L. Lescalematys et al., SURPLUS NA- HOW LOW-K+-INCUBATED LLC-PK(1) CELLS RESPOND TO K+ RESTORATION( PUMPS ), The American journal of physiology, 265(4), 1993, pp. 30000887-30000892
We have previously shown that a pig kidney cell line (LLC-PK1/Cl4) res
ponds to chronic exposure to 0.25 mM extracellular K+ by increasing th
e beta-, not alpha-, subunit mRNA levels and both alpha- and beta-abun
dance twofold over control (15). Our objective in the present study wa
s to determine how the LLC-PK1/Cl4 cells respond when returned to cont
rol (5.5 mM) medium. A 1.8-fold increase in ouabain binding establishe
d that the induced pumps were expressed at the cell surface following
24-h incubation in low K+. On restoration to 5.5 mM K+, intracellular
Na+ and K+ concentrations ([Na+]i and [K+]i, respectively) rapidly ret
urned to control levels within 15 min. The doubled pool size of pumps
in the chronic low K+ cells had no significant influence on the rate o
f ion restoration when compared with the rate in cells acutely exposed
to low K+. Despite the rapid return of ions to control values, beta-m
RNA levels remained elevated for 2 h, then sharply declined to control
levels by 6 h of K+ restoration. From these data, we estimate that th
e half-life of beta-mRNA is 2-3 h during restoration. Alpha-Subunit mR
NA remained essentially unchanged from control after return of K+ to t
he medium and restoration of intracellular ions. Both alpha- and beta-
synthesis rates remained elevated at 2 h and returned to near control
levels by 8 h of K+ restoration, parallel to the change in beta-mRNA.
Both alpha- and beta-subunit abundance and Na+-K+-adenosinetriphosphat
ase (ATPase) activity also remained elevated for 2 h, then decreased s
harply between 2 and 8 h of K+ restoration to their zero time point co
ntrol levels. Beta-Activity and Na+-K+-ATPase activity remained slight
ly elevated over the time-paired controls until 24 h of restoration. T
he rates of return in alpha, beta, and activity predict an increase in
Na+ pump subunit degradation rate in K+ restored cells. We conclude t
hat, since [Na+]i and [K+]i return to control within 15 min, whereas b
eta-mRNA and alpha- and beta-synthesis rates remain elevated for 2 h a
fter K+ restoration, these pathways do not quickly respond to these io
nic signals in LLC-PK, cells and that it is likely that both decreased
synthesis rates and increased degradation rates contribute to the eve
ntual return of Na+-K+-ATPase activity and pool size to control in Krestored cells.