**3. Function/endocrine regulation of the prostate gland**

The prostate reaches maximum secretory activity in dogs of on average four years of age [20], the secretions comprise >90% of the ejaculate; the gland continues to grow under the influence of testosterone because of stem cell differentiation, and in the aging dog will increase in size because of hypertrophy and hyperplasia. Growth and secretion are regulated by the active metabolite of testosterone (T), namely the

5α-Dihydrotestosterone (DHT). More than 95% of testosterone are converted into DHT by the enzyme 5α-reductase, after diffusion into the prostate gland cells. DHT binds stronger to the testosterone receptor than T [21, 22]. Estradiol-17ß supports the effect of DHT in a synergistic way and, in addition, causes an upregulation of testosterone receptors [11].

The prostate gland secretion supports the transport and the function of spermatozoa after ejaculation. It contains citrate, lactate, cholesterol, and enzymes; however, few sugars and phospholipids are supposed to provide additional energy. The composition of the prostate secretions was recently investigated by means of proteomics [6, 23]. The serine-protease canine prostate specific esterase (CPSE) and the lactotransferrin-precursor are the most frequently occurring proteins in the seminal plasma [23], comprising 90% of all proteins. The CPSE has a proteolytic effect, similar to chymotrypsin [24] and after binding influences spermatozoa function by its zinc-binding properties [3]. The CPSE binds to phosphorylcholine-binding protein and choline phospholipid of the membrane and induces the efflux of cholesterol from the spermatozoa membrane during ejaculation, which is essential for capacitation. The secretion of CPSE is controlled by androgens [25] and the enzyme is believed to be a reliable marker of prostate secretion [26].

The extracellular matrix of the canine prostate (noncellular stroma and fibrous tissue) supports the development of the gland and the control of cellular functions [21], supposedly via cellular transmitters like cytokines [27].
