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ABSTRACT

Binding of ¹²⁵I-labelled arginine vasotocin (AVT) was studied in isolated intact gill cells obtained from eels (Anguilla anguilla) adapted to fresh water (FW) or to sea water (SW). Experiments carried out at 20°C showed maximum and stable binding beyond 10 min of incubation. Specific binding, determined by using labelled peptide in the presence or absence of an excess of unlabelled hormone, represented 30–50% of total and was reversible, with a half-time of less than 5 min. Scatchard plot analysis revealed the presence of a single population of saturable, high-affinity sites. Maximum binding capacity (B_max: fmol AVT/10⁶ cells) and dissociation constant (K _d: nm) were respectively 5·16 and 3·21 in FW and 24·25 and 1·05 in SW. Analysis of chloride cell number and size in gills and of binding characteristics of AVT revealed parallel changes with external salinity. These results are taken as evidence for the direct intervention of neurohypophysial peptides on the gill epithelium of teleost fishes.

ABSTRACT

The presence of atrial natriuretic peptide (ANP) and the nature of its binding sites were studied in freshwater (FW)- and seawater (SW)-adapted eels using a heterologous analogue, that of the rat (rANP). Rat ANP-like immunoreactivity was demonstrated in the cardiac atria and ventricles of both FW and SW eels, and electron-dense ANP-like granules were observed. The atria and ventricles of FW eels contained significantly more granules than those of SW animals and, in both types, the atria were more granular than the ventricles.

Specific binding sites for rANP were demonstrated by displacement and uptake experiments using labelled rANP in dispersed eel branchial cell preparations, enriched in chloride cells. The concentration of rANP required to produce a 50% inhibition of binding in FW cells was significantly lower than that in SW cells. Scatchard analyses revealed the presence of two classes of binding site in SW eel branchial cells but only a single class of receptor in FW cells. The affinity of the FW receptor was not significantly different from that of the SW high affinity site.

Rat ANP stimulated the production of cyclic GMP (cGMP) in a dose-dependent manner, and both basal and stimulated levels of cGMP were significantly greater in SW branchial cells.

These studies suggest that ANP is involved in the adaptation of the euryhaline eel to differing environmental salinities; the levels of the peptide in the heart alter with changing salinity, and the nature of the receptors in the sodium chloride-transporting epithelium of the gill changes in response to the need either to eliminate or to absorb sodium chloride.

ABSTRACT

An isolated in-vitro perifused interrenal gland preparation from the dogfish Scyliorhinus canicula was used to study production of quantitatively the major corticosteroid 1α-hydroxycorticosterone (1α-OH-B), measured by radioimmunoassay. Basal secretory rates were 877·1 ± 145 (s.e.m.) fmol/mg per 15 min (n=14) and the preparation remained viable for up to 22 h, as reflected in a brisk response to 10 μm cyclic AMP (cAMP) after this time. Steroid production responded in a dose-dependent manner to porcine ACTH, with 10 μm producing a maximum stimulation of 225% above the basal secretory rate. cAMP (10 μm) produced an increase of 278% above basal, while 1 μm forskolin increased basal secretory rates by 127%. [Val⁵]- and [Ile⁵]-angiotensin II (0·1 μm) increased 1α-OH-B production by 120 and 372% respectively over basal secretory rates. Increasing the concentration of K⁺ in the perfusate from 8 mm to 12, 18, 28 and 40 mm produced a significant rise only at 28 mm. Alterations in the concentration of Na⁺ and osmolarity of the perifusion medium had inconsistent effects on steroid production. Increased concentrations of urea (from 360 to 720 mm) increased the basal secretory rate by 121%, whilst reducing the concentration of urea (from 360 to 90 mm) had no effect.