Richard J. Balment

Release of oxytocin induced by salt loading and its influence on renal excretion in the male rat.

1. The present study investigates the nature and magnitude of the renal response to plasma levels of oxytocin which might be induced by salt loading. 2. Increased plasma osmolality induced by loading with NaCl is an effective stimulus for oxytocin release in the unanaesthetized male rat. Plasma oxytocin concentration was positively correlated (r = 0‐.77) with plasma osmolality. Plasma oxytocin (muu./ml.) = 0.37 x (plasma osmolality (m‐osmole/kg) ‐297). 3. In anaesthetized Long Evans rats intra‐atrial administration of oxytocin at rates of 0.05 and 0.15 m‐u./ml. produced plasma hormone concentrations (5 +/‐ 1 and 16 +/‐ 2 mum./ml. respectively) within the range induced by salt loading. 4. Oxytocin administration at 0.15 and 1.5 m‐u./min in Long Evans rats produced dose‐related increases in urine flow and Na+ and Cl‐ excretion. Renal responses to 0.05 m‐u. oxytocin/min were equivocal. 5. Oxytocin administration at 0.15 m‐u./min was ineffective in Brattleboro rats but 1.5 m‐u./min led to increased Na+ and Cl‐ excretion and a reduction in urine flow. 6. Plasma oxytocin levels similar to those induced by severe dehydration or salt loading are effective in increasing renal Na+ and Cl‐ excretion and urine flow. These effects on water and electrolyte excretion appear to be independent of each other and both may be modified by the presence or absence of vasopressin. 7. This study provides no evidence for a major role for oxytocin in the day to day regulation of salt or water balance under conditions of normal hydration in the male rat.

Drinking behaviour in sea water and fresh water teleosts, the role of the renin-angiotensin system

Basal drinking rate and responses to administered angiotensin were examined in 12 species of fish. The responses of representative euryhaline, stenohaline marine and fresh water species to pharmacological manipulation of endogenous renin-angiotensin system (RAS) activity were also investigated.Basal drinking rates were consistently low in stenohaline and euryhaline fresh water fish, and all species examined showed an increased imbibition in response to administered angiotensin. Marine fish drank large volumes of water, rates varying considerably between species, with euryhaline species exhibiting lower rates than stenohaline groups. The extremely high drinking rates observed in the sea scorpion were associated with a high plasma osmolality. With the exception of the sea scorpion, all other species examined in sea water showed a further rise in drinking in response to exogenous angiotensin.Although the freshwater stenohaline carp showed a dipsogenic response to angiotensin, it was apparently unable to evoke this response when fish were acclimated to brackish water. The high drinking rates of both euryhaline and stenohaline fish held in sea water appeared dependent upon an activated endogenous RAS, and were lowered following inhibition of Al to All conversion by Captopril. Drinking was further stimulated in these marine species following stimulation of endogenous RAS activity by the administration of the hypotensive agent Papaverine. The study endorses a role for the RAS in the control of adaptive drinking in euryhaline and stenohaline marine teleosts.

Pituitary and plasma arginine vasotocin levels in teleost fish

The role of arginine vasotocin (AVT) in teleostean osmoregulation is unclear. Its administration may produce dose-dependent effects on renal fluid loss though the physiological significance of such effects is unknown due to the paucity of data on the normal range in pituitary and plasma AVT levels. The present study addresses these questions by examining the pituitary AVT activity in the flounder and plasma AVT concentrations in the flounder, trout, and carp following alterations in environmental tonicity. Flounder pituitary AVT content, measured by bioassay, was elevated in freshwater (FW)-acclimated compared to seawater (SW)-acclimated fish and rapidly fell or rose, respectively, following transfer from FW to SW or from SW to FW to attain a new steady state. Plasma levels of AVT were higher in FW than in SW in both flounder and trout and it would, therefore, appear that the synthesis, storage, and secretion of AVT may be enhanced in FW. In the stenohaline carp plasma levels of AVT were unchanged on acclimation from FW to 40% SW. In SW flounder there was a significant positive correlation between plasma osmolality and plasma AVT concentration. Such a relationship was not apparent in FW fish, which may reflect differing roles for the hormone in these two extremes of osmotic environment.

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.

Neurohypophysial hormones and renal function in fish and mammals.

The two major basic neurohypophysial peptides, arginine vasopressin (AVP) of mammals and arginine vasotocin (AVT) of all non-mammalian vertebrates, share common structure and major roles in regulating renal function. In this review the complexity of AVP actions within the mammalian kidney is discussed and comparisons are made with the emerging picture of AVTs renal effects in fish. It has become apparent that the antidiuretic action of the neurohypophysial hormones is an ancient phylogenetic phenomenon, although this is based upon reduced glomerular filtration in fish by comparison with predominant tubular effects in mammals. Nonetheless, there appears to be retention of AVP effects upon the functional heterogeneity of nephron populations in mammals. Preliminary evidence for the possible existence of V(2)-type (tubular) neurohypophysial hormone receptors in fish, implies possible AVT actions which parallel those in mammals on tubular ion transport. Further insight from recent mammalian tubule microperfusion studies suggests that in teleost fish both apical (tubular lumen) and basolateral (blood borne) AVT have the potential to modulate renal function, though this remains to be examined.

