Rubber-like elasticity and volume changes in the isolated spasmoneme of giant Zoothamnium sp. under Ca2+-induced contraction

Using glycerinated spasmoneme of giant Zoothamnium sp., the physical proper ties of spasmoneme before and after Ca2+-induced contraction (pCa 4.5) were investigated. The volume change of spasmoneme contraction was measured und er zero tension. The length and diameter decreased by about 50% of their in itial Value as a result of contraction, which means that contraction is nea rly isotropic. Thus the Volume of spasmoneme decreased drastically by 86% o f its original value. The swollen ratio of extended and contracted spasmone me were 0.07 and 0.37, respectively. Tension-extension relationships of ext ended and contracted spasmonemes were measured. By applying the theory of r ubber elasticity, the number of segments of a chain in originally extended spasmoneme was only 3.3, i.e., the chain was almost a straight one. On the other hand, the number of segments of a chain in contracted spasmoneme was more than 100, i.e., the chain was essentially a random one. Furthermore, t he total number of chains in single spasmoneme was the same in extended and contracted spasmoneme. This means that the interchain cross-links of chain s were not influenced by addition or removal of Ca2+. Moreover, the molecul ar weight of a chain is estimated to be at most about 50 kd. By considering ail these results, it is concluded that the contractile mechanism of spasm oneme originates in the intramolecular folding and unfolding induced by Ca2 + binding and detaching.