Heat induces intracellular acidification in human A-431 cells: role of Na(+)-H+ exchange and metabolism

Abstract

The resting intracellular pH (pHi) of A-431 cells at 37 degrees C in Na+ Hanks' solution is 7.23 +/- 0.02. In the presence of amiloride (100 microM) pHi decreases to 7.08 +/- 0.03. Hyperthermia induces a temperature- and time-dependent intracellular acidification of 0.2 pH units in either bicarbonate-free or bicarbonate-buffered solutions. After heat treatment (45 degrees C, 10 min) pHi returns to normal 1 h after incubation at 37 degrees C. The activity of the Na(+)-H+ exchanger was examined in heated and unheated cells in the absence of bicarbonate. Unheated cells recover from an acid load in a [Na+]o-dependent and amiloride-sensitive manner. The apparent Michaelis constant for extracellular Na+ is 38 +/- 9 mM, and the apparent mean affinity constant for amiloride is 11 +/- 3 microM. In heated cells the apparent affinity of the Na(+)-H+ exchanger for extracellular Na+ is not changed, but the maximal recovery rate is approximately 40% slower than that of unheated cells. The rate of recovery from acid loading returns to normal 2 h after heat treatment. [Na+]i and intrinsic buffering power in heated cells are the same as those in unheated cells. Decreases in both intracellular ATP and lactic acid are observed in heated cells. 2-Deoxy-D-glucose and sodium azide induce an intracellular acidification but prevent most of the acidification induced by heat. Heat treatment causes no further acidification in cells that are acidified by both amiloride and 2-deoxy-D-glucose together. These data are the first to suggest that thermally induced intracellular acidification is due to both an inhibition of Na(+)-H+ exchange and an inhibition of metabolic pathways.