Amelioration of heat stress-induced damage to testes and sperm quality

Heat stress (HS) occurs when temperatures exceed a physiological range, overwhelming compensatory mechanisms. Most mammalian testes are ∼4-5 °C cooler than core body temperature. Systemic HS or localized warming of the testes affects all types of testicular cells, although germ cells are more sensitive than either Sertoli or Leydig cells. Increased testicular temperature has deleterious effects on sperm motility, morphology and fertility, with effects related to extent and duration of the increase. The major consequence of HS on testis is destruction of germ cells by apoptosis, with pachytene spermatocytes, spermatids and epididymal sperm being the most susceptible. In addition to the involvement of various transcription factors, HS triggers production of reactive oxygen species (ROS), which cause apoptosis of germ cells and DNA damage. Effects of HS on testes can be placed in three categories: testicular cells, sperm quality, and ability of sperm to fertilize oocytes and support development. Various substances have been given to animals, or added to semen, in attempts to ameliorate heat stress-induced damage to testes and sperm. They have been divided into various groups according to their composition or activity, as follows: amino acids, antibiotics, antioxidant cocktails, enzyme inhibitors, hormones, minerals, naturally produced substances, phenolic compounds, traditional herbal medicines, and vitamins. Herein, we summarized those substances according to their actions to mitigate HS' three main mechanisms: oxidative stress, germ cell apoptosis, and sperm quality deterioration and testicular damage. The most promising approaches are to use substances that overcome these mechanisms, namely reducing testicular oxidative stress, reducing or preventing apoptosis and promoting recovery of testicular tissue and restoring sperm quality. Although some of these products have considerable promise, further studies are needed to clarify their ability to preserve or restore fertility following HS; these may include more advanced sperm analysis techniques, e.g. sperm epigenome or proteome, or direct assessment of fertilization and development, including in vitro fertilization or breeding data (either natural service or artificial insemination).

Testicular hyperthermia reduces testosterone concentrations and alters gene expression in testes of Nelore bulls

Increased testicular temperature reduces sperm motility, morphology and fertility. Our objectives were to characterize effects of testicular hyperthermia (scrotal insulation) on acute testosterone concentrations and gene expression in Bos indicus testes. Nelore bulls (n = 20), ∼27 mo of age, 375 kg, scrotal circumference >31 cm, with ≥30% motile sperm, were allocated into four groups (n = 5/group): non-insulated (Control) and insulation removed after 12, 24, or 48 h. Immediately after insulation, intratesticular temperatures (needle thermocouples) were coolest in Control bulls and warmest in 48-h bulls (mean ± SEM, 35.28 ± 0.31 vs 38.62 ± 0.57 °C, P < 0.05). Bulls were castrated and testes recovered. Testicular testosterone concentrations were higher in Control versus 48-h bulls (3119 ± 973.3 and 295.5 ± 122.8 ng/g of tissue, respectively, P < 0.05). Total RNA was extracted, reverse transcribed and RT-qPCR done. For STAR, mRNA abundance decreased from Control to 48 h (1.14 + 0.32 vs 0.32 + 0.5, P < 0.05). For BCL2, expression decreased from Control to 24 h (1.00 + 0.07 vs 0.70 + 0.12, P < 0.05), but then rebounded. In addition, GPX1 had a 70% increase (P < 0.05) at 48 h, whereas HSP70 had a 34-fold increase (P < 0.05) at 12 h and 2- and 14-fold increases (P < 0.05) at 24 and 48 h, respectively. HSF1, BAX, P53 and CASP 8 remained unchanged. Downregulation of STAR, critical in androgen production, was consistent with reduced testosterone concentrations, whereas increased GPX1 enhanced testicular antioxidative capability. Huge increases in HSP70 conferred protection again apoptosis and cell destruction, whereas reduced BCL2 promoted apoptosis. These findings provided novel insights into acute tissue responses (testosterone and gene activity) to testicular hyperthermia in B. indicus bulls.

Hyperthermia is more important than hypoxia as a cause of disrupted spermatogenesis and abnormal sperm

We tested the hypothesis that hypoxia replicates effects of hyperthermia on reducing number and quality of sperm produced, whereas hyperoxia mitigates effects of hyperthermia. Forty-eight CD-1 mice (∼50 d old), inspired air with 13, 21, or 95% O₂ and were exposed to ambient temperatures of 20 or 36 °C (3 × 2 factorial, six groups) twice for 12 h (separated by 12 h at 20 °C and 21% O₂), with euthanasia 14 or 20 d after first exposure. Combined for both post-exposure intervals, there were primarily main effects of temperature; mice exposed to 20 vs 36 °C had differences in testis weight (110.2 vs 96.9 mg, respectively; P < 0.0001), daily sperm production (24.7 vs 21.1 × 10⁶ sperm/g testes, P < 0.03), motile sperm (54.5 vs 41.5%, P < 0.002), morphologically normal sperm (59.9 vs 45.4%, P < 0.002), morphologically abnormal heads (7.3 vs 22.0%, P < 0.0001), seminiferous tubule diameter (183.4 vs 176.3 μm, P < 0.004) and altered elongated spermatids (2.2 vs 15.9, P < 0.001). Increasing O₂ (from 13 to 95%) affected morphologically abnormal heads (15.4, 10.8 and 17.6%, respectively; P < 0.03), seminiferous tubule diameter (175.7, 185.6 and 178.4 μm, P < 0.003) and total altered spermatids (8.3, 3.3 and 15.2, P < 0.05). Our hypothesis was not supported; hypoxia did not replicate effects of hyperthermia with regards to reducing number and quality of sperm produced and hyperoxia did not mitigate effects of hyperthermia. We concluded that hyperthermia per se and not secondary hypoxia was the fundamental cause of heat-induced effects on spermatogenesis and sperm. These findings are of interest to develop evidence-based efforts to mitigate effects of testicular hyperthermia, as efforts should be focused on hyperthermia per se and not on hyperthermia-induced hypoxia.