Hypotonicity stimulates translocation of ICln in neonatal rat cardiac myocytes

Cell volume expansion stimulates the efflux of solutes, including the amino acid taurine, to accomplish a regulatory volume decrease (RVD). One protein that may play a role in taurine efflux is the cytosolic protein ICln. In rat neonatal cardiac myocytes under isotonic conditions, ICln is found predominantly (greater than 90%) in the cytosol. However, after cell volume expansion by exposure to hypotonic medium, ICln rapidly translocates to the particulate fraction (the Triton X-114-insoluble fraction). After 2 min in hypotonic medium the percentage of ICln in the particulate fraction increases to 30%, 46% at 5 min, 40% at 10 min, and 25% at 30 min. The time course of this response is similar to that of hypotonicity-stimulated taurine efflux. Hypotonicity-stimulated taurine efflux as well as ICln translocation parallel the reduction in medium osmolarity. As osmolarity decreases, taurine efflux and ICln movement increase. The movement of ICln from the particulate back to the cytosolic fraction is accelerated when volume-expanded cells are returned to isotonic medium. When ICln is analyzed under non-denaturing conditions, a dimer is detected in the particulate fraction of volume-expanded cells, along with the monomer. This dimer is not detected in the cytosol. Treatment of the particulate fraction from volume-expanded cells with the lyotropic agent KSCN caused release of ICln but not Na-K-ATPase into the soluble fraction, indicating that translocated ICln associates with membranes in the particulate fraction rather than inserting into them.