Role of cAMP in modulating relaxation kinetics and the force-frequency relation in mitral regurgitation heart failure

A new perspective on cardiovascular drift during prolonged exercise

Prolonged exercise induces cardiovascular drift, which is characterized by decreasing mean arterial pressure (MAP), stroke volume and heart rate increase. Cardiovascular drift has been debated for a long time. Although the exact mechanisms underlying cardiovascular drift are still unknown, two theories have been proposed. The first is that increased skin blood flow displaces blood volume from central circulation to the periphery, which reduces stroke volume. According to this theory, the rise in heart rate is presumably responding to the drop in stroke volume and MAP. The alternative theory is that an increase in heart rate is due to an increase in sympathetic nervous activity causing reducing time at diastole, and therefore stroke volume. It may be difficult to determine a single robust factor accounting for cardiovascular drift, due to the broad range of circumstances. The primary focus of this review is to elucidate our understanding of cardiovascular drift during prolonged exercise through nitric oxide and force-frequency relationship. We highlight for the very first time that cardiovascular drift (in some conditions and within a specific time period) may be considered as a protective strategy against potential damage that could be induced by the intense and prolonged contraction of the myocardium.

Changes in cardiac contractility during graded exercise are greater in subjects with smaller body mass index, and greater in men than women: analyses using wave intensity and force-frequency relations

INTRODUCTION AND PURPOSE

Estimation of the contractility of the left ventricle during exercise is an important part of the rehabilitation protocol. It is known that cardiac contractility increases with an increase in heart rate. This phenomenon is called the force-frequency relation (FFR). Using wave intensity, we aimed to evaluate FFR noninvasively during graded exercise.

METHODS

We enrolled 83 healthy subjects. Using ultrasonic diagnostic equipment, we measured wave intensity (WD), which was defined in terms of blood velocity and arterial diameter, in the carotid artery and heart rate (HR) before and during bicycle ergometer exercise. FFRs were constructed by plotting the maximum value of WD (WD₁) against HR. We analyzed the variation among FFR responses of individual subjects.

RESULTS

WD₁ increased linearly with an increase in HR during exercise. The average slope of the FFR was 1.0 ± 0.5 m/s³ bpm. The slope of FFR decreased with an increase in body mass index (BMI). The slopes of FFRs were steeper in men than women, although there were no differences in BMI between men and women.

CONCLUSIONS

The FFR was obtained noninvasively by carotid arterial wave intensity (WD₁) and graded exercise. The slope of the FFR decreased with an increase in BMI, and was steeper in men than women.

Regional differences in the force-frequency relation of human left ventricular myocardium in mitral regurgitation: implications for ventricular shape

Sphericalization of the left ventricular (LV) chamber shape in patients with mitral regurgitation (MR) contributes to increased LV wall stress and energy consumption. On the basis of previous observations, we hypothesized the existence of regional differences in the force-frequency relation (FFR) within the LV that may contribute to its shape. Accordingly, in the present study, we assessed regional variation in the FFR in patients undergoing surgery for chronic, nonischemic MR with class II–III heart failure symptoms and related our findings to the in vivo LV shape. FFRs (steady-state isometric twitches, 0.2–3.4 Hz, 37°C) were evaluated in MR myocardium from the LV subepicardial free wall (MR-FW) and papillary muscle (MR-PM) and from the subepicardial free wall in coronary artery bypass graft patients with normal LV contraction patterns [nonfailing (NF)]. Ascending slope, optimal stimulation frequency, and maximal twitch tension of the FFR were depressed in MR-FW and MR-PM compared with NF ( P < 0.05). FFR depression was greater in MR-PM than in MR-FW. Between 107 and 134 beats/min, twitch tension became weaker in MR-PM, whereas it increased in MR-FW. Elevation of intracellular cAMP with forskolin eliminated FFR depression in MR-FW but not in MR-PM. MR-PM also had a 35% lower myosin heavy chain content and slowed twitch kinetics. In MR patients, the echocardiographic end-diastolic LV shape (end-diastolic eccentricity index = long axis/short axis) correlated with the ratio of ascending FFR slopes such that the end-diastolic eccentricity index increased 10% per 15% increase in slope ratio ( r = 0.88, P = 0.01). These regional differences in the frequency dependence of contractility between the free wall and papillary myocardium may contribute to changes in LV shape in MR as well as during exercise.

Differential effects of the angiotensin-converting enzyme inhibitor lisinopril versus the beta-adrenergic receptor blocker atenolol on hemodynamics and left ventricular contractile function in experimental mitral regurgitation

OBJECTIVES

The goal of this study was to determine the therapeutic efficacy of angiotensin-converting enzyme (ACE) inhibitors and beta-adrenergic receptor blockers in experimental chronic mitral regurgitation (MR), gaining knowledge using methods difficult to apply in humans.

BACKGROUND

Both ACE inhibitors and beta-blockers are cornerstones in the treatment of human congestive heart failure. However, the roles of these treatments for chronic MR is unclear.

