Strength Of Contraction Can Be Modulated In Cardiac Muscle

The force generated by cardiac muscle cells depends on the cells' contractility, sometimes called the inotropic state. An increased contractility means that for any given length of the muscle it contracts with a greater force. It is important to note that the amount of Ca2+ released from the SR with each action potential is not sufficient to fully cover all of the troponin binding sites and thus activate all of the contractile proteins. Therefore, any manipulation that leads to enhanced release of Ca2+ from the SR will result in more troponin binding sites being occupied by Ca2+ and hence more force being generated by the muscle. The Ca2+ fluxes can, in turn, be altered by a variety of physiological control systems. Three commonly encountered modulators of contractility will be considered next: (1) stimulation frequency, (2) catecholamines, and (3) cardiac glycosides.

Stimulation Frequency

When the frequency of contraction increases, so does the tension generated by each contraction. This phenomenon, known as the positive staircase effect, was first described by Bowditch, an early cardiovascular physiologist, and is illustrated in Fig. 9. When Bowditch stimulated the heart, he found that a different tension was produced for each rate of stimulation. Stimulation at low rates was associated with a low tension and when the frequency of the stimulation was increased, the tension increased as well. The tension associated with a new rate of stimulation is not achieved instantaneously; it instead takes a period of time and appears to develop in a stepwise fashion. Hence the name "staircase." The positive staircase effect is an example of a positive inotropic effect, because it is associated with increased

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