Osmotic swelling of maturing rat spermatozoa and Lysis of caput spermatozoa by acylcarnitines and acylcholines

The form that rat spermatozoa assume when swelling in hyposmotic media depends on the position of the cytoplasmic droplet, previous exposure to hypertonic media, and the stiffness of the flagellum. Bending at the end of the midpiece occurs when the swelling droplet is situated at this site; this occurs in midcaput cells, but sperm from more proximal sites do not bend in this fashion. Stiffening of caput sperm stored in vitro reduces the incidence of such midpiece bending but looping at the tip of the tail still occurs, and previous exposure of caput spermatozoa to hypertonic media also prevents hairpin bend formation. Mature sperm from the cauda are too stiff to form hairpin loops when placed in hypotonic media unless first treated with a penetrating disulphydryl-reducing agent, after which swollen spherical vesicles can result from very flexible flagella confined within an intact membrane. Long-chain acylcarnitines are more potent lytic agents than acylcholines, but, for both, chain lengths of 16 carbon atoms is optimal for preventing the swelling of rat caput sperm.