Water and sodium homeostasis in conscious rabbits: role of arginine-vasopressin

The aim of this study was to assess the role of arginine-vasopressin (AVP) on extracellular water distribution (VE) and on the urinary excretion of sodium in conscious rabbits with or without water deprivation. The study included two groups of animals. The first group involved two subgroups, both receiving an infusion of 0.9% NaCl-5% glucose (50-50, v:v) at the rate of 24 ml/hr to adequately hydrate the rabbits, but one received 2.5 ng/min/kg of AVP with the infusion; 120 min later, a bolus of inulin was injected to assess the distribution in the extracellular water. The rabbits of the second group were divided into 3 subgroups, receiving a 0.9% NaCl-5% glucose (50-50, v:v) infusion, at the rate of 6 ml/min; two subgroups received AVP, at the rate of 2.5 or 5 ng/min/kg; 40 minutes later, all animals received a bolus of inulin. In adequately hydrated rabbits, the infusion of AVP generated plasma levels of 35 +/- 7 pg/ml; the VE decreased from 204 +/- 23 to 111 +/- 10 ml/kg (p < 0.01) because of a decrease in plasma (VP) and interstitial volumes (VI). In the same group, natriuresis and osmolal clearance increased and free water clearance decreased. In rabbits deficiently hydrated, AVP reached plasma levels of 47 +/- 18 and 86 +/- 12 pg/ml following the 2.5 and 5 ng/min/kg infusions, respectively. In these animals, high plasma levels of AVP did not diminish VE but decreased the diuresis and osmolal clearance. It is concluded that in adequately hydrated rabbits, pathophysiological plasma levels of AVP shift the water from the VE to the intracellular space and increase the natriuresis, but in partially hydrated rabbits, the infusion of AVP maintains plasma volume constant because of an antidiuretic effect.

Pharmacokinetic and pharmacologic properties of antiuterotonic oxytocin analogs in the rat

The pharmacologic and pharmacokinetic properties were evaluated in a series of antiuterotonic oxytocin analogs, modified at positions 1, 2, 4, 8 and, in one case, position 9 of the oxytocin (OT) molecule. [Mpa1,D-Tyr2(Et),Val4,Orn8,desGly9]-OT, [Mpa1,Tyr2(Et),Val4,Orn8]-OT and [Mpa1,D-Tyr2,Val4,Orn8]-OT displayed similar plasma clearance rates (Clps) using the constant infusion method in rats. Two analogs, [Mpa1,D-Tyr2(Et),Val4,Orn8]-OT and, particularly, [Mpa1,D-Tyr2(Et),Thr4,Orn8]-OT, were cleared at significantly higher rates compared with the others. [Mpa1, D-Tyr2(Et), Val4, Orn8]-OT and [Mpa1, D-Tyr2(Et), Thr4, Orn8, desGly9]-OT were most potent in eliciting a short-term in vivo antiuterotonic effect, whereas the duration of effect was longest for [Mpa1, D-Tyr2, Val4, Orn8]-OT and [Mpa1, D-Tyr2(Et), Thr4, Orn8, desGly9]-OT. The Clp of [Mpa1, D-Tyr2, Val4, Orn8]-OT was similar regardless of the infusion rate. No relationship between antiuterotonic effect and Clp of the five peptides could be demonstrated, and no significant linear correlation between Clp and effect duration was found. The apparent volumes of distribution for the present analogs were 10-fold larger than the blood volume, a finding to be considered when measuring in vivo antagonistic activity. The 24-h urinary excretion ranged from 14.3 to 25.6% of the i.v. dose and was negatively correlated with peptide lipophilicity. It is concluded that, in addition to diverging pharmacologic properties, peptide analogs may differ markedly in kinetic parameters like Clp, volumes of distribution and urinary excretion despite minor molecular modifications.

