Extracellular Na+ and initiation of DNA synthesis: role of intracellular pH and K+.
Initiation of DNA synthesis in confluent quiescent 3T3 cell cultures stimulated by epidermal growth factor (EGF), vasopressin, and insulin was abolished by removing extracellular Na+. The inhibition was reversible, time- and Na+-concentration-dependent, and not due to an effect on binding or internalization of 125I-EGF. Stimulation by combinations of other growth factors with different mechanisms of action was also affected by decreasing extracellular Na+, but with different half-maximal Na+ concentrations. When choline was used as an osmotic substitute for Na+, the decrease in DNA synthesis was correlated with the decrease in intracellular K+. In contrast, when sucrose was used there was stimulation of the Na+-K+ pump and maintenance of intracellular K+ that resulted in a somewhat higher rate of DNA synthesis at lowered extracellular Na+ compared to choline. Mitogenesis induced by epidermal growth factor, vasopressin, and insulin led to cytoplasmic alkalinization as determined by an increase in uptake of the weak acid 5,5-dimethyloxazolidine-2,4-dione. Experimental decrease in extracellular Na+ blocked this cellular alkalinization. Therefore, under some conditions the supply of extracellular Na+ may limit cellular proliferation because of a reduction in the provision of Na+ to the Na+/H+ antiport and resultant failure of alkalinization. We conclude that Na+ flux and its effect on intracellular K and pH has a major role in the complex system that regulates proliferation.
D000818:Animals; D002453:Cell Cycle; D002478:Cells, Cultured; D004261:DNA Replication; D006133:Growth Substances / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D051379:Mice; D008934:Mitogens / Q000494:pharmacology; D008938:Mitosis; D009994:Osmolar Concentration; D011188:Potassium / Q000502:physiology*; D012964:Sodium / Q000502:physiology*
D006133:Growth Substances; D008934:Mitogens; D012964:Sodium; D011188:Potassium