Endogenous digoxin-like immunoactivity and diabetes mellitus: facts and hypotheses.

Substances with digoxin- and ouabain-like immunoactivity (DLIA) are specific inhibitors of Na(+)-K(+)-ATPase which increase the total amount of intracellular stored calcium (Ca2+i). In diabetic patients, DLIA levels have been reported to be increased. Although this increase is probably secondary to sodium retention and volume expansion (included in diabetic subjects by hyperinsulinemia and/or diabetic nephropathy), the question arises of whether it has pathophysiological consequences: namely, whether substances with DLIA, via their effect on Na(+)-K(+)-ATPase activity and Ca2+i stores, could in diabetic subjects facilitate development of hypertension and/or modulate insulin sensitivity or insulin secretion. Clinical findings of correlations of DLIA to blood pressure, insulin levels and to degree of insulin resistance, together with experimental findings of decreased Na(+)-K(+)-ATPase activity, increased Ca2+i and decreased Mg2+i in both diabetic and hypertensive subjects, support these hypotheses. However, the issue of whether or not these relations are causative and whether or not defects in intracellular milieu are primary or secondary to non-insulin-dependent diabetes mellitus has not been resolved yet. Moreover, pathogenesis of both diabetes mellitus and hypertension is multifactorial and includes many other factors. Therefore, further efforts should be made to elucidate the exact role of substances with DLIA in diabetes mellitus.

[What does the relation of digoxin-like immunoactivity and serum insulin levels in pregnant women signify?]. / Co signalizuje vztah digoxínu podobnej imunoaktivity k hladinám inzulínu v sére tehotných zien?

BACKGROUND: Digoxin-like immunoactivity (DLIA) reflects the presence of endogenous substances which are close to cardiac glycosides. These substances via inhibition of Na(+)-K(+)-ATPase increase intracellular calcium stores (Ca2+i) and may modulate various Ca(2+)-dependent mechanisms. Although DLIA are known primarily as hypertension and natriuresis promoting factors, several recent works have suggested that DLIA relates also to diabetes mellitus. The main stimulus for DLIA secretion represents volume-expansion. AIM OF STUDY: To assess relation of DLIA to glucose tolerance and insulin levels in pregnant women (PW). SUBJECTS AND METHODS: 1) 67 PW (DLIA measured by RIA-kit HUMA-LAB Kosice), 2) 53 PW (DLIA measured by RIA-kit ORION). PW were subdivided according to the glucose tolerance and insulin concentrations. RESULTS: 1. DLIA in hyperinsulinemic PW were significantly higher than in those with normal insulin levels. 2. DLIA significantly correlated with insulin levels as well as with insulinogenic index. 3. The increase in plasma glucose and insulinemia during OGTT was accompanied by a decrease in DLIA. These findings were independent of other measured parameters (age, body mass index, pregnancy induced weight gain, blood pressure and steroid hormones). CONCLUSIONS: These findings suggest that DLIA does not respond only to changes regarding sodium-retention and volume-expansion, but also to changes in glucose and insulin metabolism. Thus, DLIA could represent one of the markers of "specific" neurohumoral activation. However, the question of whether an elevation in DLIA may consequently modulate mechanisms of insulin secretion, insulin sensitivity, vascular reactivity and other Ca2+i-dependent mechanisms remains speculative. (Tab. 4, Fig. 4, Ref. 41).

Head-up tilt and lower body suction: comparison of hormone responses in healthy men.