The renin-angiotensin system and drinking in the euryhaline flounder, Platichthys flesus

Drinking behaviour and its possible regulation by the renin-angiotensin system (RAS) has been examined in the euryhaline flounder. Fluid intake was greater in seawater (SW)-adapted than freshwater (FW)-adapted fish, the latter having significantly lower plasma sodium, chloride, and osmotic concentrations. Oesophageal cannulation in SW-adapted fish resulted in further elevation of drinking rates, which increased proportionally with progressive body water loss as measured by the fall in body weight and rise in plasma tonicity. The influence of the RAS on drinking in SW-adapted fish was examined in animals with an intact gastrointestinal tract. Fluid intake fell markedly following administration of the converting enzyme inhibitor, Captopril. Infusions of angiotensin I (AI) and angiotensin II (AII) induced dose-related increments in the rate of drinking. The increased drinking in response to AI was inhibited, however, by the simultaneous administration of Captopril. The results are consistent with the presence in the flounder of the major elements of the RAS, including AI, AII, and a converting enzyme-like substance. The RAS appears to play an important regulatory role in the adaptative drinking behaviour associated with migration of euryhaline teleosts between FW and SW.

The influence of neurohypophysial hormones on renal function in the acutely hypophysectomized rat.

1. Renal function and the effect of neurohypophysial hormone replacement was investigated in anaesthetized, acutely hypophysectomized, male rats. 2. Although urine production was only slightly lower over the 8 h post‐operative study period in hypophysectomized rats, sodium excretion was greatly depressed reaching only 3.5 +/‐ 1.4 mumol/min compared with a peak of 13.2 +/‐ 1.0 mumol/min in intact animals. 3. In association with a decline in mean arterial blood pressure, glomerular filtration rate in hypophysectomized rats fell to 2.1 +/‐ 0.2 ml/min 8 h after operation by comparison with a mean rate in intact rats of 3.2 +/‐ 0.2 ml/min. 4. Plasma corticosterone levels were much lower in hypophysectomized (4 +/‐ 2 ng/ml) than in intact (36 +/‐ 4 ng/ml) rats, plasma aldosterone was reduced to a lesser extent (0.41 +/‐ 0.08 compared with 0.76 +/‐ 0.04 ng/ml). While oxytocin was not detectable in hypophysectomized rat plasma, trace levels of vasopressin (0.16 +/‐ 0.04 mu u./ml) were found. In intact unanaesthetized rats basal plasma levels of oxytocin were 0.32 +/‐ 0.13 mu u./ml and vasopressin were 0.85 +/‐ 0.19 mu u./ml. 5. Administration of oxytocin at 150 mu u./min, which produced plasma hormone levels (24.0 +/‐ 2.5 mu u./ml) greatly in excess of basal concentrations, increased renal sodium excretion but did not alter urine flow. Oxytocin administration at the lower rate of 15 mu u./min producing plasma hormone levels of 2.60 +/‐ 0.1 mu u./ml, did not alter renal sodium excretion. 6. Arginine vasopressin administered at 12 mu u./min induced plasma hormone levels of 1.54 +/‐ 0.09 mu u./ml and produced a large antidiuresis and small increase in the rate of sodium excretion. 7. The natriuretic response to vasopressin was potentiated by concurrent administration of oxytocin at 15 mu u./min. The peak sodium excretion of 5.8 +/‐ 1.0 mumol/min, however, remained well below that seen in intact rats. 8. It is concluded that, as restoration of posterior pituitary hormones at or above the physiological range only partially restored sodium excretion, the absence of anterior pituitary factors may also contribute directly or indirectly to the renal sodium retention of the hypophysectomized rat.

Natriuretic response of the rat to plasma concentrations of arginine vasopressin within the physiological range.

The relationship of plasma vasopressin concentrations in the physiological range to renal electrolyte excretion was investigated. Unanaesthetized rats, when normally hydrated, were found to have a plasma vasopressin concentration of 1.13 +/‐ 0.15 mu u./ml. 16 h water deprivation raised this to 1.98 +/‐ 0.21 mu u./ml. Inactin‐anaesthetized rats infused with 0.45% NaCl had a plasma vasopressin concentration of 1.19 +/‐ 0.18 mu u./ml. Administration of synthetic arginine vasopressin at 6 and 24 mu u./min raised plasma vasopressin levels to 1.88 +/‐ 0.17 and 4.26 +/‐ 0.43 microunits./ml respectively. In addition to the expected antidiuresis, vasopressin at a rate of 6 microunits./min also produced a highly significant increase in Na+ excretion from 8.9 +/‐ 0.6 to 10.5 +/‐ 0.6 mumol/min and Cl‐ excretion from 9.1 +/‐ 0.7 to 10.5 +/‐ 0.7 mumol/min. At 24 microunits./min it produced larger increases in Na+ and Cl‐ excretion. Inactin‐anaesthetized hypophysectomized rats infused with 0.45% NaCl had a plasma vasopressin concentration of only 0.17 +/‐ 0.04 microunits./ml. Administration of vasopressin at 6 and 24 microunits./ml raised plasma vasopressin levels in these animals to 0.63 +/‐ 0.17 and 2.20 +/‐ 0.11 microunits./ml respectively. Hypophysectomized rats failed to exhibit a natriuresis in response to the lower dose of vasopressin, despite exhibiting an undiminished antidiuresis. The failure of the natriuresis may be related to the lower plasma vasopressin concentration achieved. It is concluded that in the rat plasma vasopressin concentrations within the physiological range do influence Na+ and Cl‐ excretion by the kidney as well as controlling urine flow rate.

