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Introduction and Objectives: Chagas is an endemic disease in Latin America, caused by the parasite Trypanosoma cruzi, which usually affects the functioning of the heart. We have studied the regulation of intracellular calcium in cardiomyocytes isolated from chagasic patients with different degrees of heart dysfunction., Methods: Calcium selective microelectrodes were used to simultaneously measure diastolic calcium concentration ([Ca²⁺](d)) and resting membrane potential in endomyocardial biopsies obtained from chagasic patients and controls., Results: The [Ca²⁺](d) increased by 123%, 295%, and 738% in chagasic patients in functional class I, II, and III, respectively, in relation to controls. Membrane potential showed a partial depolarization of 6% in functional class I, 10% in functional class II, and 22% in functional class III, compared to control values. Alteration in the [Ca²⁺](d) was partially reverted by 1-[6-[[(17ß)-3-metoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione (U-73122), a β-phospholipase C antagonist, and by 2-aminoethoxydiphenyl-borate (2-APB), an inositol 1,4,5-trisphosphate receptor blocker. Phenylephrine, an agent that induces a rapid transient increase in 1,4,5-trisphosphate intracellular content, produced a rise in [Ca²⁺](d), higher in chagasic cardiomyocytes than in controls, and its effect was fully inhibited by 2-APB., Conclusions: In cardiomyocytes from chagasic patients there is a dysfunction of the regulation of the [Ca²⁺](d), which correlates with the cardiac abnormalities observed in the different stages of the disease. This disturbance in the regulation of intracellular calcium appears to be associated with alterations in the regulation of intracellular messenger inositol 1,4,5-trisphosphate., (Copyright © 2011 Sociedad Española de Cardiología. Published by Elsevier Espana. All rights reserved.)