The purpose of this study was to compare, in the same subjects, hormonal responses to 30-min head-up tilt (HUT) and lower body suction (LBNP) of different intensity (24 degrees and 70 degrees, and 15 and 35 mm Hg, respectively). Basal pooled individual data from -10 min (n = 32) were within normal reference limits: norepinephrine (NE) averaged 318 +/- 23 pg/ml; epinephrine, 34.0 +/- 5.5 pg/ml; plasma renin activity (PRA), 0.72 +/- 0.08 ng ATII/ml/h; aldosterone, 164 +/- 20 pg/ml; atrial natriuretic peptide (ANP), 29.9 +/- 2.0 pg/ml; cGMP, 6.29 +/- 0.59 mmol/l; cortisol, 95.7 +/- 5.8 ng/ml; and ACTH, 50.3 +/- 2.6 pg/ml. The low-level stimuli failed to induce consistent changes in hormone levels. From the onset of the stimulus (minute 0) to its termination (minute 30), norepinephrine (NE) increased by 101% with LBNP-35, and by 70% with HUT70, respectively. The NE increase with LBNP-35 was higher (p < 0.05) than with HUT70. Epinephrine rose with HUT70 (by 162%) only. PRA increased by 157% with LBNP-35, and by 119% with HUT70, respectively; these responses were not significantly different. Aldosterone rose equally (by 85 and 89%) with LBNP-35 and HUT70 but not with the low-level stimuli. No consistent changes were observed in ANP, c-GMP or ACTH concentrations. Cortisol values fell during the LBNP and HUT24 situations but rose transiently after HUT70. We conclude that the hormones investigated respond differently to head-up posture and lower body suction and in a specific manner. Greater effects of high-level stimuli (HUT70, LBNP-35) were noted as compared to low-level stimuli (HUT24, LBNP-15). The application of combined sets of models stimulating the cardiovascular system may aid in the analysis of responses of hormonal systems in man.

[The clinical significance of natriuretic hormones]. / Klinický význam nátriuretických hormónov.

Out of all until now discovered natriuretic factors it is still the atrial natriuretic factor (ANF) which is the most significant with its diuretic, natriuretic and vasodilatory effects. Its effect is antagonistic to sodium retention factors. The increase of its levels in arterial hypertension is more of secondary character, but according to some authors the functional deficit of ANF secretion can be applied also primarily in the development and maintenance of high blood pressure. ANF levels represent a good marker of the clinical severeness and are of prognostic value. Increased levels were detected also in cases of renal failure and partially in hepatic cirrhosis. Natriuretic hormone, in comparison to ANF, is a natriuretic and vasoconstrictive substance, the effect of which is based on the mechanism of sodium pump inhibition. Chemically the main candidate is represented by endogenous ouabain, or a digitalis-like activity. It increases physiologically due to the expansion of extracellular fluid during gravidity and in newborn. Its pathological increase is brought about by some forms of essential hypertension and in the diseases associated with fluid retention and edema development. Cirrhosis of the liver can reflect both the degree of sodium retention and haemodilution, as well as the severeness of hepatic lesion. (Tab. 2, Ref. 30.).

[Neurohumoral activity in liver cirrhosis]. / Neurohumorálna aktivita pri cirhóze pecene.

The authors assessed in 40 patients with cirrhosis of the liver and in 33 controls the plasma renin activity (PRA), aldosterone (PA), the atrial natriuretic factor (ANF) and the digoxin like activity (DLA) in plasma under basal conditions. In patients with cirrhosis of the liver they found significantly lower levels of PRA, PA and DLA, as compared with the control group, the ANF levels were not significantly altered. In the group with cirrhosis the highest neuroendocrine activity was recorded, in particular of PRA and PA in decompensated cirrhotics receiving diuretic treatment. Therefore it is useful to combine diuretics with preparations or measures which reduce the activity of the renin-angiotensin-aldosterone system and/or promote the activity of natriuretic substances. The authors found a negative correlation between PRA and SNa, PRA and UNaV, while ANF did not correlate with natriuresis. The main determinant of Na excretion in decompensated cirrhosis is the activity of the renin-angiotensin-aldosterone system. DLA plasma levels also correlated inversely with SNa values and Na excretion and thus also reflect the severity of fluid retention.

[Personal experience with determination of endogenous, digoxin-like substances in patients with myocardial infarct and other cardiopathies]. / Nase skúsenosti s vysetrením endogénnych, digoxínu podobných látok u chorých s infarktom myokardu a inými kardiopatiami.

A group of 65 patients with acute infarction of myocardium (IM) who were not treated with digitalis preparations were subdued to examination to the presence of digitalis-like substances in their urine by means of radioimmuno-analytic method with use of anti-digoxin antibodies. The control group was constituted of 69 healthy subjects. Patients afflicted with IM had significantly increased concentrations of DLS in serum in comparison with health subjects. No significant relations of DLS to the activity of creatinkinase, IM localisation, occurrence of dysrhythmias, heart insufficiency and IM mortality were discovered. An increase in DLS in the blood of patients with acute IM probably coincides with a decreased cardiac output, with the activation of the stress axis and retention of sodium and fluids. The second examined group of patients was constituted of 20 subjects with other severe cardiopathies (inborn and acquired heart defects, chronic ischemic heart disease, inflammatory and degenerative diseases of the heart, and hypertension), who were subdued to catheter examinations. The authors discovered no significant differences of DLS concentrations in the blood during catheterization of individual compartments of inferior vena cava, superior vena cava, and the right ventricle. They were not successful in defining the particular site of DLS secretion on the basis of this examination. The authors pay attention to interaction of DLS during the radioimmuno-analytic examination of the digoxin serum concentration.

