THE MECHANISM OF CHANGE IN RESISTANCE OF ERYTHROCYTES TO HYPOTONIC SALT SOLUTIONS

Ashby''s work on the effects of KCl and NaCl on the resistance to hypotonic hemolysis of K^•-rich and K^•-poor erythrocytes has been repeated with great attention to purity of materials and refinement of technique. The results fail to agree with those of Ashby. 1. KCl produces greater loss in resistance to hypotonic hemolysis than does NaCl, irrespective of the species of the animal from which the cells are taken. 2. While cases of an increase in resistance have been encountered in my experiments, they are either very slight, or else the particular determination is subject to very great uncertainty. The great increases in resistance found by Ashby are not even approached in any of the present series of experiments. 3. Ashby''s generalization that KCl and NaCl have opposite effects on red blood cells, and that the sense of these effects depends on whether the cell is K^•-rich or K^•-poor is not substantiated.     

New insights into the fundamental role of topological constraints as a determinant of two-way junction conformation

Recent studies have shown that topological constraints encoded at the RNA secondary structure level involving basic steric and stereochemical forces can significantly restrict the orientations sampled by helices across two-way RNA junctions. Here, we formulate these topological constraints in greater quantitative detail and use this topological framework to rationalize long-standing but poorly understood observations regarding the basic behavior of RNA two-way junctions. Notably, we show that the asymmetric nature of the A-form helix and the finite length of a bulge provide a physical basis for the experimentally observed directionality and bulge-length amplitude dependence of bulge induced inter-helical bends. We also find that the topologically allowed space can be modulated by variations in sequence, particularly with the addition of non-canonical GU base pairs at the junction, and, surprisingly, by the length of the 5' and 3' helices. A survey of two-way RNA junctions in the protein data bank confirms that junction residues have a strong preference to adopt looped-in, non-canonically base-paired conformations, providing a route for extending our bulge-directed framework to internal loop motifs and implying a simplified link between secondary and tertiary structure. Finally, our results uncover a new simple mechanism for coupling junction-induced topological constraints with tertiary interactions.     

Apparent heterogeneity in the response of quiescent swiss 3T3 cells to serum growth factors: implications for the transition probability model and parallels with "cellular senescence" and "competence"

When subconfluent, Swiss 3T3 cells made quiescent by serum deprivation are stimulated with low concentrations of serum (ca. 1%), only a proportion of them (roughly 50%) enter S phase despite daily replacement with fresh, low-serum medium. The cells that fail to enter S phase are not incapable of doing so, since most of them initiate DNA synthesis after transfer to 10% serum. It would appear that individual cells vary in their growth factor requirements. Using time-lapse cinemicroscopy a few of the cells that respond to low serum were seen to give rise to several generations of progeny, while the majority of cells failed to divide at all, or divided once at most. Despite this, differences between cells in growth factor requirements do not seem to be heritable in the long term, since attempts to enrich for responding cells by prolonged culture in 1% serum have been unsuccessful. Rather, it would appear that the capacity to respond to low serum is an unstable property lost after a few generations in low serum. The loss of responsiveness shows parallels with "cellular senescence" and could conceivably result from decay of the platelet-derived growth factor-induced state of "competence." But regardless of why some cells respond to low serum while others do not, it is clear that the kinetics of entry into S phase after serum stimulation of quiescent 3T3 cells are not strictly first-order, since the labelling index plateaus after roughly 3 days at values substantially below 100%. As such, the kinetics, though not contradicting the transition probability model, cannot be taken to support it as was previously thought.     

Neohaematotrephus arayae n. sp. (Digenea: Echinostomiformes: Cyclocoelidae) in Jacana spinosa (Aves: Charadriiformes: Jacanidae) from the Area de Conservación Guanacaste, Costa Rica

Specimens of a species of cyclocoelid digenean inhabiting Jacana spinosa from the Area de Conservación Guanacaste, Costa Rica, most closely resemble Haematotrephus facioi (Brenes and Arroyo, 1962) Yamaguti, 1971, in the same host from Aranjuez, Puntarenas Province, Costa Rica, in having confluent vitelline follicles posteriorly, diagnostic of Neohaematotrophus, and in pharynx length, ovary width, and cirrus sac on the sinistral side. The new species is also highly similar in appearance to H. gendrei Dubois, 1959, also inhabiting a jacanid (from West Africa), which has vitelline follicles confluent posteriorly, and extending anteriorly to the intestinal bifurcation and genital pore opening immediately posterior to the anterior margin of the pharynx. Like H. facioi, H. gendrei has a relatively much shorter and broader cirrus sac than does the new species. Examination of the holotype and paratype of H. facioi confirmed that the specimens from Guanacaste differ in having a longer body, a larger ovary and eggs, and smaller testes. They also have the ovary on the sinistral rather than the dextral side of the body, genital pore anterior to the pharynx rather than at or posterior to the level of the posterior margin of the pharynx, longer and thinner cirrus sac, and eggs without eyespotted miracidia. Half the eggs in both specimens of H. facioi have well-developed eyespotted miracidia, whereas the typical condition for cyclocoelids is for virtually all eggs to exhibit eyespotted miracidia. Both H. facioi and H. gendrei are transferred to Neohaematotrophus, along with the new species.