Spermatozoa secrete insulin and it has recently been shown that insulin plays a role in capacitation of mammalian spermatozoa, enhancing motility and the acrosome reaction.  In a proteomic study we found that human spermatozoa express the insulin receptor (IR) on their surface.  We thus investigated mouse tissue and found IR protein in spermatozoa by western blot and immunohistochemistry, and mRNA expression in mouse spermatocytes by in-situ hybridisation.  Insulin receptor substrate mRNA was also found in mouse spermatozoa.

When insulin was added to human sperm we found an increase in the phosphorylation of the IR, and an increase in the phosphorylation of AKT, with a corresponding increase in progressive sperm motility.  We have also found that mammalian spermatozoa produce insulin.  Insulin mRNA (by PCR) and protein (by immunohistochemistry) have been detected in human, mouse, rat and equine spermatozoa.  Importantly the insulin c-peptide was also present in these spermatozoa, indicating the protein was produced in this location.  There is evidence that the uterine tract also produces insulin, which may act as a paracrine factor on spermatozoa.

We hypothesise that insulin secreted by spermatozoa acts in an autocrine fashion to keep Akt phosphorylated, leading to the continued phosphorylation of BAD, inhibiting apoptosis.  This allows the sperm to survive longer in the female reproductive tract and avoid apoptosis while waiting to fertilise the oocyte.