Osmoregulatory role and control of secretion of lα-hydroxycorticosterone in the lesser spotted dogfish 'Scyliorhinus canicula'
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1. Groups of dogfish, Scyliorhinus canicula, were adapted to either a high or a low protein diet over a period of 30 days. The fish readily ate both diets and maintained body weights. 2. Dietary adapted fish were acclimated to 130%, 100% and 50% seawater and the metabolic clearance rates (MCR) and blood production rates (BPR) for urea and 1a-hydroxycorticosterone (1a-OH-B) determined. 3. In 100% seawater low protein diet (LPD) fish had significantly lower urea BPR and MCR than the high protein diet (HPD) fish. 4. In 50% seawater plasma osmolality, sodium, chloride and urea concentrations decreased in both dietary groups. Plasma 1a-OH-B concentration and BPR were significantly increased and in HPD fish this was accompanied by an increase in 1a-OH-B MCR. The increase in plasma 1a-OH-B concentration was significantly greater in the HPD fish than in the LPD fish. 5. In 130% seawater plasma osmolality was increased in both dietary groups. In HPD fish this was achieved by increasing plasma urea, sodium and chloride concentrations. In LPD fish this was achieved by significantly elevating plasma sodium concentration only. Plasma 1a-OH-B concentration and BPR were significantly increased and MCR significantly decreased in LPD fish, but remained unaltered in HPD fish. 6. An isolated perifused interrenal gland preparation was developed for Scyliorhinus canicula, which produced a constant 1a-OH-B basal secretion rate after two hours and remained viable for more than 22 hours. 7. ACTH, All, ANP, AVT, forskolin, dibutyryl cAMP and dibutyryl cGMP stimulated interrenal steroidogenesis. 8. ACTH-induced steroidogenesis was unaffected by the presence of dantrolene and verapamil but significantly increased in the presence of cholera toxin and significantly decreased in the absence of extracellular calcium. 9. Ile5-All-induced steroidogenesis was unaffected by the presence of verapamil but significantly decreased in the presence of dantrolene and in the absence of extracellular calcium. 10. Increases in potassium concentration were significantly steroidogenic only at 28mM and alterations in sodiimi concentration had no effect.
Thesis, PhD Doctor of Philosophy
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