Fusion of neurohypophyseal membranes in vitro

Freeze cleaving electron microscopy has shown that fusion of isolated secretory vesicles from bovine neurohypophyses was induced by Ca²⁺ in micromolar concentrations. Mg²⁺ and Sr²⁺ were ineffective. Mg²⁺ inhibited Ca²⁺-induced fusion.In suspensions containing secretory vesicles as well as sheets of cell membrane, release of vasopressin parallel to intervesicular fusion of secretory vesicles with sheets of cell membrane was observed after exposure to Ca²⁺. Mg²⁺ and Sr²⁺ were ineffective in replacing Ca²⁺ as trigger for fusion or vasopressin release.Intervesicular fusion and exocytotic profiles were observed when isolated neurohypophyses or neurosecretosome were exposed to cold.     

Crassulacean acid metabolism photosynthesis: `working the night shift'

Crassulacean acid metabolism (CAM) can be traced from Roman times through persons who noted a morning acid taste of some common house plants. From India in 1815, Benjamin-Heyne described a `daily acid taste cycle' with some succulent garden plants. Recent work has shown that the nocturnally formed acid is decarboxylated during the day to become the CO₂ for photosynthesis. Thus, CAM photosynthesis extends over a 24-hour day using several daily interlocking cycles. To understand CAM photosynthesis, several landmark discoveries were made at the following times: daily reciprocal acid and carbohydrate cycles were found during 1870 to 1887; their precise identification, as malic acid and starch, and accurate quantification occurred from 1940 to 1954; diffusive gas resistance methods were introduced in the early 1960s that led to understanding the powerful stomatal control of daily gas exchanges; C₄ photosynthesis in two different types of cells was discovered from 1965 to ∼1974 and the resultant information was used to elucidate the day and night portions of CAM photosynthesis in one cell; and exceptionally high internal green tissue CO₂ levels, 0.2 to 2.5%, upon the daytime decarboxylation of malic acid, were discovered in 1979. These discoveries then were combined with related information from C₃ and C₄ photosynthesis, carbon biochemistry, cellular anatomy, and ecological physiology. Therefore by ∼1980, CAM photosynthesis finally was rigorously outlined. In a nutshell, 24-hour CAM occurs by phosphoenol pyruvate (PEP) carboxylase fixing CO₂(HCO₃ ⁻) over the night to form malic acid that is stored in plant cell vacuoles. While stomata are tightly closed the following day, malic acid is decarboxylated releasing CO₂ for C₃ photosynthesis via ribulose bisphosphate carboxylase oxygenase (Rubisco). The CO₂ acceptor, PEP, is formed via glycolysis at night from starch or other stored carbohydrates and after decarboxylation the three carbons are restored each day. In mid to late afternoon the stomata can open and mostly C₃ photosynthesis occurs until darkness. CAM photo-synthesis can be both inducible and constitutive and is known in 33 families with an estimated 15 to 20 000 species. CAM plants express the most plastic and tenacious photosynthesis known in that they can switch photosynthesis pathways and they can live and conduct photosynthesis for years even in the virtual absence of external H₂O and CO₂, i.e., CAM tenaciously protects its photosynthesis from both H₂O and CO₂ stresses. This revised version was published online in August 2006 with corrections to the Cover Date.