Antidiuretic activity of terlipressin (triglycyl-lysine vasopressin)--role of pressure natriuresis.

Terlipressin (triglycyl-lysine vasopressin TP), a "hormonogen" analogue, was introduced in gastroenterology for its low and protracted vasopressor action, reducing bleeding from gastrointestinal tract. Its antidiuretic activity, estimated originally in ethanol-anaesthetized rats (Sawyer's method) was claimed to be equally low and protracted. We performed several series of antidiuretic tests on conscious rats (Burn's method) with the following results. TP in low doses of 0.05-1.0 micrograms/kg exhibited typical dose-dependent antidiuretic effect. In the dose of 0.2 micrograms/kg, the dynamics of urine and sodium excretion did not differ from that after equivalent dose of lysine vasopressin and equipotent dose of DDAVP. The antidiuretic potency of TP (estimated by parallel line assay) was 175.0 U/mg. TP in doses of 5.0 and 20.0 micrograms/kg exhibited limited diuresis and marked natriuresis. High osmolality and sodium content were present in all portions of excreted urine. The discrepancy between previous and our results concerning antidiuretic activity of TP and the role of pressure natriuresis for overall renal action of TP are discussed.     

Effects of melatonin and natural and synthetic analogues of arginine vasotocin on plasma prolactin levels in adult male rats

A significant elevation in plasma prolactin was observed 10 min following the intravenous injection of 100 microgram of melatonin into either estrogen-progesterone (EP) primed or into nonsteroid-treated male rats. 60 min postinjection in the EP primed rat, the groups treated with 100 microgram or 10 mg of melatonin had signficantly elevated plasma prolactin levels while no effect was observed with these same doses in the nonsteroid-treated rats. Compared to diluent-treated controls, a significant elevation in plasma prolactin was observed at 10, 20 and 60 min following the intravenous injection of either 1 microgram arginine vasotocin (AVT) or 1 mg melatonin into EP primed male rats. A consistent rise in plasma prolactin was also evident after the injection of 1 microgram of either arginine vasopressin, lysine vasopressin or AVT. Oxytocin had no effect on plasma prolactin values. The intravenous administration of 1 microgram of (deamino-1,6 dicarba, 8-arginine)-vasotocin caused a significant elevation of plasma prolactin 10 and 20 min after injection. However, the injection of another analogue of AVT, (4-leucine, 8-arginine)-vasotocin, had no effect on prolactin release at the time points measured.     

Dopamine antagonism does not potentiate the effects of oxytocin and vasopressin on prolactin secretion

The roles of oxytocin and vasopressin on prolactin secretion were studied. Adult female Sprague-Dawley rats ovariectomized for two weeks and treated with a long-acting estrogen, polyestradiol phosphate for one week were used. Hormone administration and serial blood sampling were accomplished through indwelling intra-atrial catheters which were implanted two days before the experiment. Both oxytocin (20 micrograms/rat) and vasopressin (5 micrograms/rat) stimulated prolactin secretion within 10 min after injection and the effects were diminished by 30 min. In animals pretreated with a small dose of dopamine antagonist, sulpiride (1 microgram/rat), the effect of TRH on prolactin secretion was repeatedly shown to be potentiated. Same pretreatments with two different time intervals (30 and 60 min) between sulpiride and oxytocin/vasopressin administration, however, had no effect on oxytocin- or vasopressin-stimulated prolactin secretion. A vasopressin analog, 1-deamino-[D-Arg8]-vasopressin (dDAVP), with antidiuretic but no vasopressor activity was also used in the study. It was found that unlike vasopressin, dDAVP had no effect on prolactin secretion. In conclusion, both oxytocin and vasopressin can have a stimulatory effect on prolactin secretion when given in vivo. Unlike TRH, however, the action of oxytocin or vasopressin was not augmented by pretreatments of dopamine antagonist. The action of vasopressin on prolactin secretion may be a side effect of its vasopressor activity.     

