Quantitative relation of twitch and tonic tensions to intracellular Na+ activity in cardiac Purkinje fibers

Effective scatterer diameter estimates for broad scatterer size distributions

Acoustic form factors have been used to model the frequency dependence of acoustic scattering in phantoms and tissues. This work demonstrates that a broad range of scatterer sizes, individually well represented by Faran theory or a Gaussian form factor, is not accurately described by a single effective scatterer from either of these models. Contributions from a distribution of discrete scatterer sizes for two different form factor functions (Gaussian form factors and scattering functions from Faran's theory) were calculated and linearly combined. Composite form factors created from Gaussian distributions of scatterer sizes centered at 50 µm with standard deviations of up to σ = 40 µm were fit to each scattering model between 2 and 12 MHz. Scatterer distributions were generated using one of two assumptions: the number density of the scatterer diameter distribution was Gaussian distributed, or the volume fraction of each scatterer diameter in the distribution was Gaussian distributed. Each simulated form factor was fit to a single-diameter form factor model for Gaussian and exponential form factors. The mean-squared error (MSE) between the composite simulated data and the best-fit single-diameter model was smaller with an exponential form factor model, compared with a Gaussian model, for distributions with standard deviations larger than 30% of the centroid value. In addition, exponential models were shown to have better ability to distinguish between Faran scattering model-based distributions with varying center diameters than the Gaussian form factor model. The evidence suggests that when little is known about the scattering medium, an exponential scattering model provides a better first approximation to the scattering correlation function for a broad distribution of spherically symmetric scatterers than when a Gaussian form factor model is assumed.

Improving the reproducibility of the cyclic variation of myocardial backscatter

The goal of myocardial tissue characterization is to augment information provided by two- and three-dimensional echocardiographic imaging, Doppler blood flow and speckle- or Doppler-derived tissue motion. Tissue characterization based on the systematic variation ofbackscattered ultrasound during the cardiac cycle ('cyclic variation') appears to be effective in characterizing both focal and diffuse myocardial pathologies. Unfortunately, comparison ofresults from different laboratories is difficult because of a lack of consistency among the several reported methods of analyzing the cyclic variation data. The goals of the present work are to present an improved method of analysis and to demonstrate that apparent disagreements are attributable primarily to the distinct approaches employed by different investigators. The improved automated method for determining the magnitude of cyclic variation utilizes binomial smoothing and an average deviation method and was validated using data acquired from 23 patients. This method illustrates a systematic means for resolving differences between laboratories. This resolution facilitates future comparisons between the cyclic variation of myocardial backscatter and measurements derived, for example, from strain-related approaches.

Acoustic videodensitometric parameters correlate with abnormalities of left ventricular filling in hypertensive patients assessed via Doppler Tissue Imaging

PURPOSE

Using a videodensitometric technique developed in our center we investigated whether the amplitude of variation of gray levels in the interventricular septum was related to functional parameters, especially ventricular filling as studied by doppler tissue imaging (DTI) in hypertensive patients.

METHODS AND RESULTS

We included 35 hypertensive patients from whom we recorded a two dimensional sequence of the left ventricle from the short axis parasternal view and a doppler tissue scan of the mitral ring. Two-dimensional sections were then analyzed with Iodp software to measure variations in levels of gray in the septum. We observed significant correlations between these variations and the velocity of the Ea wave on DTI of the lateral wall (r = 0.49, p < 0.002). On dividing this population into three sub-groups according to the Appleton classification of ventricular filling, we noted a tendency to a reduction in amplitude of variations of gray of the septum in cases with abnormalities of relaxation. This was even more marked in cases with a restrictive or pseudo-normal profile.

CONCLUSION

These observations point to the interest of sonographic videodensitometry in the evaluation of hypertensive cardiopathy. Further studies will be required to determine whether it can define the extent of the abnormalities of myocardial structure and the degree of fibrosis.

Integrated backscatter for the assessment of myocardial viability

PURPOSE OF REVIEW

Ultrasonic tissue characterization is a non-invasive diagnostic method that uses myocardial integrated backscatter analysis to determine contractile performance and myocardial viability independent of wall motion. This review discusses recent clinical findings regarding the application of ultrasonic tissue characterization for the assessment of myocardial viability.

RECENT FINDINGS

As this technique is non-invasive, ultrasonic tissue characterization can be used to predict the patency of infarct-related arteries in patients in the early stage of acute myocardial infarction. Several recent studies have shown that this technique is useful in identifying myocardial contractile reserve. The accuracy of ultrasonic tissue characterization for predicting functional recovery after coronary reperfusion is comparable to dobutamine echocardiography and radionuclide methods. Several studies have suggested that the cyclic variation of myocardial integrated backscatter reflects myocardial viability rather than contractile reserve. The cyclic variation of integrated backscatter is associated with myocardial viability confirmed by the integrity of the microvasculature identified by contrast echocardiography. In addition, the cyclic variation of integrated backscatter better reflects myocardial viability confirmed by the integrity of cellar metabolism than contractile reserve.

SUMMARY

Ultrasonic tissue characterization with integrated backscatter is a useful non-invasive method that can provide unique information for the assessment of myocardial viability.

Lack of Prognostic Value of Normalized Integrated Backscatter Analysis of Myocardium in Patients with Thalassemia Major: A Long-Term Follow-Up Study

BACKGROUND

Patients with beta-thalassemia major often present with severe anemia and must undergo continuous transfusion therapy, consequently developing iron overload leading to hemochromatosis. Because of these the iron deposits and/or secondary structural changes, patients develop an increase in myocardial integrated backscatter (IB).

AIM

To investigate the prognostic value of analyzing acoustic quantitative properties of the myocardium in patients with beta-thalassemia major.

PATIENTS AND METHODS

Between 1989 and 1990, 38 patients (mean age: 18 years, range: 7-26, 21 males) with beta-thalassemia major and without clinical signs of cardiac failure were enrolled prospectively. All patients were on chelation therapy (desferroxiamine). To obtain quantitative operator-independent measurement of the IB signal of the left ventricular septum and posterior wall, the ultrasonic radiofrequency signal integrated values were normalized to the pericardial interface and expressed in percentage (IB%).

RESULTS

Follow-up was 122 +/- 36 months, during which 15 events (7 cardiac deaths and 8 heart failures) occurred. The event-free survival was comparable in patients with normal and abnormal IB%. Septal IB% was 33 +/- 14 in the 15 patients with events, and 33 +/- 12 in the 25 patients without events (P = ns). The %IB had no prognostic value in this population. A prognostic value was found in multivariate analysis for patient refusal/noncompliance of chelation therapy (P = 0.02, OR: 4.37, 95% CI: 1.72-16.9) and also body mass index (P = 0.04, OR: 1.2, 95% CI: 1.0-1.4).