[Endogenous digoxin-like and ouabain-like substances]. / Endogénne, digoxínu a ouabainu podobné látky.

Despite the fact that cardioglycosides have been used in the treatment of cardiac failure for more than 2000 years, endogenous digoxin and ouabain-like substances were identified from the chemical aspect only in 1990. They are steroids, their main site of origin being the adrenals. It is assumed that they play a part in the regulation of the body fluids and sodium in the organism and participate in the regulation of cardiac activity. The stimulus which leads to their release into the blood stream are above all conditions associated with sodium and fluid retention with expansion of the intravascular volume. It is assumed that these substances participate in the pathogenesis of some types of hypertension. The authors present a general review of contemporary knowledge of endogenous digoxin and ouabain-like substances.

Blood pressure regulation in ANF-transgenic mice: role of angiotensin and vasopressin.

Transgenic mice overexpressing a transthyretin promoter-ANF structural fusion gene have a life-long reduction in arterial blood pressure compared to nontransgenic littermates. The present study was designed to test the hypothesis that the high plasma level of ANF in the transgenic mice inhibits the renin-angiotensin and/or vasopressin systems, thereby causing the hypotension. Mice were anaesthetized with Inactin and arterial pressure and heart rate were monitored before and during Saralasin infusion and vasopressin V1 receptor blockade. Effectiveness of the blockade was determined by injection of angiotensin and vasopressin before and during Saralasin and V1 receptor antagonist administration. Saralasin was associated with hypotension in both transgenic and nontransgenic mice. The decrease in blood pressure was proportionally greater in the transgenic animals. Vasopressin receptor blockade had little effect on blood pressure in either group. Heart rates were not different between the groups during any maneuver. We conclude that the chronic hypotensive effect of ANF overproduction does not involve the inhibition of either renin-angiotensin or vasopressin systems. The data, however, suggest that the renin-angiotensin system may be stimulated in the ANF-transgenic mice.

The role of endogenous digitalis-like factor in blood pressure regulation in essential hypertension.

We examined plasma renin activity (PRA), plasma aldosterone (PA), atrial natriuretic factor (ANF) and endogenous digitalis-like factor (DLF) in 15 healthy subjects and 15 patients with essential hypertension (EH) to obtain basal values and values after extracellular fluid volume (ECFV) expansion caused by infusion of isotonic saline solution over a period of 2 hours (20 mg/kg). A significant increase in diuresis and natriuresis accompanied by a decrease in PRA and PA and an increase in ANF was observed in both groups. No significant differences were observed in ANF levels between normotensive subjects and hypertensive patients. Hypertensive patients showed significantly higher basal values of DLF than normotensive subjects. However, an increase in plasma DLF following ECFV expansion was observed only in the group of healthy subjects. There was a positive correlation between ANF and natriuresis and a negative correlation between ANF and systolic and diastolic blood pressure (BP). Changes in DLF correlated positively with changes in diastolic BP in both groups, while in healthy subjects a negative correlation was recorded between PRA and DLF. We conclude that the increased diuresis and natriuresis in ECFV expansion is presumably accounted for not only by suppression of PRA and PA, but also by stimulated ANF secretion, while in healthy subjects stimulation of DLF may be involved as well. The insufficient DLF response to saline infusion may indicate an exhausted DLF reserve in hypertensive patients. As a vasoactive substance, DLF can participate in the regulation of blood pressure and play a role in the pathogenesis of arterial hypertension.

[The effect of water immersion on humoral and urinary parameters in healthy persons and in patients with liver cirrhosis]. / Vplyv vodnej imerzie na humorálne a mocové parametre u zdravých osôb a chorých s cirhózou pecene.

