Potassium secretion by human mammary epithelial cells accounts for the elevated K+ concentration in milk

The relatively high K⁺ concentration in milk provides an essential nutrient for nursing infants that is necessary for maintaining K⁺ homeostasis within the intracellular and extracellular fluids.  Its concentration in milk is tightly regulated by hormones, including glucocorticoids and prolactin that control the ion transport properties of the mammary epithelium.  Less is known however about the role autocrine signaling molecules such as ATP, which regulates electrolyte transport by activating G-protein coupled, P2Y receptors.

Although recent studies have documented the impact of P2Y receptor activation, apical epithelial Na⁺ (ENaC) channel proteins, and calcium-activated K⁺ (K_Ca3.1) channels in the secretion and absorption of electrolytes in mammary epithelial cells, there is a paucity of information on the mechanisms that underlie the secretion of K⁺ by these cells.  Thus, this study aimed to investigate the molecular mechanisms that form a basis for the constitutive secretion of K⁺ by human mammary epithelial cells.

Recently, University of Minnesota researchers: Nathan Zaidman, Peter Maniak and led by Professor Scott O’Grady in collaboration with Yotesawee Srisomboon and Chatsri Deachapunya at Srinakharinwirot University demonstrated the role of K2P channels in the basal secretion of K⁺ and ENaC-dependent absorption of Na⁺ by human mammary epithelial cells.  The study is now reported in American Journal of Physiology- Cell Physiology.

The authors observed that administration of the K2P K⁺ channel blocker bupivacaine to the apical solution triggered an instant increase in basal short-circuit current (I_sc).  This was followed by a decrease in current after the addition of ENaC inhibitor benzamil.  Similarly, apical addition of quinidine, another K2P channel inhibitor, also triggered a rapid increase in basal I_sc; this was followed by a slight decrease in current after the addition of UTP.  They also observed that pretreatment of the apical solution with a known TASK1/ TASK3 inhibitor (PK-THPP) and K_Ca3.1 inhibitor (TRAM-34) resulted in an increase in apical membrane current, consistent with inhibition of K⁺ secretion.  However, PK-THPP was approximately 20–30 times more potent than either quinidine or bupivacaine.

The research team also demonstrated that the α and γ subunits of ENaC as well as K_Ca3.1, are localized within the apical membrane.  In addition, they observed that apical stimulation with UTP triggered a biphasic change in current.  UTP produced a transient decrease that was consistent with the stimulation of K⁺ secretion, which was followed by a steady and sustained increase in current, consistent with inhibition of basal K⁺ secretion.  Furthermore, pretreatment of the apical solution with the Ca²⁺-chelating agent BAPTA-AM completely terminated the UTP-dependent decrease in current.

The authors also found that pretreatment of the apical solution with the same concentrations of quinidine or bupivacaine reduced most of the inhibitory effect of UTP on apical membrane current while pretreatment with PK-THPP completely terminated the UTP response.  Apical pretreatment of the solution with the PKC activator PMA triggered an increase in current and reduced any further change in the steady-state current caused by the subsequent addition of UTP. However, treatment with either PMA or the PKC inhibitor GF109203X did not change the decrease in apical membrane current triggered by UTP. Moreover, they observed the expression of five K2P channel subtypes in the apical membrane.

The study provides evidence that multiple subtypes of K2P channels contribute to the basal secretion of K⁺, which is coupled to ENaC-dependent Na⁺ absorption by human mammary epithelial cells. The findings elucidated by Professor Scott O’Grady and his colleagues have advanced our understanding of the epithelial transport mechanisms that control the ionic composition of milk and the purinergic signaling pathways that regulate the secretory function of the mammary epithelium.