Contrary to rat-type, human-type Na,K-ATPase is phosphorylated at the same amino acid by hormones that produce opposite effects on enzyme activity.

Renal sodium homeostasis is a major determinant of BP and is regulated by several natriuretic and antinatriuretic hormones. These hormones, acting through intracellular secondary messengers, either activate or inhibit proximal tubule Na,K-ATPase. It was shown previously that phorbol esters and angiotensin II and serotonin induce the phosphorylation of both Ser-11 and Ser-18 of the Na,K-ATPase alpha-subunit. This results in the recruitment of Na,K-ATPase molecules to the plasma membrane and an increased capacity to transport sodium ions. Treatment of the same cells with dopamine leads to phosphorylation of the Na,K-ATPase alpha-subunit Ser-18. The subsequent internalization of Na,K-ATPase molecules results in a reduced capacity to transport sodium ions. These effects are observed in cells that express the rat-type Na,K-ATPase. However, the Na,K-ATPase alpha1-subunit of several species, such as human, pig, and mouse, does not have a Ser-18 in their N-terminal region. Therefore, the possibility exists that, in those species, the Na,K-ATPase is not regulated by the hormones that regulate natriuresis. This study presents evidence that in cells that express the human-type Na,K-ATPase, dopamine inhibits and phorbol esters activate the Na,K-ATPase-mediated transport. These opposite effects are mediated by the phosphorylation of the same amino acid residue, Ser-11 of Na,K-ATPase alpha1, and the presence of alpha1 Ser-18 is not essential for the hormonal regulation of Na,K-ATPase activity in LLCPK1 cells. It was observed that, whereas the regulatory stimulation of Na,K-ATPase is mediated by protein kinase Cbeta, the regulatory inhibition is mediated by protein kinase Czeta. This is similar to what was demonstrated previously in cells that express the rat-type Na,K-ATPase.