Depressed contractility of cardiac myocytes plays a critical role in heart failure. In the mildest early stages of HF, slowing of myocyte contraction and relaxation rates and prolongation of the action potential duration have been consistently observed (1). In advanced HF, force production and shortening magnitude are normal or even greater than normal (2) at slow beating rates (<30 per minute), but decrease (or remain constant) when heart rate increases (negative force-frequency relationship). This is contrary to the relationship in nonfailing myocytes, where increasing the heart rate causes force production and shortening magnitude to increase (positive force-frequency relationship) (2). The responses of failing myocytes to high extracellular Ca²⁺ and adrenergic stimulation are also significantly reduced. These studies show that the fundamental cellular alterations of failing myocytes allow for contractile function to be maintained at rest, when the heart rate is slow, but lead to the inability of the myocytes to increase their contractile performance (contractility) when demand for additional cardiac output is increased. We and others have termed this phenomenon depressed contractility reserve.

With each heartbeat, depolarization of the myocyte surface membrane causes opening of Ca²⁺ channels that reside in the transverse tubules. The subsequent Ca²⁺ influx induces Ca²⁺ release from the sarcoplasmic reticulum (SR). The resultant increase in cytosolic [Ca²⁺] (the so-called Ca²⁺ transient) activates myofibrillar proteins to cause contraction. The force and speed of contraction are regulated by varying these Ca²⁺-dependent processes.

Myocyte contractility is depressed in CHF, with the fundamental defect being an inability to increase force much beyond that exhibited at resting levels. By studying animal models and explained failing human hearts, multiple causes of depressed myocyte contractility in heart failure have been revealed, including altered
cytoskeleton (3); changes in myofilament function human CHF (4); animal CHF (5); altered β-adrenergic signaling (6); and abnormal myocyte Ca²⁺ regulation and excitation-contraction coupling (ECC) (1). In this chapter, we will focus on alterations in myocyte electrophysiological and contractile function in CHF, with a concentration on those factors producing alterations in Ca²⁺ regulation.

One of the common observations made in failing cardiac myocytes from both human patients and animal models is that the action potential (AP) is prolonged (Fig. 2-1). There are multiple contributors to AP prolongation, with reduced repolarizing potassium (K⁺) currents playing a central role, as discussed later. Parallel to the prolonged APs, the Ca²⁺ transient is also prolonged in myocytes from explanted hearts of human HF patients (7) and animal CHF models. At very slow pacing frequencies, the relatively normal contractility seen in cardiac myocytes from human patients with end-stage cardiac disease corresponds with a maintained amplitude of the Ca²⁺ transient similar to that seen in nonfailing human myocytes. However, the amplitude of the Ca²⁺ transient is decreased [in human (7) and animal models (8)] at physiological pacing rates. In addition, diastolic Ca²⁺ concentration becomes elevated (9) and the decay of the Ca²⁺ transient is slowed (7,9). These changes contribute to diastolic dysfunction and eventually to poor systolic (pump) performance.

The regulation of the Ca²⁺ transient is abnormal in failing cardiac myocytes. In normal cardiomyocytes, increasing the beating rate causes an increase in the amplitude of the Ca²⁺ transient and the force of contraction (positive force-frequency relationship) (Fig. 2-2). In myocytes with mild to moderate hypertrophy, the peak systolic Ca²⁺ is normal under basal conditions (e.g., low stimulation frequency), but becomes depressed when conditions that increase cellular Ca²⁺ loading are imposed (faster pacing rates, high extracellular Ca²⁺ or catecholamine exposure). In severely failing myocytes (e.g., myocytes from end-stage human heart failure), peak systolic Ca²⁺ and force (or shortening) are both close to normal at very slow pacing rates. However, when the beating rate is increased there is
either no change or a decrease in peak systolic Ca²⁺ and force of contraction (negative force-frequency relationship). Furthermore, diastolic Ca²⁺ in failing myocytes increases more than in nonfailing myocytes. These results strongly support the idea that changes in myocyte Ca²⁺ handling are a final common pathway for progressive deterioration of cardiac pump function in CHF. More support for this notion comes from studies that show that the transition from compensated hypertrophy to the early stages of CHF occurs when myocytes first lose their ability to normally maintain Ca²⁺ balance, and correction of Ca²⁺ handling defects is able to delay or prevent this transition (10).

