Calmodulin antagonists inhibit secretion in Paramecium.

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Abstract
Secretion in Paramecium is Ca2+-dependent and involves exocytic release of the content of the secretory organelle, known as the trichocyst. The content, called the trichocyst matrix, undergoes a Ca2+-induced reordering of its paracrystalline structure during release; 3 stages are defined in this expansion process. The stage I, or fully condensed trichocyst, is the 4 .mu.m-long membrane-bounded form existing prior to stimulation. Stage II, the partially expanded trichocyst, is defined as an intermediate stage in the transition, preceding stage III, the fully expanded extruded form which is a 20-40 .mu.m-long needlelike structure. These stages were used to assay the effects of trifluoperazine (TFP) and W-7 [N-(6-aminohexyl)-5-chloro-1-naphthalene sulfonamide], calmodulin (CaM) antagonists, on trichocyst matrix expansion in vivo. TFP and W-7 reversibly block matrix release induced by picric acid. Ultrastructural examination reveals that one effect of this inhibition is reflected in the organelles themselves, which are prevented from undergoing the stage I-stage II transition by preincubation in 14 .mu.M TFP or 35 .mu.M W-7 before fixation. This inhibition of expansion by TFP can be moderated but not abolished by high extracellular Ca2+ (5 mM). The moderation by high Ca2+ can be eliminated by raising TFP concentration to 20 .mu.M. A possible explanation for the ability to titrate the inhibition in this manner is that TFP is acting to block expansion by binding to the Ca2+-CaM complex. Brief exposure of cells to the Ca2+ ionophore A23187 [calcimycin] and 5 mM Ca2+ following TFP treatment promotes matrix expansion, although in 14 .mu.M TFP a residual levels of inhibition remains. Following stimulation, CaM regulates secretion in Paramecium, possibly by controlling the Ca2+-dependent matrix expansion which accompanies exocytosis in these cells.