Endothelin-1 enhances responses to sympathetic nerve stimulation in endothelium-denuded and endothelium-intact rabbit ear arteries

In the rabbit isolated and endothelium-denuded ear artery, endothelin-1 (1–10 nM) elicited concentration-dependent vasoconstrictor responses. Lower concentrations of endothelin-1 (0.1, 0.3 and 1 nM) with little or no direct vasoconstrictor action significantly enhanced responses to sympathetic nerve stimulation in both endothelium-denuded and endothelium-intact arteries. The vasoconstrictor action of endothelin-1 and its enhancing effect on stimulation-induced responses were significantly decreased by the presence of the dihydropyridine-type calcium channel antagonist nicardipine (10 nM). The enhancing effect of low concentrations of endothelin-1 on responses to sympathetic nerve stimulation may play a role in the regulation of vascular tone.     

Variation in hydration forces between neutral phospholipid bilayers: evidence for hydration attraction

It is now generally recognized that hydration forces dominate close interactions of lipid hydrophilic surfaces. The commonality of their characteristics has been reasonably established. However, differences in measured net repulsion, particularly evident when phosphatidylethanolamine (PE) and phosphatidylcholine (PC) bilayers are compared, suggest there exists a variety of behavior wider than expected from earlier models of hydration and fluctuation repulsion balanced by van der Waals attraction. To find a basis for this diverse behavior, we have looked more closely at measured structural parameters, degrees of hydration, and interbilayer repulsive forces for the lamellar phases of the following lipids: 1-palmitoyl-2-oleoyl-PE (POPE), egg PE, transphosphatidylated egg PE (egg PE-T), mono- and dimethylated egg PE-T (MMPE and DMPE), 1-stearoyl-2-oleoyl-PC (SOPC), and mixtures of POPE and SOPC. POPE and SOPC bilayers differ not only in their maximum degrees of hydration but also in the empirical hydration force coefficients and decay lengths that characterize their interaction. When mixed with POPE, SOPC effects sudden and disproportionate increases in hydration. POPE, egg PE, and egg PE-T differ in their degree of hydration, molecular area, and hydration repulsion. A single methylation of egg PE-T almost completely converts its hydration and bilayer repulsive properties to those of egg PC; little progression of hydration is seen with successive methylations. In order to reconcile these observations with the conventional scheme of balancing interbilayer hydration and fluctuation-enhanced repulsion with van der Waals attraction, it is necessary to relinquish the fundamental idea that the decay of hydration forces is a constant determined by the properties of the aqueous medium. Alternatively, one can retain that fundamental idea if one recognizes the possibility that polar group hydration has an attractive component to it. In the latter view, that attractive component originates from interbilayer hydrogen-bonded water bridges between apposing bilayer surfaces, arising from correlation of zwitterionic or other complementary polar groups or from factors that affect polar group solubility. The same Marcelja and Radic formalism that accounts so well for the repulsive component also leads to an estimate of the attractive one. We suggest that the full range of degrees of hydration and of interbilayer spacings observed for different neutral bilayers results in part from variable contributions of the attractive and repulsive hydration components.(ABSTRACT TRUNCATED AT 400 WORDS)     

Modification by diacylglycerol of the structure and interaction of various phospholipid bilayer membranes

The effects of incorporating diacylglycerol (DG) derived from egg phosphatidylcholine (PC) into PC, egg phosphatidylethanolamine (PE), and bovine phosphatidylserine (PS) have been measured. In excess solution DG induces a multilamellar-to-hexagonal (L-H) structural transition in PE and PC that is temperature dependent. At 37 degrees C it begins at about 3 and 30 mol%, respectively. In PC at lower DG concentrations a modified lamellar phase is formed; at about 70 mol% DG a single primitive cubic phase forms. An L-H transition induced by 20-30 mol% DG in PS is dependent on ionic strength and degree of lipid hydration, with the appearance of crystalline acyl chains at the higher DG levels. Calcium precipitates of DG/PS (1/1) mixtures have melted chains. Structural parameters were derived for the lamellar phases at subtransition levels of DG in PE and PC. The area per polar group is increased, but by contrast with cholesterol, the polar group spreading is not accompanied by an increase in bilayer thickness. DG does not affect the equilibrium separation of PC or PE bilayers. Measured interbilayer forces as they vary with bilayer separation show that DG at 20 mol% does not effect closer apposition of PC bilayers at any separation. Spreading the polar groups may effect the binding of protein kinase C or the activation of phospholipases; the nonlamellar phases may be linked to the biochemical production of DG in cellular processes involving membrane fusion.     

Dynamic morphology of calcium-induced interactions between phosphatidylserine vesicles.

Structural changes in phosphatidylserine vesicles exposed to calcium chloride for various times have been observed by means of video-enhanced light microscopy and freeze-fracture electron microscopy. Large flat double-bilayer diaphragms form at the contacts between aggregated vesicles within milliseconds. Bilayers at and outside of diaphragms rupture and allow vesicles to collapse completely by flattening against each other within seconds. Collapse through intermediate states to a stable multilamellar phase is complete within minutes. The Ca-induced attraction energy and the resultant flattening at contacts between vesicles is far beyond that needed to stress bilayers to the point of rupture. Although the destabilizing response to this stress is preferential to the diaphragm region, 40% of adhering pairs rupture outside of the diaphragm region rather than fuse with each other. In this respect the mechanism of fusion between these vesicles may be fundamentally different from the controlled fusion process in cells.     

Properties and Partial Purification of Choline Acetyltransferase from the Nematode Caenorhabditis elegans

We have stabilized and studied choline acetyltransferase from the nematode Caenorhabditis elegans. The enzyme is soluble, and two discrete forms were resolved by gel filtration. The larger of these two forms (MW approximately 154,000) was somewhat unstable and in the presence of 0.5 M NaI was converted to a form indistinguishable from the "native" small form (MW approximately 71,000). We have purified the small form of the enzyme greater than 3,300-fold by a combination of gel filtration, ion-exchange chromatography, and nucleotide affinity chromatography. The purified preparation has a measured specific activity of 3.74 mumol/min/mg protein, and is free of acetylcholinesterase and acetyl-CoA hydrolase activities. The Vmax of the purified enzyme is stimulated by NaCl, with half-maximal stimulation at 80 mM NaCl. The Km for each substrate is also affected by salt, but in different manners from each other and the Vmax; the kinetic parameter Vmax/Km thus changes significantly as a function of the salt concentration.     

Eye movements and the enhancement of edges

A mathematical examination of retinal photochemistry leads to a hypothesis for Mach band phenomena based on eye movements. This retinal model suggests why minimally distinct borders fade under eye fixation and agrees qualitatively with subjective measures of border contrast as a function of overall field luminance.Deceased