CONCLUSION

Analysis of end-diastolic IB% of myocardium in patients with beta-thalassemia and iron overload was not predictive of adverse cardiac events during long-term follow-up in this study.

Microalbuminuria Is Associated with Reduced Cardiac Cyclic Variation of Integrated Backscatter Signal in Severe Hypertension

OBJECTIVE

Microalbuminuria (MA) as a marker of systemic vascular disease and left ventricular (LV) hypertrophy is associated with increased cardiovascular mortality and morbidity in patients with essential hypertension. The aim of this study was to investigate changes in cardiac cycle-dependent variation of integrated backscatter signals (CVIBS) in hypertensive patients with MA.

METHODS

Randomly selected 60 hypertensive patients (mean age 51 +/- 8) with uncontrolled blood pressure (BP) (>/=130 mmHg systolic and/or 85 mmHg diastolic) were included. All patients underwent urinary albumin excretion (UAE) measurements, 24-hour ambulatory BP monitoring, and LV echocardiographic examination. UAE was measured in two separate 24-hour urine collection and mean of two values was taken into consideration. Normotensive 20 healthy subjects served as controls. CVIBS values were obtained from mid-anteroseptal, mid-posterolateral, and mid-inferior areas at the papillary muscle level in the parasternal short-axis view. CVIBS was defined as the difference in integrated backscatter values between systole and diastole. CVIBS values in MA positive patients were compared with the values in MA negative patients and control subjects.

RESULTS

Twelve patients had MA (UAE 30 to 300 mg/day) while 48 patients had normal UAE (<30 mg/day). The wall thickness (at septum and posterior) and left ventricular mass index (LVMI) values were all significantly higher in hypertensive patients with MA (P < 0.01). The CVIBS values in MA positive group were significantly lower than the CVIBS values both in MA negative hypertensive patients and control subjects (P < 0.01).

CONCLUSION

This study demonstrates that in hypertensive patients a high LVMI is associated with reduced CVIBS values and MA appears to be a marker of hypertrophy.

Digoxin remains useful in the management of chronic heart failure

Despite the introduction of a variety of new classes of drugs for the management of heart failure, digoxin continues to have an important role in long-term outpatient management. A wide variety of placebo-controlled clinical trials have unequivocally shown that treatment with digoxin can improve symptoms, quality of life, and exercise tolerance in patients with mild, moderate, or severe heart failure. These benefits are evident regardless of the underlying rhythm (normal sinus rhythm or atrial fibrillation), etiology of the heart failure, or concomitant therapy (eg. ACE inhibitors). Unlike other agents with positive inotropic properties, digoxin does not increase all-cause mortality and has a substantial benefit in reducing heart failure hospitalizations. Consensus guidelines have recently been published by the Heart Failure Society of America and the American College of Cardiology/American Heart Association, and they contain the following recommendations for digoxin treatment: 1. Digoxin should be considered for the outpatient treatment of all patients who have persistent symptoms of heart failure (NYHA class II-IV) despite conventional pharmacologic therapy with diuretics, ACE inhibitors, and a beta-blocker when the heart failure is caused by systolic dysfunction (the strength of evidence = A for NYHA class II and III; strength of evidence = C for NYHA class IV). 2. Digoxin is not indicated as primary treatment for the stabilization of patients with acutely decompensated heart failure. (Strength of evidence = B). Digoxin may be initiated after emergent treatment of heart failure has been completed in an effort to establish a long-term treatment strategy. 3. Digoxin should not be administered to patients who have significant sinus or atrioventricular block, unless the block has been treated with a permanent pacemaker (strength of evidence = B). The drug should be used cautiously in patients who receive other agents known to depress sinus or atrioventricular nodal function (such as amiodarone or a beta-blocker) (strength of evidence = B). 4. The dosage of digoxin should be 0.125-0.25 mg daily in the majority of patients (strength of evidence = C). The lower dose should be used in patients over 70 years of age, those with impaired renal function, or those with a low lean body mass. Higher doses (eg, digoxin 0.375-0.50 mg daily) are rarely needed. Loading doses of digoxin are not necessary during initiation of therapy for patients with chronic heart failure. 5. Serial assessment of serum digoxin levels is unnecessary in most patients. The radioimmunoassay was developed to assist in the evaluation of toxicity, not the efficacy of the drug. There appears to be little relationship between serum digoxin concentration and the drug's therapeutic effects. 6. Digoxin toxicity is commonly associated with serum levels >2 ng/mL but may occur with lower digoxin levels if hypokalemia, hypomagnesemia, or hypothyroidism coexist. Likewise, the concomitant use of agents such as quinidine, verapamil, spironolactone, flecainide, and amiodarone can increase serum digoxin levels and increase the likelihood of digoxin toxicity. 7. For patients with heart failure and atrial fibrillation with a rapid ventricular response, the administration of high doses of digoxin (>0.25 mg daily) for the purpose of rate control is not recommended. When necessary, additional rate control should be achieved by the addition of beta-blocker therapy or amiodarone (strength of evidence = C). If amiodarone is added, the dose of digoxin should be reduced. Digitalis preparations are now entering their fourth century of clinical use for the treatment of chronic heart failure symptoms. Its clinical efficacy can no longer be doubted and its safety has been verified by the multicenter DIG trial. Future advances in pharmacogenetics should facilitate identification of those patients most likely to benefit from its pharmacologic effects.

Ultrasonic myocardial tissue characterization in patients with hypertrophic cardiomyopathy and pressure-overloaded hypertrophy by backscattered energy temporal analysis

This study measured integrated backscatter (IB) values in the subendocardium and subepicardium of patients with hypertrophy using the newly developed Backscattered Energy Temporal Analysis (BETA) system, and evaluated the differences of acoustic properties according to etiology. Twenty-one patients with hypertrophic cardiomyopathy (HCM), 16 with pressure-overloaded hypertrophy (POH), and 21 controls were studied. M-mode formatted IB images were obtained using BETA and the region of interest (ROI), automatically divided into epicardial and endocardial halves of the myocardium, was placed in the ventricular septum and posterior wall. Values for the cyclic variation of IB (CVIB) in the entire ROI and in each half of the ROI were obtained. CVIB significantly decreased in the ventricular septum in HCM and POH compared with normal subjects, but there were no significant differences between HCM and POH. In the posterior wall, the CVIB was less in the subendocardium than in the epicardium in POH, and was also less than in normal subjects (7.0+/-1.7 dB vs 8.6+/-1.9 dB and 8.8+/-2.1 dB, p<0.05, respectively). Separate ultrasonic tissue characterization of the subendocardium and subepicardium provides further etiological information of various heart diseases.

