Effects of taurine on reproductive hormones
It is well known that FSH, LH, T and E are the major hormones in male animals, which play important roles in male reproduction. In male animals, T and E are produced by testis. The secretion of T and E are regulated by FSH and LH which were produced by adenohypophysis.
In the present study, the effect of taurine on reproductive hormones was detected in male rats of different ages. The results showed that the levels of T and LH were obviously increased in rats of different ages, and the level of E was also significantly elevated in baby rats. However, β-alanine significantly lowered the concentrations of T and LH in baby rats, the concentration of E in adult rats, and the levels of four kinds of reproductive hormones in aged rats. The results were in agreement with the findings of Xiao et al. who suggested that the relative weight of the testis, and the levels of T and LH were obviously increased by taurine administration in broilers, and taurine had a promotive effect in the development of testis [24]. We have also previously identified that taurine can be biosynthesized by testis, and can stimulate the secretion of T in vivo and in vitro [18].
It has been reported that the secretion of LH had no changes by intraperitoneal injection of taurine [25]. The result is not similar to our observation about the effect of taurine on LH. The difference may be due to the concentration and the method of taurine administration.
Effects of taurine on testis biochemical indicators
Testes are the most important organs in male reproduction, which are involved in the functions of spermatogenesis and testosterone secretion. In male animals, many kinds of enzymes are closely correlated to the functions of testis [26–29]. To evaluate the effect of taurine on testis biochemical metabolism, the levels of ACP, AKP, LDH, SDH, AST, ALT, SOD, MDA, NOS and NO were detected in testes of adult and aged rats.
In testis, AKP is associated with the division of spermatogenic cells and the transportation of glucose to spermatogenic cells. ACP is one of the markers of dyszoospermia that associated with the denaturation of seminiferous epithelium and phagocytosis of sertoli cells. In the present study, the activity of ACP was significantly increased by taurine supplementation in testis of adult and aged rats, but the activity of AKP was obviously decreased in testis of adult rats. In adult rats, the activity of ACP was obviously elevated by β-alanine administration in testis, and the activity of AKP was significantly decreased. The results indicated that taurine may be important in spermatogenesis by improving the lipid and energy metabolism, increasing the spermatogenic cells division in testis.
LDH and SDH are widely distributed and located in the seminiferous tubules and germ cells, which is associated with the maturation of spermatogenic cells, testis and spermatozoa and the energy metabolism of spermatozoa. Our results showed that the activity of SDH was significantly increased by taurine administration in adult rats, and the activities of SDH and LDH were obviously elevated in aged rats. However, the activities of SDH and LDH were significantly decreased by β-alanine administration in adult and aged rats. The results suggested that taurine plays an important role in the maturation and energy metabolism of spermatogenic cells and spermatozoa.
AST and ALT are the important aminotransferase and are widely distributed in mitochondrion, which is associated with the integrality of spermatozoa acrosome and cells stress. The activities of AST and ALT increased when the membrane of spermatozoa was damaged, and the rate of intact acrosome spermatozoa decreased. The present results showed that the activities of AST and ALT were obviously decreased by taurine supplement in testis of adult and aged rats, but were significantly increased by β-alanine supplement. The results indicated that taurine is an important factor that improves the ability of antinociception and anti-stress in testis cells including spermatogenic cells, and protects spermatozoa.
NO has been shown to be an important paracrine messenger and neurotransmitter that promotes homeostasis and various functions in many tissues. In the testis, NO and NOS are thought to regulate an array of functions, including sperm motility, maturation and germ cell apoptosis, Sertoli cell tight junction dynamics, and Leydig cell steroidogenesis [30, 31]. The present results showed that the levels of NOS and NO were obviously increased by taurine administration in adult and aged rats, but had no changes after β-alanine administration. The results indicated that taurine may be essential to the function of testis.
SOD is a metalloprotein and accomplishes its antioxidant functions by enzymatically detoxifying the peroxides and superoxide anion. GSH is one of the most important compounds, which helps in the detoxification and excretion of oxygen radicals. MDA is one of several low-molecular-weight end products formed via the decomposition of certain primary and secondary lipid peroxidation products. In the present study, the levels of SOD, NOS and NO were significantly increased by taurine administration in adult and aged rats, and the activity of GSH was significantly increased in aged rats. The level of MDA was obviously increased by β-alanine supplement in experimental rats, but the activity of GSH was significantly decreased. The results indicated that taurine can improve the testis oxidative stability, inhibit lipid peroxidation, and promotes homeostasis of testis.
There were evidences that the levels of SDH, SOD, GSH, AST, ALT were significantly increased by a taurine supplement in serum, and the level of MDA decreased [32–35]. But it has been reported that the levels of NOS and NO obviously decreased in serum and many tissues [36, 37], which was not in agreement with our results. It may be attributed to the difference of the detected tissues.
Effects of taurine on the sperm quality
Taurine have been found in spermatozoa and seminal fluid of numerous species and are known to have beneficial effects on sperm characteristics in mammals [9, 38]. It has been suggested that taurine play important roles in the maintenance and stimulation of sperm motility and stimulation of capacitation and acrosome reactions in vivo and in vitro [14, 39]. It also has been found that taurine could inhibit lipid peroxidation in rabbit spermatozoa and protect the loss of motility [13]. In addition, taurine have been reported to improve either the initial post-thaw motility of ram sperm or the duration of motility of frozen–thawed ram sperm [40], and its cryoprotective effect may be attributable to its osmoregulation rather than to its antioxidant properties.
The present results showed that taurine could significantly increase the motility of sperm in adult rats, but has no obvious effects on the other semen quality. In aged rats, taurine could obviously increase the numbers and the motility of sperm, and the rate of live sperm, but has no significant effects on the rate of intact acrosome sperm. However, β-alanine could significantly decrease the motility of sperm, and increase the rate of abnormal sperm in adult and aged rats. The results indicated that taurine can improve the semen quality in male animals especially in aged male animals.
It has been found that aging resulted in a significant decline in serum and testis taurine content [19, 21]. Our unpublished data also has identified that the biosynthesis of taurine in rat testis declined as aging occurred. The results of the present study demonstrated that male reproduction can be improved by taurine administration in aged rats. The improved effects of taurine on aged male reproduction may be attributed to the stimulation of T secretion, promotion of testis homeostasis, and antioxidation.
In summary, our results indicated that there are beneficial effects of taurine on male reproduction. The mechanisms of the beneficial effects are complex and unclear but may include the modulation of calcium levels, osmoregulation, membrane stabilization and antioxidation.