Osmolytes responsible for volume reduction under isosmotic or hypoosmotic conditions in barnacle muscle cells

In numerous animal cells, experimental manipulations that increase the intr acellular free Ca2+ concentration induce cell volume reduction. This may oc cur under isosmotic conditions, e.g. when external Ca2+ (Ca-o) is replaced by Mg2+ (42) or during exposure to hypoosmotic conditions (i.e. regulatory volume decrease, RVD) in the presence of Ca-o. We determined the osmolytes responsible for volume reduction under isosmotic and hypoosmotic conditions in barnacle muscle cells. Organic osmolytes (i.e. free amino acids and met hylamines) and inorganic ions accounted for similar to 78% and 22% of the i ntracellular isosmotic activity, respectively. Isostuotic Ca-o removal indu ced a net loss of KCl (with a ratio of 1K:1C1) and free amino acids (FAA, m ainly glycine and taurine). During RVD, die same ions (but in a proportion of 2K:1C1) and FAA were lost, Since RVD was accompanied by extracellular al kalinization, the 2K:1C1 loss may be explained by the presence of a K+/H+ e xchanger (or K+-OH- co-transporter) or Cl-/OH- exchanger. The lack of RVD i n the absence of Ca-o cannot be attributed to the loss of intracellular osm olytes during isosmotic Ca-o removal because addition of Ca-o during cell s welling promoted RVD.