title: Critical density for relaying in Dictyostelium discoideum and its relation to phosphodiesterase secretion into the extracellular medium.
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 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.
mesh_terms: D015105:3',5'-Cyclic-AMP Phosphodiesterases / Q000378:metabolism* / Q000494:pharmacology; D002449:Cell Aggregation; D002465:Cell Movement; D000242:Cyclic AMP / Q000494:pharmacology; D004023:Dictyostelium / Q000201:enzymology / Q000502:physiology*; D004558:Electric Stimulation; D007700:Kinetics; D008433:Mathematics; D009235:Myxomycetes / Q000502:physiology*; D010727:Phosphoric Diester Hydrolases / Q000378:metabolism*; D013268:Stimulation, Chemical
chemical_list: D000242:Cyclic AMP; D010727:Phosphoric Diester Hydrolases; D015105:3',5'-Cyclic-AMP Phosphodiesterases