Control Of Arginine Vasopressin Release Research Articles

Abstract 020: Physiological Significance of Angiotensin AT 1A Receptors in Vasopressin-producing Cells of the Supraoptic Nucleus

Low-renin and salt-sensitive forms of hypertension are characterized by elevated activity of the brain renin-angiotensin system and secretion of arginine vasopressin (AVP). While angiotensin in the brain is a known stimulant of AVP secretion through its AT 1 receptor, the localization of relevant AT 1 receptors remains unclear. We tested whether AT 1A receptors localized to AVP-producing cells are important for AVP secretion. To examine AVP and AT 1A co-localization, mice expressing Cre-recombinase via the AVP gene (AVP-Cre) were bred with mice expressing a conditional red fluorescent ROSA-stop flox -tdTomato construct and GFP via an AT 1A BAC transgene. Dual-fluorescent cells were detected in supraoptic nuclei (SON) but not paraventricular nuclei. Mice lacking AT 1A specifically in AVP-producing cells (AT 1A AVP-KO ) were then generated by breeding AVP-Cre mice with mice harboring a conditional endogenous AT 1A gene. AT 1A AVP-KO mice exhibited normal serum (littermate n=13, 353±69 vs AT 1A AVP-KO n=7, 207±37 pg/mL, p=NS) and urine (n=26, 145±33 vs n=11, 170±54 pg/mL, p=NS) copeptin (the stable C-terminal fragment of AVP) as well as hematocrit (n=14, 46.3±0.7 vs n=7, 47.5±1.3 %, p=NS) despite increased serum osmolality (n=33, 324±1.3 vs n=19, 330±1.6 mOsm/kg, p<0.01), supporting a role for AT 1A in AVP-producing cells in modulating the osmotic control of AVP release. Systolic blood pressure (SBP) (n=18, 109±1.3 vs n=5, 107±1.2 mmHg), urine volume (n=27, 1.1±0.1 vs n=12, 0.9±0.2 mL/d), and fluid intake (n=27, 4.0±0.2 vs n=12, 3.9±0.2 mL/d) were all normal (p=NS) in AT 1A AVP-KO mice. Two-bottle choice between water and escalating concentrations of NaCl uncovered minor alterations in sodium intake behavior. Serum osmolality (n=22, 336±2 vs n=9, 333±3 mOsm/kg), SBP (n=23, +10.4±2.1 vs n=8, +12.9±2.0 mmHg), urine output (n=23, +12.7±0.8 vs n=9, +12.7±1.5 g/day), and fluid intake (n=23, +16.2±1.3 vs n=9, +14.8±2.5 mL/day) all increased normally (p=NS) in response to deoxycorticosterone acetate (DOCA)-salt treatment. Collectively these data support a role for AT 1A receptors, localized specifically to AVP-expressing cells of the SON, in the normal osmotic control of AVP secretion.

Angiotensin 1–7 Contributes to Disrupted Nitric Oxide control of Vasopressin Response to Hemorrhage during Acute Alcohol Intoxication

Acute alcohol intoxication (AAI) impairs hemodynamic counterregulation and blunts the arginine vasopressin (AVP) response to hemorrhagic shock (HS). We have demonstrated that AAI disrupts signaling mechanisms controlling AVP release through upregulation of paraventricular (PVN) nitric oxide (NO) and attenuated angiotensin (ANG) II response to HS. We hypothesized that AAI may also enhance ANG Converting Enzyme (ACE) 2 activity promoting the conversion of ANG II to the NO‐stimulating peptide ANG (1–7) further contributing to AAI‐induced suppression of AVP release. Male Sprague‐Dawley rats (250–300g) received a primed (2.5g/kg + 0.3g/kg/h) 15h intragastric infusion of alcohol or dextrose (DEX) prior to fixed‐pressure (40 mmHg) HS. AAI increased PVN ACE2 activity (41%) compared to DEX+HS. The contribution of ANG (1–7) to the AAI‐induced increase in PVN NO, was demonstrated by central administration of the Mas receptor antagonist A‐779 prior to HS. AAI increased NOS activity (30%) and NO levels (40%) in the PVN compared to DEX controls. Administration of A‐779 decreased NOS activity (66%), NO levels (27%; p=NS) and partially restored AVP levels (20%) in AAI‐treated animals. These results suggest that increased hypothalamic Mas receptor activation partially contributes to the alcohol‐induced disruption of NO control of AVP release during HS. DOD PR‐054196, NIAAAAA7577, APS Porter Fellowship.

Role of endogenous nociceptin in the regulation of arginine vasopressin release in conscious rats.

The effects of central administration of the opioid-like peptide nociceptin (also known as orphanin FQ) were investigated on the secretion of arginine vasopressin (AVP) in response to dehydration and hyperosmolar or hypovolemic stimulation in conscious rats. Intracerebroventricular (i.c.v.) administration of nociceptin suppressed plasma AVP concentration in a dose-dependent manner (0.1-10 microg/rat) in dehydrated rats, and the maximum effect was obtained 10 min after the administration (dehydration with 10 microg/rat nociceptin, 3.11 +/- 0.27 pg/ml vs. control, 10.32 +/- 0.96 pg/ml). The plasma AVP increase in response to either hyperosmolality [i.p. injection of hypertonic saline (HS) (600 mosml/kg)] or hypovolemia [i.p. injection of polyethylene glycol (PEG)] was also significantly blunted when nociceptin was injected i.c.v. (HS with 10 microg/rat nociceptin, 1.16 +/- 0.09 pg/ml vs. control, 1.82 +/- 0.30 pg/ml; PEG with 10 microg/rat nociceptin, 0.91 +/- 0.16 pg/ml vs. control, 2.41 +/- 0.26 pg/ml). Pretreatment with a selective opioid kappa-receptor antagonist, nor-binaltorphimine (1 microg/ rat, i.c.v.) or naloxone (2.5 mg/rat, s.c. injection) did not reverse the inhibitory effects of nociceptin on AVP release. Moreover, when plasma AVP was suppressed by acute water loading, immunoneutralization of endogenous nociceptin by antinociceptin-antiserum i.c.v. significantly reversed the suppression (0.57 +/- 0.12 pg/ml vs. control, 0.25 +/- 0.04 pg/ml). These results suggest that central nociceptin is physiologically involved in the control of AVP release through an inhibitory action.

