Dose-response of female llamas to ovulation-inducing factor from seminal plasma

The present study was designed to determine if the dose of purified ovulation-inducing factor (OIF) from llama seminal plasma required to provoke an ovulatory response is physiologically relevant in terms of the proportion present in a normal ejaculate and to test the hypothesis that corpus luteum (CL) form and function are affected by OIF in a dose-dependent manner. Female llamas were assigned randomly to five groups (n = 10 per group) and given a single i.m. dose of 500, 250, 125, or 60 μg of purified OIF (representative of the amount present in 1/25th to 1/200th of a normal ejaculate) or 1 ml of PBS (control). Ovulation and CL development were monitored by transrectal ultrasonography. Blood samples were taken to measure plasma progesterone concentrations and to determine changes in plasma concentrations of luteinizing hormone (LH). The high dose of OIF (500 μg) was associated with the highest incidence of ovulation (P < 0.05), the greatest maximum CL diameter (P < 0.05), and the largest day-to-day profiles of CL diameter (P < 0.05) and plasma progesterone concentrations (P < 0.01). A rise in plasma LH concentration was apparent in all llamas that ovulated and was most rapid and highest in the high-dose group (P < 0.01). The low dose of OIF (60 μg) was minimally effective for induction of ovulation and the least luteotrophic, as evidenced by the smallest maximum CL diameter and the smallest day-to-day profiles for CL diameter and plasma concentrations of progesterone and LH. Responses were intermediate for the middle-dose groups (125 and 250 μg). We conclude that OIF from llama seminal plasma has a dose-dependent effect on ovulation rate and CL form and function in llamas and that the biological effect of OIF is evident at physiologically relevant doses (i.e., as little as 1/100th of that present in an ejaculate).