Reaction of the contractile apparatus in Physarum to injected Ca++, ATP, ADP and 5'AMP

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. The replacement of endoplasm offers the possibility to study the effects of physiologically active substances upon the contraction activity of cytoplasmic actomyosin under in vivo conditions. Using different methods for measuring the force output as indicator of the effects of injected substances, the influences of free Ca++-ions and different nucleotides were studied. The injection of Ca++ buffers revealed that force output of the veins increased with ascending Ca++ concentrations between 2 x 10(-7) and 10(-6) M. Half maximal response was found to lie in the range of 4 x 10(-7) M, the saturation value at 6 x 10(-7) M Ca++. 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 Ca++. From a threshold concentration of 2 x 10(-7) M Ca++, force output increased up to 10(-5) M Ca++. In presence of 0.2 mM ATP, the value for half maximal contraction response to Ca++ was 10(-6) M Ca++. It is concluded that 0.2 mM ATP and 2 to 4 x 10(-7) M Ca++, are the conditions, under which the cytoplasmic actomyosin system of Physarum performs its function in vivo. The implication of the results and the possibilites of the new method for further investigations of the contraction physiology of Physarum as a model system for cytoplasmic actomyosin are discussed.