The principal physiological function of the prostate gland is the synthesis, accumulation and secretion of the anion citrate . Citrate may be used as an important energy source for spermatozoa, or involved as a buffer or chelator of cations in seminal fluid . Na+ dependent uptake of aspartate from plasma is achieved by two kinetically distinct Na+-dependent transport systems to create a high cytosolic aspartate concentration . Aspartate provides the intra-mitochondrial source of oxaloacetate while glucose provides the source of acetyl-CoA for citrate biosynthesis . Ouabain-sensitive Na, K-ATPase-mediated transport is critical for aspartate uptake, citrate production and prostatic fluid formation since the inward Na+ gradients generated by Na, K-ATPase are utilized for the Na+ dependent uptake of aspartate. Na+ and K+ also represent a large component of prostatic fluid osmolarity  and their levels are finely regulated by plasma membrane transport systems that have yet to be identified. Furthermore, androgen activation of Na, K-ATPase serves as a metabolic pacemaker in the prostate [6, 7] exerting transcriptional control over the expression of Na, K-ATPase subunits, which play a critical role in the biogenesis of Na, K-ATPase in prostate cancer [8, 9]. In normal prostate, citrate concentrations in prostatic fluid range from 40 to 150 mM. In prostate cancer however, citrate production levels are significantly reduced as a result of altered cellular metabolism and bioenergetics .
Na, K-ATPase is an important regulator of intracellular electrolyte levels in almost all mammalian cells [10, 11]. It is a Mg2+-dependent P-type transport pump responsible for maintaining the low intracellular Na+:K+ ratio that is essential for cell homeostasis and physiological function. It catalyzes the active uptake of K+ and extrusion of Na+ at the expense of hydrolyzing ATP with a stoichiometry of 3Na+ for 2K+. The active form of Na, K-ATPase is an integral membrane protein complex primarily composed of two non-covalently attached subunits; a 110-kDa catalytic α subunit and a 45–55-kDa glycosylated β subunit. The α subunit has binding sites for Na+, K+, ATP and cardiac glycosides (digitalis and ouabain) . Four α isoforms encoded by different genes have been identified which are ~85% identical at the protein level [13, 14]. The β subunit is a complex type II glycoprotein with a short cytoplasmic NH2 terminus, a single transmembrane domain and a large globular COOH ectodomain containing three disulfide bridges and sites for N-linked glycosylation.
Renal Na, K-ATPase consists of an additional small component known as the γ subunit [15, 16]. The γ subunit is a member of the FXYD family of small ion transport regulators  and is believed to be responsible for fine regulation of Na+ transport in the nephron by modulating the transport function of renal Na, K-ATPase [18, 19].
We have previously shown that human and rat prostatic epithelial cells express the α1, β1 and β2 isoforms of Na, K-ATPase [20, 11]. Despite the importance of Na, K-ATPase function for citrate production there is no information about its expression patterns in hyperplastic or neoplastic prostate. The objective of this study was to determine the localization of Na, K-ATPase and to compare expression of Na, K-ATPase isoforms in normal canine prostate, benign prostatic hyperplasia (BPH) and prostatic adenocarcinoma (PCa) in order to determine whether reduced citrate levels in PCa are also accompanied by changes in Na, K-ATPase expression.