Metabolic clearance rate of arginine vasopressin in severe chronic renal failure

1. The metabolic clearance rate of arginine vasopressin was determined using a constant infusion technique in normal subjects and patients with chronic renal failure immediately before commencing dialysis. Endogenous arginine vasopressin was suppressed in all subjects before the infusion with a water load. 2. Plasma arginine vasopressin concentrations were determined using a sensitive and specific radioimmunoassay after Florisil extraction. The detection limit of the assay was 0.3 pmol/l, and intra- and inter-assay coefficients of variation at 2 pmol/l were 9.7% and 15.3%, respectively. 3. In normal subjects, the metabolic clearance rate was determined at two infusion rates producing steady-state concentrations of arginine vasopressin of 1.3 and 4.4 pmol/l. In the patients with renal failure, a single infusion rate was used, producing a steady-state concentration of 1.5 pmol/l. 4. At comparable plasma arginine vasopressin concentrations, metabolic clearance rate was significantly reduced in patients with renal failure (normal 1168 +/- 235 ml/min versus renal failure 584 +/- 169 ml/min; means +/- SD; P < 0.001). 5. Free water clearance was significantly reduced in normal subjects during the arginine vasopressin infusion from 8.19 +/- 2.61 to -1.41 +/- 0.51 ml/min (P < 0.001), but was unchanged in the patients with renal failure after attaining comparable plasma arginine vasopressin concentrations. 6. In normal subjects there was a small but significant fall in metabolic clearance rate at the higher steady-state arginine vasopressin concentration (1168 +/- 235 ml/min at 1.3 pmol/l versus 1059 +/- 269 ml/min at 4.4 pmol/l; P = 0.016).(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic clearance rate of arginine vasopressin in patients with cirrhosis

Metabolic clearance rate and half-time of arginine vasopressin were measured in 43 cirrhotic patients and 10 control subjects. Synthetic arginine vasopressin was infused intravenously at a rate of 500 pg/min/kg of body weight for 75 min. The metabolic clearance rate was significantly reduced, and the half-time of arginine vasopressin after stopping the infusion was significantly increased in patients with cirrhosis, particularly in those with ascites and in those with moderate or severe liver dysfunction. Changes in metabolic clearance rate and half-time of arginine vasopressin correlated with the score of the liver dysfunction, prothrombin activity and levels of serum albumin and bilirubin but not with parameters of kidney function (serum creatinine levels and clearance of creatinine). We conclude that reduced metabolic clearance rate and prolonged half-time of vasopressin in plasma are frequent findings in cirrhotic patients with poor liver function. This impaired catabolism of antidiuretic hormone may contribute to maintaining elevated plasma levels of this hormone in these patients and may be an additional factor leading to fluid retention and to dilutional hyponatremia.

Transport, uptake, and metabolism of blood-borne vasopressin by the blood-brain barrier

Transport, binding, and metabolism of [phenylalanyl-3,4,5,-3H(N)]arginine vasopressin (AVP) by the blood-brain barrier (BBB) was studied in adult guinea-pigs by means of a novel vascular brain perfusion (VBP)/capillary depletion technique and HPLC. A time-dependent, progressive brain uptake of 3H-radioactivity was measured over the 10 min period of VBP both in brain homogenates and in brain tissue depleted of cerebral microvessels. The unidirectional blood-to-brain transport constant, K(IN), estimated by multiple-time tissue uptake analysis of the homogenate and postcapillary supernatant, indicated that the BBB transfer rate for [3H]AVP (K(IN) = 2.37 +/- 0.25 microliters min-1 per gram brain homogenate) was almost 10 times higher than for simultaneously perfused [14C]sucrose, a cerebrovascular space marker. In contrast to homogenate and postcapillary supernatant, the [3H]radioactivity determined in the vascular pellet after dextran density centrifugation of the brain homogenate was very low and only somewhat higher than for [14C]sucrose. HPLC analysis of the perfused brain tissue revealed time-dependent degradation of the blood-borne neuropeptide. The percentage of intact [3H]AVP as determined in the postcapillary supernatant progressively declined during brain perfusion, from 49% at 1 min to 11.9% at 10 min. The major detectable labeled metabolite was [3H]phenylalanine, the labeled amino acid residue of [3H]AVP. The aminopeptidase inhibitor bestatin (0.5 mM), perfused simultaneously with [3H]AVP by the VBP technique, did not alter tissue uptake of [3H]AVP, indicating that there was no significant hydrolysis of peptide by the luminal BBB surface.(ABSTRACT TRUNCATED AT 250 WORDS)

Prolonged systemic delivery of peptide drugs by long-circulating liposomes: illustration with vasopressin in the Brattleboro rat

