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The response of an intertidal bivalve heart to hypo-osmotic and ionic stress at high pressure

Authors:
  • Clarke Life Support Consulting

Abstract

1.1. Recovery of beating in ventricles of the bivalves Modiolus demissus demissus (Mdd) and Modiolus demissus granosissimus (Mdg), arrested by hypoosmotic seawater, is accelerated by 340 atm.2.2. Pressure does not accelerate recovery of rhythmicity by accelerating the volume regulatory amino acid efflux.3.3. Pressure greater than 200 atm accelerates onset of quiescence in Mdd exposed to Ca-Mg-free artificial seawater.4.4. Tachycardia, induced by divalent ion-free seawater at 1 atm, is suppressed by 340 atm.
Article
The structure of the heart of Geukensia demissa, a common object of physiological and biochemical investigation, is described by scanning, transmission and freeze-fracture electron microscopy. A single-cell epithelial layer covers the ventricle, but an endothelium is lacking. Myofibers are small (6–7 μm diam.), mononucleate, and tapered. Glycogen is concentrated peripherally. Mitochondria are particularly concentrated under the sarcolemma, near the ends of the nucleus, and in rows between bundles of myofilaments. The myofilaments (6–8nm thin, 30–35 nm thick filament diam.) are loosely arranged into sarcomeres (2–4 μm) by Z bodies. Many of these Z bodies interconnect, and some anchor to the sarcolemma forming attachment plaques. Cells are joined by intercalated discs consisting of fascia adherentes, spot desmosomes, and gap junctions. The gap junctions include intramembrane particles. T tubules are absent. The sarcolemma is coupled to the junctional sarcoplasmic reticulum (JSR) over 357ndash;40% of the cell surface. Tubules extend from the JSR deep into and throughout the cell as an irregularly dispersed network. The SR occupies 1% of the cell volume. A few, small (0.1–1.0 μm) unmyelinated nerves are present, but no neuromuscular junctions were seen. The auricles have fewer and smaller myocytes than the ventricle. The auricles also contain podocytes with pedicels having 20–35 nm slits and containing sieve-like projections. The morphology of the Geukensia heart is similar to that of other bivalves.
Article
Molluscan cells volume regulate in dilute salinities by releasing amino acids from an intracellular pool. The magnitude of this efflux depends upon external divalent cation concentration. Incubation of isolated Modiolus demissus ventricles in a K⁺ solution isosmotic to 100% seawater (SW) results in membrane depolarization and an increased amino acid efflux. In the presence of Ca²⁺-Mg²⁺-free SW, the myocardial cell membrane is depolarized and an amino acid efflux occurs. Additional removal of Na⁺ repolarizes the membrane and reduces the amino acid release. Cyanide or 2,4-DNP in 50% SW result in a prolonged amino acid efflux from the isolated hearts. Decreased temperature markedly reduces the hypoosmotically induced amino acid efflux. These results indicate that the control mechanism of the amino acid release from molluscan cells stressed by hypoosmotic salinities depends upon external divalent cation concentration, membrane permeability, and ATP.
Article
SYNOPSIS. Actomyosin was extracted from skeletal muscle of Coryphaenoides, a benthic fish living at 2,200 meters depth, at a temperature of 2°C, or less, and at pressure of 3,000 psi. On SDS-urea electrophoresis on acrylamide gel, the actomyosin extracts yield components of apparent molecular weight 210,000 (myosin heavy chains), 47,000 (actin), 35,000 (tropomyosin and/or troponin subunits), and 13,000 (myosin light chains). The Mg2+-ATPase of Coryphaenoides actomyosin shows a complex Arrhenius plot, with marked denaturation at temperatures above 30°C, and diminished temperature sensitivity at temperatures below 15°C. Mg2+-ATPase is inhibited by pressure, with activation volumes of + 160 cc/mole at 25°C, and + 230 cc/mole at 2°C. Ca2+-ATPase of actomyosin exhibits the same pH, temperature, and pressure dependence as Ca2+-ATPase of myosin. The overall data would be consistent with a positive activation volume that is independent of temperature (to first approximation) and is related to the interaction of actin and myosin, and a negative activation volume that is temperature dependent and is related directly to activation of myosin ATPase. The net effect appears to be an adaptive mechanism whereby Mg2+-ATPase of Coryphaenoides actomyosin is relatively insensitive to pressure and temperature under conditions encountered by the living fish.
Article
1. The alteration of the Ca2+ requirements of the ATPase activity of fibrils from rabbits and crabs at varying ionic strength, pH and concentration of MgATP (i.e. MgATP2− + MgHATP−) was investigated.2. Under physiological conditions, it was found that the ATPase activity of rabbit and crab fibrils after an initial increase decreased steeply when the Ca2+ concentration is raised above 1×10−4 M. This is a primary effect of the over-optimal Ca2+ concentration and not a secondary one caused by the influence of accompanying ions.3. The Ca2+ requirements for ATP splitting by rabbit fibrils remain constant at an ionic strength from 0.1 to 0.2 and for a MgATP concentration in the range from 0.5 to 10 mM. At I = 0.05 it is about 5 times smaller than at 0.1. When the pH is decreased from 8 to 7, the Ca2+ requirements are increased some 10 times but only 3 times when the pH is varied between 7 and 6.4. In crab fibrils, there is no alteration of the Ca2+ requirements when the ionic strength is varied between 0.05 and 0.2, but a reduction of the pH from 8.0 to 6.0 raises the Ca2+ requirements for half activation and for threshold by a factor of 10. Changing the MgATP concentration increases the Ca2+ requirements only in the range from 1 to 5 mM, while the concentration required in 0.5 mM is identical with that at 1 mM, and 10 mM corresponds to 5 mM.5. It can be deduced from the experimental results that at a pH above 6.0 maximal activation is always obtained if the Ca2+ concentration is 5×10−5 M. By contrast, relaxation is only achieved when the Ca2+ concentration is below 1×10−7 M for pH 7.0 and I > 0.1 or below 1×10−8 for pH > 7.0 or I < 0.1.6. To achieve complete relaxation, an ethyleneglycoldiaminotetraacetate (EGTA) concentration of 1 mM is sufficient, even when there is a large degree of contamination by Ca2+ as long as the pH stays above 6.5.
Article
1. Molluscan cells maintain constant volume in decreasing salinities by releasing intracellular free amino acids as osmotic solute. 2. During a salinity decrease both osmotic pressure and ion concentrations decline simultaneously. Either condition might initiate the amino acid release. 3. Under constant osmotic conditions a decrease in Na+, K+ or Cl- concentration in the medium surrounding the isolated bivalve heart, did not result in an amino acid efflux. 4. On the other hand, removal of Ca2+ and Mg2+ resulted in a vigorous amino acid efflux which continued until the divalent ion concentration was restored. 5. The hypo-osmotically induced amino acid efflux could be partially blocked by raising the external divalent ion concentration. Similarly, lanthanum proved to be an effective blocking agent, both in low osmotic pressures and in Ca2+- and Mg2+-free SW. 6. These results show that salinity-induced free amino acid regulation is initiated by a decrease in external osmotic pressure. The time course of the efflux is dependent on external divalent ion concentration. 7. Finally, the site of divalent ion action is at the external membrane surface. 8. A hypothetical membrane model is proposed.