Role of antidiuretic activity in the inhibition of renin secretion by vasopressin in anesthetized dogs

The nature of the activity of vasopressin which is responsible for the inhibition of renin secretion was studied by comparing the effects of vasopressin (AVP) and analogs of AVP in anesthetized water-loaded dogs. Infusion of AVP (1.0 ng/kg/min) increased mean arterial pressure (MAP) and decreased heart rate (HR) and free water clearance (CH2O). Plasma renin activity (PRA) decreased from 11.9 +/- 4.7 to 3.8 +/- 1.7 ng/ml/3 hr (p less than 0.05). A selective antidiuretic agonist, 1-deamino-8-D-arginine vasopressin (1.0 ng/kg/min), which had no effect on MAP or HR but was effective as AVP in decreasing CH2O, decreased PRA from 13.5 +/- 4.6 to 7.0 +/- 2.9 ng/ml/3 hr (p less than 0.05). Infusion of a selective vasoconstrictor agonist, 2-phenylalanine-8-ornithine oxytocin (1.0 ng/kg/min), increased MAP and decreased HR but did not decrease CH2O or PRA. A vasoconstrictor antagonist, d(CH2)5Tyr(Me)AVP (10 micrograms/kg), completely blocked the MAP and HR responses to AVP but did not block the decrease in CH2O or PRA (5.9 +/- 1.8 to 2.9 +/- 1.6 ng/ml/3 hr) (p less than 0.001). Infusion of the 0.45% saline vehicle had no significant effect on MAP, HR, CH2O or PRA. These results indicate that the inhibition of renin secretion by vasopressin in anesthetized water-loaded dogs is due to its antidiuretic activity.     

A model for the study of the oral administration of peptide hormones.

The intragastric administration of lysine vasopressin (LVP) to rats is used as a model to study the biological activity of orally administered peptide hormones. Using a modification of the antidiuretic assay of Sawyer, LVP given by stomach tube caused a significant antidiuresis that was dose dependent in doses of 300 to 2000 mU. The simultaneous administration of the protease inhibitor, Trasylol, increased the antidiuretic effect of LVP. The synthetic peptide (1-deamino, 4 valine)-8-D-arginine-vasopressin also caused a dose-dependent prolonged and significant antidiuresis. No pressor effect was observed after intragastric administration of LVP in doses up to 40 U/rat. We are now using this model to test other procedures for enhancing the activity of lysine vasopressin administered in the gastrointestinal tract such as encapsulation into liposomes. The information gained with vasopressin will then be applied to insulin with the ultimate goal of making oral administration practical.     

Effect of ethanol in preovulatory periods on LH, FSH, prolactin and ovulation in rats

The effect of ethanol (4 g/kg) as well as the role of serotoninergic neurons on the rate of ovulation and plasma LH, FSH and prolactin secretion have been studied in rats at preovulatory periods (18th hour of diestrus). It has been found that administration of ethanol in preovulatory periods decreased the number of ovules per rat (p less than 0.001), the number of ovulating rats and LH levels (p less than 0.001). These effects were accompanied by an increase in prolactin concentration (0.05 greater than p greater than 0.02), which was followed by a diffuse luteinization in the ovarian tissue. These results showed that ethanol had an effect of central depression in preovulatory periods. These effects could be mediated through the hypothalamic releasing factors. Under previous serotonin depletion with p-chlorophenylalanine (PCPA: 300 mg/kg), ethanol caused similar effects on LH and FSH levels as compared with the control group with PCPA. However, prolactin concentration was not increased. These results showed that serotoninergic neurons could be mediated in changes caused by ethanol on prolactin secretion, but do not affect directly in changes caused on LH and FSH secretion.     

Heroin self-administration is under control of vasopressin

Heroin self-administration in rats is attenuated by daily intracerebroventricular treatment with desglycinamide⁹, arginine⁸, vasopressin (50 ng/animal). Removal of vasopressin by injection of antivasopressin serum into the cerebrospinal fluid, leads to facilitation of self-administering behavior. Treatment with oxytocin or human growth hormone antiserum fails to influence this behavior, while prolactin antiserum exhibits similar effects as observed with antivasopressin serum. These results indicate that pituitary hormones are physiologically involved in heroin self-administration.     

Antagonistic properties of tetra-substituted analog of vasopressin with selective antidiuretic effects]

Biological properties of a novel vasopressin analogue were investigated. It was found that this analogue has no vasopressor and oxytocic activities but it exhibits a selective antidiuretic effect which is weaker than that of adiuretin (DDAVP). Novel analogue inhibits vasopressor, oxytocic and antidiuretic effects caused by arginine--vasopressin. The usefulness of novel compound as a pharmacological tool--vasopressin antagonist is suggested.     