Ca²⁺ transients (dynamic cytosolic Ca²⁺ changes) are brought about by a combination of Ca²⁺ influx and SR Ca²⁺ release. When a cardiac myocyte is depolarized, an action potential is elicited and Ca²⁺ enters the myocyte down its transmembrane electrochemical gradient through the L-type calcium channel (LTCC). This Ca²⁺ influx, termed the Ca²⁺ current, I_Ca, then triggers a larger amount of Ca²⁺ to be released from the SR by activating the Ca²⁺ release channel (ryanodine receptor, RyR) through a mechanism termed calcium-induced calcium release (CICR) (11,12). The efficacy of I_Ca as a trigger to induce SR Ca²⁺ release is termed the ECC gain. These processes cause the free cytosolic Ca²⁺ concentration to increase rapidly and Ca²⁺ binds to the thin filament protein troponin C to induce contraction. The rate and magnitude of myocyte contraction (force and shortening) are largely determined by the amount of Ca²⁺ entering the cytoplasm with each systole. The duration of contraction and the rate of relaxation are primarily determined by processes that reduce cytosolic [Ca²⁺]. Cytosolic Ca²⁺ is lowered by three principal processes: sequestration back into the SR via SR Ca²⁺-ATPase (SERCA), transport out of the cell via the sarcolemmal Ca²⁺-ATPase, and transport out of the cell by the Na⁺/Ca²⁺ exchanger (NCX) (Fig. 2-3, ECC). In a steady state, the Ca²⁺ that enters with each action potential is removed from the cell primarily by the NCX, and the Ca²⁺ that is released from the SR is resequestered there via SERCA. The amplitude and duration of the Ca²⁺ transient are regulated in the normal heart to orchestrate changes in myocyte contractility. In CHF this regulation is abnormal and results in Ca²⁺ transients of reduced size and prolonged duration. Decreased SR Ca²⁺ release is now known to be the principal reason for the reduced size of the Ca²⁺ transient in HF. Significantly, it appears that multiple cellular defects can contribute to the reduced SR Ca²⁺ release of the failing myocyte.

The ECC process is abnormal in failing myocytes from human and most animal models and contributes to the reduced size of the Ca²⁺ transient. In a rat hypertrophy/HF model, it has been shown that I_Ca-1, is a less effective trigger of SR Ca²⁺ release in hypertrophied and failing myocytes. This defective signaling could be rescued in hypertrophied (but not failing) myocytes by exposure to β-adrenergic agonists, which increase I_Ca and SR Ca²⁺ load. We and others have also shown that Ca²⁺ release from the SR in failing cardiomyocytes is disorganized and this contributes to reduced Ca²⁺ transient size (13,14). Furthermore, in some animal CHF models where there is no change of SR Ca²⁺ load, the Ca²⁺ transients are smaller than in normal cardiac myocytes (8). Cannell et al. (15) and Gomez et al. (8) found that in spontaneously hypertensive rats (SHRs) the ability of I_Ca-L to trigger calcium release from the SR in both hypertrophied and failing hearts was reduced (a reduction in ECC gain). Several models were proposed to explain these ECC defects. These include a decreased density of T-tubules, an increased distance between the sarcolemmal LTCC and RyR on the face of the junctional SR membrane, and mismatch of the numbers or properties of the LTCC and RyR. He et al. (16) reported a decrease in T-tubule density in tachycardia-induced failing canine myocytes. However, this is not supported by a recent, preliminary report in failing human myocytes. In addition, reduced ECC gain does not adequately explain the fact that Ca²⁺ transients are similar in normal and failing human myocytes at slow pacing rates and only become significantly different when the pacing rate increases. In this regard, a frequency-dependent abnormality in ECC, decrease in the size of I_Ca-L, or its efficacy as a trigger of SR Ca release could be responsible for the reduction in SR Ca²⁺ release at fast heart rates in failing human myocytes. The respective roles of the SR Ca²⁺ load and the L-type calcium current in abnormal ECC in failing myocytes will be discussed in some detail below.