Ultrasonic tissue characterization in acute myocarditis: a case report

A 25-year-old woman was admitted because of acute myocarditis. Echocardiogram revealed hypokinesis of the left ventricle with increased wall thickness, but on day 7, the wall motion normalized. Cyclic variation of myocardial integrated backscatter on day I was reduced to 1.8 dB (normal range, 2.9-5.3 dB) and normalized to 3.2 dB on day 3. The normalization of the cyclic variation of integrated backscatter in the myocardium preceded the recovery of the left ventricular wall contractility, suggesting the ability of tissue characterization to predict recovery of cardiac function.

Novel ultrasonic fusion imaging method based on cyclic variation in myocardial backscatter

Quantitative ultrasonic tissue characterisation of the myocardium based on integrated backscatter (IB) has the potential of becoming an effective method for detecting and evaluating myocardial ischaemia. To facilitate IB-based clinical applications, a new imaging method has been developed that combines the anatomical information of a B-mode image with the contractile performance of a selected myocardial region. To produce such a fusion image, a region of interest (ROI) in a B-mode cardiac image was first selected by the user. Algorithms for detection of the endocardium and epicardium were developed, and the resulting mean distance between the computer-detected curve and the manually traced curve was 0.83mm for the endocardium and 0.58mm for the epicardium. The cyclic variation of IB (CVIB) of each myocardial tissue element within the ROI was then calculated over one cardiac cycle. Finally, a grey-scale B-mode image at the end of diastole was displayed as a still image, and the pixels representing the myocardial tissue in the ROI colour-coded according to the corresponding CVIB over the past heart cycle. Both the B-mode image and the colour-coded region were refreshed (up-dated) at the next end-of-diastole. Preliminary results from normal (CVIB= 10-12dB) and ischaemic (CVIB = 5-7 dB) canine hearts are presented that demonstrate the utility of this new imaging method.

Determination of successful reperfusion after thrombolysis for acute myocardial infarction: a noninvasive method using ultrasonic tissue characterization that can be applied clinically

BACKGROUND

The aim of the present study was to determine the use of cyclic variation in ultrasonic integrated backscatter (IBS), which is reduced in ischemic myocardium, to predict an occluded infarct-related artery (IRA) after thrombolysis for acute myocardial infarction (AMI). This is important, because patency of the IRA 90 minutes after thrombolysis has been shown to predict outcome.

METHODS AND RESULTS

One hundred thirteen patients with AMI had peak-to-peak cyclic IBS measured in the myocardial territory supplied by their IRA as well as a remote territory with normal function from the parasternal long- or short-axis view. This analysis took 5 to 10 minutes. Wall motion score index was assessed, and coronary angiography, to determine patency of the IRA, was performed in all patients. Cyclic IBS in the IRA territory was much lower in segments supplied by an occluded IRA (3.3 versus 4.6 dB, P<0.00001). Using a difference in cyclic IBS between infarcted and normal segments of 15% (or 1.5 dB) as a cutoff, the sensitivity, specificity, positive and negative predictive values to determine an occluded IRA were 92%, 75%, 81%, and 89%, respectively.

CONCLUSIONS

The difference in cyclic IBS between IRA and remote normal segments, which can be analyzed rapidly, can be used to predict patency of the IRA in patients with AMI. This provides a noninvasive method to determine those patients who may require urgent invasive investigation.

Ultrasonic integrated backscatter in early assessment of myocardial injury during open heart surgery

OBJECTIVES

The recovery of cyclic variation of ultrasonic integrated backscatter in myocardial ischemia provides early assessment of myocardial injury and is useful in assessing myocardial injury during open heart surgery.

METHODS

We studied 25 patients with valvular disease undergoing cardiac surgery--7 with aortic stenosis, 7 with aortic regurgitation, 6 with mitral stenosis, and 5 with mitral regurgitation. All underwent transesophageal echocardiography (before aortic cross-clamping: T-pre and 60 minutes after aortic declamping: T-60). The short-axis view at the papillary muscle level of the left ventricle was recorded and anterior areas were assessed.

RESULTS

The magnitude of cyclic variation at T-pre and T-60 was 9.4 +/- 2.5 dB and 8.8 +/- 3.0 dB, and the ratio was 97 +/- 32%. Fractional shortening at T-pre and at T-60 was 27 +/- 7% and 20 +/- 9%, and the ratio was 79 +/- 44%. Recovery of magnitude was ahead of recovery of fractional shortening. The percent recovery of magnitude at T-60 did not correlate with aortic cross-clamping time (p = 0.91), postoperative peak creatine kinase-MB (p = 0.4), or catecholamine dosage (p = 0.13), but correlated with preoperative left ventricular mass index (p < 0.01). In patients with aortic stenosis, the percent recovery of magnitude at T-60 (66 +/- 4%) was significantly lower than in those with other types of valvular disease.

CONCLUSIONS

The recovery of magnitude of cyclic variation of ultrasonic integrated backscatter provides early assessment of myocardial injury, particularly in severely hypertrophied hearts, during reperfusion after aortic declamping in open heart surgery.

Resting Ultrasonic Tissue Characterization and Dobutamine Stress Echocardiography for Prediction of Functional Recovery in Chronic Left Ventricular Ischemic Dysfunction

This study was designed to assess the diagnostic accuracy of the percentage of resting systolic wall thickening (WT), dobutamine stress echocardiography (DSE), resting cyclic variation of integrated backscatter (IBS-CV), and low-dose dobutamine stress IBS-CV (DSE-IB) for the prediction of regional function recovery (RFR) in patients with chronic left ventricular (LV) ischemic dysfunction. The study also evaluated whether or not global LV function affected the diagnostic accuracy. All studies were conducted before percutaneous transluminal coronary angioplasty (PTCA) and RFR was assessed after PTCA (mean interval, 10 months) in 30 patients with chronic LV ischemic dysfunction. Patients were divided into 2 groups according to the LV ejection fraction (LVEF): group A, LVEF<40%, n=14; group B, LVEF> or =40%, n=16. Of a total of 480 segments, 37 initially demonstrating akinetic wall motion before PTCA were analyzed. The wall motion of 24 of the 37 segments improved on visual analysis after PTCA. In the prediction of RFR, resting WT, DSE, resting IBS-CV and DSE-IB had sensitivities of 79%, 79%, 92% and 62%, and specificities of 54%, 84%, 83% and 69%, respectively. In particular, the resting IBS-CV in group A, as well as DSE, was an excellent predictor of RFR (sensitivity, 100%; specificity, 86%; vs sensitivity, 82%; specificity, 78%; respectively). Therefore, both resting IBS-CV and DSE are useful predictors for RFR in patients with chronic LV ischemic dysfunction.

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.