METHODS

Mitral regurgitation was created in 11 closed-chest dogs. Three months after the creation, the ACE inhibitor lisinopril 20 mg was given orally daily. After three months of lisinopril therapy, the beta-blocker atenolol was added to lisinopril for another three months. Atenolol was begun at a dose of 12.5 mg daily and increased gradually to 100 mg daily. Hemodynamics and left ventricular (LV) function were assessed throughout the study.

RESULTS

Regurgitant fraction was consistently >50% over the course of this study. Pulmonary capillary wedge pressure and LV end-diastolic pressure were significantly increased after three months of MR and decreased during both lisinopril and the combined therapy in which it was not different from baseline. Left ventricular contractility measured by the end-systolic stiffness constant was depressed from 3.66 +/- 0.20 to 2.65 +/- 0.12 (p < 0.05) at three months of MR and rose insignificantly after lisinopril treatment (2.99 +/- 0.17). When atenolol was added, it rose significantly and returned to normal (3.50 +/- 0.22, p < 0.05).

CONCLUSIONS

Although lisinopril significantly reduced preload, its effect on LV contractility was insignificant in experimental MR. Conversely, atenolol, when added to lisinopril, achieved maximum hemodynamic benefit and also restored LV contractility.

Rate-dependent changes of twitch force duration in rat cardiac trabeculae: a property of the contractile system

1. We examined the mechanisms for rate-dependent changes in twitch force duration by simultaneously measuring force and [Ca2+]i in rat cardiac trabeculae. 2. Peak force decreased when the rate of stimulation was increased from 0.2 to 0.5 Hz, whilst it increased from 1 to 2 Hz. Over the same range of frequencies, peak [Ca2+]i transients increased monotonically, whilst both force and [Ca2+]i transient duration were abbreviated. 3. Changes in peak force or peak [Ca2+]i transients were not responsible for the changes in force or [Ca2+]i transient duration. 4. The changes in twitch force and [Ca2+]i transient duration were completed roughly within one beat following an abrupt change in the rate of stimulation. 5. Rate-dependent changes resembled those observed with isoproterenol (isoprenaline) application. However, kinase inhibitors (i.e. K252-a, H-89, KN-62 and KN-93) had no effect on the rate-dependent changes of twitch force and [Ca2+]i transient kinetics, suggesting that protein kinase A (PKA), protein kinase PKG) and Ca2+-calmodulin-dependent protein kinase II (CaM/kinase II) were not responsible for these kinetic changes. 6. Despite the changes in twitch force and [Ca2+]i transient kinetics, the rate-limiting step for the rate-dependent force relaxation still resides at the level of the contractile proteins. 7. Our results suggest that rate-dependent changes in force and [Ca2+]i transients do not depend on PKA or CaM/kinase II activity but might result from intrinsic features of the contractile and/or Ca2+-handling proteins.

Effect of dopamine and atrial pacing on stunned myocardium

OBJECTIVE

Post-ischaemic stunned myocardium shows an impaired function at restored coronary blood flow, but performance can be normalized by positive inotropic stimulation. The power of stunned myocardium, however, is not augmented with increasing heart rate by atrial pacing, which is in contrast to intact areas. This pathological response is mitigated by inhibiting the degradation of cyclic AMP. The present experiments studied the effect of stimulating cyclic AMP formation by dopamine on the response of stunned myocardium to atrial pacing.

METHODS

In anaesthetized (piritramide) open chest pigs, heart rate, left ventricular and aortic pressure, left descending (LAD) and circumflex (LCX) coronary artery and aortic blood flow, myocardial systolic shortening in the LAD and LCX area were monitored, and myocardial power was calculated. The LAD region was subjected to ischaemia and reperfused for 2 h. Subsequently, heart rate was raised by right atrial pacing before and during intravenous infusion of dopamine (10 microg/kg per min). The ischaemic/reperfused area was sliced post mortem and stained by triphenyl tetrazolium chloride to exclude myocardial infarction. Data from 11 experiments are presented.

RESULTS

After 2 h LAD reperfusion, LAD blood flow and power were 100% and 36% of pre-ischaemic control, respectively, indicating myocardial stunning. The power of the intact area was not changed significantly (111% of control). Increasing heart rate by +36 and +70 from 94 beats/min increased the power of the intact area to 161% and 183% of control; the power of the stunned myocardium decreased to 34% and 19% of pre-stunning control. Dopamine increased the power of the stunned region to 143% of the pre-stunning level and the power of the intact area to 206% of control. Increasing heart rate by +34 and +70 from 113 beats/min during dopamine, increased the power of the intact myocardium to 288% and 344% of control and the power of the stunned region to 177% and 174% of the pre-stunning level.

CONCLUSIONS

The data confirm the pathological response of stunned myocardium to atrial pacing and the recruitment of a functional reserve by catecholamines. The adverse effect of pacing on the function of stunned myocardium is abolished by positive inotropic stimulation. Possibly, the cyclic AMP system is involved in the normal response to pacing and this pathway is disturbed in stunned myocardium; other defects are not excluded or supported, however. Physiologically increased heart rate by an increased activity of the sympathetic nervous system, is probably not accompanied by a reduced power of stunned myocardium, due to the associated positive inotropic stimulation.