Acetylcholine sensitivity of the spine-test articular capsule of the sea urchin Eucidaris tribuloides.

1. The changes in the consistence of the spine-test articular capsule, or ligament, of the primary spines of Eucidaris tribuloides induced by acetylcholine (ACh) have been studied. Two complementary techniques were used: (a) "forced-vibration", which detects variations in the stiffness of the ligament along a single diametral plane; and (b) "forced-rotation" which records the spatial distribution of those changes. 2. ACh (1 microM to 1 mM) caused a rapid increase in the resistive force opposed by the ligament to passive stretching. Similar effects were elicited by several monoquaternary, N-substituted derivatives of trimethylammonium. 3. The opposite effect, i.e. softening, was induced by decamethonium, dimethylphenylpiperazine, and 2-ketoamyltrimethylammonium. 4. The involvement in these effects of ACh-binding groups with pharmacological properties similar to those of the "anionic sites" of nicotinic ACh receptors is suggested.