The myocyte is made up of myofibrils surrounded by sarcoplasmic reticulum (for calcium release and uptake, as described earlier) and mitochondria (for generation of ATP, the energy source for contraction). Each myofibril is made up of a number of sarcomeres longitudinally connected by dense attachments between sarcomeres at the Z band (the insertion site of the actin filament). A portion of a sarcomere is depicted in Figure 3-7 . Each sarcomere contains thick myosin filaments (a longitudinal helix of myosin molecules). Each myosin molecule has a head that when energized by hydrolysis of ATP (by an ATPase that is part of the myosin molecule) stands out at right angles to the myosin molecule like a “cocked oar” (see Fig. 3-7A ). Each myosin filament is surrounded by six actin filaments (light chains). There are a number of active sites on the actin filaments that can interact with the myosin heads. In the resting state these active sites on the actin filaments are covered by another protein, tropomyosin. The tropomyosin covering of the actin active sites is altered by yet another protein, troponin. When calcium combines with one of the protein subunits of troponin, the tropomyosin molecule is altered such that the actin active sites become available to the myosin head. Calcium therefore leads by this mechanism to uncovering of the actin active sites, which then interact with the “cocked oars” of the myosin heads. The energy stored in the myosin heads is utilized in a conformational change that pushes the actin filament longitudinally along the myosin filament. Each oar stroke leads to a movement of 5 to 10 nm. At the end of the stroke an uncoupling of the myosin head from the actin site occurs, and the myosin head is then reenergized and moved back into a cocked position by hydrolysis of another molecule of ATP (see ( Fig. 3-7B ). The myosin heads then act in a wavelike fashion sweeping the actin filaments along, moving the Z lines closer together. When calcium is rapidly removed from the cell at the termination of the action potential (by the huge number of ATP-dependent calcium pumps in the sarcotubular system described earlier), a conformational change again occurs between troponin and tropomyosin leading to recovering of the actin active sites with disengagement of the actin filament from the myosin filament and relaxation of the sarcomere. The removal of calcium from the cytosol is an energy-dependent process. Relaxation therefore requires energy. [4 ] , [28 ] – [30 ]
The basic elements of the excitation-contraction coupling sequence were described earlier in descriptions of calcium channels and calcium pumps, but a synthesis of this sequence is appropriate at this point. Depolarization of the sarcolemmal membrane (and the T-tubule extensions of this membrane into the middle of the myocyte) leads to influx, through the sarcolemmal calcium channels, of calcium ions into the cell. This influx occurs in close proximity to the foot protein. These foot proteins are a portion of the calcium channels of the subsarcolemmal cisternae in which a large quantity of intracellular calcium is concentrated. These channels open, and calcium floods from the subsarcolemmal cisternae into the cytosol surrounding the contraction elements. This calcium channel subsequently closes (in about 200 ms), and the huge number of ATP-dependent calcium pumps in the sarcotubular system rapidly remove the calcium from the cytosol. As described in the preceding section, this pulse of increased cytosolic calcium ions leads to an actin-myosin interaction that converts chemical energy into mechanical movement. Energy is expended as the myosin heads are reenergized after each oarlike movement, and energy is expended in the removal of calcium from the cytosol back into the sarcotubular system. [10 ] , [18 ] , [29 ]
The strength of the myocardial contraction appears to be mediated primarily by the degree of uncovering of the actin active sites as tropomyosin is pulled away from the active sites by the combination of troponin and calcium. The magnitude of this effect depends on the affinity of troponin for calcium and the availability of calcium ions (i.e., the magnitude of the systolic pulse of calcium). The magnitude of the initial calcium ion influx through the sarcolemmal calcium channels is altered by cyclic AMP levels, by stimulatory G-proteins from beta-adrenergic receptors, and by inhibitory G-proteins from adenosine and acetylcholine receptors. This change in the magnitude of the calcium trigger leads to a change of the magnitude of the cytosolic calcium pulse. The rate of uptake of calcium from the cytosol (and thereby the duration of the calcium pump) is altered by cyclic AMP, as depicted in Figure 3-6 . In addition to this mechanism, cyclic AMP can lead to phosphorylation of a portion of the troponin molecule in a manner that allows more rapid release of calcium from troponin, thereby increasing the rate of relaxation of the actin-myosin complex. [10 ] , [29 ]
In addition to regulation of the strength of contraction by the primary mechanism of cytosolic calcium levels, the rate at which the myosin heads are reenergized can alter the speed and strength of the contraction. The rate of this reaction can be altered by phosphorylation of the myosin molecule during beta-adrenergic stimulation leading to an increased rate of the myosin ATPase reaction. Changes in the rate of cross-bridge cycling also may be caused by alterations in the myosin heavy chains that are a consequence of the synthesis of various isoforms of myosin (regulated by gene expression). [3 ] , [18 ]
In the heart, as in skeletal muscle, a relationship exists between resting sarcomere length and the strength of contraction. In skeletal muscle this relationship is bell-shaped, with maximum contraction at a sarcomere length of 2.2 µm. It has been proposed that at a greater sarcomere length force declines because there is a decreased overlap of actin and myosin and thereby a decreased availability of cross-bridging sites. In the heart, a decrease in contractility related to decreased overlap of the filaments does not seem to occur, since the resting length of the cardiac sarcomere rarely exceeds 2.2 to 2.4 µm. As the heart attempts to dilate beyond this state, a stiff parallel elastic element prevents dilatation. Even if dilatation does occur, there appears to be primarily slippage of fibers or myofibers rather than stretching of sarcomeres. [2 ]
The increase in contractility that is associated with stretching of the myocardium appears to be related to an increased sensitivity of the contractile elements to cytosolic calcium. A calcium pulse that will cause approximately 50 percent maximal tension if the initial sarcomere length is 1.95 µm will lead to more than 75 percent maximal tension if the initial sarcomere length is 2.4 µm. [2 ] This poorly understood but dramatically increased sensitivity to calcium as length is increased is an important part of the ascending limb of the Starling curve observed in the intact ventricle.