The LTCC plays multiple critical roles in the function of cardiac myocytes. The LTCC is responsible for the upstroke of the action potential (AP) in the sinoatrial node (SA) cells and contributes to the pacemaker activity of these cells. The current through the LTCC is also responsible for the conduction of the AP through the AV node and the plateau phase of the AP in the atrial, ventricular, and His-Purkinje systems (60). The major function of the LTCC in cardiac myocytes is to control the influx of Ca²⁺ ions that serve as the trigger for SR Ca²⁺ release (ECC). Ca²⁺ influx through the LTCC elevates the [Ca²⁺] in a diffusion-limited subsarcolemmal space between the T-tubules and junctional SR, which are in close proximity. Accumulation of subsarcolemmal Ca²⁺ promotes Ca²⁺ binding to and opening of the Ca²⁺ release channel (RyR) in the junctional SR. The Ca²⁺ influx through LTCC must be balanced by Ca²⁺ efflux through the NCX and sarcolemmal Ca²⁺-ATPase. Unbalanced fluxes either load or unload the SR until flux balance is restored in a new steady state.

Changes in the abundance, isoform type, regulation, or localization of the LTCC could cause abnormal Ca²⁺ handling in CHF. These topics have been well-studied, but the results are far from conclusive. In general, the consensus of these studies is that in mild to moderate hypertrophy, LTCC abundance and Ca²⁺ influx are either increased or unchanged (61,62,63). These changes are associated with a transient increase in myocyte contractility and the initiation of pathological cardiac hypertrophy. In severe left ventricular hyperthrophy (LVH) and CHF, available evidence suggests that LTCC abundance and Ca²⁺ influx are either unchanged (64,65,66) or reduced (67). These changes are associated with a progressive reduction in cardiac contractility and dilation of the ventricular wall. The data from animal models fall into this generalized scheme very well, but the data from terminal human heart failure patients are less uniform, possibly due to a larger variation in disease etiology and to the fact that some human studies have been performed using very poor-quality isolated myocyte preparations. Supporting our hypothesis are studies showing that dihydropyridine (DHP) binding is unchanged in idiopathic dilated hearts but is decreased in ischemic cardiomyopathy. A study performed by Takahashi et al. (67) examined DHP binding sites in CHF myocardium with four etiologies (ischemic, idiopathic, hypertensive, and congenital) and found that LTCC expression was down by ∼47% at both mRNA and DHP binding (protein) levels in all groups. These results are consistent with a study in which I_Ca was measured in nonfailing and failing human myocytes from our group, in which we suggest a significant reduction in LTCC density in end-stage human heart failure (68).

One study in human myocardium has explored the idea that LTCC isoform shifts occur in CHF. This study revealed the switching of splicing isoforms at segment 3 in domain IV in failing human hearts using competitive RT-PCR (69). The functional significance of these isoform shifts is largely unknown and deserve further study.

In our view, the most reliable approach to determine the density of functional LTCCs is with biophysical techniques. LTCC abundance and function have been studied at the whole-cell and single-channel levels (61,62,63). These studies suggest that in mild to moderate hypertrophy, I_Ca-L increases or remains constant and I_Ca-L kinetics do not change, whereas in severe cardiac hypertrophy and heart failure I_Ca-L density decreases but there are few, if any, changes of channel properties (61,62,63). In this regard, a few studies have found that I_Ca-L decay is slowed in HF and have suggested that this slower decay might result in larger Ca²⁺ influx and contribute to the prolongation of action potentials in failing cardiac myocytes (70). In a pacing-induced dog model of CHF, I_Ca-L density was not changed but the inactivation at positive potentials was slowed, which resulted in a higher plateau of APs (71). These ideas need further study.