Ultrasonic tissue characterization predicts myocardial viability in early stage of reperfused acute myocardial infarction

BACKGROUND

The aim of the present study was to characterize temporal changes in cyclic variation of ultrasonic integrated backscatter (IBS), which reflects intrinsic contractile performance, in patients with reperfused acute myocardial infarction (AMI) and to elucidate the clinical value of tissue characterization in predicting myocardial viability.

METHODS AND RESULTS

We recorded short-axis IBS images before and 3, 7, and 21 days after reperfusion in 26 patients with AMI and obtained the cyclic variation of IBS in the normal and infarct zones. When cyclic variation showed synchrony and asynchrony, we expressed its magnitude as positive and negative values, respectively, called the phase-corrected magnitude. We also measured average wall motion score (dyskinesis, 4; normal, 0) of the infarct segments. The phase-corrected magnitude was lower in the infarct zone than in the normal zone before reperfusion (0.3+/-2.5 versus 5.2+/-1.7 dB, P<.05). At day 3, the phase-corrected magnitude increased by 2.1+/-2.6 dB despite no improvement in wall motion. Improvement in wall motion was observed only at day 21. The patients with the phase-corrected magnitude of > or =2.0 dB at day 3 showed significantly lower wall motion score at day 21 than did the other patients (1.7+/-0.6 versus 2.4+/-0.5, P<.01).

CONCLUSIONS

In patients with AMI, cyclic variation of IBS is blunted during ischemia but recovers much faster after reperfusion than the improvement in wall motion. The greater phase-corrected magnitude at day 3 may be a predictor of better functional improvement.

[Status of digitalis in therapy of acute and chronic heart failure]

Although supported by more than 200 years of experience and anecdotal clinical evidence, the efficacy of digitalis in the management of heart failure has been questioned until the past decade. The idea to improve contractility of the diseased myocardium with an inotropic agent is fundamental in the management of left ventricular dysfunction. The majority of clinical trials published since 1980, most of which examined patients with mild to moderate heart failure, indicate that digitalis alone or in combination with vasodilators may improve the clinical outcome particular in those patients with more advanced symptoms and poorer left ventricular function. Aside from its action as an inotropic drug the pharmacology and the mechanisms by which digitalis influence the diseased myocardium and peripheral circulation in heart failure has gained more complexity within the last years, raising the idea of other mechanisms that might be involved in its action. Particular for ACE inhibition multiple clinical trials have conclusively demonstrated its impact on survival and morbidity in congestive heart failure. Improvement of clinical outcome as measured in terms of fewer hospitalizations and improvement of symptoms in patients receiving digitalis seems to be comparable to patients receiving beta-blockers additional to diuretics and ACE inhibitors, an entirely different approach to the treatment of heart failure. Despite initial improvement of hemodynamics it now appears that there is no survival benefit found for digitalis in the management of heart failure.

Transesophageal Echocardiographic Evaluation of Patients Receiving Mechanical Assistance from Ventricular Assist Devices

Ventricular assist devices (VADs) have been successfully used to provide circulatory support for patients with cardiogenic shock after cardiac surgery. Transesophageal echocardiography plays a vital role in the evaluation of patients on VAD support: it is used to verify the correct positioning of the VAD hardware; to diagnose complications, including intracardiac thrombus and tamponade; and to assess right and left ventricular function when VAD flow is decreased to determine the correct timing for weaning from the device. Future developments may allow us to prospectively identify patients with stunned but viable myocardium who would benefit the most from VAD support.

Ultrasonic Tissue Characterization: Review of a Noninvasive Technique for Assessing Myocardial Viability

The determination of myocardial perfusion and myocardial viability has prognostic and therapeutic implications, particularly in the current era of percutaneous transluminal coronary angioplasty and thrombolytic therapy. Several modes of investigation, including positron emission tomography, thallium-201 scintigraphy, and nuclear magnetic resonance imaging are used to differentiate viable from nonviable myocardium. Though these noninvasive tests are useful diagnostic modalities, they are expensive, time consuming, and too cumbersome to be used in the acute setting. Expeditious distinction between viable and nonviable myocardium, during acute coronary syndromes, is of great importance since reperfusion can minimize the extent of ischemic injury and infarction. An expanding body of evidence confirms that ultrasonic tissue characterization has great potential to become a practical bedside diagnostic tool in the search for salvageable myocardium. Further clinical investigative studies would help accomplish a better understanding of the complex interaction between ultrasound and myocardium. (ECHOCARDIOGRAPHY, Volume 13, July 1996)

On-Line Myocardial Tissue Characterization with a New Commercially Produced Software

Myocardial tissue characterization has been performed using various ultrasonic techniques, one of which is the cyclic variation of integrated backscatter, a method that analyzes the acoustic properties of the myocardium using backscattered radiofrequency signals to provide information about myocardial structure and function. Previous studies using prototype equipment have demonstrated a reduction in the cardiac cycle variation of integrated backscatter in various pathologic states. Recently, a commercially produced software package that allows online analysis of cyclic variation of integrated backscatter has been made available for testing by various investigators. To evaluate this new commercially produced software, we compared integrated backscatter results in three groups of patients: a control group; an end-stage cardiomyopathy group; and a heart transplant recipient group. Integrated backscatter of the septum and posterior walls in the parasternal long axis and 12, 3, 6, and 9 o'clock regions in the short axis was performed using a commercially produced program (Hewlett-Packard Sonos 1500). In the control group, the mean cyclic variation of integrated backscatter was 5.04 +/- 1.60 dB in the septum and did not significantly vary from the rest of the regions studied. In comparison, cyclic variation of integrated backscatter in every region studied was reduced in the cardiomyopathy and heart transplant groups. Intraobserver variability, interobserver variability, and reproducibility over a 3-month interval was found to be 6.5%, 5.7%, and 7.5%, respectively. These results indicate that: (1) online analysis of cardiac cyclic variation of integrated backscatter is possible utilizing commercially produced software; (2) results obtained are consistent with a low intraobserver and interobserver variability and are reproducible over time; and (3) as observed in the comparison between the transplant and control groups, this information may detect changes in cardiac structure even in the absence of changes in function. (ECHOCARDIOGRAPHY, Volume 13, May 1996)

Effects of in vivo vanadate administration on calcium exchange and contractile force of rat ventricular myocardium

The aim of this study was to evaluate the effect of a 14-day intragastric administration of Na3VO4 (0.03 mmol/kg daily) on calcium metabolism and contractile force of rat myocardium. Left ventricular pressure as well as its first derivative (dP/dt) were registered with a balloon inserted to this ventricle and the cellular content of exchangeable calcium with the aid of 45Ca2+. Left ventricular pressure of the hearts exposed to vanadium was 11.8 +/- 0.2 kPa and was lower by 23% in comparison with control hearts (without vanadate), and its first derivative was 131.01 +/- 2.8 kPa/s (lower by 36.4%). The cellular content of exchangeable calcium in myocardium of rats treated with vanadium was 1.305 +/- 0.173 and 2.019 +/- 0.231 mmol/kg of wet weight (w.w.) in the stimulated and in the rested (for the last 10 min) ventricles, respectively. Accordingly, in the control group, the Ca content in stimulated hearts was lower by 0.384 mmol/kg w.w., and in the rested ones it was lower by 0.715. This indicates that the myocardial contractile force decreases, in spite of the fact that the content of calcium is considerably higher than that in the control group. These results also show a toxic effect of vanadate on the myocardium, probably due to large intracellular accumulation of calcium and cell damage.

