REACTION OF CONTRACTILE APPARATUS IN PHYSARUM TO INJECTED CA++, ATP, ADP AND 5'AMP

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  • 1 January 1978
    • journal article
    • research article
    • Vol. 18  (1) , 76-94
Abstract
After replacement of endoplasm by artificial media, the ectoplasmic tube of the plasmodial veins survives and contraction automaticity continues. Motive force generation for plasmodial locomotion is widely independent of the presence of endoplasm, i.e., force generation is located within the ectoplasmic tube. Endoplasm replacement offers the possibility to study the effects of physiologically active substances on cytoplasmic actomyosin contraction activity under in vivo conditions. Using different methods for measuring the force output as indicator of the effects of injected substances, free Ca2+ influences and different nucleotides were studied. The injection of Ca2+ buffers revealed that force output of the veins increased with ascending Ca2+ concentrations between 2 .times. 10-7 and 10-6 M. Half maximal response was found to lie in the range of 4 .times. 10-7 M, the saturation value at 6 .times. 10-7 M Ca2+. The injection of adenosine nucleotides induced optimal contraction responses at 0.2 mM ATP, 0.5 mM ADP and 2.5 mM 5''AMP, respectively. The contraction response induced by 0.2 mM ATP depends strongly on the concentration of the available Ca2+. From a threshold concentration of 2 .times. 10-7 M Ca2+, force output increased up to 10-5 M Ca2+. In presence of 0.2 mM ATP, the value for half maximal contraction response to Ca2+ was 10-6 M Ca2+. Conditions under which the Physarum cytoplasmic actomyosin system performs its function in vivo are 0.2 mM ATP and 2 to 4 .times. 10-7 M Ca2+. The implication of the results and the possibilities for further investigations into the Physarum contraction physiology as a model system for cytoplasmic actomyosin are discussed.