Single nephron structure and function, and renal effects of catecholamines in the dogfish, Scyliorhinus canicula
1. The organisation of the kidney, and nephron structure of Scyliorhinus canicula, was examined using light and scanning electron microscopy. 2. Visceral epithelium of the glomerulus was composed of podocytes, primary processes and interdigitating pedicels. Morphological variations were common, with areas of flattening and broadening of these structures. 3. The nephron was found to be composed of Bowman's capsule, neck segment, first, second and third proximal segments, and distal segment, identified by characteristic cytological features and dimensions. 4. Renal vasculature was examined using scanning electron microscopy of corrosion casts. 5. Morphological evidence of smooth muscle sphincters, intra-arterial cushions and glomerular bypass shunts in renal vasculature was found. The function of these structures is discussed in relation to patterns of renal blood flow, blood pressure and glomerular perfusion. 6. Plasma levels of the catecholamines adrenaline, noradrenaline and dopamine were determined in resting fishes, and following stress induced by MS222 (Tricaine methane sulphonate) anaesthesia and surgery. 7. Infusion of adrenaline increased systolic and diastolic systemic blood pressure in conscious and anaesthetised fishes. Heart rate was elevated in conscious fishes only. Haemodynamic effects of adrenaline were found to be more potent than those of noradrenaline. 8. Adrenaline induced a marked urinary diuresis, and increased glomerular filtration rate. Urine to plasma inulin concentration ratios, relative clearance of osrnolytes and relative free water clearance were unchanged by adrenaline. 9. Anaesthesia significantly reduced urinary output. 10. Micropuncture techniques were used to determine single nephron glomerular filtration rates (SNGFR). Infusion of adrenaline increased mean SNGFR, and increased the tubular fluid to plathtia inulin concentration ratio. This is discussed in relation to tubular water secretion. 11. Patterns of glomerular perfusion were determined. Adrenaline decreased the proportion of filtering nephrons. 12. The role of circulating catecholamines is discussed in relation to the regulation of renal circulation.