Vascular And Renal Actions Of Salmon Calcitonin In Freshwater- and Seawater-Adapted European Eels (Anguilla Anguilla)

Fish calcitonin was first demonstrated from the ultimobranchial (suprapericardial) body of dogfish by Copp, Cockroft & Kueh (1967), who later showed that calcitonin of salmon origin was a more potent hypocalcaemic agent in mammalian systems than were naturally produced mammalian calcitonins (Copp et al. 1969). However, in spite of this potency, the role(s) of fish calcitonin in teleosts is equivocal.

There are conflicting reports on both its ability to affect significantly the ionic composition of the body fluids and also the possible target tissues involved (Louw, Sutton & Kenny, 1967; Chan, Chester Jones & Smith, 1968;Pang, 1971; Copp et al. 1972; Lopez et al. 1976; Yamauchi, Matsuo, Yoshida & Orimo, 1978; Milhaud, Bolis & Benson, 1980; Hirano, Hasegawa, Yamauchi & Orimo, 1981).

In a preliminary study, Wales & Barrett (1983) have shown that in 1 h after intraperitoneally administered salmon calcitonin, albeit in higher doses of 50 or 100 i.u. kg⁻¹ body weight, there was a significant depression of sodium, chloride and calcium ions in the plasma of goldfish and freshwater- and seawater-adapted eels.

The present series of experiments seeks to establish whether a similar response could be detected in eel plasma following intravenous injections of more physiologically related amounts of hormone whilst extending the investigation to include possible actions on blood pressure and renal handling of electrolytes.

Freshwater-adapted eels (Anguilla anguilla L.) were held for 2 months at 14°C in running Sheffield tap water containing 0·50 mmol l⁻¹ Na⁺, 0·51 mmol l⁻¹ Cl⁻ and 0·25 mmol 1⁻ Ca²⁺, whilst seawater-adapted animals were maintained for 2 months at 12°C in aerated recirculated artificial sea water (Natura Sea Aquariums Ltd, Jungle Laboratories Corporation, Sanford, Florida, U.S.A.) containing 450 mmol l⁻¹ Na⁺, 530 mmol l⁻¹ Cl⁻ and 9 mmol 1⁻¹ Ca²⁺. The eels did not feed after their arrival.

Prior to surgery, animals were anaesthetized in a solution of MS 222 (Tricaine methanesulphonate, Sigma Chemical Company, Poole, England) and weighed. A ventral incision was then made alongside the liver to expose both the pneumogastric artery and vein. Details of this method have been given before (Henderson & Wales, 1974). Implantation of polythene cannulae (Laboratoire Portex SA, Berck-sur-Mer, France) enabled arterial blood sampling at selected intervals during otherwise continuous recording of blood pressure in the dorsal aorta (Harvard Model 377 Pressure Transducer, Harvard Apparatus Ltd, Edenbridge, Kent, England) as well as a slow venous infusion of saline (20μ1 min⁻¹ 0·9 % w/v NaCl). The urinary bladder was also cannulated as a device for collecting urine samples. This cannulation was achieved with non-compressible autoanalyser tubing (Altec Laboratory Supplies & Systems Ltd, Alton, England) of 0·9 mm internal diameter. The tubing was held in place by a purse-string suture around the cloaca and by stitching the rigid orange collar to the skin surface. The operated fish were then transferred to individual black polythene tanks designed in such a way that when the tank lid was fitted the cannulae could be freely manipulated from the outside without disturbing the animals but with minimal dead space.

Some 48 h following surgery, the eels were given a priming intravenous injection of 5 μCi ³H-inulin (Amersham International PLC, Amersham, England) followed by a sustained infusion of 2μCiml^{−1 3}H-inulin in 0·9% w/v saline solution (Meltic Infusion Pump, Meltec Ltd, Buckingham, England) at a rate of 20μ1 min⁻¹. This infusion was maintained throughout the experiment. The renal clearance of inulin was taken as an index of glomerular filtration rate. Urine samples were collected at intervals in tared polythene vials, whilst 150μ1 mid-point blood samples were withdrawn from the arterial cannula and immediately centrifuged. Blood remaining in the cannula was returned into the body with heparinized isotonic saline. The samples, when collected, were stored deep frozen for subsequent analysis. When stable rates of urine flow and blood pressure were evident, a single acute injection of 0, 5 or 15 i.u. kg⁻¹ of synthetic salmon calcitonin (Calsynar, Armour Pharmaceutical Company Ltd, Eastbourne, England) was given into the venous cannula by microlitre Syringe and the infusion restored. Acetate saline similar to that in which the calcitonin was supplied was also tested for vascular activity. The animals were usually monitored continuously for up to 5 h or more following injection.

