Lammel, E. M., Barbieri, M. A., Wilkowsky, S. E., Bertini, F., and Isola, E. L. D. 1996.Trypanosoma cruzi:Involvement of intracellular calcium in multiplication and differentiation.Experimental Parasitology83,240–249. The possible role of intracellular Ca2+level onTrypanosoma cruzidifferentiation was explored. The addition to epimastigotes of aTriatoma infestansintestinal homogenate, which that triggers off the differentiation to the infective metacyclic form, induced a sudden rise in Ca2+ifrom the basal value, 94 ± 28 to 584 ± 43 nmole/liter. This increase was not affected by the presence of EGTA in the medium. Trypsin-treated intestinal homogenate did not alter the Ca2+iof epimastigotes. Calmodulin inhibitors (Calmidazolium, Trifluoperazine, and Chlorpromazine) blocked differentiation. Although the calcium ionophore ionomycin increased Ca2+ito 342 ± 29 nmole/liter, it was unable to induce differentiation by itself. BAY K8644 and Methoxyverapamil (agonist and antagonist of Ca2+channels, respectively) were unable to affect Ca2+iby themselves, or when added to stimulated parasites, and did not exert a stimulatory or inhibitory effect on morphogenesis. BAPTA/AM, a Ca2+chelator, partially blocked the rise in Ca2+iand morphogenesis; this effect was reversed by ionomycin. The requirement of intracellular Ca2+on epimastigote multiplication was also evaluated. The addition of EGTA to the culture medium led to a decrease in epimastigote multiplication till it practically ceased in the sixth passage. When such parasites were transferred to LIT they partially recovered the growth rate. Parasites from passages III, IV, and V in the Ca2+-depleted medium maintained their basal Ca2+i, but when treated with the intestinal homogenate, the rise in Ca2+iwas abrogated. Accordingly, the differentiation percentages of such parasites dropped significantly compared with controls.