The authors subjected 10 healthy subjects and 13 patients inflicted by liver cirrhosis in the stage of vascular decompensation (ascites and/or oedemas) to water immersion (WI). The group of healthy subjects responded during WI by a significant increase of diuresis and sodium diuresis at its maximum in the third hour, which was accompanied by a decrease of plasma aldosterone (PA) and a decrease of atrial natriuretic factor (ANF) and digoxin-like activity (DLA). The decrease of plasma vasopressin (PAVP) was not statistically significant. In patients with liver cirrhosis a significant increase of diuresis and sodium diuresis took place, whilst the response was significant already in the first hour of WI. Only 3 non-respondents were among the ill patients. In the group of liver cirrhotics also a significant decrease of AVP, PA, and an increase of ANF and DLA were recorded, the response of humoral factors was not significant due to great variability. A long term administration of diuretics (Spirolakton+Furosemid) did not decrease the serum K in ill patients. Hence, WI by means of supervention of volume expansion evokes an increase of diuresis and natriuresis in both healthy and ill subjects, the latter with decompensated liver cirrhosis which is associated with a controversial reaction of sodium diuresis and sodium retention factors. It is possible to use it as a supplementary method in the therapy of oedemas due to liver cirrhosis besides diuretic therapy. (Tab. 2, Fig. 3, Ref. 22.)

Effect of a low salt diet on blood pressure and vasoactive hormones in the hereditary hypertriglyceridemic rat.

In hereditary HTG rats, basal systolic blood pressure using tail-cuff sphygmomanometry was significantly higher (122.1 +/- 2.1 mm Hg; n = 16) than that in NTG animals (107.1 +/- 1.52; n = 16). A low salt diet did not influence blood pressure in NTG rats during the consecutive 4 weekly periods. However, in the second week blood pressure in HTG rats rose significantly in both the control rats on a normal salt diet and those on a low salt diet (132.5 +/- 1.89, n = 8, and 132.6 +/- 1.93, n = 8). No further changes were registered in the third and fourth week in control HTG rats. On the other hand, blood pressure fell significantly in HTG rats on a low salt diet in the third week in comparison with the second week (119.5 +/- 3.2, n = 8), and it increased again in the fourth week (123.0 +/- 2.35, n = 8). Hormones in plasma were determined at the end of the experiment. Plasma levels of norepinephrine were not influenced by differences in salt intake and were significantly higher by about 45% in HTG than in NTG animals. The lowest concentration of corticosterone in plasma was found in control HTG rats (1.2 +/- 0.2 vs 4.6 +/- 0.8 micrograms/100 ml in control NTG rats). Nevertheless, corticosterone concentration increased in HTG rats on a low salt diet at comparable values found in NTG rats on a low salt diet (3.1 +/- 0.8 vs 4.3 +/- 1.5). Plasma renin activity and plasma aldosterone concentrations were not different in the NTG and HTG groups and were uninfluenced by the diets (Table 1). We conclude that the elevated blood pressure in HTG rats and its variations during the experiment may reflect more pronounced sympathetic activity in HTG rats rather than blood pressure dependency on different salt intake.

On the role of digoxin-like substances, ANP, and AVP in natriuresis induced by hypertonic saline infusion in dogs.

The increase of sodium concentration in cerebrospinal fluid or in plasma triggers the osmoregulatory mechanism, namely, the enhancement of renal free-water reabsorption and natriuresis. The increase of free-water reabsorption has been recognized for many years as a consequence of the osmotically released vasopressin (AVP). However, the control of renal sodium excretion in the mechanism of osmoregulation has not been clarified It has been suggested to be, at least in part, of hormonal nature, implying the decreased release of aldosterone and the increased release of atrial natriuretic peptide (ANP), digoxin-like substances (DLIS), and AVP. Neither of these factors, however, has been unequivocally linked to the mechanism of immediate natriuresis caused by an acute increase in cerebrospinal fluid or plasma sodium concentration. It was reconfirmed in our present experiments in anesthetized dogs that aldosterone, ANP, and DLIS could hardly play a role in the immediate natriuresis after the i.v. infusion of hypertonic saline (20% NaCl solution infused in 20 min in an amount that was 0.13% of body weight). However, the role of AVP in this type of natriuresis seems more promising as a V1/V2 receptor antagonist applied i.v. before the hypertonic saline loading completely prevented the increase of renal sodium excretion. Natriuresis after the isotonic saline load was not impaired by the same antagonist of vasopressin receptors.

Hydroosmotic activities of arginine-vasopressins modified either in positions 1, 2 and 4 or at N-terminal extensions.

