APLN and type 1 diabetes mellitus: For instance, it has been shown that Apelin-13 can reduce testosterone release by inhibiting the secretion of luteinizing hormone;29 another study shows that APLN expression is increased in the testes of the T1DM mouse model, and testes cultured with ML221 significantly increased testosterone secretion30, however, whether APLN/APJ is involved in diabetic-associated spermatogenesis dysfunction remains unclear.