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Short-term changes in the cytoplasmic pH in the plasmodium of Physarum polycephalum were measured with special reference to its contraction-relaxation cycle. The measurement was done photometrically in a plasmodial strand segment loaded with fluorescein diacetate. To eliminate possible errors caused by volume changes of the plasmodial strand in the course of its contraction-relaxation cycle, fluorescence intensities at two excitation wavelengths, 493 nm (pH sensitive) and 448 nm (pH insensitive), were recorded alternately. Simultaneously, tension production of the same strand segment was recorded under isometric conditions. The results were : 1) The cytoplasmic pH averaged 6.6 ± 0.5, for 34 specimens. 2) The cytoplasmic pH changed cyclically with the same period as the tensile force. 3) The average peak-to-peak amplitude of pH change was 0.13 ± 0.06 pH unit. 4) The phase of the pH oscillation was just opposite to that of the force production cycle; i.e., the cytoplasmic pH decreased during the contraction phase and increased during the relaxation phase. The plasmodia of myxomycetes, especially of Physarum polycephalum, are among the most extensively investigated materials in cell motility research. However, only a few reports have appeared on the pH of Physarum plasmodia. Seifriz and Zetzmann (12) first estimated the cytoplasmic pH in Physarum by using the color changes of its intrinsic pigments as an indicator, with particular reference to its fruiting and sclerotium formation. The change in the cytoplasmic pH of Physarum in relation to its mitotic cycle was studied by Gerson and Burton (3) and Morisawa and Steinhardt (9). According to these authors, the pH rises at or near the stage of mitosis and reaches a low point at the mid-interphase of the mitotic cycle. Hirose et al. (5) used fluorometry of the intrinsic fluorescent pigments of Physarum to study the role of intracellular pH in chemotactic transduction and suggested that the degree of cytoplasmic acidification paralleled the relaxing phase of the tension cycle. However, there has been no report on the short-term pH changes in relation to the contractionrelaxation cycle. In the present work, we simultaneously measured cytoplasmic pH and isometric tensile force production in a plasmodial strand segment of Physarum. The cytoplasmic pH was measured using a fluorescent pH-indicator dye, fluorescein diacetate, which shows pH-sensitive or -insensitive fluorescence excitation depending on the wavelength of the excitation light.

Oscillation of cytoplasmic pH of Physarum plasmodium in relation to motility.