Secretory Pattern of Inhibin During Estrous Cycle and Pregnancy in African (Loxodonta africana) and Asian (Elephas maximus) Elephants

Abstract

The ovary of female elephants has multiple corpora lutea (CL) during the estrous cycle and gestation. The previous reports clearly demonstrated that inhibin was secreted from lutein cells as well as granulosa cells of antral follicles in cyclic Asian elephants. The aim of this study is to investigate the inhibin secretion during the pregnancy in African and Asian elephants. Two African elephants and two Asian elephants were subjected to this study. Circulating levels of immunoreactive (ir-) inhibin and progesterone were measured by radioimmunoassay. Four pregnant periods of an African elephant and three pregnant periods of an Asian elephant were analyzed in this study. Circulating levels of ir-inhibin started to increase at 1 or 2 week before the ovulation and reached the peak level 3 or 4 weeks earlier than progesterone during the estrous cycle in both African and Asian elephants. After last luteal phase, the serum levels of ir-inhibin remained low throughout pregnancy in both an African and an Asian elephant. The mean levels of ir-inhibin during the pregnancy were lower than the luteal phase in the estrous cycle despite high progesterone levels were maintained throughout the pregnancy. These results strongly suggest that CL secrete a large amount of progesterone but not inhibin during the pregnancy in elephants.

Assessment of Pregnancy Status of Asian Elephants <i>(Elephas maximus)</i> by Measurement of Progestagen and Glucocorticoid and Their Metabolite Concentrations in Serum and Feces, Using Enzyme Immunoassay (EIA)

“…The serum progestagen concentration profile in our study was similar to the serum concentration profiles of progesterone, 5α-P-3OH and 5α - DHP (progesterone metabolites) in pregnant African elephants that were reported previously [15, 16, 24, 25]. Moreover, the serum progestagen concentration profile in our research could indicate that luteal activity was at its highest level in mid-gestation (in order to maintain pregnancy) and then declined to its lowest level from 7 to 2 days prior to calving, which is supported by the serum progesterone concentration profiles of pregnant elephants found in other studies [5, 6, 16, 17, 19,20,21, 24, 25].…”

“…Moreover, the serum progestagen concentration profile in our research could indicate that luteal activity was at its highest level in mid-gestation (in order to maintain pregnancy) and then declined to its lowest level from 7 to 2 days prior to calving, which is supported by the serum progesterone concentration profiles of pregnant elephants found in other studies [5, 6, 16, 17, 19,20,21, 24, 25]. The average fecal progestagen concentration in this study was lower than the 5α-P-3OH (major progesterone metabolite in elephant feces) concentration that was found previously [11].…”

Zoo elephant research: contributions to conservation of captive and free‐ranging species

“…Inhibin follows a distinct pattern in elephants (Brown et al ., ; Kaewmanee et al ., ; Lueders et al ., ; Yamamoto, Yuto et al ., ), and increases after follicle luteinization post LH1. Luteinized follicles and acCLs are sources of inhibin, which also increases during the luteal phase.…”

“…5–6 weeks after conception (the normal luteal phase lifespan), but then rebound and grow significantly larger than CL during a non‐conceptive luteal phase, in conjunction with a marked secondary rise in progestagen concentrations and presumed implantation (Meyer et al ., ; Lueders et al ., ). Inhibin is elevated during the first 8 weeks of gestation, but then declines to baseline for the rest of gestation (Yamamoto, Yuto et al ., ), suggesting a role for inhibin in pregnancy is unlikely. After 5–7 months, relaxin and prolactin markedly increase (Meyer et al ., ; Yamamoto, Yamamoto et al ., ); prolactin remains elevated throughout, whereas relaxin concentrations start to decline after mid‐gestation, with a secondary surge at parturition (Meyer et al ., ).…”

Immunohistochemical localization of inhibin/activin subunits in adult Asian elephant (<i>Elephas maximus</i>) testes

“…Although research regarding the role of inhibin in both African and Asian female elephants has been previously reported [ 7 , 13 , 24 ], only males in the African species have been studied [ 8 , 12 ]. The present study determined the immunolocalization of inhibin/activin subunits in the testes of adult male Asian elephants.…”