The value of novel systemically long-circulating liposomes to prolong the duration of an antidiuretic hormone, arg8-vasopressin (VP), was investigated as a representative of low molecular weight peptides with rapid clearance. Cholesterol content was found to have a controlling effect on VP release in serum. Three types of liposomes were selected for urine production measurements in VP deficient Brattleboro rats. One contained phosphatidylserine (PS), which was rapidly cleared from the circulation. In the other two liposomes, the PS component was replaced by either phosphatidylglycerol or a novel phospholipid derivatized with polyethylene glycol (PEG); both showing prolonged circulation. Free VP (up to 8 micrograms/kg) gave reduced urine production for less than 24 hr. The PG formulation exhibited a dose-dependent prolonged duration of bioactivity of up to 4 days. Substitution of PEG-PE resulted in a 2-day delay followed by a prolonged duration of bioactivity for over 4 days. The duration of the prolonged bioactivity was not dose dependent but the amplitude was. This is attributed to VP release from liposomes which have distributed intact to another compartment without having been taken up by the RES. By balancing liposome circulation time, release rate, and dose, long-circulating liposomes can be applied to prolong the biological activity of a therapeutic peptide.

Localization and quantification of nonapeptide binding sites in the kidney of Xenopus laevis: evidence for the existence of two different nonapeptide receptors

The distribution and properties of nonapeptide binding sites in the kidney of the anuran Xenopus laevis were investigated using quantitative in vitro autoradiography. The binding studies were performed with [3H]arginine vasopressin (AVP) as ligand because [125I]arginine vasotocin (AVT) lacks biological activity. Specific binding sites for [3H]AVP are located in the glomeruli of the kidney. [3H]AVP binding results in a steady state of association and dissociation between ligand and binding sites. Scatchard and Hill analyses of saturation experiments showed that [3H]AVP binds to a single class of binding sites with a dissociation constant (Kd) of 430 +/- 109 pM and a maximum binding capacity (Bmax) of 5.306 +/- 1.379 fmol/mm2 (n = 8). Displacement studies demonstrated the same affinity of these [3H]AVP binding sites to [3H]AVP, unlabeled AVP, and AVT, whereas mesotocin possesses only weak affinity. Further nonapeptides like oxytocin and isotocin or the mammalian-specific V1 receptor antagonist [1-beta-mercapto-beta,beta-cyclopentamethylene propionic acid)-2-(O-methyl)-tyrosine)-AVP or the V2 receptor agonist (1-deamino-8-D-arginine)-vasopressin or unrelated peptides did not alter the binding of [3H]AVP. The localization of nonapeptide binding sites in the glomeruli with the same affinity to AVP as to AVT agrees with the finding that AVT causes antidiuresis in Xenopus laevis. An earlier study demonstrated Xenopus laevis interrenal tissue to possess a higher sensitivity for AVT than AVP which points to a nonapeptide receptor with a higher affinity for AVT than AVP.(ABSTRACT TRUNCATED AT 250 WORDS)

Oral-nasal membranes are not the major route for fetal absorption of amniotic fluid arginine vasopressin

Intraamniotically injected substances such as arginine vasopressin and digoxin have been found to rapidly appear in the ovine fetal circulation, irrespective of whether the fetal esophagus has been ligated or occluded. To determine if the ovine fetal oral-nasal membrane plays a significant role in this fetal absorption of amniotic substances, we used two groups of chronically catheterized fetal sheep with a surgical glove sewed over the fetal head to prevent access of the head to amniotic fluid. In the first group 22.5 micrograms of arginine vasopressin was injected into the amniotic cavity; in the second group 22.5 micrograms was injected into the glove over the fetal head. We found that, after injection into the amniotic cavity, there were rapid and highly significant increases in amniotic fluid arginine vasopressin concentrations, from 6.1 +/- 1.3 to 51,249 +/- 18,182 pg/ml (mean +/- SE) (p less than 0.00001). Concurrently there was a rapid increase in fetal plasma arginine vasopressin concentrations from 4.5 +/- 1.3 to 93.8 +/- 18.9 pg/ml (p less than 0.00001). The increase was significant within 15 minutes and reached a maximum at 60 minutes after the injection. Fetal arterial pressure increased by 10 +/- 2 mm Hg, whereas heart rate decreased by 30 +/- 5 beats/min (p less than 0.00001). In contrast, after the injection into the glove covering the fetal head, there were no significant changes in any of the measured parameters. This suggests that the ovine fetal oral-nasal membrane is not a significant route of absorption of amniotic fluid arginine vasopressin and that the most likely route of absorption is the vascularized fetal surface of the placenta and vascularized fetal membranes, i.e., the intramembranous pathway.

