Abstract
Marine elasmobranchs maintain their body fluids hyperosmotic to sea water, continually taking up water through their gills and integument, while maintaining water balance through the renal excretion of a urine which is hypo-osmotic to both sea water and plasma. An increased uptake of water occurs when these fish migrate to fresh water or are placed in dilute sea water. In these circumstances plasma osmolality becomes reduced, urine flow rate and glomerular filtration rate increase, and the ratio of urine osmolality to plasma osmolality decreases (1). A similar response to dilution is observed in higher vertebrates (2-4) and it has been repeatedly suggested that endocrine control of water excretion exists in the elasimobranchs as it does in higher forms. In teleosts the caudal neurosecretory system of the spinal cord has been implicated in the control of water excretion, and the neurosecretory cells in the spinal cords of elasmobranchs have been considered as having analogous functions (5,6). However, despite many conjectures no experimental evidence has supported the presence of a humoral factor capable of influencing water excretion in elasmobranchs, nor is there evidence directly implicating the caudal neurosecretory cells or any other elasmobranch tissue as the source of these postulated factors. In a search for such fmtors we have examined the effect of injections of extracts of various tissues taken from the smooth dogfish, Mustelus canis, on the renal function of other Mustelus undergoing water diuresis while in dilute sea water.
Methods. Donor fish, (male and female, weighing between 1 and 4 kg) were placed in fresh running dilute sea water (7040%) for 2-3 days, a period sufficient to promote development of the maximum drop in plasma osmolality and the maximum water diuresis abtaimble in this salinifty.
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