Abstract:
The insulin-like peptide 3 (INSL3; previously known as relaxin-like factor) and testosterone are major
circulating hormones secreted by Leydig cells in mammalian testes. Along with testosterone, serum INSL3
concentrations have emerged as a novel clinical marker of Leydig cell function in humans. However, its
role in dogs in relation to the age is scanty. The objectives were to: (1) to measure INSL3 and testosterone
concentrations in male German shepherd dogs and to compare the changes with age (2) to identify the
relationships among 1NSL3, testosterone, age and body weight. Blood samples were takemfrom normal
male German Shepherd dogs from prepubertal age to advanced age (1.5 months to 11 years; n=46), and
from pre-pubertal (n=5), pubertal (n=5), post-pubertal (n=15) middle age (n=17) and advanced age (n=4).
German shepherd serum INSL3 and testosterone were measured using enzyme immunoassays. The
detection ranges of the INSL3 and testosterone assays were 0.08 to 80ng/ml and 0.01 to 40ng/ml,
respectively. Serum INSL3 concentrations increased (P<0.05) from pre-pubertal age to post-pubertal age,
remained similar during post-pubertal and middle ages, and significantly reduced (P<0.05) from middle to
advanced age. Testosterone concentrations increased (P<0.05) drastically from pre-pubertal age to
pubertal age reaching a plateau, and no significant alteration was observed from pubertal to advanced age.
The coefficients of determination of best regression curves between serum INSL3 and testosterone levels,
INSL3 and age, INSL3 and body weight, testosterone and age, and testosterone and body weight were
0.065 (n=46, P>0.15), 0.11 (n=46, P=0.075), 0.055(n=46, P>0.15), 0.53 (n=46, P<0.0001) and 0.59 (n=46,
PcO.0001). In conclusion, INSL3 concentrations showed a clear decline with increasing old age of German
shepherd dogs whereas testosterone did not show such trend. Different serum INSL3 and testosterone
dynamics were found in relation with age and body weight.