Effect of V2 antagonist on clearance of arginine vasopressin by isolated perfused rat kidneys

Isolated rat kidneys were perfused with Krebs-Henseleit-bovine serum albumin solution at a mean pressure of 99 +/- 2.6 mmHg. After control periods, arginine vasopressin (AVP) was added to the perfusate at a final calculated concentration of 25 pg/ml (2.5 x 10(-11) M). Urine and perfusate samples were collected at 15-min intervals for the following 60 min to measure kidney function and the renal clearance of immunoreactive AVP (irAVP). At 15-30 min after the addition of AVP, total renal clearance of irAVP was 1,623 +/- 190 microliters.min-1.g kidney wt-1. Glomerular filtration accounted for 35 +/- 3.0% of the total clearance, and 65 +/- 10.3% was cleared by peritubular pathways. Of the filtered irAVP, 48 +/- 4.8% was recovered in the urine. To investigate the importance of V2 receptors in the metabolism of AVP, clearance measurements were made in the presence of the V2 antagonist [d(CH2)5,D-Ile2,Ile4,Arg8]AVP (5 x 10(-9) M). Total renal clearance of irAVP was reduced by 48% to 848 +/- 79 microliters.min-1.g-1. This reduction was entirely accounted for by the complete inhibition of peritubular clearance of irAVP. In the presence of the V2 antagonist, irAVP was cleared only by filtration. The proportion of filtered AVP recovered in the urine (53 +/- 8.7%) was not significantly altered by the presence of the V2 antagonist. We conclude that a major component of the renal clearance of AVP depends on receptor-mediated uptake of AVP in the kidney cells.

Rapid intramembranous absorption into the fetal circulation of arginine vasopressin injected intraamniotically

Recently an intramembranous pathway was reported in the ovine fetus as a route for the movement of a significant volume of water from the amniotic cavity directly into the fetal blood, which perfuses the fetal membranes and fetal surface of the placenta. To test whether this pathway could be an avenue for the movement of arginine vasopressin from the amniotic cavity into the fetal circulation, we injected 1 to 25 micrograms of arginine vasopressin into the amniotic cavity of two groups of chronically catheterized fetal sheep: a control group of seven animals and a group of seven animals with surgical ligation of the fetal esophagus. We found similar and highly significant increases of arginine vasopressin concentrations in both control and surgically ligated fetuses in amniotic fluid (p less than 0.00001), fetal plasma (p less than 0.0001), and fetal urine (p less than 0.0001). Both groups had similar increases in arterial (p less than 0.0001) and venous (p less than 0.003) pressures with simultaneous decreases in urine flow (p less than 0.001) and heart rate (p less than 0.0001) after the intraamniotic injection of arginine vasopressin. We conclude that amniotic arginine vasopressin can be rapidly absorbed in its biologically active form directly into the fetal circulation through the intramembranous pathway. Furthermore, the observation that esophageal ligation did not alter this absorption suggests that the intramembranous pathway may be important in the regulation of amniotic fluid volume and composition.

Kinetics of arginine-vasopressin uptake at the blood-brain barrier

Uptake of arginine-vasopressin, VP, at the luminal side of the blood-brain barrier (BBB) was studied by means of an in situ brain perfusion technique in the guinea-pig. Kinetic experiments revealed a saturable peptide influx into the parietal cortex, caudate nucleus and hippocampus with Km between 2.1 and 2.7 microM, and Vmax ranging from 4.9 to 5.6 pmol.min-1.g-1. The non-saturable component, Kd, was not significantly different from zero. Influx of VP into the brain was not altered by the presence of the peptide fragments: VP-(1-8), pressinoic acid and [pGlu4,Cyt6]VP-(4-9) at 4.5 microM, nor yet by the aminopeptidase inhibitor, bestatin (0.5 mM) and the L-amino acid transport system substrates, L-tyrosine and L-phenylalanine at 5 mM. At a perfusate concentration of 4.5 microM, the V1-vasopressinergic receptor antagonist, d(CH2)5[Tyr(Me)2]VP, reduced VP influx; regional Ki values, assuming that the observed inhibitions were purely competitive, ranged between 4.7 and 8.5 microM. It is concluded that there is an apparent cerebrovascular permeability to circulating VP due to the presence of a carrier-mediated transport system for the peptide located at the luminal side. The mechanism for VP BBB uptake exhibits no affinity for peptide fragments and large neutral amino acids, but requires reception of the intact molecule, which may be the same initial step for both the BBB VP transporter and the V1-receptor.