After appropriate dilutions, the sodium levels were measured by flame emission using a Corning 400 spectrophotometer. Chloride levels were assayed using a Corning 925 chloride analyser. Calcium analysis was performed on a Perkin-Elmer 360 atomic absorption spectrophotometer using an acetylene/air flame. The method for calcium determination was that described by Trudeau & Freier (1967) using a lanthanum chloride/acetic acid releasing solution to remove interference from phosphate and protein, giving a value for total plasma calcium. The urinary and plasma levels of ³H-inulin used to give an estimate of the glomerular filtration rates were measured by liquid scintillation (Kontron MR 300) from 20μl of sample in 2 ml of scintillation fluid (Cocktail T, BDH Chemicals Ltd, Poole, England).

Freshwater-adapted eels responded to calcitonin injections with an increase in dorsal aortic blood pressure whilst seawater-adapted eels did not. No vascular responses were seen when control injections of either saline or acidified saline were given. The increase in dorsal aortic systolic pressure in response to 5 i.u. calcitonin kg⁻¹ body weight was from 24·8 ± 0·42 to 27·94 ± 0·46mmHg (mean±s.D.; N = 6), whilst that to 15 i.u. calcitonin kg⁻¹ body weight was from 23·9 ± 0·39to31·32 ± 0·94 mmHg (mean ± S.D. ; N = 6). The duration of these responses was between 8 and 15 min respectively.

The results in Table 1 are values of renal function recorded from freshwater- and seawater-adapted eels both before and 1 h after either calcitonin or acetate saline injection.

The freshwater eels showed a significant increase in urine volume coupled to an increase in glomerular filtration rate during the 1 h collection period after calcitonin injection. At the same time there were no indications of changes in the relative handling of electrolytes. Injections of 5 i.u. calcitonin kg⁻¹ body weight failed to elicit any significant changes in the plasma electrolyte composition of either freshwater-or seawater-adapted eels. However, the results summarized in Table 2 show that administration of 15 i.u. calcitonin kg⁻¹ body weight did significantly affect plasma electrolyte levels in freshwater-adapted eels within 1 h of injection when there was a significant decrease in plasma sodium, chloride and calcium levels coupled with a significant increase in plasma potassium. These changes did not persist beyond this 1 h interval and did not occur at all either in seawater-adapted fish or in acetate saline injected control animals. However the seawater-adapted fish did show a response to 15 i.u. calcitonin kg⁻¹ body weight some 3h after injection, when there was a mean 40% increase in plasma calcium levels from 3·14 ± 0·4 to 4·38 ± 0·4 mmol l⁻¹ as well as a significant reduction in relative calcium clearance from 1·51 ±0·12 to 1·06 ±0·33 (mean ± S.D. ; N = 6). This effect was continued for at least 5h whilst no similar Response was seen in freshwater-adapted fish.

The ability of calcitonin to induce diuresis in freshwater eels has been reported before (Hirano et al. 1981), although these workers did not find corresponding actions on either blood pressure or plasma electrolyte composition. This apparent discrepancy in calcitonin activity could be due to the fact that Hirano et al. (1981) were using 1 i.u. kg⁻¹ of synthetic eel calcitonin given via the caudal vein whilst the results presented here were taken after giving 5 and 15 i.u. kg⁻¹ of synthetic salmon calcitonin injected almost directly into the heart. The lack of any immediate vascular or renal responses by seawater-adapted eels when compared with their freshwater-adapted counterparts may point to a difference in the degree of sensitivity to calcitonin which is related to external salinity. The electrolyte changes seen in freshwater fish following injection of 15 i.u. kg⁻¹ calcitonin might further indicate a branchial role of calcitonin as relative ion clearances remained unchanged at a time when plasma levels of sodium, chloride and calcium were falling and potassium levels rising. This suggestion would certainly find some support (see Milhaud et al. 1980), although significant falls in plasma calcium levels could give rise to altered membrane permeabilities to other electrolytes.

In an earlier paper, Wales & Barrett (1983) showed a depression of plasma sodium chloride and calcium ions in both freshwater- and seawater-adapted immature eels following intraperitoneal injections of 50 and 100 i.u. of salmon calcitonin kg⁻¹ body weight. If such responses are taken as being physiological rather than pharmacological, it could be that calcitonin plays a role which alters as the eel at first increases its skeletal growth and then later faces hypoosmotic challenges prior to catadromous migration. The apparent hypercalcaemia which was seen in seawater-adapted eels was quite unexpected. The fish used in these experiments were large female eels and there is evidence that oestrogens may increase circulating plasma calcium levels (Woodhead, 1969) presumably for oocyte development. One explanation of the delayed hypercalcaemic action of calcitonin in sea water might be that it is mediated via a stimulatory action on gonadal steroid production.

The author would like to thank Professor J. N. Ball of the Department of Zoology, University of Sheffield, for the use of the scintillation counter facilities and also Dr Rankin of the Department of Zoology at the University of Bangor for supplying the eels used in these experiments.