HYPOTONICITY STIMULATES TRANSLOCATION OF ICLN IN NEONATAL RAT CARDIACMYOCYTES

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 pr otein ICln. In rat neonatal cardiac myocytes under isotonic conditions , ICln is found predominantly (greater than 90%) in the cytosol. Howev er, after cell volume expansion by exposure to hypotonic medium, ICln rapidly translocates to the particulate fraction (the Triton X-114-ins oluble fraction). After 2 min in hypotonic medium the percentage of IC ln 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-stimu lated taurine efflux as well as ICln translocation parallel the reduct ion 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 a re returned to isotonic medium. When ICln is analyzed under non-denatu ring conditions, a dimer is detected in the particulate fraction of vo lume-expanded cells, along with the monomer. This dimer is not detecte d in the cytosol. Treatment of the particulate fraction from volume-ex panded cells with the lyotropic agent KSCN caused release of ICln but not Na-K-ATPase into the soluble fraction, indicating that translocate d ICln associates with membranes in the particulate fraction rather th an inserting into them.