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Evidence for the action of bovine follicular fluid factor(s) other than inhibin in suppressing follicular development and delaying oestrus in heifers

Summary. The aim of this study was to investigate the importance of inhibin in the delay in return to oestrus in heifers induced by steroid-stripped bovine follicular fluid (bFF). Oestrous activity was synchronized in 18 Hereford x Friesian heifers with two injections of prostaglandin (PG) 12 days apart. At the time of the second PG injection (time 0), the animals were assigned at random to one of three experimental groups and received i.v. injections of 20 ml saline (controls, n = 6), whole bFF (FF group, n = 6) or bFF in which the bioactive inhibin content had been reduced by >95% by immunoaffinity chromatography (-INH group, n = 6; inhibin content 0·8 ml whole bFF) every 8 h for 2 days. In a dose–response study, 2·5 ml whole bFF was insufficient to delay oestrus consistently following a similar synchronization regimen. Blood samples were taken every 8 h, initially before each injection and then subsequently for a further 9 days for hormone analysis. Animals were observed every 8 h throughout the experiment for signs of behavioural oestrus. The ovaries of all animals were examined using real-time ultrasonography about 30 h after the second PG injection.

Treatment failed to suppress peripheral follicle-stimulating hormone (FSH) concentrations, although a significant increase was observed in both treatment groups after cessation of injections. Progesterone concentrations fell immediately after the second PG injection in all animals and remained below minimum detectable concentrations in all treated animals for the remainder of the experiment. In control animals, progesterone rose above minimum detectable concentrations by day 6 and continued to rise until the end of the experiment. Analysis of samples taken from treated animals several days after observed oestrus revealed that all had apparently ovulated. Mean daily luteinizing hormone (LH) concentrations did not differ between treatment groups before ovulation, but after ovulation, mean daily LH was significantly reduced in control animals as progesterone concentrations rose.

Follicular development, as assessed by the mean antral diameter of the largest follicle on a pair of ovaries at ultrasound examination, was significantly suppressed in treated animals compared with controls (P < 0·01) and there was no significant difference (P = 0·397) between the two treatment groups. Control animals displayed oestrus 68 h (± 8 SEM) after the second PG injection, but oestrus was delayed in treated animals to 186 h ± 5 (FF group) and 191 h ± 6 (-INH group).

We conclude that the suppression of follicular development and the subsequent delay in return to oestrus associated with bFF treatment of cattle is not due to inhibin. Furthermore, the data suggest that the rebound of FSH concentrations following bFF treatment is similarly due to a factor other than inhibin.

Keywords: inhibin; bovine follicular fluid; FSH; oestradiol; oestrus; cattle

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