348 Specialia EXPERIENTIA 31/3 Na+7. We found in this study that 15 mM CaCI~ did not significantly increase the activity of either the ValS-A II or IleS-A II (Table II) Previous caution that our results could be influenced by the intrinsic activity of Ca++ in modifying the bp activity appears to be unfounded, at least at the CaC12 concentration used. Moreover, the fact that CaC12 enhanced the bp activity of ASh 1, VaP-A II but failed to significantly enhance the bp activity of the ValS-A II and IIeS-A II, is further evidence against the possibility that the peptides reach the vascular receptors in a 'bolus', not unlike the results of an earlier investiga- tion 7. This investigation shows that the degree of enhance- ment of activity of a given angiotensin II peptide varies with the ion present. In addition it is apparent that the activity of the naturally occuring angiotensins is affected by a given ion in a different manner than the amide derivative. These results lend further support to the hy- pothesis of a selective interaction of each ion with the hormone or its analogs leading to an alteration of the equilibrium of the preferred conformation which is.de- tected by changes in the bp activity 5, s. Zusammen/assung. Nachweis, dass die Blutdruck- aktivit~t yon Val 5-, Ile ~- und Asni,VaP-Angiotensin II durch NaC1 gesteigert wird. Die AktivitS.t der natfirlichen Hormone wird jedoch yon CaC12 nicht signifikant erhSht, obwohl das Ion die AktivitXt yon Asnl,ValS-Angiotensin II stark vermehrt. G. SCHAECHTELIN, DASHA SUROVEC and 1~. WALTER 9 Department ol Physiology and Biophysics, Mount Sinai School o I Medicine o/ the City University oI New York, Filth Avenue and 700th Street, New York (N.Y. 10029, USA), 12November 1974. s H. GLOSSMANN, A. BAUKAL and IK. J. CATT, Science 185, 281 (1974). 9 Present address: Department of Physiology and Biophysics, University of Illinois Medical Center, P.O. Box 6998, Chicago, Illinois 60680, USA. Morphological Aspects of the Functional Synchronization of Supraoptic Nucleus Neurons It is well-known that the neurons of the supraoptic nucleus are involved in the synthesis, transport and libera- tion of the neurohormones ADH and oxitocinl, ~. The secretory activity of the hypothalamic-neurohipophyseal system in vertebrates has been studied in both normal and experimental conditions, the latter producing hypo- and hyperfunctionK To evaluate the functional state of the neurosecretory neurons, the nuclear and nucleolar volumes and the development of Nissl substance were used as parameters by ZAMBRANO and MORDOH 4. They demonstrated periods of asynchronic secretory activity in neurosecretory neurons of normal rats, during which different zones in the supraoptic nucleus could vary their functional state, although the degree of activity within each zone was maintained constant. This functional zonal asynchrony is maintained in animals stimulated by dehydration, although a certain degree of synchronization of all neurosecretory neurons during rehydration can be observed ~, ~. In our study we wish to analyze some ultra- structural aspects of the neurosecretory neurons in the supraoptic nucleus, suggesting the existence of functional coordination and synchronization processes among these neurons. Materials and methods. The ultrastructure of hypo- thalamic supraoptic nucleus in both sexes of Wistar rats was studied. The rats were fixed by perfusion of 3% glutaraldehyde and the hypothalamic blocks were post- fixed in 2% osmium tetroxide. Both fixatives were buffered at pH 7.4 in 0.12 M phosphate buffer. The hypothalamic blocks were embedded in Durcupan (Fluka) as usual and stained with 1% aqueous uranyl- acetate and then lead citrate. Results. The present ultra-structural study of normal rat supraoptic nucleus neurons is in accord with studies previously described ~, s. Briefly, the neurons have a large nucleus with a prominent nucleolus. The cytoplasm presents 2 well-defined regions: a perinuclear region with a very well developed Golgi apparatus, containing lysosomes and neurosecretory vesicles; and a marginal cytoplasm, very rich in free ribosomes and cisterns of granular endo- plasmic reticulum. Different functional states of the neurons of the supra- optic nucleus are reflected by the variations seen in the Fig. 1. Wide zone of apposition without interposition of a glial barrier in 2 neurosecretory neuron somas. Numerous granular endoplasmie reticulum cisterns are seen in the marginal cytoplasm of these neurons. • 12.000. 1 W. BARGMANN, Z. Zellforseh. 34, 610 (1949). 2 ]~. SCHARRER and B. SCHARRER, Recent Progr. Horm. Res. 707, 183 (1954). R. ORTMANN,Am. Physiological Soe., Washington 1960, p. 1039. D. ZAMBRANO and J. MORDOH, Z. Zellforsch. 73, 405 (1966). 5 D. PICARD, ~V[. MICHEL-BECHET, A. M. ATHOUEL and S. RUA, Expl. Brain Res. 74, 331 (1972). 6 C. PILGRIM, Ergebn. Anat. EntwGesch. 47, i (1969). 7 L. RECHARDT, Aeta physiol, seand. Suppl. 329, 1 (1969). 8 D. ZAMBRANO and E. DE ROBERTIS, Z. Zellforsch. 73, 414 (1966).