Total protein synthesis (as precipitable [3H] leucine incorporation) was determined under various experimental conditions to examine the relationship between cellular K+ and protein synthesis in human umbilical vein endothelial cells (HUVEC). We found that under normal external K+ concentration (5 mM), total protein synthesis was inhibited by cycloheximide and ouabain, with estimated values of IC50 of 0.41 microM and 0.60 mM, respectively. Cellular K+ concentrations were determined (102 +/- 4 mM for control cells) and found to be significantly increased (P < 0.01) by high external K+ (25 mM) and significantly decreased (P < 0.001) by low external K+ (0.5 mM) as well as by ouabain (2 mM). Under high external K+, total protein synthesis and the inhibitory responses of cycloheximide and ouabain were not altered. On the contrary, cellular K+ and protein synthesis were both further reduced by about half (P < 0.001) under low external K+. While ouabain further reduced cellular K+ by half (P < 0.001), protein synthesis was only slightly reduced (P < 0.05) under low external K+ and thus the relative reduction on protein synthesis was much less than that for cellular K+. These results indicate that while elevated intracellular K+ did not alter protein synthesis, reduced intracellular K+ correlated with a reduced cellular protein synthesis in HUVEC such that K+ may play a permissible role in the regulation of protein synthesis.