Single exposure to heat induces stage-specific germ cell apoptosis in rats: Role of intratesticular testosterone on stage specificity

Short term exposure of the testis to heat causes degeneration of germ cells . However, the mechanisms underlying this process are poorly understood. Th e major objectives of this study were to determine whether the heat-induced loss of germ cells in the adult rat occurs via apoptosis, to document its stage-specific and cell-specific distribution, and to examine whether intra testicular testosterone (T) plays any role in the stage specificity of heat -induced germ cell death. Testes of adult male Sprague-Dawley rats were exp osed to 22 C (control) or 43 C for 15 min. Animals were killed on days 1, 2 , 9, and 56 after heat exposure. Germ cell apoptosis was characterized by D NA gel electrophoresis and in situ terminal deoxynucleotidyl transferase-me diated deoxy-UTP nick end labeling assay. The incidence of germ cell apopto sis [apoptotic index (AI)] was quite low in control rats(AI = 0.04-0.1). Mi ld hyperthermia within 1 or 2 days resulted in a marked activation (AI = 4. 7-5.6) of germ cell apoptosis predominantly at early (I-IV) and late (XII-X IV) stages. Stages V-VI and VII-VIII were relatively protected from heat-in duced apoptosis. Spermatocytes, including pachytenes at stages I-IV and IX- XII, diplotene and dividing spermatocytes at stages XIII-XIV, and early (st eps 1-4) spermatids, were most susceptible to heat. On day 9, the majority of the tubules were severely damaged and displayed only a few remaining apo ptotic germ cells. By day 56, spermatogenesis was completely recovered, and the incidence of germ cell apoptosis was compatible with the control level s. To determine whether intratesticular T plays a role in protecting germ c ells at stages VII-VIII against heat-induced cell death, adult rats were ex posed to local testicular heating on day 2 or were given a daily sc injecti on of GnRH antagonist (GnRH-A) for 4 days with and without a single exposur e of testes to heat applied on day 2. By day 4, the incidence of increased germ cell apoptosis at stages other than VII-VIII were not different betwee n heat-treated and GnRH-A- plus heat-treated groups, whereas the control gr oup and the group treated with GnRH-A alone showed minimal apoptosis. GnRH- A addition to heat resulted in a further increase in apoptosis (by 3.2-fold ) at stages VII-VIII over the values measured in the heat-treated group, an d it became comparable to that at all other stages. Collectively, these res ults provide evidence that 1) heat induces germ cell apoptosis in a stage-s pecific and cell-specific fashion; and 2) intratesticular T plays a pivotal role in protecting germ cells at stages VII-VIII against heat-induced cell death. However, the possible involvement of various other factors, includi ng growth factors, thermoprotectants, cytokines, and various death-related proteins, in protecting germ cells against heat-induced apoptosis cannot be ruled out.