the RFT was most affected; in hypothyroidism it increased to 123.9 +/- 25.2 (P less than 0.01) and in hyperthyroidism it decreased to 71.5 +/- 21.3 msec (P less than 0.01). In hyperthyroid patients the IVRT, although shorter than in control subjects, was not significantly altered, but it showed a significant increase after treatment. The RFT also returned toward normal after therapy in both groups (116.7 +/- 14.6 msec in hypothyroid patients and 89.0 +/- 23.1 msec in those with hyperthyroidism).

Frequency-dependent changes in the cardiac sarcolemmal ATPase

1 Effects of various frequencies (0.25, 0.5, 1.0, 1.5 or 2.0 Hz) of stimulation for various durations (2, 5, 10 or 15 min) on the contractile force of trabecular or papillary muscles of dog myocardium were investigated. 2 Effects of various frequencies (0, 0.25, 0.5, 1.0, 2.0 Hz) of various stimulus strengths (0.5, 1, 10 V) for various durations (2,5,10 or 15 min) on the Mg2+/-dependent Na+/--K+/--adenosinetriphosinetriphosphatase (ATPase) of isolated sarcolemmal fraction of dog myocardium were determined. 3 There was a frequency-dependent increase in the contractility and inhibition of the Na+/--K+/--ATPase within 2 minutes. 4 Frequency-dependent increase in the contractility and inhibition of Na+/--K+/--ATPase decreased as the duration of stimulation was increased. 5 The diminution in the inhibition of ATPase was associated with a decrease in the contractility with prolonged stimulation. 6 These results suggest that the frequency-dependent increase in the myocardial contractility might be mediated through an inhibition of the sarcolemmal ATPase.

A possible molecular mechanism of the action of digitalis: ouabain action on calcium binding to sites associated with a purified sodium-potassium-activated adenosine triphosphatase from kidney

Calcium binding at 0 degrees C to a purified sheep kidney Na+,K+-ATPase was described by linear Scatchard plots. Binding at saturating free calcium was 65-80 nmol/mg of protein, or 30-40 mol of calcium/mol of enzyme. Aqueous emulsions of lipids extracted from Na+,K+-ATPase yielded dissociation constants and maximum calcium-binding values that were similar to those for native Na+,K+-ATPase. Phospholipase A treatment markedly reduced calcium binding. Pretreatment of native Na+,K+-ATPase with ouabain increased the dissociation constant for calcium binding from 131 +/- 7 to 192 +/- 7 muM without altering maximum calcium binding. Ouabain pretreatment did not affect calcium binding to extracted phospholipids, ouabain-insensitive ATPases, or heat denatured Na+,K+-ATPase, Na+ and K+ (5-20 mM) increased the dissociation constants for calcium, which suggests competition between the monovalent cations and calcium for the binding sites. At higher concentrations of monovalent cations, ouabain increased the apparent affinity of binding sites for calcium. Extrapolation to physiological cation concentrations revealed that the ouabain-induced increase in apparent affinity for calcium may be as much as 2- to 3-fold. These results suggest: (1) calcium binds to phospholipids associated with Na+,K+-ATPase; (2) ouabain interaction with Na+,K+-ATPase induces a perturbation that is transmitted to adjacent phospholipids, altering their affinity for calcium; and (3) at physiological concentrations of Na+ or K+, or both, ouabain interaction with Na+,K+-ATPase may lead to an increased pool of membrane-bound calcium.

The effect of heart rate on indices of myocardial contractility in the dog

1. Changes in heart rate were evoked by atrial pacing in anaesthetized dogs with no pretreatment and in dogs given reserpine or guanethidine for 72 h. The effect of alterations in heart rate were related to two indices of myocardial contractility: the maximal rate of change of left ventricular pressure (dp/dt), and an index which was independent of initial fibre length (dp/dt)/IIT, where IIT is integrated isometric tension. 2. An increase in heart rate in control dogs was accompanied by a rise in both dp/dt and (dp/dt)/IIT confirming that the Bowditch staircase does exist in the intact ventricle. The regression line relating heart rate to (dp/dt)/IIT was significantly steeper than that relating heart rate to dp/dt because the reduction in left ventricular preload at high heart rate tends to attenuate the rise in dp/dt. 3. Reserpine, but not guanethidine pretreatment was accompanied by either a slight decrease or no change in (dp/dt)/IIT during pacing. 4. Acute elevation of (dp/dt)/IIT by either calcium or isoprenaline infusion in reserpine pretreated dogs did not restore the Bowditch effect. 5. Acute depression of (dp/dt)/IIT by propranolol and pentobarbitone was accompanied by a greater rise in (dp/dt)/IIT with pacing in control dogs and a rise rather than a fall in reserpine-pretreated dogs.

