Occurrence of Na+-Ca2+ exchange in the ciliate Euplotes crassus and its role in Ca2+ homeostasis

Ciliates possess diverse Ca2+ homeostasis systems, but little is known abou t the occurrence of a Na+-Ca2+ exchanger. We studied Na+-Ca2+ exchange in t he ciliate Euplotes crassus by digital imaging. Cells were loaded with fura -2/AM or SBF1/AM for fluorescence measurements of cytosolic Ca2+ and Na+ re spectively. Ouabain pre-treatment and Na+, substitution in fura-2/AM-loaded cells elicited a bepridil-sensitive [Ca2+](i) rise followed by partial rec overy, indicating the occurrence of Na+-Ca2+ exchanger working in reverse m ode. In experiments on prolonged effects, ouabain, Na-o(+) substitution, an d bepridil all caused Ca-o(2+)-dependent [Ca2+](i) increase, showing a role for Na+-Ca2+ exchange in Ca2+ homeostasis. In addition, by comparing the e ffect of orthovanadate (affecting not only Ca2+ ATPase, but also Na+-K+ ATP ase and, hence, Na+-Ca2+ exchange) to that of bepridil on [Ca2+](i), it was shown that Na+-Ca2+ exchange contributes to Ca2+ homeostasis. In electroph ysiological experiments, no membrane potential variation was observed after bepridil treatment suggesting compensatory mechanisms for ion effects on c ell membrane voltage, which also agrees with membrane potential stability a fter ouabain treatment. In conclusion, data indicate the presence of a Na+- Ca2+ exchanger in the plasma membrane of E. crassus, which is essential for Ca2+ homeostasis, but could also promote Ca2+ entry under specific conditi ons.