Alterations in ultrasonic backscatter during exercise-induced myocardial ischemia in humans

BACKGROUND

Experimentally induced myocardial ischemia in animals causes tissue modifications that alter characteristics of the ultrasonic beam backscattered from the myocardial muscle. Alterations of backscatter parameters have been evidenced in human subjects with acute or remote myocardial infarction and during ischemia induced by angioplasty balloon occlusion or pharmacological stimuli. The effects of transient effort ischemia in humans have not been reported. The purpose of this study is to assess ultrasonic backscatter parameter changes induced by transient effort myocardial ischemia in human subjects.

METHODS AND RESULTS

Nineteen patients with single left anterior descending coronary stenosis and 15 healthy subjects underwent ultrasonic backscatter analysis (parasternal long-axis view) at rest, immediately after a supine stress test, and 30 minutes later. Two windows were selected in each ultrasonic study: one encompassing the septum; the other, the posterior wall. Integrated backscatter was computed throughout the cardiac cycle, yielding a power curve relative to the midmyocardial region of the myocardial wall (excluding pericardial and endocardial borders). Five parameters were computed from the backscatter power curve: the maximum-minimum difference, amplitude and phase of the first harmonic Fourier fitting, phase-weighted amplitude, and time-averaged integrated backscatter difference from rest (an index of overall myocardial reflectivity). This protocol allowed comparison of the backscatter data from a region at risk of ischemia (the septum) with that from a region normally perfused (posterior wall) and a comparison with the same regions of the control group during the three ultrasonic studies. All backscatter indexes in the septum were altered significantly by exercise compared with rest values, whereas no changes were found in the normally perfused posterior wall or in the septum of the control group. All modified parameters returned to baseline values at the time of the recovery study.

CONCLUSIONS

These data indicate that transient, exercise-induced ischemia is associated with reduction of the cardiac cycle-dependent variation of the integrated backscatter power curve, a temporal shift in the nadir of the power curve with respect to the R wave (phase increase), and a small but detectable increase of myocardial reflectivity. These changes may be detected noninvasively in humans with ultrasonic backscatter analysis.

Relation of ultrasonic backscatter and acoustic propagation properties to myofibrillar length and myocardial thickness

BACKGROUND

Ultrasonic backscatter demonstrates a cardiac cycle-dependent modulation. The exact mechanism of the modulation is under debate. The objective of the present study was to test the hypothesis that a change in size and configuration of myofilaments from systole to diastole alters acoustic propagation properties and backscatter.

METHODS AND RESULTS

In vivo measurements were made of integrated backscatter at 5 MHz (IBR5), followed by in vitro measurements of ultrasonic attenuation, speed, and heterogeneity index using a scanning laser acoustic microscope at 100 MHz. Studies were performed in canine hearts (16) arrested in systole (8) with calcium chloride or arrested in diastole (8) with potassium chloride. Sarcomere length was measured with a calibrated eyepiece on a Ziess microscope. Wall thickness was measured with calipers. The attenuation coefficient of 220 +/- 34 dB/cm during systole was significantly higher than the coefficient of 189 +/- 24 dB/cm during diastole (P < .01); the IBR5 of -44.7 +/- 1.2 dB during systole was significantly greater than the IBR5 of -47.0 +/- 1.0 dB during diastole (P < .01); the ultrasonic speed of 1591 +/- 11 m/s during systole was higher than the speed of 1575 +/- 4.2 m/s during diastole (P < .01); and the heterogeneity index of 7.4 +/- 1.8 m/s during systole was significantly lower than the index of 9.0 +/- 2.0 m/s during diastole (P < .02). The sarcomere length of 1.804 +/- 0.142 microns during diastole was significantly higher than the length of 1.075 +/- 0.177 micron during systole (P < .01). Wall thickness was significantly greater during systole than during diastole (20 +/- 3 versus 9 +/- 3 mm, P < .01).

CONCLUSIONS

Ultrasonic backscatter and propagation properties are directly related to sarcomere length and myocardial thickness and may be responsible for cardiac cycle-dependent variation in backscatter.

Acoustic propagation properties of normal, stunned, and infarcted myocardium. Morphological and biochemical determinants

BACKGROUND

Identification of viable but stunned myocardium remains a major problem. Since stunned myocardium results in impairment of myocardial function without any structural damage and infarcted myocardium causes major structural disruption, we postulated that acoustic properties could distinguish between the two insults.

METHODS AND RESULTS

Anesthetized open-chest dogs underwent a total occlusion of the left anterior descending coronary artery for 15 minutes (stunned, n = 7) and 90 minutes (infarcted, n = 8), followed by reperfusion for 3 hours. Circumflex coronary artery perfusion territory (n = 15) served as normal control tissue. Regions of myocardium were quantitatively evaluated with a scanning laser acoustic microscope operating at 100 MHz and a research ultrasound system operating at 4 to 7 MHz. Four ultrasonic parameters were determined: attenuation coefficient (an index of loss per unit distance), speed of propagation, a spatial variation of propagation speed called the heterogeneity index (HI), and ultrasonic backscatter at 5 MHz (IBR5). Myocardial water, lipid, and protein contents of normal, stunned, and infarcted myocardium were also determined. The attenuation coefficient of normal myocardium (179 +/- 20 dB/cm) was significantly greater than that of stunned (136 +/- 7 dB/cm, P < .001) and infarcted (130 +/- 8 dB/cm, P < .001) myocardium. The propagation speed of normal myocardium (1597 +/- 6 m/s) was similar to that of stunned (1600 +/- 6 m/s) and significantly higher than that of infarcted (1575 +/- 7 m/s, P < .001) myocardium. The HI for specimen thicknesses of 75 to 100 microns showed an increase of 33% between normal (5.0 +/- 0.8 m/s) and stunned (7.5 +/- 2.3 m/s, P < .05) myocardium. However, for the infarcted myocardium (5.8 +/- 2.0 m/s), the HI was essentially the same as that of the normal myocardium (5.0 +/- 0.8 m/s). The IBR5 of normal (-47.1 +/- 1.0 dB) was not significantly different from that of stunned myocardium (-46.8 +/- 0.9 dB). The IBR5 of infarcted myocardium (-42.4 +/- 1.0 dB) was significantly greater than that of normal myocardium. Myocardial water and protein contents were similar in the normal and stunned myocardium. Water content in the infarcted myocardium (80.8 +/- 2%) was significantly greater (P < .05) than in the normal (72.7 +/- 1.3%), and protein content of 18.5 +/- 0.7% was significantly lower (P < .05) than the normal (21.4 +/- 0.8%). Lipid content was increased in the stunned (8.5 +/- 0.5%) and virtually absent in the infarcted myocardium (0.8 +/- 0.3%) compared with normal (5.5 +/- 0.6%).

