The epithelial cells lining the ducts of the human mammary gland are responsible for modifying sodium and potassium concentrations in milk by actively absorbing sodium from and secreting potassium into the ductal fluid. In the present study we show that adenosine triphosphate (ATP) and uridine triphosphate (UTP) can stimulate sodium absorption and potassium secretion by a mechanism that involves increasing intracellular calcium and activation of calcium-dependent potassium channels. We discovered that addition of ATP or UTP to the luminal surface stimulates potassium secretion, whereas addition of the same concentrations to the epithelial surface normally exposed to the blood produces an increase in sodium absorption. These results provide a better understanding of the signalling mechanisms that control the concentrations of sodium and potassium present in milk. Abstract Human mammary epithelial (HME) cells express several P2Y receptor subtypes located in both apical and basolateral membranes. Apical UTP or ATP-γ-S stimulation of monolayers mounted in Ussing chambers evoked a rapid, but transient decrease in short circuit current (I sc), consistent with activation of an apical K + conductance. In contrast, basolateral P2Y receptor stimulation activated basolateral K + channels and increased transepithelial Na + absorption. Chelating intracellular Ca 2+ using the membrane-permeable compound BAPTA-AM, abolished the effects of purinoceptor activation on I sc. Apical pretreatment with charybdotoxin also blocked the I sc decrease by >90% and similar magnitudes of inhibition were observed with clotrimazole and TRAM-34. In contrast, iberiotoxin and apamin did not block the effects of apical P2Y receptor stimulation. Silencing the expression of K Ca3.1 produced ~70% inhibition of mRNA expression and a similar reduction in the effects of apical purinoceptor agonists on I sc. In addition, silencing P2Y 2 receptors reduced the level of P2Y 2 mRNA by 75% and blocked the effects of ATP-γ-S by 65%. These results suggest that P2Y 2 receptors mediate the effects of purinoceptor agonists on K + secretion by regulating the activity of K Ca3.1 channels expressed in the apical membrane of HME cells. The results also indicate that release of ATP or UTP across the apical or basolateral membrane elicits qualitatively different effects on ion transport that may ultimately determine the [Na +]/[K +] composition of fluid within the mammary ductal network.
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