Variation of pH of mammary tumour cytosols: potential influence on steroid receptor assay.

Mammary tumour cytosols prepared for steroid receptor assay showed pH values above or below 7.3-7.5 in 50% of the samples. An increase of 0.6 pH units was observed after the addition of dextran coated charcoal. To test the sensitivity of the assay systems to pH changes, breast cancer cytosols were adjusted to pH between 6.0-9.0 before incubation with ligand at 0 degrees C. ER was then separated by isoelectric focusing in a pH gradient between 9.5-3.5 in a routine assay including a dextran coated charcoal step before focusing. PgR was assayed by electrofocusing as well as by a conventional DCC method and Scatchard plot. Unadjusted control cytosols were run in parallel. At pH above or below 7.0-8.0 recovery of receptor was reduced, the loss being most prominent at low pH. At low pH levels Scatchard plots appeared to be of curvo-linear shape. The effect of low intracellular tumour pH on receptor-ligand affinity in vitro may be of great importance as well in vivo, and should be taken into consideration in the therapeutic situation.