Lysozyme acts as a chemorepellent and secretagogue in Paramecium by activating a novel receptor-operated Ca++ conductance

Using combined intracellular recordings and behavioral bioassays, it was found that lysozyme has two different effects in Paramecium, depending upon the concentrations used. At low concentrations (0.5 nm to 1.0 m) it acts as an effective chemorepellent that causes reliable electrophysiological changes. Lysozyme-induced somatic depolarizations, isolated by blocking K⁺ channels with Cs-TEA, showed concentration dependencies that were well correlated with chemorepulsion. Ion dependency experiments showed that these were Ca⁺⁺ based depolarizations. Addition of either Na⁺ or Mg⁺⁺ improves chemorepulsion by providing additional depolarizations. Both the depolarizations and chemorepulsion were blocked by 10 m neomycin, suggesting that the depolarization is necessary for this chemosensory transduction event. At higher concentrations (100 m), lysozyme is a secretagogue. A transient inward current, recorded in Ca⁺⁺ alone solutions with Cs-TEA present, was seen in response to high lysozyme concentrations. The amplitude of this inward current was well correlated with exocytosis. Addition of neomycin (1.0 mm) eliminated both the inward current and exocytosis, suggesting a causal relationship. Neither amiloride or W-7, compounds previously suggested to affect the electrophysiological responses to secretagogues, had any significant effects. The mucopolysaccharide hydrolysis activity of lysozyme was not required for any of these responses. We propose that Paramecium have a high affinity receptor on the body plasma membrane that responds to either lysozyme or a related compound to cause an increase in a novel body Ca⁺⁺ conductance. This receptor-operated Ca⁺⁺ conductance causes membrane depolarization and chemorepulsion at low concentrations and triggers a sufficient Ca⁺⁺ influx at high concentrations to cause exocytosis.We thank Drs. C. Kung and R. Preston for sharing mutants and Drs. H. Machemer, A. Turkewitz and K. Clark for their comments on the first draft of this work. This was supported by NSF grants BNS8916228 and MCB9410756 to TMH and a grant from the American Diabetes Association to BHS.