Showing papers by "Mount Desert Island Biological Laboratory published in 1996"

Cytokinesis in Animal Cells

Abstract: The organizers of this meeting probably placed cytokinesis first because it has long been an object of curiosity, speculation and investigation and because it appears to be a relatively simple event. It has been studied for so long because it is obviously important in the life of cells; and, for an activity that is controlled and carried out within the resources of a single cell, it is comparatively easy to see. The observer of cytokinesis in a large, relatively transparent cell like a cleaving sea urchin egg may think he has seen all the important aspects of the process, but appearances are deceptive. He can see the overall cell dimensions, the general form of the mitotic apparatus and the distance between the astral centers, the deformation of the surface contour that comprises division and (with the aid of natural or introduced particles) local changes in surface behavior. He cannot see (but may think he can) the location and nature of the constraints and forces that reshape the cell, the outermost limits of the mitotic apparatus and the location of regions where the action of the mitotic apparatus upon the surface is most intense. Whether cytokinesis can serve as a useful example for analysis of other kinds of cell deformation and shape change is not now clear. It seems evident that all shape changes have similar requirements but they may not be carried out by the same processes. Shape change requires that a forceproducing mechanism be created and used. The cell must put the mechanism in the right place and in the right orientation, and it must activate and deactivate it at the right times.

Regulatory phosphorylation of the secretory Na-K-Cl cotransporter: modulation by cytoplasmic Cl

Abstract: The effect of cytoplasmic Cl concentration ([Cl]i) on the activation state ([3H]benzmetanide binding rate) and phosphorylation state (32P incorporation) of the Na-K-Cl cotransporter was evaluated i...

Swelling-activated anion conductance in skate hepatocytes: regulation by cell Cl- and ATP

Abstract: Cell swelling activates an outwardly rectifying anion conductance in mammalian cells. The channel responsible for this conductance mediates volume-regulatory efflux of organic osmolytes such as tau...

Characterization and function of ATP receptors on hepatocytes from the little skate Raja erinacea.

Abstract: Hormonal regulation of hepatocytes, via cytosolic Ca2+ signaling, is well established in higher life forms but has not been investigated in elasmobranchs. We therefore examined Ca2+ signaling in hepatocytes isolated from the little skate, Raja erinacea. In hepatocyte populations, ATP induced a rapid, biphasic increase in Ca2+, as it does in mammalian hepatocytes. Other hormones that act on mammalian hepatocytes, such as vasopressin, angiotensin, and phenylephrine, induced no such Ca2+ increase. The initial phase of the ATP-induced Ca2+ increase was seen even in Ca(2+)-free medium, whereas the late sustained phase of the increase was not. Similar dose-response curves were obtained by stimulation with ATP, ADP, UTP, and 2-methylthio-ATP. In contrast, AMP, adenosine, beta, gamma-methyl-ATP, CTP, and GTP induced little or no Ca2+ increase. In single hepatocytes, ATP, ADP, UTP, and 2-methylthio-ATP each induced a sustained increase in Ca2+ at high concentrations, but at low concentrations induced Ca2+ oscillations. A maximal concentration of ATP (100 microM) caused a marked, transient increase in bile flow in the isolated perfused skate liver, whereas 100 microM adenosine had no such effect. These findings demonstrate that skate hepatocytes possess P2 nucleotide receptors that link to intracellular plus extracellular Ca2+ mobilization, which in turn regulates bile secretion. The broad specificity of the response to ATP and related compounds suggests either that multiple types of P2 receptors are expressed by skate hepatocytes or else that these cells possess a single primitive nucleotide receptor from which other P2 subtypes subsequently evolved.

Erythropoietin stimulates tyrosine phosphorylation and taurine transport in skate erythrocytes

Abstract: Taurine, a β amino acid, is a primary osmolyte in nucleated skate erythrocytes and is involved in the regulation of cell volume. Growth factors may be involved in the regulation of cell volume which occurs during cell division. Erythropoietin (EPO) is the primary growth factor controlling erythropoiesis. To investigate its mechanism of action, we used nucleated skate erythrocytes. EPO stimulates Na+-independent uptake of taurine in a concentration-dependent manner. The uptake was inhibited by the tyrosine kinase inhibitor genistein. Concomitantly, EPO stimulates tyrosine phosphorylation of a number of proteins, particularly ones of molecular masses 145, 120, 100, 80, 65, and 35 kDa. Using specific antibodies, the 145 kDa protein is identified as phospholipase C γ-1 (PLC γ-1) and the 100 kDa protein as the skate homolog of the anion exchanger band 3. Since PLC γ-1 is activated, turnover of membrane lipids was determined. EPO increased 1,2-diacylglycerol formation from phosphatidylinositols (phosphatidylinositol-4-monophosphate and 4,5-biphosphate) during an early phase and later preferentially from phosphatidylcholine. The early hydrolysis of phosphoinositides was confirmed measuring generation of inositol-1,4,5-trisphosphate, demonstrating an activation of PLC γ-1 activity. To determine if phospholipase D (PLD) stimulation also occurred, ethanol was included in the reactions. Phosphatidylethanol, synthesized by PLD-mediated transphosphatidylation, appeared at times longer than 5 min, suggesting delayed activation of PLD. These results demonstrate that EPO, via simulation of tyrosine phosphorylation, stimulates taurine transport in skate erythrocytes. © 1996 Wiley-Liss, Inc.

Electromyogram as a Measure of Heavy Metal Toxicity in Fresh Water and Salt Water Mussels

Abstract: The response of bivalves to heavy metals and other toxins has usually been determined by observing valve position (e.g., Davenport and Manley 1958; Borcherding 1992). Since mussels close their valves to avoid noxious stimuli, experimental delivery of chemicals is uncertain. To obtain constant results, Preston (1994 and personal communication) employed plastic spacers to hold the valves apart. This obviates the observation of valve position as an index of response, and some other method is required. Electromyography of intact mussels is one such index, and is shown to be a simple, effective and quantitative measurement of activity. Experiments are reported on the effects of added mercury on salt water and fresh water species. Parts of this work have appeared in brief form (Kidder and McCoy 1995).