Abstract
A model of morphogenetic regulation in Dictyostelium discoideum (1) is based on the assumption that NH 3 inhibits the synthesis and/or release of extracellular 3′,5′-cyclic AMP and that by topographical restriction of c-AMP production to specified zones within the cell aggregate, NH 3 is presumed to set up the conditions for apical dominance and directed morphogenetic movements. This study indicates that: exposure of preaggregative cells to exogenous NH 3 + NH 4 + inhibits the acquisition of c-AMP-induced properties associated with aggregation competence (accumulation of specific contact sites required to form EDTA resistant aggregates and the synthesis of extracellular and membrane-bound c-AMP phosphodiesterase); exposure of aggregation competent cells which are actively producing extracellular c-AMP to exogenous NH 3 + NH 4 + is followed by the immediate cessation of extracellular c-AMP release. The pH dependence of these effects suggests that the active species is NH 3.
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