Effect of hypothalamic surgery on prolactin release induced by 5-hydroxytryptophan (5-HTP) in rats.

Intravenous injections of varying doses of 5-HTP (1, 3 and 5 mg/100 g body wt), a precursor of serotonin, caused a significant and dose-related increase in plasma prolactin concentrations in urethane-anesthetized rats. Increases in plasma prolactin concentrations caused by 5-HTP (1 mg/100 g body wt iv) were abolished by the concomitant administration of L-DOPA (2 mg/100 g body wt iv). Plasma prolactin levels were also significantly elevated following the injection of 5-HTP in rats with complete hypothalamic deafferentation, whereas 5-HTP had no significant effect on plasma prolactin levels in rats with extensive hypothalamic ablation. These results suggest that 5-HTP causes prolactin secretion by stimulating the serotoninergic mechanism in the hypothalamus.     

Effect of serotonin depletion by p-chlorophenylalanine on serum prolactin levels in estrogen-treated ovariectomized rats: insights concerning the serotoninergic, dopaminergic and opioid systems.

The stimulatory effect of serotonin on prolactin secretion is well documented, and the administration of an inhibitor of serotonin synthesis (p-chlorophenylalanine - pCPA) has the expected inhibitory action on prolactin release in most experimental situations. However, there is evidence that in certain physiological or experimental conditions, activation of the serotoninergic system can also determine inhibition of prolactin secretion. The aim of the present study was to investigate the ability of estrogen to modify the effect of pCPA on prolactin secretion and to evaluate the participation of opioid and/or dopaminergic systems in regulating pCPA-induced prolactin secretion in estradiol-treated rats. We observed that pCPA administration (200 mg/kg/day, s.c., 2 days) to ovariectomized (OVX) female rats treated with estradiol benzoate (300 microg/week for 2 weeks, or 50 microg/week for 4 weeks, s.c.) causes a significant increase in serum prolactin, whereas no effect is observed in intact rats or in OVX rats without treatment. Bromocriptine administration completely reversed prolactin values previously increased by estradiol and by pCPA [OVX rats + estradiol = 86.50 ng/ml (68.90-175.02), OVX + estradiol + pCPA = 211.30 ng/ml (142.03-311.00), OVX + estradiol + pCPA + bromocriptine = 29.35 ng/ml (23.01 - 48.74), p<0.05. Naloxone administration partially reduced estrogen-induced high prolactin concentrations, but did not affect prolactin secretion stimulation determined by pCPA. Overall, the data from this report confirm the involvement of the dopaminergic system and, to a lesser degree, of endogenous opioids in prolactin secretion stimulation determined by estradiol. Furthermore, our results suggest that the stimulatory action of pCPA on prolactin secretion in estradiol-treated OVX rats is mediated by serotonin, which may also act indirectly on dopamine neurons.     

Effects of beta-endorphin and D-alanine2 enkephalinamide on urine production and urinary electrolytes in the rat.

The intracerebro-ventricular administration of human β-Endorphin (β-EP, 0.1–3 μg/rat) or D-alanine₂ methionine enkephalinamide (D-ala, 0.3–30 μg/rat) caused a dose dependent reduction in the urine volume. The oliguria was associated with a decrease in the concentration of Na⁺ and K⁺ in the urine of rats previously hydrated by oral administration with 25 ml/kg tap water plus 50 ml/kg 0.5% NaCl. On a molar basis, β-EP proved to be about 5–7 times more potent than D-ala. The effects caused by the peptides were antagonized by the simultaneous intraperitoneal administration of 1 mg/kg naloxone. In rats treated chronically with morphine, no cross-tolerance was demonstrated to the antidiuretic effect of β-EP, but clear cross-tolerance was evident to the changes in urine electrolytes induced by β-EP. Results suggest that morphine and the opiate peptides share a similar mechanism of action.     

Antidiuretic action of prolactin in the rat with diabetes insipidus.