Vasopressin and its synthetic analogs were studied for their effect on transepithelial water flux in frog urinary bladder. As compared with AVP, 1-deamino-8-D-arginine vasopressin (dDAVP) was about 40 times less effective in stimulating osmotic water flow. The vasopressin analogs obtained by modification in positions 1 and 2 were: [1-(1-mercapto-4-tert-butylcyclohexaneacetic acid)] AVP (I); [1-(1-mercapto-4-methylcyclohexaneacetic acid)]AVP (II); [1-(1-mercapto-4-methylcyclohexaneacetic acid)-2-O-methyltyrosine]AVP (III); and those modified in position 4 were: [1-(1-mercaptocyclohexaneacetic acid)-4-arginine] AVP (IV); [1-(2-mercaptopropionic acid)-4-arginine]AVP (V). Any of the above analogs did not influence basal, but antagonized vasopressin-stimulated water flux. N-terminally extended analogs of AVP: Ala-AVP (VI); Ser-Ala-AVP (VII) and Thr-Ser-Ala-AVP (VIII) stimulated osmotic water flux to the same extent in concentration 200 times higher as that of AVP. We conclude from these studies that vasopressin analogs (I-V) competitively antagonize vasopressin-stimulated hydroosmotic activity in frog urinary bladder probably at the epithelial vasotocin V1 and/or V2 receptor site. N-terminal extension of the vasopressin molecule did not influence the capacity of AVP to induce V2 receptor-mediated action, even when used at higher concentrations.

Changes of hormones regulating electrolyte metabolism after space flight and hypokinesia.

The changes of hormones in plasma involved in the body fluid regulation were studied in human subjects during and after space flights in relation to redistribution of body fluids in the state of weightlessness. Since hypokinesia was used as a model for simulation of some effects of the stay in microgravity the plasma hormone levels in rats exposed to hypokinesia were also investigated. Plasma aldosterone values showed great individual variations during the first inflight days, the increased levels were observed with prolongation of space flights. The important elevation was found in the recovery period, however it was interesting to note, that in some cosmonauts with repeated exposure to space flight, the postflight plasma aldosterone levels were not elevated. The urine excretion of aldosterone was increased inflight, however in postflight period the decrease or increase were found in the first 1-5 days. The increase of plasma renin activity was observed in flight and postflight period. The rats were exposed to hypokinesia (forced restriction of motor activity) for 1, 7 and 60 days and urine was collected during last 24 hours. The animals were sacrificed and the concentration of electrolytes and of levels of corticosterone, aldosterone (A), ANF and plasma-renin activity (PRA) were determined in plasma. In urine excretion of sodium and potassium were estimated. An important increase of plasma renin activity and aldosterone concentration was found after short-term hypokinesia (1 day). These hormonal values appear to decrease with time (7 days) and are not significantly different from controls after long-term hypokinesia (60 days). A decrease of values ANF in plasma was observed after 1 and 7 days hypokinesia. After prolonged hypokinesia a decrease of sodium plasma concentration was observed. The excretion of sodium in urine was higher in long-term hypokinetic animals. There were no significant changes of plasma potassium levels in rats exposed to hypokinesia, however the urinary excretion of potassium was elevated. In rats exposed to hypokinesia for 7 and 60 days an increase of urine osmolality was observed. The results of hormone and electrolyte determination in plasma of cosmonauts after space flight and in experimental animals after hypokinesia suggested that in evaluation of relations between the changes of hormone levels and electrolyte in plasma and urine other factors like emotional stress working load; altered diurnal cycles should be considered in interpretation of homeostatic response of fluid and electrolyte metabolism to space flight conditions.

Endogenous digitalis-like factor in patients with acute myocardial infarction.

The authors examined endogenous digitalis-like factor (DLF) concentrations in the serum and urine in 65 patients with acute myocardial infarction. Radioimmunoassay was used for the examination and patients' data were analyzed in detail in relation to sex, risk factors and acute myocardial infarction complications. The concentrations of digitalis-like factor found in the serum of men (0.317 +/- 0.026 micrograms/l) and women (0.256 +/- 0.057 micrograms/l) with acute myocardial infarction were much higher compared with values of healthy men (0.009 +/- 0.004 micrograms/l) and women (0.015 +/- 0.012 micrograms/l). This finding is in agreement with data published by others and suggests a role of DLF in the pathogenesis of myocardial infarction.

Periventricular lesions block natriuresis to hypertonic but not isotonic NaCl loads.

