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

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 X 10(4)/cm2 less than N less than 7.5 X 10(5)/cm2 and saturates for N more than 1.0 X 10(6)/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) X 10(-9) cm2/s, and the ratio of relaying threshold concentration to signal pulse size, C*/eta = (1.4 +/- 0.05) X 10(5) cm-3. N* has also been measured in the presence of various amounts of added beef heart PDE. The cAMP relaxation rates, I/tauo, due to beef heart PDE were calculated from the N* measurements and found to be proportional to amounts of added PDE for (I/tauo)max less than (10s-1). Finally, two kinds of inhibition have been observed in the PDE secretion. The PDE activity per cell is constant for N less than 8.0 X 10(4)/cm2, and decreases for larger N. It depends only on N for I/tau less than 10 s-1 and is strongly inhibited by extracellular PDE activity above this relaxation rate.