Rats with hereditary hypothalamic diabetes insipidus, devoid of endogenous ADH, exhibited a prompt antidiuresis when injected subcutaneously or intraarterially with ovine prolactin. The antidiuresis was accompanied by a decrease in free water clearance and an increase in urine osmolality without a change in osmolal clearance or creatinine excretion. Measurement of PAH and insulin clearances indicated that prolactin had no effect on renal plasma flow or glomerular filtration rate. Prolactin injection caused a transient decrease in urinary sodium excretion, but proximal tubular sodium reabsorption, estimated by lissamine green transit time, was unaffected. The antidiuretic effect of prolactin could not be attributed to ADH contamination of the ovine prolactin preparation. Kidney cyclic AMP content was increased significantly 5 min after injection of prolactin. Thus, prolactin has an antidiuretic effect similar to that which occurs as a result of ADH action on the kidney and does not require either the release or the presence of ADH in order to cause the antidiuresis. Further, the impaired water excretion cannot be attributed to an increase in proximal tubular sodium reabsorption or to alteration of renal hemodynamics. It is suggested that prolactin has a direct ADH-like action on the kidney resulting in antidiuresis.     

Physiological responses to low dose infusions of atrial peptide in conscious dogs

Mongrel dogs prepared with chronic catheters in their femoral artery and vein and urinary bladder received 60 minute infusions of atrial peptide ranging from 5 to 100 ng/kg/min. Infusion of atrial peptides caused dose dependent increases in plasma atrial peptide concentration with doses of 25 ng/kg/min or less increasing plasma concentrations to levels observed in normal animals during stimulation of endogenous atrial peptide secretion. Atrial peptide infusion at doses of 10 ng/kg/min and above caused significant decreases in mean arterial pressure which were not accompanied by statistically significant changes in heart rate. Atrial peptide infusion at doses of 25 ng/kg/min and above increased urinary sodium excretion and urine flow rate. Atrial peptide infusion was without effect on plasma vasopressin, ACTH and corticosterone concentrations. However, atrial peptide infusion resulted in dose dependent decreases in plasma aldosterone concentration and plasma renin activity, but the decreases were only significant with the high physiologic (25 ng/kg/min) and pharmacologic doses (50 & 100 ng/kg/min). These data show that atrial peptide infusions in conscious dogs have minimal effects when infused in small doses that mimic endogenous atrial peptide release. At higher doses, significant effects on the cardiovascular, renal and endocrine systems can be observed but their physiological significance is unclear.     

Effects of thyroid deficiency on the vasopressin receptors in the kidney of developing and adult rats. A comparative study of hormonal binding and adenylate cyclase activation

The effects of propylthiouracil (PTU) treatment on renal vasopressin sensitive adenylate cyclase in young and adult rats were studied by measuring the binding of tritiated vasopressin and adenylate cyclase activation by vasopressin in kidney medulla plasma membranes. Thyroxine therapy completely corrected the effects of PTU treatment on the vasopressin-adenylate cyclase system. Thus, the abnormalities observed after a such treatment are directly related to thyroid deficiency and not to toxic effects of PTU. The inability of the kidney to normally concentrate urine in developing and adult animals with induced hypothyroidism was mainly related to the reduction of the number of binding sites without significant changes in the basal and guanylyl-imidodiphosphate (Gpp(NH)p)-stimulated adenylate cyclase activities, the apparent dissociation constant (Kbind) of labeled vasopressin from its specific receptor and the apparent activation constant (Kact) of vasopressin for adenylate cyclase. These results also show that thyroid deficiency has more effect on the ontogenesis of receptors than on their turnover, and demonstrate that a normal antidiuretic response occurs at very low receptor occupancy. Since, on the one hand, the hypothyroidism-induced abnormalities in renal medulla responsiveness to vasopressin were reversible and, on the other, only a permanent therapy consisting of two daily physiological doses of thyroxine from birth to the age of sacrifice fully restored them, the responsiveness of developing kidney to thyroid hormones appears to be fundamentally different from that of the CNS.     

Secretin receptors in the rat kidney: adenylate cyclase activation and renal effects

In previous studies it has been demonstrated that pharmacological administration of secretin can alter urine output. Whether the effect is due to a direct action on kidney was investigated by examining the effect of secretin on renal output, and determining whether there were secretin receptors and a secretin sensitive adenylate cyclase in the kidney. Secretin had an antidiuretic action on kidney when administered intravenously to anesthetized hydrated rats. In addition, binding sites for (125I)-secretin, and a secretin sensitive adenylate cyclase were identified in rat kidney. Binding was saturable and reversable and was half maximally inhibited by 1 X 10(-7) M synthetic porcine secretin. Autoradiographic studies revealed a high density of secretin binding sites in the outer medulla of the kidney, a region that is composed mainly of the thick ascending limb of the loop of Henle, and is also the major site of action for the antidiuretic hormone, vasopressin. The data indicate that a functional secretin receptor system exists in kidney which may have a physiological role in regulating urine output.     

