Critical density for relaying in Dictyostelium discoideum and its relation to phosphodiesterase secretion into the extracellular medium

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Abstract
The critical cell density for relaying in D. discoideum, N*, has been measured as a function of cell density, N, and time after harvesting, t. It has logarithmic dependence on N for 2·5 × 104/cm2 < N < 7·5 × 105/cm2 and saturates for N > 1·0 × 106/cm2. N* is an increasing function of time after harvesting. The phosphodiesterase (PDE) secretion rate on which N* depends is a constant. Expressions were derived which relate N* to PDE secretion and diffusion. They have been fitted to the data from time delay experiments yielding values of the PDE diffusion constant in 2% buffered agar, Dp = (2·25 ± 0·15) × 10−9 cm2/s, and the ratio of relaying threshold concentration to signal pulse size, C*/η = (1·4 ± 0·05)× 105cm-3. N* has also been measured in the presence of various amounts of added beef heart PDE. The cAMP relaxation rates, 1/τ0, due to beef heart PDE were calculated from the N* measurements and found to be proportional to amounts of added PDE for (1/τ0)max < (10 s-1). Finally, two kinds of inhibition have been observed in the PDE secretion. The PDE activity per cell is constant for N < 8·0 ×104/cm2, and decreases for larger N. It depends only on N for 1/τ0 < 10 s-1 and is strongly inhibited by extracellular PDE activity above this relaxation rate.