Transport of desglycinamide-arginine vasopressin across the blood-brain barrier in rats as evaluated by the unit impulse response methodology

The pharmacokinetic characteristics of desglycinamide-arginine vasopressin (DGAVP) with respect to its transport across the blood-brain barrier (BBB) were studied with the use of serial CSF sampling in an individual animal and the unit impulse response methodology. Transport rate is determined as BBB clearance, the volume of plasma per unit time cleared of the peptide by BBB transport, and the extent of transport as the percentage of the administered dose transported into the central nervous system. Plasma kinetics of DGAVP were shown to be linear within the dose range studied (50-150 micrograms), plasma mean residence time (MRT) being 18 +/- 4 min (mean +/- SE; n = 9). Elimination of DGAVP from CSF after icv administration was linear, with an MRT of 10 +/- 1 min (n = 9). After iv administration of 100 micrograms DGAVP, CSF concentrations were detectable for 90 min. Transport from plasma to the central nervous system was linear. The BBB transport clearance value was 1.0 +/- 0.3 microliters/min, and 0.026 +/- 0.007% of the administered dose was transported into the central nervous system. Results demonstrate that, within the concentration range studied, DGAVP is transported across the BBB by passive diffusion, although to a very low extent.

Effects of sodium intake, furosemide, and infusion of atrial natriuretic peptide on the urinary and metabolic clearances of arginine vasopressin in normal subjects

Arginine vasopressin (AVP) and atrial natriuretic peptide (ANP) have important influences on water and electrolyte metabolism, and studies on the interactions between these hormones may have important implications. We have investigated the effects of sodium intake, furosemide, and infusion of ANP on the urinary and metabolic (nonurinary) clearances of AVP in hydrated normal subjects. On a high sodium diet there was an increase in urine volume, sodium excretion, osmolal clearance, plasma ANP concentration, and urinary clearance and fractional excretion of AVP, with a decrease in PRA. The infusion of furosemide increased urine volume, sodium excretion, osmolal clearance, and PRA, but decreased circulating ANP levels and urinary clearance and fractional excretion of AVP. Since there was a positive correlation between circulating ANP and urinary clearance of AVP in these experiments, we infused human alpha ANP in physiological amounts and found increases in the urinary and metabolic (nonurinary) clearances of AVP. The changes in urinary clearance of AVP in all three experiments occurred even in relation to creatinine clearance. These observations demonstrate that urinary clearance of AVP does not correlate with urine volume, sodium or solute excretion, or PRA. The observations support a physiological role for ANP in modulating the renal action of AVP, probably at the level of the renal tubules, and indicate a need for caution when using plasma or urinary AVP as an indicator of AVP release from the neurohypophysis.

Rectal absorption enhancement of cefoxitin and desglycinamide arginine vasopressin by sodium tauro-24,25-dihydrofusidate in conscious rats

The effects of sodium tauro-24,25-dihydrofusidate (STDHF), an enhancer of nasal insulin absorption, on the rectal absorption of cefoxitin and desglycinamide arginine vasopressin (DGAVP) were evaluated in the rat. Cefoxitin and DGAVP proved to be poorly absorbed rectally without STDHF, but their bioavailability was considerably increased by STDHF in concentrations of 0.15 to 8% w/v. Both rectal infusion and rectal bolus delivery resulted in complete cefoxitin absorption at 4% w/v of STDHF. Delivery rate appeared to be an important factor in the effect of 4% w/v of STDHF on DGAVP bioavailability; on infusion a mean DGAVP bioavailability (+/- S.D.) of 47 +/- 12% was obtained, whereas after bolus delivery it amounted to 27 +/- 6%. For both compounds the effect of STDHF was significant at 0.5% w/v. It is concluded that STDHF is capable of actively enhancing the rectal absorption of poorly absorbed drugs, including small peptides.

Habenular nuclei specifically bind synthetic arginine vasotocin

The antidiuretic activity of an arginine vasotocin (AVT) solution (10 microU/ml) was studied in comparison with an arginine vasopressin (AVP) solution (10 microU/ml) after their incubation for 1 h at 37 degrees C with the supernatants (S) or the pellets (P) of the habenular nuclei (HN) or frontal cortex (FC) fragments homogenized in saline solution, removed from adult or newborn rats. Both SHN and SFC from adults inactivated about 5 microU/ml AVT and 10 microU/ml AVP, PHN and not PFC inactivated only about 7 microU/ml. In newborns the phenomena were in the same direction, but the specific inactivation of AVT by the PHN was of about twice greater than in adults. The results are interpreted as suggesting that, in contrast to an unspecific inactivation from S, probably of an enzymatic nature, the PHN specifically bind AVT and that this activity is greater in newborn than in adult rats.

