The influence of ions on the contractility of the ventricle of the mollusc, Helix pomatia
Başlık:
The influence of ions on the contractility of the ventricle of the mollusc, Helix pomatia
Yazar:
Loudon, James Robert, author.
ISBN:
9780438058507
Yazar Ek Girişi:
Fiziksel Tanımlama:
1 electronic resource (181 pages)
Genel Not:
Source: Dissertation Abstracts International, Volume: 76-08C.
Advisors: I. A. Boyd.
Özet:
1. By means of the superfusion technique (Lamb & McGuigan, 1966), and intracellular microelectrodes, the influence of ions and drugs upon baseline (resting) and contracture tension, and upon the membrane potential of the ventricle of the vineyard snail. Helix pomatia, has been examined. 2. When the ventricle is superfused by saline of physiological composition - 'K4 Mg 14 saline' (see Methods) - the baseline actually represents a small degree of activity in the contractile elements, the membrane potential exceeding the tension threshold. Thus, the ventricle is in a state of threshold contracture, and therefore does not represent the true zero contractile activity from which contracture tensions should be measured. This also explains why spontaneous beating is a rare event in K4 Mg 14 saline in this preparation. The baseline tension is very sensitive to ionic variation as follows (normal values: Ca 8mM, Na 80mM, Mg 14mM, K 4mM) (i) Contracture tension is induced by lowered calcium, zero magnesium, and raised sodium concentrations. Nominally calcium-free solutions which contain a normal magnesium concentration (14mM) also produce a contracture, which is abolished by raising the magnesium concentration to 56mM. (ii) The baseline tension actually drops when the sodium concentration is lowered, or when manganese is added (5mM). Raised magnesium concentrations also cause a drop in baseline tension, but in addition induce spontaneous beating. (iii) There is no effect when the calcium concentration is raised. 4. Addition of excess potassium chloride induces contracture - the potassium (K) contracture. Two unusual features are noted in relation to the K contracture. (i) The 'rebound' contracture - which occurs with the re-introduction of K4-Mgl4 saline, following a poorly sustained K contracture. (ii) A rapid decay of control K contractures in an experiment, of 40% in two to three hours (c.f. frog heart - 10% decay (Lamb & McGuigan, 1966) ). 5. Both acetylcholine, and 5-hydroxytryptamine (5H.T.) are able to induce contracture; and acetylcholine can also reduce the K contracture in certain conditions. Therefore, the possibility of transmitter release from nerve endings in this preparation (Cottrell & Osbourne, 1969) has been considered as an explanation of the threshold contracture, the rebound contracture, and the rapid experimental decay of the K contracture. 6. The influence of agents which block the action of acetylcholine, and 5H.T., upon the baseline and K contracture (including the rebound contracture), and also of eserine, which potentiates acetylcholine action, have therefore been examined, and found to have no effect. Therefore, there is no evidence from these experiments that transmitter release plays a significant part in any of the observed effects. 7. Three possible factors are suggested to explain the state of threshold contracture in K4Mg 14 saline. (i) A low membrane potential (60mv), (ii) A high concentration ratio of calcium (8mM) to magnesium (14mM) in the environment. (iii) The use of the superfusion technique, which largely eliminates the intraluminal stretch, normally occurring in vivo, or in cannuloted hearts. 8. The Influence of added potassium chloride on the membrane potential has been examined. Ten fold Increase In potassium induces a -38mv depolarisation, while potassium-free fluids hyperpolarlse the membrane by +9.mv These results are combined with the earlier results relating potassium to tension, in order to relate membrane potential to tension development. The results indicate that depolarisation induces tension, and are supported by the fact that acetylcholine concentrations, which produce contracture, also depolarise the membrane. 9. The Influences of other Ions on the membrane potential are as follows. (I) A small depolarisation is produced by a lowered calcium concentration, and nominally calcium-free solutions containing either normal (14mM) or high (56mM) concentrations of magnesium. (ii) A small hyperpolarisation occurs when the concentration of calcium is raised, or that of sodium lowered. Addition of manganese also has a small effect In this direction. (ill) There Is no demonstrable effect when the sodium concentration Is raised, or when the magnesium concentration is raised or lowered.
Notlar:
School code: 0547
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Yer Numarası | Demirbaş Numarası | Shelf Location | Lokasyon / Statüsü / İade Tarihi |
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XX(684664.1) | 684664-1001 | Proquest E-Tez Koleksiyonu | Arıyor... |
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