The plasmodium of reacts to external stimuli tactically. Positive and negative taxes correspond to relaxation and contraction, respectively. Variations of intracellular cAMP and cGMP concentrations in response to chemical stimuli were examined in relation to the regulation of contractility. Concentrations of the two cyclic nucleotides oscillated, with a gradual shift, for some time after stimulation. The period of oscillation was 4-5 min, with phases being the same in response to repellents, but different when attractants were tested. Therefore, changes in the accumulation of the nucleotides summed over 15 min were taken as a quantitative measure of the external stimuli. Attractants (glucose, 2-deoxyglucose, galactose, maltose) induced decreases both in cAMP and in cGMP concentration, the latter being larger than the former. Repellents (KCl, CaCl, MgCl, sucrose) induced decreases both in cAMP and in cGMP concentration, the former being larger than the latter. Variations of the intracellular cAMP concentration for repellents and those of cGMP concentration for attractants, took place at similar concentrations of stimulants as variations of contraction and relaxation, respectively. Microinjection of cAMP and cGMP into the plasmodial strands induced contraction, cGMP being about 10 times as effective as cAMP. The results indicate that both cAMP and cGMP regulate the ability to contract, not antagonistically, but cooperatively, in the sensory transduction of the plasmodium.


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