Influences of pH on human platelet metabolism.

The intracellular pH of human platelets is affected by external pH and by the addition of metabolic substrates and analogues. Acetate and propionate decrease pH in a rapid concentration-dependent manner, whereas glucose decreases the internal pH at a slower rate which is independent of concentration above 0.3 mM. The mechanisms of these effects is discussed. The rate of metabolism of glucose to lactate in human platelets was strongly pH-dependent, with higher rates at more alkaline pH values. This effect was found for several different buffer systems. Addition of acetate caused an inhibition of glycolysis, whereas addition of propionate had little effect. The rate of the oxidative pentose phosphate pathway also increased with increasing pH and this pathway was inhibited by both acetate and propionate. It is proposed that the effect of acetate on glycolysis required the metabolism of the acetate, whereas the effect of both acetate and propionate on the pentose phosphate pathway are directly due to the decrease in internal pH. The oxidation of acetate to carbon dioxide showed only small pH-dependent changes in rate unless glucose was also present: glucose inhibited oxidative metabolism (the 'Crabtree Effect'), but this inhibition was only apparent at higher pH values when glycolytic rates were high.