INSL3 Variation in Dogs Following Suppression and Recovery of the HPG Axis

Simple Summary Insulin-like peptide 3 (INSL3) is a hormone produced by the steroidogenic Leydig cells of the testes and in most mammals acts as a robust blood biomarker for pubertal development and gonadal function. Using a new immunoassay to measure INSL3 in adult male dogs, we show that it is much lower than in other species. When a cohort of male beagle dogs is implanted with a GnRH agonist to induce contraceptive suppression of male fertility, the average circulating INSL3 is first reduced and then recovers slowly when the implant is removed. Importantly, however, and unlike in other species, in dogs there is considerable variation in INSL3 expression, with some dogs showing little or no initial reduction of INSL3, and similarly, there is often only a poor response during the recovery phase. We conclude that in dogs, unlike in most other mammals, the Leydig-cell production of INSL3 appears to be less tightly coupled to the functioning of the hypothalamic-pituitary-gonadal (HPG) hormone axis.Abstract Insulin-like peptide 3 (INSL3) is a constitutive product of mature, adult-type Leydig cells of the testes and consequently in most mammals is an ideal biomarker with which to monitor pubertal development. A new heterologous time-resolved fluorescence immunoassay was developed and validated to measure circulating INSL3 in the blood of adult male dogs. Compared to other species, INSL3 concentration is low with marked variation between individuals, which appears to be independent of breed, age, or weight. A model system was then used in which a cohort of beagle dogs was subject to a GnRH-agonist implant to suppress the HPG axis and spermatogenesis, followed by implant removal and recovery. Unlike testosterone, INSL3 levels were not fully suppressed in all animals by the GnRH agonist, nor was the recovery of Leydig cell function following implant removal uniform or complete, even after several weeks. In dogs, and dissimilar from other species (including humans), Leydig-cell INSL3 appears to be quite variable between individual dogs and only weakly connected to the physiology of the HPG axis after its suppression by a GnRH-agonist implant and recovery. Consequently, INSL3 may be less useful in this species for the assessment of testis function.