CONCLUSIONS

We conclude that acoustic propagation properties can identify stunned and infarcted myocardium and may be related to biochemical/morphological differences.

Effects of in vivo manganese administration on calcium exchange and contractile force of rat ventricular myocardium

The purpose of this study was to investigate the effect of prolonged (14 days) intragastric administration of Mn2+ (0.25 mmol/kg daily) on Ca2+ exchange and contractility of rat ventricular myocardium. Left-ventricular pressure and its first derivative (dP/dt) were recorded by means of a balloon catheter inserted via the left atrium into the left ventricle of the rat heart perfused by Langendorff method. Ca2+ exchange in the stimulated and rested ventricular myocardium was investigated with the aid of 45Ca under the conditions of complete equilibration of preparations with a solution containing 45Ca2+. The "cellular" 45Ca2+ content was calculated by subtraction of 45Ca2+ dissolved in the free water of extracellular space from the total tissue 45Ca2+ content. The cellular 45Ca2+ content in the stimulated (60/min) ventricles of control rats (without Mn2+) was 0.83 +/- 0.09 mmol/kg wet weight (w.w.). Ten minutes of rest resulted in a gain of 0.06 mmol 45Ca/kg w.w. (not statistically significant). Fourteen days' exposure to Mn2+ resulted in an increase of the mean 45Ca content to 1.61 +/- 0.09 mmol/kg w.w. in the stimulated preparations and to 1.35 +/- 0.06 mmol/kg w.w. in the rested ones (p less than 0.001). Thus, the control rest preparations did not change their Ca2+ content, while in the rats treated with Mn2+ the rest resulted in an increase at exchangeable Ca by 52%. The maximal ventricular developed pressure (Pmax) after 14 days of Mn2+ administration was increased by 35% and dP/dtmax was 228% of the value in the control group.(ABSTRACT TRUNCATED AT 250 WORDS)

Aging: stimulation rate on cardiac intracellular Na+ activity and developed tension

Previous reports suggested that Na,K-ATPase activity and Na(+)-pump capacity decrease with senescence in left atrial myocardium of F344 rats. Current experiments were designed to determine if this reduction in the Na(+)-pump affects free intracellular Na+ levels. Mean intracellular Na+ ion activity (aiNa) was measured with Na-selective microelectrodes in left atrial muscle isolated from hearts of 4-, 14- and 25-month-old F344 rats. Preparations were stimulated randomly at frequencies between 0 and 12 h. There were no age-associated differences in aiNa measured at any frequency or in the decay of Na+ activity following discontinuation of electrical stimulation. These data indicate that the aging-related decline in Na,K-ATPase does not result in elevated aiNa even at extremely high stimulation frequencies, thus suggesting that other routes of Na+ influx and efflux are also altered in atrial muscle.

Digitalis sensitivity of Na+,K(+)-ATPase, myocytes and the heart

Cardiac Na+,K(+)-ATPase, the receptor molecule for digitalis glycosides, have isoforms with different intrinsic affinities for the glycosides. Expression of these isoforms are under developmental and hormonal regulation. Switching in isoforms to those with lower intrinsic affinity may decrease digitalis sensitivity of the heart. In addition to the intrinsic affinity of the cardiac Na+,K(+)-ATPase for the glycoside, increases in the rate of Na+ influx and decreases in extracellular K+ concentrations increase glycoside sensitivity of the heart and also reduces the margin of safety by reducing reserve capacity of the sodium pump. Reserve capacity of the sodium pump is also reduced by pathological conditions or aging, resulting in reduced margin of safety for the glycoside. Events that follow sodium pump inhibition also affect sensitivity of the heart to digitalis toxicity. These are hypercalcemia and magnesium depletion. It is now feasible to predict digitalis sensitivity of the heart, not empirically but based on the understanding of the mechanisms responsible for the positive inotropic and toxic actions of the glycoside.

Excitation-contraction coupling in the heart

There has been dramatic progress in our understanding of normal cardiac excitation-contraction coupling and in control of contraction strength, as the result of the new patch pipette method of voltage clamping of single cells and the new methods for monitoring Cai. Several abnormalities have been shown to exist in hypertrophied muscle; the action potential is changed and the contraction is slower. A kinetic change appears to exist in the L-type Ca current, associated with a slower decay of Cai. The next few years should bring a much improved understanding of the molecular and cellular basis for the changes of hypertrophy.

Relation between Na+-K+ pump, Na+ activity and force in strophanthidin inotropy in sheep cardiac Purkinje fibres

1. The effects of different concentrations of strophanthidin on intracellular sodium activity (aiNa), membrane potential and contractile force have been studied in cardiac sheep Purkinje fibres under conditions (overdrive) that stimulate Na+-K+ pump activity. 2. In fibres driven at 1 Hz, a 5 min overdrive at 2 Hz in the steady state increased force by +74.2%, aiNa by +10.9% and the maximum diastolic potential (Emax) by 3.32 +/- 0.52 mV. 3. During the recovery from overdrive (the fibres being driven again at 1 Hz), both contractile force and aiNa transiently undershot the control value by -10.5 and -3.7%, respectively. When the fibres were quiescent during the recovery from overdrive, no aiNa undershoot was present. 4. During overdrive, force and aiNa were closely correlated when plotted either on linear (correlation coefficient, R = 0.98) or logarithmic (R = 0.98) co-ordinates. 5. A low concentration of strophanthidin (0.01 microM) decreased force (-31.7%) and aiNa (-7.2%): overdrive increased force and Emax more and aiNa less than in the absence of strophanthidin. During the recovery, the undershoot in force (-12.9%) and aiNa (-5.4%) was larger and longer than in the absence of strophanthidin. 6. An intermediate concentration of strophanthidin (0.05 microM) increased force (+43.5%) and aiNa (+6.4%): overdrive increased force and aiNa as usual, but during the recovery the force remained above the value prior to overdrive and there was no aiNa undershoot. 7. A high concentration of strophanthidin (0.1 microM) increased force (+91.4%) and aiNa (+11.7%): overdrive further increased force and aiNa more than in control but there was no increase in Emax. During the recovery, both force and aiNa remained well above the values prior to overdrive. 8. Force and aiNa were closely correlated whether aiNa decreased in 0.01 microM-strophanthidin (R = 0.99 both on linear and logarithmic co-ordinates) or increased in 0.05-microM- (R = 1.00 on both co-ordinates) and in 0.1 microM- (R = 0.98 and 0.99, respectively) strophanthidin. The two parameters were well correlated also during overdrive in the three strophanthidin solutions. However, the slope of the relation was less steep in the low- than in the higher-strophanthidin solutions. 9. For a 1 mM change in aiNa, force decreased less in the low- than it increased in the intermediate-strophanthidin solution. Also, in low-strophanthidin solution, at the end of overdrive the aNao/aNai ratio was similar to that in Tyrode solution but force was well above control (+73.2%).(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of ouabain and low-Na+ perfusion on rest-decay and post-rest recovery of cellular Ca content in ventricular muscle of guinea-pig heart

