Heat stress disrupts the ovarian microenvironment in cattle: An in vivo analysis of hormonal alterations, oxidative stress-induced apoptosis, and reduced pregnancy outcomes.

Abstract

Heat stress during summer impairs reproductive function in cattle. Since previous studies have largely employed in vitro models to examine heat stress effects on oocytes and mural granulosa cells (MGCs), comprehensive in vivo research is required to elucidate its multifaceted impact on cattle fertility. This study aimed to clarify the mechanisms of heat stress-induced reproductive changes by comparing ovarian microenvironment, hormone levels, and pregnancy rates in cattle across seasons. Compared with the control group (temperature-humidity index [THI] <72, n&#x202f;=&#x202f;87), the heat-stressed group (HS; THI &#x2265;72, n&#x202f;=&#x202f;86) exhibited significant alterations in reproductive physiology, including increased serum cortisol concentration, and a reduced pregnancy rate (60.3&#x202f;% vs. 43.5&#x202f;%). In the HS group, MGCs showed reduced expression of steroidogenic-related proteins (StAR, LHR, and aromatase), accompanied by reduced estradiol concentrations in follicular fluid. A significant upregulation of oxidative stress-related proteins (SOD1 and SOD2) and the apoptosis marker (caspase-3) was observed, with a positive correlation identified between these factors. These findings suggest that MGCs exposed to heat stress undergo late-stage apoptosis driven by oxidative stress. In the HS group, gene expression in dominant follicle-derived cumulus-oocyte complexes presented an impaired oocyte maturation (decreased GDF9 and BMP15), while immunofluorescence analysis revealed increased HSP70 signal intensity in oocytes. Collectively, these findings suggest that heat stress triggers molecular and hormonal alterations in the ovarian microenvironment, leading to impaired oocyte maturation and reduced fertility. This study provides essential insight for the development of effective strategies to mitigate heat stress and enhance reproductive outcomes in livestock.