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 is maintained by a marginal band (MB) of microtubules. When hemolymph (or "blood") is withdrawn from the animal, its erythrocytes change, within minutes, from the normal smooth-surfaced, flattened ellipsoids (N-cells) to spheroids with folded surfaces (X-cells). This alteration can be prevented by rapidly diluting the hemolymph with physiological medium, yielding N-cells for use in studying the transformation to X-cells. Bioassays showed that shape transformation was induced by a hemolymph activity (Hx) and was a function, 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 Hx; recovery was correlated with loss of Hx. The X-cells contained an intact but highly deformed MB, but this was not the effector of the transformation. Erythrocytes made to lack MBs still 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 When clams were cooled before hemolymph was withdrawn, the concentration of Hx was reduced. Hx was retained after dialysis of hemolymph, and initial filtration and chromatography indicated that its Mr was greater than 500,000. Shape transformation was blocked by EGTA, by serine protease inhibitors, and by sodium azide; the last indicates ATP-dependence. Although the mechanism responsible for shape transformation remains to be determined, the data suggest that the change is triggered by a coagulation-related activity in response to the removal of hemolymph from the animal.