Ca2+ shifts in isolated, perfused ventricular muscle of guinea-pig hearts were investigated with the aid of 45Ca under the conditions of complete equilibration of preparations with isotope-containing containing solutions. The content of 45Ca in stimulated preparations (rate 60/min) was 1.30 +/- 0.12 mmol/kg of wet weight (w.w.). 6 min rest resulted in the drop of this content to 0.37 +/- 0.05 mmol/kg w.w. despite continued perfusion with 45Ca containing solution. The difference of 0.93 mmol/kg w.w. is equivalent to fraction Ca2 (15) and is labelled accordingly. Ouabain (1 microM) increased the 45Ca content to 1.53 +/- 0.15 mmol/kg w.w. in the stimulated and to 1.12 +/- 0.23 mmol/kg w.w. in the rested muscle. The respective values after low (50 mM) sodium perfusion were 1.70 +/- 0.11 and 1.07 +/- 0.13 mmol/kg w.w. The differences between the stimulated and rested preparations (Ca2 fraction) were 0.41 and 0.63 mmol/kg, respectively. In the control experiments the force of the first post-rest beat dropped to 20 +/- 5% of the force of steady-state beats. During ouabain and low-sodium perfusion, the force of the first contraction increased markedly and its peak was larger than that of the few subsequent beats. It is concluded that Na-Ca exchange is the important factor in the rate-dependent control of Ca2 fraction content and of contractile force.

Inotropic response to hypothermia and the temperature-dependence of ryanodine action in isolated rabbit and rat ventricular muscle: implications for excitation-contraction coupling

We have used the sarcoplasmic reticulum (SR) inhibitor ryanodine to assess the contribution of the SR to the increase in twitch tension seen on cooling the mammalian myocardium. To select a suitable concentration of ryanodine, i.e., one that will exert a maximal effect at all temperatures studied, concentration-response curves for ryanodine action were constructed at 37 degrees, 29 degrees, and 23 degrees C in ventricular muscle from rabbit and rat. Using a concentration of ryanodine (1 microM) that exerted a maximal effect at all temperatures studied, the ability of ryanodine to inhibit SR function at 37 degrees, 29 degrees, and 23 degrees C was then confirmed by using rapid cooling contractures (RCCs) to provide an indirect assessment of the SR calcium content. To estimate the rest decay of the SR calcium content in the absence and presence of ryanodine (1 microM), RCCs were initiated after a range of rest intervals (0.3-300 seconds) in rabbit muscles maintained at 37 degrees, 29 degrees, or 23 degrees C. In the absence of ryanodine, low temperatures elevated RCCs at all rest intervals studied. In the presence of ryanodine, RCCs were only seen at rest intervals shorter than 2.0 seconds, even at 23 degrees C, the lowest temperature studied. Thus, even at 23 degrees C, ryanodine appears to be effective at inhibiting SR calcium release in muscles stimulated at 0.5 Hz (i.e., after 2 seconds rest). Therefore, using this concentration of ryanodine (1 microM) and a stimulation rate of 0.5 Hz, we have investigated the contribution of the SR to the positive inotropic response to hypothermia. Under these conditions, the positive inotropic response to cooling in rabbit ventricle was almost unaffected by the inhibition of the SR with ryanodine. In rat ventricle, a tissue in which SR calcium release may dominate excitation-contraction (EC) coupling, the inotropic response to hypothermia was still observed, although developed tension was strongly depressed at all temperatures. These results suggest that a change in SR function is not the principal mediator of the large (400-500%) increase in force associated with cooling mammalian ventricular muscle from 37 degrees to 25 degrees C. The ryanodine-sensitive fraction of tension development was greatest at 37 degrees C, suggesting that the relative contribution of the SR to tension development in rabbit ventricle is reduced at temperatures below 37 degrees C.(ABSTRACT TRUNCATED AT 400 WORDS)

Strophanthidin and force regulation by intracellular sodium activity in cardiac Purkinje fibers

The role of intracellular sodium activity (aiNa) in the inotropy of a low concentration of strophanthidin (5 X 10(-8 M) was studied in sheep cardiac Purkinje fibers by recording contractile force, aiNa and transmembrane potentials under conditions that vary aiNa. High [Na]O, strophanthidin and tetrodotoxin (TTX) changed force and aiNa in a closely related manner: on logarithmic coordinates, the data were well fitted by a single line obtained through the regression equation F = b (aiNa)s where b represents the intercept and s the slope of the relation. With low strophanthidin, force increases as a linear function of (aiNa) approximately 5 and with high [Na]O as a linear function of (aiNa) approximately 6. However, the combined administration of high [Na]O and strophanthidin results in a potentiated inotropic effect as force becomes a linear function of (aiNa) approximately 14. This potentiation and its abolition by TTX suggests that factors other than aiNa powerfully modify the inotropy of a low strophanthidin concentration.

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.

Dissociation between force and intracellular sodium activity with strophanthidin in isolated sheep Purkinje fibres

1. We have recorded membrane potential, intracellular Na activity (aiNa) and force of contraction in sheep Purkinje fibres. Force and aiNa were recorded continuously and simultaneously during exposure to strophanthidin and its subsequent washing off. 2. Exposure to strophanthidin in concentrations of 1.5 X 10(-7), 5 X 10(-7) and 10(-5) M caused an increase in force of contraction which was temporally dissociated from the increase of aiNa. 3. There was hysteresis in the relationship between force and aiNa when the period of increasing force was compared to the period of decreasing force. When force increased on exposure to strophanthidin, the same aiNa was always associated with a higher force than when force was decreasing while washing off the drug. 4. For the same rise of aiNa higher doses of strophanthidin produced larger rises of force than lower doses. 5. When diphenylhydantoin was present in the bathing solution at concentrations of 10(-5) and 10(-4) M, the relation between force and aiNa with 10(-5) M-strophanthidin had a less steep slope, but still displayed hysteresis. 6. The relationship between force and aiNa during changes of the bathing K concentration also displayed a hysteresis, which was in the same direction as that found with strophanthidin. 7. These results are discussed in relation to proposed mechanisms of action of strophanthidin and more generally in relation to the factors linking force of contraction and aiNa. We conclude that in sheep Purkinje fibres the increase of force caused by strophanthidin is not solely due to an increase of aiNa, and that other interventions can also result in hysteresis between force and aiNa.

