Detection and identification of plasma progesterone metabolites in the female Florida manatee (Trichechus manatus latirostris) using GC/MS/MS

Florida manatees (Trichechus manatus latirostris) have relatively low peripheral concentrations of progesterone (P4). The objective of this study was to determine if these relatively low P4 concentrations are associated with a high ratio of progestin metabolites and to document metabolite concentrations from individual blood samples obtained from manatees during diestrus or pregnancy. Metabolites known to exist in elephants—terrestrial manatee relatives—were targeted. These included 5α-reduced progestins (5α-pregnane-3,20-dione [5α-DHP] and 3α-hydroxy-5α-pregnan-20-one [5α-P3-OH]) and 17α-hydroxyprogesterone (17α-OHP), which occurs in Asian elephants. An additional, inactive metabolite, 20α-hydroxyprogesterone (20α-OHP), indicative of P4 overproduction, was also targeted. Progesterone itself was the predominant progestin detected in pregnant and nonpregnant manatee plasma (n = 10) using gas chromatography–mass spectrometry with tandem quadrupole detectors (GC/MS/MS). Progesterone concentrations in pregnant females varied from early (moderate to high) through mid and late (low) pregnancy. Progesterone concentrations ranged from low to high in nonpregnant, nonlactating females. The most commonly detected metabolite was 5α-P3-OH (n = 7), which occurred in pregnant (lower limit of detection [LLOD] to high) and nonpregnant (trace to high) females. The 5α-DHP metabolite was also detected in pregnant (LLOD to moderate) and nonpregnant (low) females. The 17α-OHP metabolite was not detected in any tested female. The 20α-OHP metabolite was detected in one nonpregnant, nonlactating, captive female (LLOD). Metabolites were most prevalent during early pregnancy, concurrent with maximum P4 concentrations. Based on their concentrations in peripheral circulation, we inferred that these metabolites may have, opposite to elephants, a limited physiologic role during luteal, pregnant, and nonpregnant phases in the manatee.