High-calcium diet modulates effects of long-term prolactin exposure on the cortical bone calcium content in ovariectomized rats

High physiological prolactin induced positive calcium balance by stimulating intestinal calcium absorption, reducing renal calcium excretion, and increasing bone calcium deposition in female rats. Although prolactin-induced increase in trabecular bone calcium deposition was absent after ovariectomy, its effects on cortical bones were still controversial. The present investigation, therefore, aimed to study the effect of in vivo long-term high physiological prolactin induced by either anterior pituitary (AP) transplantation or 2.5 mg/kg prolactin injection on cortical bones in ovariectomized rats. Since the presence of prolactin receptors (PRLR) in different bones of normal adult rats has not been reported, we first determined mRNA expression of both short- and long-form PRLRs at the cortical sites (tibia and femur) and trabecular sites (calvaria and vertebrae) by using the RT-PCR. Our results showed the mRNA expression of both PRLR isoforms with predominant long form at all sites. However, high prolactin levels induced by AP transplantation in normal rats did not have any effect on the femoral bone mineral density or bone mineral content. By using (45)Ca kinetic study, 2.5 mg/kg prolactin did not alter bone formation, bone resorption, calcium deposition, and total calcium content in tibia and femur of adult ovariectomized rats. AP transplantation also had no effect on the cortical total calcium content in adult ovariectomized rats. Because previous work showed that the effects of prolactin were age dependent and could be modulated by high-calcium diet, interactions between prolactin and these two parameters were investigated. The results demonstrated that 2.0% wt/wt high-calcium diet significantly increased the tibial total calcium content in 9-wk-old young AP-grafted ovariectomized rats but decreased the tibial total calcium content in 22-wk-old adult rats. As for the vertebrae, the total calcium contents in both young and adult rats were not changed by high-calcium diet. The present results thus indicated that the adult cortical bones were potentially direct targets of prolactin. Moreover, the effects of high physiological prolactin on cortical bones were age dependent and were observed only under the modulation of high-calcium diet condition.     

Biological half-life and organ distribution of [3H]8-arginine vasopressin following administration of vasopressin receptor antagonist OPC-31260

The effects of the antidiuretic (V(2)) non-peptide receptor antagonist OPC-31260 on the plasma vasopressin level and the biological half-life and organ distribution of radiochemically pure, biologically active [(3)H]8-arginine vasopressin [spec. act.: 15.9 mCi/mmol (588 GBq/mmol)] were studied in Wistar rats. The plasma vasopressin level increased significantly throughout the whole experimental period (24 h). There was no change in the fast phase of the curves of total radioactivity disappearance from the plasma after the administration of [(3)H]arginine vasopressin (control: 1.51+/-0.17 min, OPC-31260-treated: 1.42+/-0.12 min, n=10). The fast phase of the disappearance curves of intact [(3)H]arginine vasopressin did not change either following the administration of OPC-31260 in a dose of 30 mg/kg p.o. (control: 1.06+/-0.19 min, OPC-31260-treated: 1.00+/-0.15 min, n=6). The slow phase of the biological half-life, which is characteristic for the examined compound, proved to be significantly longer (total radioactivity control: 9.29+/-0.61 min, OPC-31260-treated: 12.33+/-0.42 min, P<0.05, n=10; [(3)H]arginine vasopressin radioactivity: control: 5.96+/-0.58 min, OPC-31260-treated: 8.90+/-0.37 min, P<0.05, n=6). In the control rats, the radioactivity was accumulated to the greatest extent in the neurohypophysis, adenohypophysis and kidney. Following OPC-31260 administration, significantly more radioactive compounds accumulated in the kidney (control: 0.30+/-0.052 total radioactivity %/100 mg organ weight, OPC-31260-treated: 0.50+/-0.133 total radioactivity %/100 mg organ weight, P<0.05, n=10) and neurohypophysis (control: 0.37+/-0.053 total radioactivity %/100 mg organ weight, OPC-31260-treated: 0.52+/-0.076 total radioactivity %/100 mg organ weight, P<0.05, n=10). Our results permit the conclusion that the antidiuretic antagonist OPC-31260 not only blocks the V(2) receptors, but also increases the biological half-life of vasopressin. The longer biological half-life of vasopressin following OPC-31260 administration may play a role in the elevation of the plasma vasopressin level.     