The renal excretion of Na and water after an intravenous load of hypertonic or isotonic saline was studied in conscious sheep in which periventricular tissue in the region of the lamina terminalis had been ablated. Hypertonic saline (3.4-4.2 mmol/l) was infused at 0.06 mmol.kg-1.min-1 for 40 min into the jugular vein. Plasma Na concentration increased 5 mmol/l, and in normal sheep a natriuresis and increase in glomerular filtration rate ensued during the next hour. Such a natriuretic effect did not occur in sheep with periventricular lesions. By contrast, intravenous infusion of isotonic saline (30 ml/kg body wt, i.e., 0.23 mmol.kg-1.min-1 for 20 min) caused similar increase in renal Na excretion in normal sheep and sheep with periventricular lesions. When the same intravenous load of NaCl (0.23 mmol.kg-1.min-1 for 20 min) was administered as hypertonic 20% NaCl, ablation of periventricular tissue greatly impaired the excretion of this Na load. We suggest that the periventricular tissue in the region of the lamina terminalis has a role in regulation of renal Na excretion in conditions where the plasma Na concentration increases. This tissue is also involved in osmoregulatory thirst and vasopressin secretion. We further propose that increased renal Na excretion in response to hypernatremia is another cerebrally mediated osmoregulatory response.

Failure to demonstrate natriuretic activity in the posterior pituitary after immunoneutralization of the vasopressin content.

The attempt to demonstrate the presence of a natriuretic substance in the posterior pituitary after immunoneutralization of the AVP content failed. Rats infused i.v. AVP-immunoneutralized posterior pituitary extract did not respond by natriuresis to a saline infusion, in contrast to those infused untreated posterior pituitary extract. Thus, vasopressin seems to be the natriuretic substance in the posterior pituitary extract.

Suppression of natriferic activity of the vasopressin molecule by modifications in positions 1, 2 and 4.

The capacity of five synthetic analogs of [8-arginine] vasopressin (AVP) to stimulate frog skin sodium transport (natriferic activity) was characterized electrophysiologically using the method of short-circuit current, and compared to that of synthetic AVP. The analogs used were [8-arginine] vasopressins modified in positions 1 and 2: [1-(1-mercapto-4-tert-butylcyclohexaneacetic acid)] AVP (I); [1-(1-mercapto-4-methylcyclohexaneacetic acid)] AVP (II); [1-(1-mercapto-4-methylcyclohexaneacetic acid)-2-O-methyltyrosine] AVP (III); and in position 4: [1-(1-mercaptocyclohexaneacetic acid)-4-arginine] AVP (IV); [1-(2-mercaptopropionic acid)-4-arginine] AVP (V). The addition of synthetic vasopressins I, II and V to the frog skin resulted in a weaker stimulation of the skin sodium transport, measured as the level of the short-circuit current (Isc), as compared to that induced by synthetic AVP. In relation to natriferic activity, analogs III and IV did not change the electrical parameters of the skin. It is concluded that introduction of cyclic structure at the beta-carbon in position 1 of the vasopressin molecule decreased its natriferic activity by about 70%. The same reduction of the activity was caused by the replacement of the glutamine residue in position 4 with arginine, and deamination in position 1. Cyclic structure bound in position 1 together with methylation of tyrosine in position 2 resulted in a full suppression of natriferic activity. Similarly, introduction of cyclic group in position 1 in combination with substitution of glutamine in position 4 with arginine totally abolished natriferic activity.

Stimulation of the sodium transport across the frog skin by three N-terminally extended arginine-vasopressins.

The standard Ussing method was used to electrophysiologically characterize the effects of three analogs of arginine-vasopressin (AVP) on the frog skin, a model Na-transporting epithelium. The analogs tested were N-terminally extended Arg8-vasopressins: Ala-AVP, Ser-Ala-AVP and Thr-Ser-Ala-AVP; synthetic Arg8-AVP was used as the reference agent. The vasopressins were applied to the basolateral side of the frog skin in concentrations ranging between 10(-8) to 10(-5) mol.l-1. All the three analogs increased both the short-circuit current (Isc) and the open-circuit transepithelial potential (Voc), and decreased the transepithelial d.c. resistance (Rt) similarly as did synthetic Arg8-AVP. The results show that N-terminal extension of the Arg8-AVP did not alter the natriferic properties of AVP.