Prolactin reference range and pulsatility in male dogs

Little is known about serum prolactin (PRL) concentrations and secretion patterns in male dogs. Blood samples (n = 65) were collected from crossbred dogs and from Beagles and German Shepherd dogs to describe the PRL reference range, and from five male Beagles at 15-min intervals for 6 h (n = 24 samples/dog) to describe the ultradian rhythm of this hormone. Serum PRL was measured by a homologous endpoint enzyme immunometric assay. The reference range was established from nondetectable to 6.0 ng/mL. There was an effect of breed; serum PRL concentrations in Beagles were higher (P < 0.05) than in crossbreeds and German Shepherds. However, there was no significant correlation between PRL concentration and age of the dog. During the ultradian study, PRL was characterized by a fluctuating baseline with occasional distinct elevations, indicating a pulsatile secretion. The mean basal PRL concentration was 1.4 ± 0.6 ng/mL and the mean AUC was 9.9 ± 2.7 ng/mL/6 h. Prolactin pulse frequency ranged from one to two peaks/6 h, pulse duration between 15 and 75 min, and amplitude from 1.7 to 2.4 ng/mL. In conclusion, this reference range, pulsatility and breed differences should be taken into account when interpreting serum PRL concentrations, for clinical or research purposes.