Cardiac muscle contracts when cytosolic [Ca²⁺] rises above about 100 nmol/L. This rise in [Ca²⁺] couples the action potential (AP) to contraction, and the mechanisms involved are referred to as excitation–contraction coupling. The relationship between cardiac muscle force and stretch is discussed in Chapter 17. The ability of cardiac muscle to generate force for any given fibre length is described as its contractility. This depends on cytosolic [Ca²⁺], and to a lesser extent on factors that affect Ca²⁺ sensitivity of the contractile apparatus. The contractility of cardiac muscle is primarily dependent on the way that the cell handles Ca²⁺.

Initiation of Contraction

During the plateau phase of the AP, Ca²⁺ enters the cell through L-type voltage-gated Ca²⁺ channels (Figure 12). L-type channels are specifically blocked by dihydropyridines (e.g. nifedipine) and verapamil. However, the amount of Ca²⁺ that enters the cell is less than 20% of that required for the observed rise in cytosolic [Ca²⁺] ([Ca²⁺]_i). The rest is released from the sarcoplasmic reticulum (SR), where Ca²⁺ is stored in high concentrations associated with calsequestrin. APs travels down the T tubules which are close to, but do not touch, the terminal cisternae of the SR (Figure 12a). During the first 1–2 ms of the plateau Ca²⁺ enters and causes a rise in [Ca²⁺] in the gap between the T tubule sarcolemma and SR. This rise in [Ca²⁺] activates Ca²⁺-sensitive Ca²⁺ release channels in the SR, through which stored Ca²⁺ floods into the cytoplasm. This is called calcium-induced calcium release (CICR) (Figure 12a). The amount of Ca²⁺ released depends both on the content of the SR and size of the activating Ca²⁺ entry, and modulation of the latter is the major way by which cardiac function is regulated (see Regulation of contractility below). Ca²⁺ release and entry combine to cause a rapid increase in [Ca²⁺]_i, which initiates contraction. Peak [Ca²⁺]_i normally rises to ∼2 µmol/L, although maximum contraction occurs when [Ca²⁺]_i rises above 10 µmol/L.

Generation of Tension

The arrangement of actin and myosin

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