Myonemal contraction of spirostomum. II. Some mechanical properties of the contractile apparatus

In several respects, notably the high velocity of shortening, Ca²⁺ dependence, and ATP independence, contraction of Spirostomum resembles the spasmonemal mechanism of the peritrich ciliates. In this report further mechanical properties of the contractile apparatus are described that extend this comparison. The velocity-load characteristic is more appropriate to an elastomer than to a muscle where contraction force is load-dependent. Active tension is found to relate linearly to cell length for extensions up to and beyond resting length (l_r), an elastic limit is reached around 1.5 l_r. At resting length this tension, measured by the deformation of a glass microbalance, is similar to that predicted from consideration of the hydrodynamic forces normally resisting shortening. The tension-length relation for the unstimulated (passive) cell is also linear between l_r and the elastic limit, but is displaced from the active tension-length curve and is of reduced stiffness. Kinetic studies suggest that maximum tension and maximum velocity coincide. Calculations are presented that support a model of contraction in Spirostomum in which the myonemes behave as a mechanochemical engine powered directly by the chemical potential of Ca²⁺.