Ion Transport

Abstract

The principal physiological function of the prostate is synthesis, accumulation and secretion of citrate, which is achieved by selective uptake of aspartate from plasma, transamination to oxaloacetate and delivery to the citric acid cycle (Kavanagh, 1985). Ouabain-sensitive Na+,K+-ATPase-mediated transport is implicated in aspartate uptake, citrate production and prostatic fluid formation . Na+ and K+ together with Cl-represent a large component of prostatic fluid osmolarity (Smith, 1969). Androgen activation of Na +,K + -ATPase serves as a metabolic pacemaker in the rat prostate (Farnsworth , 1993) and androgen also exerts transcriptional control over the expression of Na +,K + -ATPase fJ subunits, which play a critical role in the biogenesis of the pump (Blok et al. 1998).In this study we identified the isoform composition and cellular localization of Na+,K+-ATPase in the rat prostate. Young Spargue-Dawley rats weighing between 200 and 550 g were killed by dislocation of the neck. The prostate was snap frozen by immersion in liquid nitrogen-cooled isopentane and 7 flm sections were cut in a temperaturecontrolled cryostat. Well characterised isoform -specific antibodies against Na+,K+-ATPase isoforms were then employed to establish their specific expression and localization using immunofluorescence and immunoperoxidase techniques.The results showed the presence of epitopes of Na+,K+-ATPase a1, fJ 1, fJ2 and fJ 3 isoforms in epithelial cells. We found no evidence for a 2 and a3 expression in epithelial cells suggesting that epithelial pumps consist of al l fJ1 , al l fJ2and a1 I fJ3 isozymes. However, nominal a2 and a 3 expression was detected in the smooth muscle and stroma. Previous pharmacological studies in the rat prostate indicate that 10-5 M ouabain inhibits less than 30% of Na+,K+-ATPase confirming the dominance of ouabai n-resistant a1 (Hirano et al. 1994). Digital imaging and analysis confirmed that Na+,K+-ATPase isoforms were localised primarily to the lateral and basolateral membrane domains of columnar epithelial cells but in some cases fJ subunits were also observed in juxta-nuclear and internal membranes. Similar immunostaining of epithelia and stroma was observed when sections were probed with a1 -specific or pan-specific (reactive towards all a isoform s) antibodies suggestin g that cd predominates in all cell types within the rat prostate.Prostatic Na+,K+-ATPase may regulate the concentration of lumenal Na+ and K +, major counter ions for citrate. The presence of several fJ isoforms in the prostatic epithelium may influence the catalytic properties of Na+,K+-ATPase isozymes as has been shown in other epithelia. An elevation in the intracellular haemoglobin concentration in sickle cells (SCs) leads to an increase in red blood cell sickling (Bookchin et al. 1976). This can be achieved by a decrease in cell volume mediated by the KCl cotransporter (KCC l). Unlike in normal human red cells, KCC1 in SCs remains active at low O 2 tensions (Po,s) (Gibson et al. 1998). Thus KCC1 in SCs can be stimulat...