Ca2+ and the contractile proteins.

Troponin regulation can be divided into two categories, primary and secondary. While the former underlies the processes common to troponin-regulated muscles, the latter varies between different types of muscle. One example of secondary regulation is the Ca2+ dependent interaction of troponin T and troponin C, which tends to suppress the myosin-actin-ATP interaction at relatively high Ca2+ concentrations, say, 10(-4)M. This interaction is marked in fast skeletal muscle, but weak in cardiac muscle. Since the Ca2+ concentration in fast skeletal muscle can physiologically reach a high level, this interaction may be considered as a kind of self-defense mechanism to avoid excess contraction. Cardiac muscle, which carries out its contractile cycle at lower Ca2+ concentrations, does not require such a mechanism, but under pathological conditions where the Ca2+ concentration could reach a high level, the lack of this mechanism might be detrimental to the contractile system.