Abstract
High environmental temperatures induce detrimental effects on various reproductive processes in cattle. According to the predicted global warming the number of days with unfavorable ambient temperatures will further increase. The objective of this study was to investigate effects of acute heat stress during the late pre-ovulatory phase on morphological, physiological and molecular parameters of dominant follicles in cycling cows during lactation. Eight German Holstein cows in established lactation were exposed to heat stress (28°C) or thermoneutral conditions (15°C) with pair-feeding for four days. After hormonal heat induction growth of the respective dominant follicles was monitored by ultrasonography for two days, then an ovulatory GnRH dose was given and follicular steroid hormones and granulosa cell-specific gene expression profiles were determined 23 hrs thereafter. The data showed that the pre-ovulatory growth of dominant follicles and the estradiol, but not the progesterone concentrations tended to be slightly affected. mRNA microarray and hierarchical cluster analysis revealed distinct expression profiles in granulosa cells derived from heat stressed compared to pair-fed animals. Among the 255 affected genes heatstress-, stress- or apoptosis associated genes were not present. But instead, we found up-regulation of genes essentially involved in G-protein coupled signaling pathways, extracellular matrix composition, and several members of the solute carrier family as well as up-regulation of FST encoding follistatin. In summary, the data of the present study show that acute pre-ovulatory heat stress can specifically alter gene expression profiles in granulosa cells, however without inducing stress related genes and pathways and suggestively can impair follicular growth due to affecting the activin-inhibin-follistatin system.
Highlights
It has been reported in several studies that fertility in lactating dairy cows is depressed during the summer months in warm areas of the world
In a retrospective study evaluating the effects of heat stress that was defined as temperatures !29°C on the conception rates after artificial insemination, it has been reported in lactating Holstein cows that the conception rate was negatively affected by HS exposition prior, but not after artificial insemination, suggesting that in particular, HS prior to and immediately after artificial insemination should be avoided [6]
The P4/E2 ratios were clearly above 1 in all follicles, which is indicative for a late pre-ovulatory, E2-inactive stage after the luteinizing hormone (LH) surge [13]
Summary
It has been reported in several studies that fertility in lactating dairy cows is depressed during the summer months in warm areas of the world Heat stress generally deteriorates the body condition score, affects reproductive parameters such as follicle differentiation and alters the composition of the follicular fluid in particular the concentrations of fatty acids, suggestively leading to inferior oocyte quality and compromised granulosa cell functions [4]. It was shown in a bovine cell culture model that increased concentrations of oleic acid affect morphological and physiological features and gene expression levels of granulosa cells altering their functionality [5]. The proportion of pregnancy losses was not affected suggesting that HS is especially deleterious during late folliculogenesis
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