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Aquaporins (AQPs) are small transmembrane proteins that facilitate transport of water and other small molecules across biological membranes. In mammals, 13 isoforms of these proteins have been discovered so far, 9 of them are localized in kidneys (Michalek, 2016). Aquaporin-2 (AQP2) is of particular importance in renal water excretion in response to the body’s needs (Holmes, 2012; Kwon et al., 2013; Kitchen et al., 2015). Our previous studies confirmed the presence of AQP2 in the renal tubules in young beef cattle, which, as in humans and laboratory animals, is located mainly in the apical membrane and intracellular vesicles of renal collecting duct principal cells (Michałek et al., 2014b). We have also confirmed in our studies on calves that AQP2 in cattle is excreted in urine, similarly as in other mammals. However, the mechanisms regulating renal excretion of this protein, especially during the postnatal period, seem to be slightly different in humans and laboratory animals. In fact, the observed increase in AQP2 excretion in calves in response to additional water losses along with faeces in the course of diarrhoea was not accompanied by a typical increase in plasma vasopressin (AVP) concentration (Michałek et al., 2014a). ABSTRACT. Aquaporin-2 (AQP2) is a small, transmembrane protein, active in the principal cells of the renal collecting duct system. Phosphorylation and dephosphorylation of this protein regulate its redistribution in the cell, thereby influencing renal water reabsorption. The aims of the study were to identify and analyse the location of AQP2 phosphorylated at position 261, and to determine its potential role in renal water balance regulation, because data on AQP2 in cattle are scarce, and the mechanisms regulating its expression and cellular localization remain unclear. The analyses were carried out on kidneys collected from eight Polish Holstein-Friesian Black-and-White male calves, aged 5–7 months. It was found, using immunohistochemistry and commercially available phosphospecific antibodies, that in cattle AQP2 Ser-261 is located exclusively in the apical plasma membrane of the principal cells of the collecting duct. This atypical location of AQP2 Ser-261 allows to conclude that it may play a significant role in the process of renal water retention. The results of our previous studies on the regulation of renal total AQP2 excretion with urine in calves, as well as the bioinformatic analysis of available data presented in this study, seem to support this assumption. In addition, bioinformatic tools predicted mitogen activated protein kinases (MAPK) as a possible vasopressin-independent factor involved in AQP2 Ser-261 phosphorylation and accumulation in the apical plasma membrane. Received: 7 June 2018 Revised: 27 November 2018 Accepted: 18 February 2019

Cellular localization and putative role of aquaporin-2 Ser-261 in the bovine kidney