Effects of monovalent cations on cardiac Na+, K+-ATPase activity and on contractile force

The relationship between Na+, K+-ATPase inhibition by monovalent cations and their inotropic effect was studied in guinea pig hearts. The activity of partially purified cardiac enzyme was assayed in the presence of 5.8 mM KC1 and either 20 or 150 mM NaCl. Rb+ and Tl+ inhibited Na+, K+-ATPase activity, the magnitude of the inhibition by these cations being greater in the assay media containing lower Na+ concentrations. Tl+ produced a dose-dependent inhibition of Na+, K+-ATPase activity in the presence of 20 mM Na+ and 75 mM K+, a cationic condition similar to that of intracellular fluid. Other monovalent cations such as K+, Cs+, NH4+, Na or Li+ produced essentially no effect on the Na+, K+-ATPase activity or slightly stimulated it. In left atrial strips stimulated with field electrodes and bathed in Krebs-Henseleit solution (5.8 mM K+ and 145 mM Na+), addition of Cs+ failed to alter the isometric contractile force significantly. NH4+ and K+ caused a transient positive inotropic effect which was partially blocked by propranolol. The positive inotropic response to K+ was followed by a negative inotropic response. Rb+ produced a sustained, dose-dependent inotropic response reaching a plateau at 1-2 min, whereas Tl+ produced a dose=dependent positive inotropic effect which developed slowly over a 30-min period. The positive inotropic effects produced by Rb+ and Tl+ were insensitive to propranolol pretreatment. Concentrations of Tl+ and cardiac glycosides which produce similar inotropic effects appear to cause the same degree of Na+-pump inhibition. The onset of the positive inotropic response to Rb+ or Tl+ was not dependent on the number of contractions which is in contrast to the cardiac glycoside-induced inotropic response. Substitution of 20 mM LiCl for an equimolar amount of NaCl in Krebs-Henseleit solution produced a significantly greater inotropic response than that observed when sucrose was substituted for NaCl. It appears that, among monovalent cations, only sodium pump inhibitors produce a sustained positive inotropic response.

Influence of calcium on the inotropic actions of hyperosmotic agents, norepinephrine, paired electrical stimulation, and treppe

To analyze the interaction of calcium ion concentration with hypertonic agents and with other inotropic interventions, isolated right ventricular cat papillary muscles were studied under isometric conditions in Krebs-Ringer bicarbonate solution. Extracellular calcium concentrations were varied between 2.5 and 11.0 mM. Maximal inotropic effects occurred between 5 and 8.0 mM calcium and further elevation to 11.0 mM was without additional influence. The effect of hyperosmotic sucrose and mannitol on papillary muscle performance was compared with that of 10(-6) M norepinephrine at calcium concentrations of 2.5 and 10.0 mM and with paired electrical stimulation in 10.0 mM calcium. Both norepinephrine and the hyperosmotic agents produced significant increases in developed tension and in the maximal rate of tension rise (dT/dt) in Krebs-Ringer in 2.5 and 4.0 mM calcium. In 10 mM calcium norepinephrine increased developed tension and dT/dt, but sucrose and mannitol caused no change or small reductions in both. Paired electrical stimulation, like hyperosmolality, caused no increase in dT/dt in 10 mM calcium. The presence of a potent pharmacological inhibitor of systolic calcium transfer across the cell membrane (D600, 10(-6) M) reduced developed tension and dT/dt by 76+/-2.7 and 74+/-2.0%, respectively, and prevented and in fact reversed the expected increase in dT/dt associated with an increase in rate of stimulation (treppe). However, hypertonic mannitol and paired pacing persisted in causing marked increases in developed tension and dT/dt even in the presence of D600, suggesting that their inotropic effects are not dependent on increased intracellular transfer of calcium during systole through cell membrane channels in which D600 acts as a competitive inhibitor. The results of these studies suggest that apparent functional saturation of intracellular calcium receptor sites eliminates any additional inotropic effect of hyperosmolality or paired pacing. The data are compatible with the hypothesis that the inotropic effects of hyperosmolality and of paired pacing result from an increase in calcium concentration at the myofilaments during contraction. The increase induced by hyperosmolality might occur because of an increase in the total amount of calcium released into the cytosol with each action potential and/or as a passive consequence of cellular dehydration. Norepinephrine has the capacity to increase contractility even when intracellular calcium receptor sites appear to be functionally saturated, suggesting that it may act at least in part by a mechanism that is independent of changes in net intracellular calcium concentration.

A dissociation of positive staircase (Bowditch) from ouabain-induced positive inotropism

