Metabolic consequences of positive inotropy in rat and guinea-pig hearts

The metabolic and physiological responses of hearts from male rats and guinea-pigs to isoprenaline and SK&F 94120 (a phosphodiesterase III inhibitor), have been studied. Doses which gave similar chronotropic stimulation gave different inotropic responses. In both species, isoprenaline generated a greater increase in developed tension than SK&F 94120. With both drugs the inotropic response in the rat was less than than in the guinea-pig. 31P-NMR investigation of high-energy phosphate levels showed reduction in PCr concentration and an accompanying acidosis in the isoprenaline-perfused rat heart only. In both species, lactate production was stimulated by SK&F 94120 but not by isoprenaline. These results are discussed with reference to G-protein activation of ion channels and differences in Ca2+ handling by the two species.     

Regulation of the phosphate (Pi) concentration in UMR 106 osteoblast-like cells: Effect of Pi,Na+ and K+

Osteoblast-like cells possess Na-dependent transporters which accumulate orthophosphate (Pi) from the extracellular medium. This may be important in bone formation. Here we describe parallel measurements of Pi uptake and cellular [Pi] in such cells from the rat (UMR 106–01 and UMR 106–06) and human (OB), and in non-osteoblastic human fibroblasts (Detroit 532 (DET)). In UMR 106–01, cellular [Pi] was weakly dependent on extracellular [Pi] and higher than expected from passive transport alone. [³²Pi]-uptake was inhibited by Na deprivation, but paradoxically increased on K deprivation. With Na, 87 per cent of cellular ³²P was found in organic phosphorus pools after only 5 min. Na deprivation also decreased cellular [Pi], in both UMR 106–01 and DET, but the decrease was smaller than that in [³²Pi]-uptake. Ouabain decreased [³²Pi]-uptake and cellular [Pi] in DET, but not in UMR 106–01. Regulation of cellular [Pi] is therefore at least partly dependent on Na/Pi co-transport, but this does not seem to be an exclusive property of osteoblasts.     

Digestion in the cattle-tick Boophilus microplus: light microscope study of the gut cells in nymphs and females

The gut caeca of B. microplus were studied by light microscopy using paraffin and methacrylate embedded material. It has been shown that during feeding of nymphs and adults, the midgut consists of five cell types, stem cell, digest cell, secretory cells (s₁) and (s₂) and basophilic cell. The stem cell differentiates into any of the other cell types. The digest cell matures through a series of stages and has up to three generations during feeding on the host. The final generation has two distinct cell types, the first type is thought to be capable of both phagocytosis and pinocytosis. Cells of the second type are predominant at the end of feeding, and may be specialized to ingest and digest haemoglobin. The final stage of the digest series is the spent digest cell which discharges its content into the gut lumen or is excreted whole. The basophilic cell has structures which suggest that one of its functions is to transport digested materials, water and ions across the gut. Secretory cell (s₁) secretes a glycoprotein which may be a haemolysin and secretory cell (s₂) secretes the gut “colloid” mass, an acid mucopolysaccharide, which may function as an anticoagulant. Intracellular digestion leads to the breakdown of host blood and storage of lipid and glycogen in the digest cells.     

Ameloblastic secretion and calcification of the enamel layer in shark teeth

Tooth primordia at early stages of mineralization in the sharks Negaprion brevirostris and Triaenodon obesus were examined electron microscopically for evidence of ameloblastic secretion and its relation to calcification of the enamel (enameloid) layer. Ameloblasts are polarized with most of the mitochondria and all of the Golgi dictyosomes localized in the infranuclear end of the cell toward the squamous outer cells of the enamel organ. Endoplasmic reticular membranes and ribosomes are also abundant in this region. Ameloblastic vesicles bud from the Golgi membranes and evidently move through perinuclear and supranuclear zones to accumulate at the apical end of the cell. The vesicles secrete their contents through the apical cell membrane in merocrine fashion and appear to contribute precursor material both for the basal lamina and the enameline matrix. The enamel layer consists of four zones: a juxta-laminar zone containing newly polymerized mineralizing fibrils (tubules); a pre-enamel zone of assembly of matrix constituents; palisadal zones of mineralizing fibrils (tubules); and interpalisadal zones containing granular amorphous matrix, fine unit fibrils, and giant cross-banded fibers with a periodicity of 17.9 nm. It seems probable that amorphous, non-mineralizing fibrillar and mineralizing fibrillar constituents of the matrix are all products of ameloblastic secretion. Odontoblastic processes are tightly embedded in the matrix of the palisadal zones and do not appear to be secretory at the stages investigated. The shark tooth enamel layer is considered homologous with that of other vertebrates with respect to origin of its mineralizing fibrils from the innerental epithelium. The term enameloid is appropriate to connote the histological distinction that the enamel layer contains odontoblastic processes but should not signify that shark tooth enamel is a modified type of dentine. How amelogenins and/or enamelins secreted by amelo- blasts in the shark and other vertebrates are related to nucleation and growth of enamel crystallites is still not known.     

Inhibition of energy-transducing reactions by 8-nitreno-ATP covalently bound to bovine heart submitochondrial particles: direct interaction between ATPase and redox enzymes

The photoaffinity label 8-azido-ATP has been used to study the effect of inhibition of ATP synthase on ATP-driven reverse electron transfer from succinate to NAD+ ('reversal'), succinate- and NADH-driven ATP synthesis and ATP-Pi exchange. In reversal, where ATPase functions as primary proton pump, inactivation by covalently bound nitreno-ATP results in an inhibition that is proportional to the inactivation of ATP hydrolysis, or, consequently, with the concentration of inactivated ATP synthases. Up to 60% inactivation of the reversal rate does not lead to a decrease in delta mu H+. Inhibition of ATP synthase as secondary proton pump results in case of NADH-driven ATP synthesis in a proportional inhibition, but with succinate as substrate ATP synthesis is less than proportionally inhibited, compared with inactivation of ATP hydrolysis. Inhibition of one of the primary pumps of NADH-driven ATP synthesis, the NADH:Q oxidoreductase, with rotenone also resulted in an inhibition of the rate of ATP synthesis proportional to that of the NADH oxidation. ATP-Pi exchange is much more affected than ATP hydrolysis by photoinactivation with 8-azido-ATP. Contrary to reversal and NADH-driven ATP synthesis the rate of ATP-Pi exchange does not depend linearly, but quadratically on the concentration of active ATP synthases. The observed proportional relationships between inhibition of the primary or secondary pump and the inhibition of the overall energy-transfer reactions do not support the existence of a pool intermediate in energy-transduction reactions. However, the results are consistent with a direct transfer of energy from redox enzymes to ATP synthase and vice versa.     

Structure study of osteostatin PTHrP[Thr107](107-139).

The structure of chicken osteostatin or parathyroid hormone-related protein (PTHrP) (residues 107-139) containing an Ala/Thr substitution at the N-terminus was studied using two-dimensional proton NMR spectroscopy in an aqueous environment. Osteostatin is a separate circulating domain responsible for a range of activities related to the modulation of bone formation as well as keratinocyte proliferation. Anti-mitogenic properties of osteostatin have been detected in breast cancer cells and cytosolic calcium is used by osteostatin to signal in some neurons through a non-PTH receptor, unlike the separate circulating N-terminal domain. A structural basis for the activity is presented with particular emphasis given to the conformation of the bioactive segment 107-111, forming part of a finger-like projection capable of binding to the non-PTH receptor both in the presence and absence of the remainder of the molecule which appears simply to act as a largely globular carrier.