Responses of vasopressin release in patients with cardiopulmonary bypass anesthetized with enflurane and morphine

Changes in plasma level of arginine vasopressin (AVP), arterial pressure, and urine flow were studied before, during and after cardiopulmonary bypass (CPB) in 11 patients with congenital heart disease. Anesthesia was induced with thiopental sodium (3-5 mg/kg) and was maintained with enflurane (1.0-1.5%), 50% N2O in O2 and morphine (0.5 mg/kg). Concentration of plasma AVP increased slightly from 3.8 +/- 1.5 pg/ml after induction and increased 3-fold after sternotomy. Plasma AVP level increased to 132 +/- 26 pg/ml and 218 +/- 54 pg/ml after 5 and 60 min on CPB, respectively. When the circulation returned to normal, plasma AVP level decreased gradually but was still significantly higher at 24 hr (13.4 +/- 2.5 pg/ml). Marked osmolar diuresis was induced with mannitol in the priming solution used during the CPB: increases in urine flow, Na excretion and osmolar clearance. Possible mechanisms of marked increase in AVP release and differences of AVP responses during CPB reported by other investigators are discussed.

Transport of vasopressin fragments across the blood-brain barrier: in vitro studies using monolayer cultures of bovine brain endothelial cells

A well established in vitro blood-brain barrier (BBB) model, consisting of bovine cerebrovascular endothelial monolayers from primary cultures, was used to study the transport profile of vasopressin and its fragments across the BBB and to assess the metabolic properties of the BBB for the behaviorally active vasopressin fragment arginine vasopressin (AVP)1-8 (desglycinamide-AVP). All vasopressin fragments crossed the in vitro BBB to a measurable extent. Endothelial permeabilities were (in 10(-3) cm/min): AVP1-6, 3.0 +/- 0.2; AVP1-7, 4.6 +/- 0.4; AVP1-8, 2.0 +/- 0.5 and AVP1-9, 2.4 +/- 0.4. A significant effect of molecular size on endothelial permeability was seen. Transport rate of AVP1-8, expressed as BBB-clearance, was not affected by luminal concentration change and proved to be symmetrical. These findings suggest that, in the concentration range studied, vasopressin-like peptides can cross the BBB mainly by paracellular transport and that no relevant carrier mediation is involved. AVP1-8 was metabolized slowly (half-life, 6.5 hr) by a 60 cm2 confluent monolayer to AVP1-7, which was not broken down further, suggesting that carboxypeptidases are responsible for AVP1-8 metabolism in the BBB.

Mechanisms underlying the cardiovascular responses to intrathecal vasopressin administration in rats

Vasopressinergic pathways within the spinal cord have been implicated in the control of cardiovascular function. This study was undertaken to determine the mechanisms whereby intrathecally administered arginine vasopressin (AVP) increases blood pressure and heart rate in anesthetized rats. The cardiovascular responses to intrathecal AVP administration were significantly attenuated after intravenous administration of the ganglionic blocking agent, chlorisondamine chloride, as were the pressor responses following alpha-adrenergic receptor blockade with phentolamine and the heart rate responses following beta-receptor blockade with propranolol. Intrathecal administration of the V1 vasopressin receptor antagonist d(CH2)5Tyr(Me)AVP completely blocked the cardiovascular responses to intrathecal AVP injections, but did not significantly alter the responses to intrathecal substance P injections. There was no evidence for the involvement of the renin-angiotensin system in the pressor responses to intrathecal AVP, as (i) an angiotensin II receptor blocking agent, [Sar1, Val5, Ala8]angiotensin, failed to significantly alter the responses to intrathecal AVP, and (ii) plasma renin levels did not change following administration of the peptide. Intrathecal injections of [3H]AVP suggest that only small amounts of the peptide may cross into the plasma during the time in which the cardiovascular variables are changing. These data provide evidence that intrathecally administered AVP discretely activates the sympathetic outflow to the heart and vasculature, and confirm the neurally mediated nature of the response.

