Inhibition of Na+,K+ pump affects nucleic acid synthesis and smooth muscle cell proliferation via elevation of the [Na+]i/[K+]i ratio: possible implication in vascular remodelling

OBJECTIVES : Na+,K+ pump inhibition is known to delay the development of apoptosis in vascular smooth muscle cells (VSMC). This study examines Na+,K+ pump involvement in the regulation of VSMC macromolecular synthesis and proliferation. METHODS : DNA, RNA and protein synthesis in VSMC from the rat aorta was studied by the incorporation of 3H]-labelled thymidine, uridine and leucine. Cell cycle progression was estimated by flow cytometry. Intracellular Na+ and K+ content and Na+,K+ pump activity were quantified as the steady-state distribution of 22Na and 86Rb and the rate of ouabain-sensitive 86Rb uptake in Na+-loaded cells, respectively. RESULTS : Ouabain inhibited the Na+,K+ pump with a Ki of 0.1 mmol/l. At concentrations less than 0.1 mmol/l, neither Na+]i nor K+]i was affected by ouabain; elevation of ouabain concentration sharply increased the Na+]i/K+]i ratio with a K0.5 of approximately 0.3 mmol/l. At concentrations higher than 0.1 mmol/l, ouabain time- and dose-dependently activated RNA and DNA syntheses in serum-deprived VSMC and inhibited cell cycle progression triggered by serum. In quiescent VSMC, ouabain did not affect protein synthesis, total cell number, but slightly increased the percentage of cells in the S-phase (4.25 versus 1.46%) and attenuated cell death assessed by staining with trypan blue and lactate dehydrogenase release. CONCLUSIONS : Elevation of the Na+]i/K+]i ratio caused by Na+,K+ pump inhibition markedly enhances nucleic acid synthesis in quiescent VSMC and blocks cell cycle progression in serum-supplied VSMC. The relative contribution of this phenomenon as well as the anti-apoptotic action of increased Na+]i/K+]i ratio to vascular remodelling under augmented content of endogenous Na+,K+ pump inhibitors, seen in volume-expanded hypertension, should be investigated by in-vivo studies.