Direct contact is required between serum albumin and hamster spermatozoa for capacitation in vitro

A dialysis unit was used to test whether direct physical contact between serum albumin and hamster spermatozoa is required for capacitation and/or the acrosome reaction. Sperm and bovine serum albumin (BSA) were incubated cither together (direct incubation) or separated by a dialysis membrane (indirect incubation). Sperm viability was supported with “sperm motility factors” (hypotaurine and epinephrine) and polyvinylalcohol (PVA). Spermatozoa became capacitated and underwent acrosome reactions when directly incubated in medium containing BSA (TALP‐PVA), but did not undergo acrosome reactions when indirectly incubated with BSA (medium TLP‐PVA). When sperm were first incubated for 4 hr indirectly with BSA, followed by 4 hr direct incubation with BSA, capacitation did not occur during indirect incubation. These findings indicate that an “intimate association” is necessary between serum albumin and spermatozoa to support capacitation under in vitro incubation conditions. The data are consistent with the concept of direct transfer of compounds from sperm to albumin and/or vice versa during sperm capacitation.