Proceedings of the Third Congress of the European Society for Comparative Physiology and Biochemistry, August 31-September 3, 1981, Noordwijkerhout, Netherlands
Proceedings of the Third Congress of the European Society for Comparative Physiology and Biochemistry, August 31-September 3, 1981, Noordwijkerhout, Netherlands
Exogenous and Endogenous Influences on Metabolic & Neural Control, Volume 2 presents lectures and posters on feeding; respiration; reproduction; and activity and energy supply in muscles. The book also contains lectures and posters on ion and osmoregulation, perception, and orientation.
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GOALS OF IONIC REGULATION
R.F. BURTON, Institute of Physiology, University of Glasgow, Glasgow, U.K.
Publisher Summary
Ionic regulation may lead to near-constancy of Ca concentration, pH, etc.: however, it is unlikely that homeostatic goals are ever strictly definable in terms of single ionic species. This chapter describes regulated patterns of ionic interrelationships and optimum internal environments with examples taken mostly from the vertebrates and molluscs. No ionic species can alter in concentration independently of others so that where the correction of one perturbation involves secondary disturbances that are deleterious, the evolution of homeostatic mechanisms may involve compromise. Thus, better intracellular buffering in mammals would be at the expense of enhanced Na and K redistribution in acid-base disturbances. Vertebrates differ from marine invertebrates in having less Ca and more phosphate in their extracellular fluid, and the resulting Ca/phosphate ratio of extracellular fluid is close enough to that of hydroxyapatite for it to have comparable roles in the homeostasis of both Ca and phosphate. The relative levels of ions may influence the amount of compromise required in homeostasis. Clues to some of the goals of homeostasis may be sought in patterns of ionic balance within particular taxonomic groups.
Though ionic regulation may lead to near-constancy of Ca concentration, pH etc., it is unlikely that homeostatic goals are ever strictly definable in terms of single ionic species. This talk is about regulated patterns of ionic interrelationships and optimum internal environments, with examples taken mostly from the vertebrates and molluscs.
For several reasons no one ionic species can alter in concentration independently of others so that, where the correction of one perturbation involves secondary disturbances that are deleterious, the evolution of homeostatic mechanisms may involve compromise. Thus better intracellular buffering in mammals would be at the expense of enhanced Na and K redistribution in acid-base disturbances. Vertebrates differ from marine invertebrates in having less Ca and more phosphate in their extracellular fluid and the resulting Ca/phosphate ratio of extracellular fluid (between 0.5 and 5) is close enough to that of hydroxyapatite (1.67) for this to have comparable roles in the homeostasis of both Ca and phosphate. In H. pomatia that ratio is near 100, so that significant augmentation of haemolymph phosphate from stored Ca phosphate would leave haemolymph Ca barely changed. The ratio of Ca to HCO3 in H. pomatia (about 0.5) gives CaCO3 equal roles in Ca and acid-base regulation. Thus the relative levels of ions may influence the amount of compromise required in homeostasis.
Sometimes the secondary ionic disturbances could be beneficial in restoring an optimum balance of antagonistic and synergistic ions. This seems to be the case in Helix. The infusion of KCl into Helix leads to immediate muscular contractions that are preventable by the simultaneous infusion of CaCl2 or MgCl2. If only the KCl is infused, the snail itself transfers Ca into the blood and in normal, uninfused snails there is a correlation between the levels of Ca and K. This suggests that a goal of homeostasis here is not the independent regulation of Ca, but of Ca/K balance and hence of nerve and muscle excitability.
Clues to some of the goals of homeostasis may be sought in patterns of ionic balance within particular taxonomic groups. Several groups share a general correlation between intracellular K and total extracellular ionic concentration (implying some constancy in the ratio of the contributions of other cell constituents to osmolality and to net anionic charge). In marine invertebrates this correlation extends to nerve cells but not muscle. In several groups there is also a general correlation between total extracellular ionic concentration (or just Na) and extracellular K. The associated correlation between intracellular and extracellular K presumably relates to membrane potentials. However, excitability is also strongly influenced by Ca and Mg. Within the vertebrates K, Na, Ca and Mg vary in normal extracellular concentration according to a distinctive pattern that is explicable in terms of excitability. The same cations conform to another clear pattern in non-marine Prosobranchia and this could again relate to excitability: Ca and Mg apparently act synergistically with each other and in opposition to K.
ON THE FUNCTIONAL SIGNIFICANCE OF ION CIRCULATION INDUCED BY ELECTROGENIC TRANSPORT
J. KĂźppers and U. Thurm, Lehrstuhl fĂźr Neurophysiologie, Zoologisches Institut, HĂźfferstr. 1, D-4400 MĂźnster
Publisher Summary
This chapter discusses the functional significance of ion circulation induced by electrogenic transport. A purely electrogenic transport mechanism for monovalent cations is found in insects. Any electrogenic transport can be considered as a voltage source within a current circuit. The physiological significance of the transport mechanism is finally determined by the route and the composition of the current that depend on the permeability of the membranes and the organization of the tissues in series and in parallel. In Malpighian tubules, in salivary glands, and in the midgut, the current circuit energized by this transport seems to be predominantly closed by the passive co-movement of anions or the counter-movement of other cations. This leads to a net flow of solutes for the purpose of excretion or resorption. In other insect organs, the actively transferred ions close the current circuit by their passive reflux via a membrane region of the same or another cell. The chapter discusses two such cases: (1) In epidermal sensilla of insects, at least one of the epithelial cells accompanying the sensory cell bears the electrogenic cation transport at its folded apical membrane. (2) The uniform, monolayered epithelium of the posterior rectum of Lepismatidae is known to transfer water against an osmotic pressure gradient of more than 108 Pa...
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Table of Contents
European Society for Comparative Physiology and Biochemistry Third Congress
