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The movement and the distribution of water, sodium, potassium, chloride, magnesium and calcium have been studied in slices prepared from rabbit placentae at different periods of gestation, incubated in condition of depressed metabolism (0-1 degree C). In these conditions the tissue takes up water from the external medium up to a maximum of about 2.0 kg/kg d. wt., which represents 30 per cent of the initial H2O content of the fresh tissue. The extracellular compartment swells progressively and proportionally to the age of the placenta. The sodium and chloride content of the tissue increases while that of potassium decreases and their intracellular concentration reaches, after 120 min, that of the external medium. Magnesium does not show appreciable changes and calcium, too, despite its extreme variability, does not seem to undergo significant variations during the cold incubation. The results obtained show that placenta, like most of other mammalian cell systems, possesses specific metabolism-dependent mechanisms responsible for the maintenance of water distribution and ion gradients among the different tissue compartments. The characteristics and the regulation of these mechanisms are discussed in detail.

The choice of a cardiac prosthesis for mitral valve replacement remains controversial, and thromboembolic complications are still a major cause of morbidity and mortality in patients with mechanical valves. Because of this, permanent anticoagulation with its risks and constraints on daily life is necessary. Bioprostheses, however, are associated with a lower rate of thromboembolic events. Therefore, the need for long‐term anticoagulation is minimized. These advantages are counterbalanced by the limited durability of tissue valves.1 In an effort to give some perspective to this balance, we compared the long‐term results of three commonly used mitral valve prostheses. Three hundred patients operated on in the same institution January 1974 to December 1978 form the basis of this evaluation.

Rabbit placenta slices incubated at 38 degrees C in oxygenated medium, in the presence of endogenous substrates +5 mM glucose, after a period of preincubation of 120 min at 0--1 degrees C, show a net accumulation of potassium and extrusion of water, sodium and calcium. The movement of potassium, sodium and calcium appears to be related to the rate of oxygen consumption of the slices and to be inversely proportional to the age of the tissue. The efficiency of cations movement is also dependent on the nature and composition of the incubation medium. In this respect the potassium concentration of the medium plays a major role. The movement of water, on the other hand, seems to be completely independent from both the rate of respiration and the age of the tissue. The results are discussed in terms of relation between placenta age and senescence, and efficiency of the mechanisms devoted to the cell regulation of water and ions content.