Non-crossbridge forces in activated striated muscles: a titin dependent mechanism of regulation?

When skeletal muscles are stretched during activation in the absence of myosin-actin interactions, the force increases significantly. The force remains elevated throughout the activation period. The mechanism behind this non-crossbridge force, referred to as static tension, is unknown and generates debate in the literature. It has been suggested that the static tension is caused by Ca2+-induced changes in the properties of titin molecules that happens during activation and stretch, but a comprehensive evaluation of such possibility is still lacking. This paper reviews the general characteristics of the static tension, and evaluates the proposed mechanism by which titin may change the force upon stretch. Evidence is presented suggesting that an increase in intracellular Ca2+ concentration leads to Ca2+ binding to the PEVK region of titin. Such binding increases titin stiffness, which increases the overall sarcomere stiffness and causes the static tension. If this form of Ca2+-induced increase in titin stiffness is confirmed in future studies, it may have large implications for understating of the basic mechanisms of muscle contraction.