Determination of tissue and plasma concentrations of PTHrP in fish: development and validation of a radioimmunoassay using a teleost 1–34 N-terminal peptide

A specific and sensitive radioimmunoassay (RIA) for the N-terminus of sea bream (Sparus auratus) and flounder (Platichthys flesus) parathyroid hormone-related protein (PTHrP) was developed. A (1-34) amino-terminal sequence of flounder PTHrP was synthesized commercially and used as the antigen to generate specific antiserum. The same sequence with an added tyrosine (1-35(Tyr)) was used for iodination. Human (1-34) parathyroid hormone (PTH), human (1-34) PTHrP, and rat (1-34) PTHrP did not cross-react with the antiserum or displace the teleost peptide. Measurement of PTHrP in fish plasma was only possible after denaturing by heat treatment due to endogenous plasma binding activity. The minimum detectable concentration of (1-34) PTHrP in the assay was 2.5 pg/tube. The level of immunoreactive (1-34) PTHrP in plasma was 5.2+/-0.44 ng/ml (mean+/-SEM, n=20) for flounder and 2.5+/-0.29 ng/ml (n=64) for sea bream. Dilution curves of denatured fish plasma were parallel to the assay standard curve, indicating that the activity in the samples was indistinguishable immunologically from (1-34) PTHrP. Immunoreactivity was present, in order of abundance, in extracts of pituitary, oesophagus, kidney, head kidney, gills, intestine, skin, muscle, and liver. The pituitary gland and oesophagus contained the most abundant levels of PTHrP, 37.7+/-6.1 ng/g wet tissue and 2.3+/-0.7 ng/g wet tissue, respectively. The results suggest that in fish PTHrP may act in a paracrine and/or autocrine manner but may also be a classical hormone with the pituitary gland as a potential major source of the protein.

A synergistic effect of oxytocin and vasopressin on sodium excretion in the neurohypophysectomized rat.

1. Renal function and the effect of oxytocin and vasopressin replacement have been examined in anaesthetized male neurohypophysectomized rats. 2. Rates of urine flow were higher but sodium excretion markedly lower in neurohypophysectomized rats than in intact animals receiving hypotonic saline infusion (33.8 +/‐ 2.3 vs. 27.0 +/‐ 0.7 ml and 472 +/‐ 84 vs. 1946 +/‐ 124 mumol respectively for the third to sixth hour of study). 3. In intact animals, mean arterial blood pressure stabilized at 106 mmHg. Haematocrit (46%) remained stable but glomerular filtration rates declined slightly over the 8 h of study to 2.5 +/‐ 0.2 ml/h. These values in neurohypophysectomized rats did not differ significantly from those in intact rats. 4. Although plasma corticosterone levels (54 +/‐ 13 ng/ml) did not differ significantly from those in intact rats, neurohypophysectomy was associated with greatly reduced aldosterone concentration (0.12 +/‐ 0.03 vs. 0.76 +/‐ 0.04 ng/ml). Trace levels of vasopressin (0.17 +/‐ 0.03 microunit/ml) were found in neurohypophysectomized rat plasma. 5. Oxytocin administration at 15 microunits/min, which produced plasma hormone levels of 1.62 +/‐ 0.19 microunit/ml, had no detectable effect on sodium excretion but increased urine flow. Arginine vasopressin administration (12 microunits/min) inducing plasma levels of 1.24 +/‐ 0.08 microunit/ml, reduced urine flow by 80% and produced a small increase in sodium excretion. 6. Concurrent administration of oxytocin (15 microunits/min) potentiated the natriuretic response to vasopressin (12 microunits/min). Total sodium excretion during the 3 h combined hormone infusion (1256 +/‐ 149 mumol) greatly exceeded that in animals receiving vasopressin alone (549 +/‐ 132 mumol) and approached that observed in intact animals (1946 +/‐ 124 mumol). Combined hormone administration at the lower rate of 5 microunits/min oxytocin and 4 microunits/min vasopressin produced a similar large increment in sodium excretion. 7. It is concluded that replacement of both neurohypophysial hormones, at plasma levels within the physiological range, largely reverses the renal sodium retention of neurohypophysectomized rats, oxytocin considerably potentiating the natriuretic action of vasopressin. This synergism between the two neurohypophysial peptides to promote salt excretion may be an important component of the non‐steroidal management of sodium.