Identification of insulin-like growth factor I in bovine seminal plasma and its receptor on spermatozoa: Influence on sperm motility

Insulin-like growth factor I (ICF-I) has been identified in human seminal plasma. This study was conducted to determine whether ICF-I is present in bovine seminal plasma, whether sperm cells express the IGF-I receptor (IGF-IR), and whether ICF-I affects sperm motility. Semen samples were collected from bulls by electroejaculation and maintained at 37 degrees C, and motility of sperm was assessed. After centrifugation to separate sperm cells from seminal plasma, the seminal plasma was submitted to a validated heterologous RIA for IGF-I. Significant concentrations of IGF-I (116.29 +/- 40.83 ng/ml expressed as mean +/- SD) were measured in bovine seminal plasma. Sperm cells were washed with buffer and subjected to either radioreceptor assay (RRA) or immunocytochemistry (IC). RRA revealed a single high affinity for the IGF-IR with a K-d of 0.83 nM as determined by the computer program LIGAND. IC, using three monoclonal antibodies, localized the ICF-IR to the acrosomal region of the sperm. Computer-assisted sperm-motion analysis was used to determine the effects of IGF-I and ICF-II on bovine sperm motility parameters. Both IGF-I and IGF-II increased sperm motility and straight-line velocity (p < 0.05) relative to the control. The presence of ICF-IR on sperm, the presence of IGF-I in semen, and the ability of IGF-I to stimulate sperm motility provide evidence that the ICF system may be involved in the fertilization process in the bovine species.