The effects of verapamil on myocardial contractility were examined in perfused rabbit ventricular preparations. Verapamil (0.2 µ M ) decreased the time derivative of developed force (dF/dt) of the preparations paced at 90 beats/min to 39 ± 3% ( SE ) of the control value. Partial reversal to 76 ± 5% of the control value was achieved by subsequent perfusion with drug-free Krebs solution. Verapamil hastened the decline and delayed the return of contractility in response to a cycle of calcium washout and reperfusion. It converted the response to increased heart rate from positive (Bowditch effect) to negative inotropism. Moreover, the sensitivity to the drug was frequency related; the drug concentration required to induce a 50% decrease in dF/dt was an inverse function of heart rate, suggesting that the drug altered contractility by interfering with a phenomenon associated with systole. Verapamil did not decrease the magnitude of the positive inotropic response to 0.5 µ M ouabain, although a significant delay in the development of the response was observed. After 8 minutes of ouabain perfusion dF/dt had increased 35 ± 2% in the absence of verapamil but only 13 ± 1% in the presence of 1.0, µ M verapamil. The maximum response to ouabain was a 39 ± 4% increase at 9 minutes in the absence of verapamil and a 34 ± 2% increase at 23 minutes in the presence of verapamil. These results suggest that the responses to increased frequency and to cardiac glycosides are not explained by a common mechanism such as sodium-calcium exchange occurring during diastole or any other diastole-related mechanism for calcium influx. However, the possibility that both events are mediated by some common mechanism occurring during systole has not been eliminated.

Contribution of quantitative assay technics to the understanding of the clinical pharmacology of digitalis

Despite recent advances in understanding of the pharmacokinetics and electrophysiologic effects of cardiac glycosides, digitalis toxicity remains distressingly common in clinical practice. Another substantial group of patients is markedly underdigitalized, failing to gain the full therapeutic benefits of optimal use of these drugs. Since cardiac digitalis toxicity is a dose-related phenomenon, and serum or plasma digoxin and digitoxin concentrations rise with increasing doses, at least a statistical correlation between circulating levels and clinical state might be expected. Increasing availability of serum or plasma digitalis concentration measurements thus offers the clinician a potential means of improving the patient's chances of benefiting from treatment with cardiac glycosides. Assay methods in current use include a double-isotope dilution derivative method (digitoxin), red cell 86 Rb-uptake inhibition (digitoxin and digoxin), Na + -K + ATPase inhibition (digitoxin), ATPase enzymatic displacement (digitoxin and digoxin), gas chromatography (digoxin), and radioimmunoassay (digitoxin, digoxin, and ouabain).

The rapidly expanding literature reporting clinical experience with these technics reflects general agreement that mean serum or plasma digoxini and digitoxin levels are significantly higher in patients with clinical evidence of toxicity compared with nontoxic patients. Nevertheless, multiple factors influence individual responses, and blood level data must be interpreted in the overall clinical context. Hypokalemia, hypercalcemia, hypomagnesemia, acid-base disturbances, hypoxemia, and hypothyroidism all tend to decrease tolerance to any given digitalis dose or blood level. Autonomic nervous system tone and other drugs concurrently received must also be considered. Advanced heart disease in general, and coronary artery disease in particular, appear to predispose patients to apparent digitalis toxicity at relatively lower serum or plasma levels.

Cardiac glycoside assay technics have also proven useful in various studies of the clinical pharmacology of digoxin, digitoxin, and ouabain. Handling of digoxin by patients on cardiopulmonary bypass has been assessed, and gastrointestinal absorption has been evaluated in normal subjects; poor and erratic absorption of the drug has been documented in patients with malabsorption syndromes. Potentially important drug-drug interactions of agents such as phenobarbital and phenylbutazone with digitoxin have been studied, as well as the effects of steroid-binding resins on digoxin and digitoxin metabolism. Studies of ouabain pharmacokineties by radioimmunoassay have demonstrated a plasma half-life of 21 hours, indicating that, as in the case of digoxin and digitoxin, half-life of serum or plasma concentration after establishment of blood-tissue equilibrium bears a close relationship to duration of clinical effect.

Ouabain-specific antibodies: immunochemical properties and reversal of Na + , K + -activated adenosine triphosphatase inhibition

Antibodies with high affinity and specificity for the cardiac glycoside ouabain were raised in rabbits. The antigen used was a conjugate of ouabain linked through its rhamnose moiety to terminal alpha-amino groups of poly D,L alanyl-human serum albumin. Ouabain-specific antibodies were present as early as 3 wk, and rose steadily in titer over the initial 20-33 wk of immunization. Levels as high as 6.5 mg specific immunoglobulin per ml antiserum were reached in one rabbit at the end of 45 wk. The average intrinsic association constants for ouabain were 1.3 x 10(9) M(-1) and 1.6 x 10(9) M(-1) in antisera studied in detail, and there was evidence of restricted heterogeneity of binding site affinities. A high degree of specificity was demonstrated. Significant cross-reactivity occurred only with other cardioactive steroid compounds such as acetyl strophanthidin, digoxin, and digitoxin, while endogenous steroids did not cross-react even when present in 1000-fold excess. A rapid and convenient radioimmunoassay procedure for plasma or urine ouabain concentrations was developed using these antibodies. Competition between ouabain-(3)H tracer and unlabeled ouabain for specific antibody binding sites allowed the measurement of ouabain concentrations as low as 0.1 ng/ml or less without need for extraction procedures. The high association constants observed in these studies permit antibody reversal of established myocardial effects of ouabain. Both blockade and reversal of ouabain inhibition of canine myocardial microsomal Na(+), K(+)-activated ATPase by antibody were documented, suggesting a possible mechanism for reversal of cellular effects.