pH gradients were measured with microelectrodes in cellular spheroids of human glioma (U-118 MG) and thyroid carcinoma (HTh7) origin. pH decreased outside the spheroids and then continuously decreased when the electrode was moved through the spheroid towards the center. The lowest pH values inside the spheroids were in the range 6.7-6.8 when grown under standard conditions with F10 medium. When medium with stronger buffer capacity (Dulbecco's minimum essential medium or Locke's) was used, the gradients in both types of spheroids were less steep. Less steep pH gradients were also obtained in both types of spheroids when the concentration of glucose was lowered to 0.1 g/liter in the medium. In the case of HTh7 spheroids the low pH inside the spheroids under standard culture conditions seemed toxic because the growth rate increased when the spheroids were cultured under conditions giving higher central pH values (high buffer capacity or low glucose concentration). No such growth-stimulating effects could be seen for the U-118 MG spheroids. The growth rate of both types of spheroids was retarded when they were grown in medium with very high glucose concentration (10 g/liter). The thickness of the viable cell layer increased for HTh7 spheroids when the concentration of glucose was lowered to 0.1 g/liter. A decrease in the thickness of the viable layer of U-118 MG spheroids was observed when they were grown at a high glucose concentration (10 g/liter).
