A function that relates protein synthetic rates to potassium activity in vivo.

A newly developed experimental system allows the controlled alteration of intracellular K+ activity (aK) and the measurement of amino acid incorporation rates in a single cell, the Xenopus oocyte. We found that as aK is increased by microinjecting a K+ salt, [3H]leucine incorporation (R) varies over a 100-fold range, first stimulated and then inhibited as it passes through four response regions (A-D). In region A (aK approximately 60-100 mM), R is at a nongrowth or maintenance level and is stimulated weakly by increasing aK. In region B (aK approximately 100-130 mM), R is stimulated intensely by increasing aK, roughly tripling with every 10 mM increase. In region C (aK approximately 130-160 mM), R is inhibited intensely by increasing aK. Finally, in region D (aK greater than 160 mM), R is inhibited weakly as aK increases. Collectively, the four response regions constitute the oocyte's R/aK response function. The function provides a comprehensive description of how K+ activity influences the rate of protein synthesis in an intact cell. In the subsequent discussion, we compared the oocyte response function with the K+ response determined in cell-free translational systems. While in vivo and in vitro functions are similar, differences exist that may be important in a cellular control system. We then considered the relevance of the oocyte R/aK response function to "normal" processes in the oocyte and in somatic cells, i.e., those in which aK is varied by physiological changes in the plasma membrane. We concluded that the intensely stimulatory region B is importantly involved in hormonal action and other growth-activating processes and that the entire R/aK response function may play a role in control of protein synthesis during the cell cycle.