Expression of the long form of the prolactin receptor in magnocellular oxytocin neurons is associated with specific prolactin regulation of oxytocin neurons

Magnocellular neurons of the supraoptic (SON) and paraventricular nuclei (PVN) show considerable plasticity during pregnancy and lactation. Prolactin receptors (PRL-R) have been identified in both these nuclei. The aim of this study was to investigate the cell type(s) expressing mRNA for the long form of prolactin receptor (PRL-R(L)) and to determine whether patterns of expression change during pregnancy and lactation. In addition, we examined effects of prolactin on excitability of oxytocin and vasopressin neurons. Sections from brains of nonpregnant, pregnant, and lactating rats were hybridized with an 35S-labeled probe to label PRL-R(L) mRNA together with digoxigenin-labeled probes to detect either oxytocin or vasopressin mRNA. In the SON, PRL-R(L) mRNA was predominantly colocalized with oxytocin mRNA, with over 80% of oxytocin neurons positive for PRL-R(L) mRNA. Very few (<10%) vasopressin neurons expressed PRL-R(L) mRNA. In the PVN, PRL-R(L) mRNA was also predominantly found in oxytocin neurons, and the proportion of PRL-R(L)-positive oxytocin neurons increased significantly during pregnancy and lactation. As in the SON, relatively few vasopressin cells contained PRL-R(L) mRNA. For in vivo electrophysiology, nonpregnant rats were anesthetized, and then extracellular single neuron activity was recorded in identified oxytocin and vasopressin neurons. After a period of baseline recording, the effect of prolactin (1 microg i.c.v.) on firing rate was examined. Prolactin treatment of nonpregnant rats induced a significant decrease in firing rates of oxytocin neurons. There was no effect of prolactin on the activity of vasopressin neurons. Together, these data provide strong evidence that prolactin directly and specifically regulates activity of oxytocin neurons.     

Vasopressin and oxytocin reduce plasma prolactin levels of conscious rats in basal and stress conditions. Study of the characteristics of the receptor involved

Subcutaneous administration of arginine vasopressin (AVP) to conscious rats induced a dose-dependent increase of plasma ACTH and beta-endorphin levels and decrease of plasma prolactin (PRL) levels 30 min later. AVP similarly reduced PRL increase induced by exposure to a novel environment stress. Oxytocin (OT) was also active but 5-fold less potent than AVP. The study of several analogs with specific agonistic and antagonistic activity on the oxytocic, vasopressor and antidiuretic receptors of OT and AVP suggests that the receptor involved in this effect does not fit into this classification.     

Comparison of efficacy of cysteamine in depleting prolactin immunoreactivity in different hyperprolactinemic animal models

We have examined the effects of cysteamine on its ability to deplete prolactin in various states of hyperprolactinemia. Administration of subtoxic doses of cysteamine (75 and 150 mg/kg,sc) dramatically reduces serum prolactin levels as well as pituitary prolactin content in a dose-dependent manner in estrogen-primed brown Irish ACI female rats. A similar dose-dependent decrease in anterior pituitary prolactin levels was observed in two ectopic prolactin secreting pituitary tumor models (MtTW15 and 7315a). However, a significant reduction in serum prolactin levels was seen in these same tumor bearing animals at only the 150 mg/kg dose of cysteamine. Interestingly, the prolactin content of each of the prolactin secreting tumors, although reduced by cysteamine administration, the effect was neither dose-dependent nor as dramatic as that observed in the anterior pituitary gland proper. These data demonstrate that cysteamine can significantly lower prolactin concentrations in hyperprolactinemia. Further, ectopic prolactin secreting pituitary tissue appears less sensitive to the prolactin-depleting effects of cysteamine. This latter finding may explain, in part, why serum prolactin levels were not as severely reduced in the ectopic tumor bearing female rats as in estrogen-induced hyperprolactinemic animals.