Calcium-stimulated myofibrillar ATPase activity correlates with shortening velocity of muscle fibres in Xenopus laevis

Summary The iliofibularis muscle ofXenopus laevis is reported to contain five types of fibres which have different force—velocity relationships. Ten fibres of each type were selected on the basis of succinate dehydrogenase activity, cross-sectional area and location in the muscle, in order to assess the validity of the fibre type classification.Maximum calcium-stimulated myofibrillar ATPase activity (V _max) and apparent Michaelis constant (K _m) for ATP were determined for these 50 fibres from serial sections. The values obtained varied according to the type of fibre. Type 1 had the highest and type 5 the lowest values forK _m andV _max.In a separate experiment, single freeze-dried fibres were used to determine the relationship between their ATP content and apparentK _m for ATP. There was a tendency for high ATP concentrations in fibres with highK _m values.When myofibrillar ATPase activity was related to the maximum velocity of shortening of the five fibre types, a significant correlation was found. It is concluded that calcium-stimulated myofibrillar ATPase histochemistry allows an estimate of the maximum shortening velocity of muscle fibres fromXenopus laevis.     

N-acetyl-sphingenine-1-phosphate is a potent calcium mobilizing agent.

Calcium mobilization induced by phosphorylated sphingoid bases was analyzed in calf pulmonary artery endothelial cells by confocal microscopy. A sphingenine-1-phosphate (SeP) analogue, N-acetyl-sphingenine-1-phosphate (N-C2-SeP), exogenously added to these cells, caused a fast and transient intracellular rise in calcium and was as potent as SeP. A minimal concentration of 0.6 nM for N-C2-SeP versus 1 nM for SeP was determined. The N-C2-SeP-induced Ca2+-signaling, like the response to SeP, was due to a release from thapsigargin-sensitive, ryanodine-insensitive, intracellular Ca2+-stores and not to a Ca2+-influx. N-C2-SeP can be considered as a truncated ceramide-phosphate, a lipid already reported to be mitogenic (Gomez-Munoz, A., Duffy, P.A., Martin, A., O'Brien, L., Byun, H.S., Bittman, R. and Brindley, D.N. (1995) Mol. Pharmacol. 47, 833-839), an effect that might be secondary to Ca2+-mobilization.     

Study on the enzymatic 5′-deiodination of 3′,5′-diiodothyronine using a radioimmunoassay for 3′-iodothyronine

A radioimmunoassay for 3′-iodothyronine has been developed. All iodothyronine analogues (except 3,3′-diiodothyronine) showed very little (0.02% at most) cross-reactivity, and the assay was sensitive to 1 pg 3′-iodothyronine/ tube. We have studied the 5′-deiodination of 3′,5′-diiodothyronine by rat liver microsomal fraction in the presence of dithiothreitol. Production of 3′-iodothyronine at 37°C was found to be linear with time of incubation up to 30 min and with concentration of microsomal protein up to 100 μg/ml. The reaction rate reached a limit on increasing 3′,5′-diiodothyronine concentration to 10 μM. The effect of pH on 3′-iodothyronine production was found to depend on 3′,5′-diiodothyronine concentration. Increasing 3′,5′-diiodothyronine concentration from 0.1 to 10 μM resulted in a shift of the pH optimum from 6–6.5 to 7.5. Similar effects on the 5′-deiodination of 3,3′,5′-triiodothyronine were observed, supporting the hypothesis that these reactions are catalysed by a single enzyme (iodothyronine 5′-deiodinase).     

Presymptomatic visualization of plant-virus interactions by thermography.

Salicylic acid (SA), produced by plants as a signal in defense against pathogens, induces metabolic heating mediated by alternative respiration in flowers of thermogenic plants, and, when exogenously applied, increases leaf temperature in nonthermogenic plants. We have postulated that the latter phenomenon would be detectable when SA is synthesized locally in plant leaves. Here, resistance to tobacco mosaic virus (TMV) was monitored thermographically before any disease symptoms became visible on tobacco leaves. Spots of elevated temperature that were confined to the place of infection increased in intensity from 8 h before the onset of visible cell death, and remained detectable as a halo around the ongoing necrosis. Salicylic acid accumulates during the prenecrotic phase in TMV-infected tobacco and is known to induce stomatal closure in certain species. We show that the time course of SA accumulation correlates with the evolution of both localized thermal effect and stomatal closure. Since the contribution of leaf respiration is marginal, we concluded that the thermal effect results predominantly from localized, SA-induced stomatal closure. The presymptomatic temperature increase could be of general significance in incompatible plant-pathogen interactions.     

Two-dimensional crystallization of bovine rhodopsin

Bovine rhodopsin has been clustered into two-dimensional crystals in highly purified native rod disk membranes and studied with negative staining and transmission electron microscopy. The lattice is P2(1) with dimensions of 8.3 X 7.9 nm and interaxis angles of 86 +/- 3 degrees. 110 images of ordered areas were digitized and aligned with computer-correlation methods to calculate an average image with diffraction to the fourth order. The images were computer-filtered and reconstructed to approx. 2 nm resolution. When crystals appeared they covered 20-40% of the surface of the preparation and, since rhodopsin is at least 95% of the protein, there is no doubt that the crystals were due to rhodopsin. There appear to be two rhodopsin dimers per unit cell. Each rhodopsin molecules takes up about 7.5 nm2 of membrane area and is estimated to be associated with about 12 lipids on each side of the membrane. The membrane area found for bovine rhodopsin supports the rhodopsin origin of rarely seen but more highly ordered two-dimensional crystals found in detergent-treated frog rod membranes (Corless, J.M., McCaslin, D.R. and Scott, B.L. (1982) Proc. Natl. Acad. Sci. USA 79, 1116-1120). Furthermore, the rhodopsin membrane area is close to that of bacteriorhodopsin and is consistent with a seven transmembrane helix structure proposed for rhodopsin (for references see Dratz, E.A. and Hargrave, D.A. (1983) Trends Biochem. Sci. 8, 128-131). Crystallization was accomplished by lowering the pH to 5.5 near the isoelectric point of rhodopsin, raising the salt concentration of 2 M (NH4)2SO4, adding 5% glucose and 0.02% Hibitane (Ayerst), a cationic amphipathic antiseptic that favored crystal growth.