Cyclic AMP-induced pH changes in Dictyostelium discoideum and their control by calcium

Abstract

During differentiation of Dictyostelium discoideum the extracellular pH was measured in unbuffered cell suspensions. 1. 1. Stimulation of the cells by cyclic AMP pulses in the nanomolar range resulted in two successive increases of the extracellular proton concentration. 2. 2. Cyclic AMP analogs were effective in the order of their chemotactic activity, indicating that the pH changes were mediated by cyclic AMP receptors present at the cell surface. 3. 3. Cyclic AMP binding to the receptors was, however, not sufficient to elicit the pH changes; for glutaraldehyde-treated cells which exhibit normal binding did not respond. Moreover, the proton : cyclic AMP ratio was in the order of 3 · 10³. 4. 4. The second proton peak was not due to cyclic AMP secretion or subsequent hydrolysis and persisted in the presence of dithiothreitol, an inhibitor of cyclic AMP phosphodiesterases. 5. 5. Removal of extracellular calcium increased the proton peaks, particularly the first one, up to 10-fold. A half maximal response was obtained with 3 · 10^-10 M cyclic AMP. 6. 6. Only at cyclic AMP doses higher than 0.1 μ M did hydrolysis of the added cyclic AMP contribute significantly to the pH changes.