The effect of dietary protein restriction on the secretory dynamics of 1 alpha-hydroxycorticosterone and urea in the dogfish, Scyliorhinus canicula: a possible role for 1 alpha-hydroxycorticosterone in sodium retention.

The putative osmoregulatory role of the unique elasmobranch corticosteroid, 1 alpha-hydroxycorticosterone (1 alpha-OH-B), was investigated using dietary protein restriction as a means of limiting urea biosynthetic ability. Groups of dogfish (Scyliorhinus canicula) were adapted to either a high or a low protein diet (HPD and LPD respectively) and the secretory dynamics of urea and 1 alpha-OH-B were determined following acclimation to normal (100%), 130% and 50% sea water. In normal sea water, LPD fish showed significantly decreased blood production of urea compared with fish fed a HPD (P < 0.05), and the plasma urea concentration required to maintain iso-osmolality was achieved only by a substantial decrease in urea clearance from the plasma. Unlike HPD fish, LPD fish in 130% sea water had no apparent ability to increase plasma urea concentration. An alternative strategy adopted by these animals was the retention of high plasma concentrations of Na+ and Cl-, which increased plasma osmolality and tended to decrease osmotic water loss. Concomitant with the increased ion concentrations, plasma 1 alpha-OH-B concentration was also greatly elevated in LPD fish indicating that the steroid may be acting to minimize Na+ (and Cl-) excretion at osmoregulatory sites such as the rectal gland, kidney and gills. This and a previous study have also demonstrated that 1 alpha-OH-B concentration is elevated in 50% sea water. Decreases in plasma Na+ concentration are tolerated down to 75% sea water, whereafter Na+ is preferentially retained and further decreases in osmolality are achieved by reductions in plasma urea concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms of ACTH- and angiotensin II-stimulated 1 alpha-hydroxycorticosterone secretion in the dogfish, Scyliorhinus canicula.

An isolated perifused interrenal gland preparation from the lesser-spotted dogfish, Scyliorhinus canicula, was used to investigate the mechanisms of action of ACTH and angiotensin II (AII) on elasmobranch adrenocortical cells. ACTH-stimulated 1 alpha-hydroxycorticosterone secretion was unaffected by dantrolene and significantly decreased in the absence of extracellular calcium. Dibutyryl cyclic AMP produced a dose-dependent increase in 1 alpha-hydroxycorticosterone secretion. The results suggest that the mechanism of ACTH action in elasmobranchs may be similar to that reported for mammals and amphibians, involving the synergistic action of calcium with the cyclic AMP messenger system. AII-stimulated 1 alpha-hydroxycorticosterone secretion was significantly inhibited in the presence of dantrolene and in the absence of extracellular calcium, indicating that both extracellular and intracellular calcium are required for the full action of AII. These results are consistent with results in mammals and amphibians where AII stimulates phosphatidylinositol 4,5-bisphosphate hydrolysis and changes in intracellular calcium concentration, and they suggest that AII may operate via this mechanism to stimulate 1 alpha-hydroxycorticosterone secretion in elasmobranchs.

Renin and angiotensin converting enzyme in elasmobranchs.

Renin-like activity (RLA) and angiotensin I converting enzyme-like activity (ACELA), the two key enzymes of the renin-angiotensin system (RAS), were sought in the elasmobranch Scyliorhinus canicula. Renal extracts were desalted in a G-25 and eluted in a G-100 Sephadex column (calibration 15,000-70,000). The fractions were concentrated in a vacuum device. A 48,000-MW fraction incubated with synthetic and porcine angiotensiongen generated angiotensin I estimated by RIA. This same fraction was vasopressor in rats and dogfish. ACELA was sought in gill, heart, liver, spleen, pancreas, intestine, kidney, gonads, brain, skin, and muscle of dogfish using a spectrophotometric assay. The highest level of ACELA was found in the gills followed by spleen, kidney, and brain (33.79 +/- 2.3, 29.56 +/- 1.0, 14.62 +/- 1.0, and 13.80 +/- 2.3 nmol hippurate/min/mg protein, respectively). Intestine, gonads, skin and muscle contained no measurable amounts of ACELA. Captopril inhibited enzymatic activity from all ACELA containing tissues.

Secretory patterns of 1 alpha-hydroxycorticosterone in the isolated perifused interrenal gland of the dogfish, Scyliorhinus canicula.

An isolated in-vitro perifused interrenal gland preparation from the dogfish Scyliorhinus canicula was used to study production of quantitatively the major corticosteroid 1 alpha-hydroxycorticosterone (1 alpha-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 microM cyclic AMP (cAMP) after this time. Steroid production responded in a dose-dependent manner to porcine ACTH, with 10 microM producing a maximum stimulation of 225% above the basal secretory rate. cAMP (10 microM) produced an increase of 278% above basal, while 1 microM forskolin increased basal secretory rates by 127%. [Val5]- and [Ile5]-angiotensin II (0.1 microM) increased 1 alpha-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.

The renin-angiotensin system and vascular and dipsogenic regulation in elasmobranchs.

The role of a renin-angiotensin-like system (RAS) in the regulation of blood pressure and drinking has been investigated in the elasmobranch, Scyliorhinus canicula. Injection of exogenous angiotensin II produced, as expected, a vasopressor response, though injection of the converting enzyme inhibitor, Captopril, alone produced little change in resting blood pressure. Papaverine, a smooth muscle relaxant, reduced blood pressure which completely recovered within 30 min. A subsequent injection of Captopril produced a rapid vasodepressor response with no recovery over 2 hr. The low basal levels of drinking in dogfish were not altered by Captopril injection but angiotensin II-induced increased drinking and papaverine administration resulted in markedly stimulated water intake, which was inhibited by coadministration with Captopril. Captopril inhibition of the recovery in blood pressure and associated dipsogenic response following the papaverine-induced hypotension is consistent with the activation of a RAS-like system in the dogfish. This and other evidence supporting the presence of a RAS-like system in elasmobranchs are discussed in relation to other vertebrates.