Changes in the metabolic clearance of vasopressin and in plasma vasopressinase throughout human pregnancy

Metabolic clearance rates (MCR) of arginine vasopressin (AVP) were measured serially in five women starting before conception, during gestational weeks 7-8 (early), 22-24 (middle), and 36-38 (late pregnancy), and again 10-12 wk postpartum. Hormonal disposal rates were determined after water loading to suppress endogenous AVP release using a constant infusion method designed to achieve three different steady-state concentrations of plasma AVP (PAVP) on each test occasion. Dose schedules were altered in mid- and late pregnancy to obtain comparable AVP levels at each stage of the protocol. Prehydration decreased plasma osmolality sufficiently to suppress AVP release, as circulating AVP-neurophysin measured serially in three of the women was undetectable. The MCR of AVP was similar before conception (0.75 +/- 0.31, 0.79 +/- 0.34, and 0.76 +/- 0.28 liters/min at PAVP of 2.6 +/- 1.9, 4.7 +/- 2.4, and 8.3 +/- 3.9 pg/ml), in early pregnancy (0.89 +/- 0.34, 0.97 +/- 0.04, and 0.95 +/- 0.40 liters/min at PAVP of 2.2 +/- 2.1, 3.9 +/- 3.2, and 7.9 +/- 3.4 pg/ml), and postpartum (0.70 +/- 0.21, 0.69 +/- 0.24, and 0.75 +/- 0.20 liters/min at PAVP 3.5 +/- 1.8, 5.1 +/- 3.7, and 9.1 +/- 4.2 pg/ml). Values at mid-pregnancy (2.8 +/- 1.3, 3.0 +/- 1.2, and 2.7 +/- 1.2 liters/min at PAVP 2.3 +/- 2.2, 4.0 +/- 3.6, and 7.7 +/- 3.9 pg/ml) and late pregnancy (3.2 +/- 1.4, 3.3 +/- 1.4, and 2.9 +/- 1.2 liters/min at PAVP 1.9 +/- 2.0, 3.8 +/- 2.6, and 7.4 +/- 4.1 pg/ml) increased 3-4-fold (all P less than 0.01). Plasma vasopressinase, undetectable at 7-8 gestational wk, increased markedly by mid- and slightly more by late gestation. Finally, relationships between PAVP and urine osmolality were similar before, during, and after pregnancy. We conclude that marked increments in the MCR of AVP occur between gestational weeks 7 and 8 and mid-pregnancy, which parallel the period of greatest rise in both trophoblastic mass and plasma vasopressinase. There was no evidence of a renal resistance to AVP during gestation.

Clearance and urinary excretion of vasopressin in conscious dogs

The purpose of this study was to determine the relationship between plasma arginine vasopressin (PAVP) levels and the urinary clearance rate, the nonurinary plasma clearance rate, and the urinary excretion rate of AVP. Female conscious dogs were given an intravenous infusion of p-aminohippuric acid and inulin, and, after a 90-min control period, the infusion was either continued or AVP was added to the infusate to produce doses of 0, 40, 100, and 200 microU.kg-1.min-1 for the last 90 min of the experiment. The resulting PAVP levels were 1.2, 8.2, 11.4, and 21.6 microU/ml, respectively. The urinary clearance rate of AVP nearly doubled between the infusion rates of 0 and 100 microU.kg-1.min-1. Likewise, the nonurinary plasma clearance rate of AVP also nearly doubled between the infusion rates of 40 and 100 microU.kg-1.min-1. However, at the rate of infusion of 200 microU.kg-1.min-1, both the urinary and nonurinary clearance rates were unchanged compared with the 100-microU.kg-1.min-1 rate. The largest incremental rise in PAVP was observed between the doses of 100 and 200 microU.kg-1.min-1, when the clearance rates were apparently plateaued. The urinary excretion rate of AVP was linearly correlated with PAVP (R = 0.91) under the conditions of this study. The results suggest that the measurement of the urinary excretion rate of AVP is a reliable method of assessing PAVP and that the clearance of AVP increases when PAVP is elevated.

Chlorpropamide alters AVP-receptor binding of rat renal tubular membranes

The effects of chlorpropamide on AVP-receptor binding in rat renal tubular basolateral membranes were investigated utilizing [3H][Arg8]vasopressin (AVP). Our data indicate that chlorpropamide alters AVP-receptor binding in a competitive manner.

