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The Response of Physarum polycephalum to Extracellular Ca2+: Studies on Ca2+ Nutrition, Ca2+ Fluxes and Ca2+ Compartmentation

Summary: The calcium requirement for optimal growth of Physarum polycephalum is between 0.05 and 2.5 mM. Media without Ca2+ will not support growth though Ca2+-chelating agents, such as EDTA and EGTA, do not have any marked effect on Ca2+ availability at the low pH used to culture this organism. The intracellular Ca2+ concentration varies approximately 10-fold when the external Ca2+ concentration is varied between 0.05 and 25 mM. Ca2+ is concentrated in the mitochondrial and microsomal fractions, but the high concentration found in the cytosol suggests that considerable redistribution of Ca2+ occurs during disruption and fractionation of the plasmodia. 45Ca2+ uptake by intact microplasmodia occurs at rates which are comparable to those required for Ca2+ uptake during growth and generation of new cell mass. This uptake process has two components: one non-saturable with Ca2+, and the other saturable with an apparent maximum velocity of 67 nmol Ca2+ h-1 (mg protein)-1 and an apparent K
m of 1.9 mM-Ca2+. As judged by [3H]inulin uptake, part of the 45Ca2+ uptake may be endocytotic.
Ionophores A23187 and Br-X537A stimulate 45Ca2+ uptake. Uptake is inhibited by KCl, Mn2+ and Sr2+ at high concentrations, but is not sensitive to La3+ at concentrations similar to Ca2+ in the uptake medium. KCN and iodoacetic acid inhibit the transport, while cyto-chalasin B, verapamil, tetracaine and procaine have little or no effect.