Reversible alteration of morphology in an invertebrate erythrocyte: Properties of the natural inducer and the cellular response

The normal shape of the erythrocytes of the bivalves known as blood clams i s maintained by a marginal band (MB) of microtubules. When hemolymph (or "b lood") is withdrawn from the animal, its erythrocytes change, within minute s, from the normal smooth-surfaced, flattened ellipsoids (N-cells) to spher oids with folded surfaces (X-cells). This alteration can be prevented by ra pidly diluting the hemolymph with physiological medium, yielding N-cells fo r use in studying the transformation to X-cells. Bioassays showed that shap e transformation was induced by a hemolymph activity (H-x) and was a functi on, in part, of cell responsiveness to this activity. Eventually the shape of the cells spontaneously returned to normal, at a rate dependent upon the concentration of the cells and of H-x; recovery was correlated with loss o f H-x. The X-cells contained an intact but highly deformed MB, but this was not the effector of the transformation. Erythrocytes made to lack MBs stil l changed shape, although they did not recover as completely as did the MB- containing controls. When clams were cooled before hemolymph was withdrawn, the concentration of H-x was reduced. H-x was retained after dialysis of h emolymph, and initial filtration and chromatography indicated that its M-r was greater than 500,000. Shape transformation was blocked by EGTA, by seri ne protease inhibitors, and by sodium azide; the last indicates ATP-depende nce. Although the mechanism responsible for shape transformation remains to be determined, the data suggest that the change is triggered by a coagulat ion-related activity in response to the removal of hemolymph from the anima l.