Radioimmunoassay of desglycinamide-arginine vasopressin and its application in a pharmacokinetic study in the rat

The purpose of this investigation was to develop a sensitive and selective radioimmunoassay for Desglycinamide-Arginine Vasopressin (DGAVP). DGAVP was extracted from rat plasma after protein precipitation, using Sep-Pak C18 cartridges and 50 mM glycine buffer/methanol (10:90) solution. Extraction recovery was 73 +/- 14% (mean +/- S.D.; n = 11) and good linearity was achieved in the concentration range of 0.25-128 pg/tube. Instantaneous tracer addition resulted in a detection limit of 250 fg/tube, whereas 24 hours preincubation and delayed tracer addition resulted in a detection limit of 100 fg/tube. Intra-assay variation ranged between 7.4% and 10.0% depending on the peptide concentration and inter-assay variation was 13.2%. Using this procedure, plasma pharmacokinetics of DGAVP in the rat were determined after IV administration. DGAVP plasma concentration showed a rapid distribution phase (t1/2 = 1.0 +/- 0.2 min) and a somewhat slower elimination phase (t1/2 = 7.2 +/- 2.1 min). High clearance values (CLss = 97 +/- 30 ml.min-1) suggest rapid metabolism by amino- and carboxy-peptidases.

Nasal delivery of a vasopressin antagonist in dogs

The dosage form (drop or spray) and site of administration (dorsal or ventral surface of the nostril) profoundly affect the distribution and clearance of a gamma-emitting 99mTc-labeled diethylenetriamine pentaacetic acid (99mTc-DTPA) solution in dogs. The slowest nasal clearance was observed for dorsally administered drops. Administration of drops to the ventral surface or sprays to either dorsal or ventral surface results in rapid clearance and little deposition in the turbinates. The octapeptide vasopressin antagonist, SKF 101926, was administered intravenously (0.3, 1.0, 3.0, and 10 micrograms/kg) and then on separate occasions intranasally (10, 25, and 50 micrograms/kg as a drop to the ventral surface) to four conscious, trained, female, water-loaded, vasopressin-infused dogs. SKF 101926 reversed the antidiuretic response to vasopressin after administration by either the intravenous or intranasal route in a dose-dependent fashion. Peak dilution of urine occurred within 50- to 60-min postdosing by both routes. Estimated doses to reduce vasopressin antidiuresis by 50% were 1.4 micrograms/kg intravenously and 23 micrograms/kg intranasally. After recovery to at least 70% of antidiuretic base line, and then administration of a second dose of SKF 101926 (3 micrograms/kg), subsequent dilution of urine osmolality was inversely related to the magnitude of the previously administered dose. It is concluded that the estimated relative effectiveness of intranasally administered SKF 101926 is 3-21%, compared with intravenous administration. Acute tachyphylaxis to repeated dosing was observed. The mechanism of the apparent tachyphylaxic response was not elucidated. No tachyphylaxis to less frequent (weekly) dosing was observed.

Calcium-mobilizing hormones and phorbol myristate acetate mediate heterologous desensitization of the hormone-sensitive hepatic Na+/K+ pump

The Na+/K+ pump in rat hepatocytes is stimulated in response to Ca2+-mobilizing hormones such as [arginine]vasopressin (AVP), angiotensin II and adrenaline, as well as tumour promoters such as 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA). The ability of these agents to increase cellular contents of diacylglycerol and activate protein kinase C may be necessary to observe this response. In the present work, ouabain-sensitive 86Rb+ uptake was studied in isolated rat hepatocytes to help to explain why stimulation of the Na+/K+ pump by Ca2+-mobilizing hormones and tumour promoters is not temporally sustained relative to other hormone responses. A transient stimulation (3-4 min) of the Na+/K+ pump was observed in hepatocytes exposed to high (10 nM), but not low (0.1 nM), concentrations of AVP. Experiments with the Ca2+ chelator EGTA and the Na+ ionophore monensin indicate that the rapid secondary decrease in Na+/K+-pump activity which occurs after AVP stimulation is not due to changes in cytosolic Ca2+ and Na+ concentrations. When added after the stimulation and rapid decrease in Na+/K+-pump activity induced in hepatocytes by a high concentration of AVP, a second challenge with AVP or PMA failed to stimulate the pump. Similarly, previous exposure of hepatocytes to angiotensin, adrenaline or PMA attenuated the subsequent Na+/K+-pump responses to AVP and PMA. In contrast, previous exposure to AVP had no significant effect on subsequent stimulation of the Na+/K+-pump by monensin, glucagon, forskolin or 8-p-chlorophenylthio cyclic AMP. In addition, exposure to monensin had no effect on subsequent responses to AVP and PMA. These data indicate that high concentrations of Ca2+-mobilizing hormones and PMA result in heterologous desensitization of the hepatic Na+/K+ pump to subsequent stimulation by Ca2+-mobilizing hormones and PMA, but not by cyclic-AMP-dependent agonists or monensin.