Effect of norepinephrine and cyclic AMP on intracellular sodium ion activity and contractile force in canine cardiac Purkinje fibers

The effect of norepinephrine on the Na+-K+ pump was investigated by simultaneously measuring intracellular sodium ion activity (aiNa) and contractile force of canine cardiac Purkinje fibers driven at 1.0 Hz in K+-free solution, high K+ solution, and in the presence of tetrodotoxin. In Tyrode solution containing 5.4mM [K+]o, 10(-6) M norepinephrine decreased aiNa, whereas in K+-free solution 10(-6) M norepinephrine did not lower aiNa. 16.2 mM [K+]o decreased aiNa from 8.8 +/- 0.9 mM to 6.5 +/- 0.5 mM (mean +/- SD, n = 5). Exposure to 10(-6) M norepinephrine in the presence of high [K+]o further decreased aiNa by 0.7 +/- 0.4 mM. This further decrease was prevented by exposure to 2.5 X 10(-6) M strophanthidin (n = 4). Blockade of the fast sodium channel with 5 X 10(-6) M tetrodotoxin lowered aiNa from 8.5 +/- 1.3 mM to 7.4 +/- 1.1 mM (n = 4). Exposure to 10(-6) M norepinephrine in the presence of tetrodotoxin further lowered aiNa by 0.9 +/- 0.2 mM. We also studied the effects of the analogues of adenosine 3':5'-cyclic monophosphate, N6, 2'-0-dibutyryladenosine 3':5'-cyclic monophosphate, and 8-(4-chlorophenylthiol)-adenosine 3':5'-cyclic monophosphate on aiNa and twitch tension. Both analogues lowered to aiNa and increased twitch tension mimicking the effects of norepinephrine. Our results support the hypothesis that norepinephrine lowers aiNa by stimulating the Na+-K+ pump in this tissue. This stimulation appears to be mediated by adenosine 3':5'-cyclic monophosphate and does not appear to be due to intercellular K+ accumulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular sodium and the positive inotropic effect of veratridine and cardiac glycoside in sheep Purkinje fibers

Veratridine is a sodium channel toxin that exerts a powerful positive inotropic effect and prolongs the action potential duration in the heart. To determine the basis of the inotropic action of veratridine and to examine the effects of dissimilar methods of raising intracellular sodium activity on contractility, we measured twitch tension and intracellular sodium activity using sodium-sensitive microelectrodes in stimulated sheep Purkinje fibers exposed to veratridine and in voltage-clamped fibers exposed to veratridine and cardiac glycoside. In stimulated fibers, veratridine (0.1-1 microM) produced coincident increases in intracellular sodium activity, action potential duration, and tension. In voltage-clamped fibers, veratridine (1-2 microM) and acetylstrophanthidin (0.1 microM) raised intracellular sodium activity and tension to a comparable degree. Tetrodotoxin (10 microM) abolished the mechanical, electrophysiological, and ionic changes produced by veratridine. The relationship between intracellular sodium activity and tension in voltage-clamped fibers (n = 6) was indistinguishable for veratridine and acetylstrophanthidin and could be fitted either with a linear function with slopes of 122.8% and 124.2%, respectively, or with a power function with slopes of 4.60 and 4.54, respectively, where the slope represents the exponential power of intracellular sodium activity to which tension is proportional. These results indicate that the positive inotropic action of veratridine is entirely accounted for by accumulation of intracellular sodium, which increases intracellular calcium available for contraction by sodium-calcium exchange. This study is the first direct demonstration that veratridine or any other sodium channel toxin affects intracellular sodium activity and suggests that the inotropic potency of veratridine and cardiac glycoside rely on the same mechanism, namely, elevation of intracellular sodium.

Lidocaine's negative inotropic and antiarrhythmic actions. Dependence on shortening of action potential duration and reduction of intracellular sodium activity

The mechanisms by which lidocaine brings about negative inotropic effects and antiarrhythmic actions in the heart have been examined. Using sheep cardiac Purkinje fibers, we studied the effects of "therapeutic" concentrations of lidocaine (20 microM lidocaine = 5.4 micrograms/ml) on electrical activity, intracellular sodium activity, and tension. For the preparation with a normal, physiological level of intracellular sodium activity (5-9 mM), the application of lidocaine leads to the following actions: action potential duration is decreased, intracellular sodium activity falls, and twitch tension is reduced. If intracellular sodium activity is first elevated (e.g., by sodium pump inhibition) so that arrhythmogenic transient depolarizations (and the underlying transient inward current are seen) then lidocaine has the following actions: The action potential duration is reduced. There is a reduction of the magnitude of the arrhythmogenic transient depolarization (or the underlying membrane current transient inward current). The magnitude of the aftercontraction that accompanies the transient depolarization (or transient inward current) is reduced. If, after sodium pump inhibition, intracellular sodium activity is still rising-then, the application of lidocaine leads to a reduction of the rate of rise of intracellular sodium. From these results, parallel voltage-clamp experiments, and recent work by others, we conclude that lidocaine leads to the reduction of the arrhythmogenic transient depolarization and a reduction of twitch tension by decreasing the inward sodium current. These actions are mediated by a reduction in action potential duration and a reduction of intracellular sodium activity and of intracellular calcium activity (by the sodium-calcium exchange mechanism).

Cellular mechanism of action of cardiac glycosides

It has long been known that cardiac glycosides can inhibit the membrane sodium-potassium (Na+-K+) pump, raising intracellular Na+. However, at clinical concentrations of cardiac glycosides, a change in intracellular Na+ that correlates with a change in cardiac contraction has been very difficult to demonstrate. The recent use of Na+-sensitive microelectrodes in the experimental laboratory has made intracellular Na+ measurements possible. A doubling of contraction strength in vitro is associated with a change of only approximately 1 mM intracellular Na+. Another membrane transport system, the Na+-Ca2+ exchange system, exchanges extracellular Na+ for intracellular Ca2+. If this system is responsible for regulating intracellular Ca2+, then it would be very sensitive to the transmembrane Na+ concentration gradient. This influence of intracellular Na+ on Na+-Ca2+ exchange is though to be the cellular basis of the positive inotropic action of digitalis. However, a number of issues remain unresolved, such as the extent of Na+-K+ pump inhibition by the level of cardiac glycoside achieved clinically.