Relationship between internal calcium and outward current in mammalian ventricular muscle; a mechanism for the control of the action potential duration?

In sheep and calf ventricular bundles, increasing the internal calcium by increasing the frequency of voltage-clamping to plateau range potentials increased the time-independent outward current. This effect was more marked with higher [Ca]o, and was reduced if the Ca current blockers Verapamil or D 600 were used. 2. If the internal Ca was increased by the addition of cyanide and reduction of external sodium the outward current was also increased. The frequency-dependent increase in outward current also occurred in this Na-poor (12 mM) solution. 3. Tension measurement on the ventricular bundles showed that a Na-free solution with cyanide did not cause a contracture. On changing from Tyrode to a Na-free solution containing cyanide, and on changing back to Tyrode there was a potentiation of the twitch. 4. In Na-poor solution with cyanide, although no contracture was found, ECa was less positive, suggesting that under these circumstances Ca accumulates at the inner side of the membrane, but not around the myofibrils. 5. The prolongation of the action potential in Cl-free solution is frequency-dependent. A greater prolongation is seen at lower frequencies suggesting that Cl current is relatively more important for repolarization at lower frequencies of stimulation. 6. It is suggested that calcium at the inner side of the membrane sets the level of the background outward current. A feed-back mechanism on this basis is proposed for the control of the action potential duration. Various factors that could influence this basic mechanism are discussed.     

Comparison of ras-p21 bound to GDP and GTP: differences in protein and ligand dynamics

This paper documents the first essential dynamics analysis of ras protein ligands and of the protein itself, showing important features of their dynamic properties. Essential dynamics analysis of 300 ps of full solvent molecular dynamics simulations revealed differences in structure and dynamics between GDP- and GTP-bound forms of H-ras-p21. Regions in the protein which exhibited a structural shift correspond to the switch regions described previously. Differences in dynamics between H-ras-p21 GDP and H-ras-p21 GTP may be related to interactions of ras with GAP and its receptor and effector. Molecular dynamics of free GDP (in the absence of protein) were performed in water for 2 ns and analysed using essential dynamics. The conformations of GDP and GTP when bound to the protein were compared with free GDP, revealing that the ligands bind to the protein in an energetically unfavourable conformation. GDP and GTP molecules from various other protein crystal structures were also analysed. These ligands adopt similar conformations to those seen in H-ras-p21.      

Induction of rabbit renal mixed-function oxidases by phenobarbital: cell specific ultrastructural changes in the proximal tubule

Administration of phenobarbital (60 mg/kg) daily for 4 days to male rabbits resulted in induction of renal cytochrome P-450 (3.5-fold) and a corresponding increase in ethoxycoumarin-O-deethylase and benzphetamine-N-demethylase activity (17- and 4-fold, respectively). Kidney weight to body weight ratio and renal ethoxyresorufin-O-deethylase were not affected by phenobarbital pretreatment. Numerous focal areas of proliferation of smooth endoplasmic reticulum (SER) were evident in proximal tubule cells from phenobarbital treated rabbits while proximal tubular cells from control rabbits had only small and sparcely located aggregates of SER. Phenobarbital-induced SER proliferation was specifically localized to the S3 segment of the proximal tubule. Proliferation was not observed in S2 cells of the proximal tubule, cells of Henle's loop, distal tubules, or collecting tubules in rabbits pretreated with phenobarbital. These data demonstrate the biochemical heterogeneity of cell types within the proximal tubules of rabbits. Furthermore, induction of mixed-function oxidases specifically in S3 cells of the proximal tubule may be of toxicological significance in the metabolic activation of certain nephrotoxicants.     

Isolation, structure elucidation, and biological activity of the steroid oligoglycosides and polyhydroxysteroids from the Antarctic starfish Acodontaster conspicuus

A total of 19 steroids, of which 13 steroidal oligoglycosides (nine new and four known) and six polyhydroxylated steroids (four new and two known), has been isolated from the Antarctic starfish Acodontaster conspicuus. The mixture is dominated by glycosides composed of steroidal aglycons having the hydroxyl groups typically disposed on one side of the tetracyclic nucleus, i.e., 3 beta,4 beta,6 alpha,8,15 beta-, with some having a sulfate at C-6, and differing in the side chains and/or in the disaccharide moieties that are usually attached at C-26, with some at C-28 and C-29. Those compounds are accompanied by minute amounts of glycosides with a delta 8(14)-double bond in the steroid, which is a structural feature not previously found among polyhydroxysteroids derived from starfish. Small amounts of six related unglycosidated polyhydroxysteroids and three higher-molecular-weight asterosaponins complete the composition of the mixture. The structures of the new compounds were determined by interpretation of their spectral data and by comparison with spectral data of known compounds. Eighteen of these compounds were evaluated for their ability to inhibit growth in Antarctic marine bacteria isolated from either the water column or the surfaces of benthic marine invertebrates. Of these compounds, 50% were active against at least one Antarctic marine bacterium. This suggests that these compounds may play an important role in deterring microbial fouling.