REVERSIBLE HISTOCHEMICAL MODIFICATIONS OF ENDOPLASMIC-RETICULUM FOLLOWING ARGININE-VASOPRESSIN STIMULATION OF GRANULAR CELLS OF TOAD BLADDER

The endoplasmic reticulum is generally absent from schematic represent ations of transport phenomena, although it shows a well-organized netw ork in most transport epithelial cells. In order to examine the correl ation between this organelle and cellular activity, bladders of Bufo m arinus were studied under different experimental conditions and fixed by immersion in glutaraldehyde, followed by OsO4 impregnation for 3 da ys. Normal granular and mitochondria-rich cells showed a rich cytoplas mic network of canaliculi, well-impregnated by osmium deposits. Follow ing a 2 to 15-min stimulation (serosal bath) with arginine vasopressin , the V-2 receptor agonist dD-arginine-vasopressin or cyclic AMP (cAMP ), the staining of endoplasmic reticulum in granular cells disappeared . After washing out of the hormone or the agonist, impregnation of the endoplasmic reticulum could be observed once again. Arginine vasopres sin did not modify the impregnation of endoplasmic reticulum of either mitochondria-rich or basal cells. Our data indicate a correlation bet ween the reactivity of endoplasmic reticulum to osmium, and a cAMP-dep endent effect of arginine vasopressin through its V-2 receptors. Incub ation of toad bladders carried out with agents interfering with cellul ar calcium (calcium ionophores, high or low bath calcium) or with calc ium release from the endoplasmic reticulum (TMB-8, thapsigargin) sugge sted that an early step in the cAMP-dependent effect of arginine vasop ressin must involve the release of intracellular calcium from the endo plasmic reticulum. However, calcium ATPases in this organelle do not s eem to participate in the hormonal effect. The reversible loss of osmi um impregnation induced by arginine vasopressin may represent protein changes in the endoplasmic reticulum accompanying a cAMP-dependent cal cium release, from the organelle.