Cholesterol-to-phospholipid ratio in whole sperm and seminal plasma from fertile stallions and stallions with unexplained subfertility

Semen samples were collected from six fertile stallions and seven stallions with unexplained infertility. Percentages of motile sperm (77.5 ± 11.3 versus 67.5 ± 12.2, P = 0.2), and progressively motile sperm (70.8 ± 13.6 versus 60.7 ± 14.0, P = 0.2) were similar between fertile and subfertile stallions, respectively. Morphologic characteristics in ejaculates of control and affected stallions (% normal: 60.2 ± 18.2 versus 52.9 ± 11.3, P = 0.4; % abnormal heads 7.3 ± 4.8 versus 12.1 ± 5.0, P = 0.11; and % abnormal acrosomes 1.6 ± 2.1 versus 3.0 ± 3.4, P = 0.4) did not differ. After incubation with the calcium ionophore A23187, acrosome reaction rate of sperm from fertile stallions was 96 ± 2.8% whereas only 2.9 ± 2.5% of sperm from stallions with unexplained subfertility had acrosome reacted (P < 0.001). Molar amounts of cholesterol and phospholipid in whole sperm and seminal plasma did not differ (P > 0.1) between fertile and subfertile stallions. However, the molar ratio of cholesterol-to-phospholipid was 2.5 times greater in the seminal plasma (P = 0.09) and 1.9 times greater (P = 0.009) in whole sperm of subfertile stallions compared to fertile stallions.