Role of Endogenous Nociceptin in the Regulation of Arginine Vasopressin Release in Conscious Rats

The effects of central administration of the opioid-like peptide nociceptin (also known as orphanin FQ) were investigated on the secretion of arginine vasopressin (AVP) in response to dehydration and hyperosmolar or hypovolemic stimulation in conscious rats. Intracerebroventricular (icv) administration of nociceptin suppressed plasma AVP concentration in a dose-dependent manner (0.1–10 μg/rat) in dehydrated rats, and the maximum effect was obtained 10 min after the administration (dehydration with 10 μg/rat nociceptin, 3.11± 0.27 pg/ml vs. control, 10.32 ± 0.96 pg/ml). The plasma AVP increase in response to either hyperosmolality [ip injection of hypertonic saline (HS) (600 mosml/kg)] or hypovolemia[ ip injection of polyethylene glycol (PEG)] was also significantly blunted when nociceptin was injected icv (HS with 10 μg/rat nociceptin, 1.16 ± 0.09 pg/ml vs. control, 1.82 ± 0.30 pg/ml; PEG with 10 μg/rat nociceptin, 0.91 ± 0.16 pg/ml vs. control, 2.41 ± 0.26 pg/ml). Pretreatment with a selective opioid κ-receptor antagonist, nor-binaltorphimine (1 μg/rat, icv) or naloxone (2.5 mg/rat, sc injection) did not reverse the inhibitory effects of nociceptin on AVP release. Moreover, when plasma AVP was suppressed by acute water loading, immunoneutralization of endogenous nociceptin by antinociceptin-antiserum icv significantly reversed the suppression (0.57 ± 0.12 pg/ml vs. control, 0.25 ± 0.04 pg/ml). These results suggest that central nociceptin is physiologically involved in the control of AVP release through an inhibitory action.

Galanin as a physiological neurotransmitter in hemodynamic control of arginine vasopressin release in rats.

Previous neuroanatomical studies have revealed a localization of galanin in several nuclei in the brain stem which are involved in the hemodynamic control of arginine vasopressin (AVP) release. The present study, therefore, investigates the contribution of endogenous galanin to the plasma volume-mediated control of AVP release in conscious rats. Injection of synthetic rat galanin (12.5-50 pmol/rat) into the cisterna magna (i.c.s.) suppressed plasma AVP increased by polyethylene glycol-induced hypovolemia (2.45 +/- 0.24 pg/ml at 50 pmol/rat vs. the vehicle group 5.72 +/- 0.69 pg/ml, p < 0.01). In contrast, when plasma AVP was suppressed by isotonic plasma volume expansion, immunoneutralization of endogenous galanin by antigalanin-antibody i.c.s. significantly reversed the suppression (1.02 +/- 0.07 pg/ml vs. vehicle group 0.63 +/- 0.05 pg/ml, p < 0.01) without altering the mean arterial blood pressure. These results suggest that endogenous galanin is physiologically involved in the plasma volume-mediated control of AVP release through an inhibitory action on this pathway.

Release of vasopressin from the rat hypothalamo-neurohypophysial system by angiotensin-(1-7) heptapeptide.

We have recently shown that hydrolysis of labeled angiotensin I in canine brainstem homogenate causes a rapid accumulation of the heptapeptide angiotensin-(1-7) [Ang-(1-7)]. Although this angiotensin fragment has no vasopressor activity, its consistent generation in brain homogenate led us to study its potential neurosecretory effects in the rat hypothalamo-neurohypophysial system (HNS) in vitro. Ang-(1-7) or angiotensin II (Ang II) was added to HNS perifusate in concentrations of 0.04, 0.4, and 4 microM, and release of arginine vasopressin (AVP) during each treatment was quantified as a percentage of the AVP release detected in the preceding collection period. Base-line release of AVP averaged 281 +/- 47 pg per 15 min (mean +/- SEM) in HNS explants (five experiments, five explants per chamber) perifused in Krebs solution at 37 degrees C, after a 1-hr equilibration period. At 0.04 microM, Ang II or Ang-(1-7) did not stimulate AVP release. Ang II increased AVP release over the control value by 172% +/- 44% and 268% +/- 66% at 0.4 and 4 microM, respectively; the same concentrations of Ang-(1-7) increased AVP release by 134% +/- 12% and 216% +/- 45%. The responses to Ang II and Ang-(1-7) at the highest concentration were both significant (P less than 0.05), and comparison by two-way analysis of variance indicated that Ang II and Ang-(1-7) were equipotent in stimulating AVP release over the range of concentrations studied. In the presence of the competitive Ang II antagonist [Sar1,Thr8]Ang II (20 microM), the release of AVP increased approximately equal to 2-fold. Neither Ang II nor Ang-(1-7) (4 microM) caused a further enhancement of AVP release in the presence of [Sar1,Thr8]Ang II. These data suggest that a hydrophobic residue in position 8 of the angiotensin peptide is not essential for activation of angiotensin receptors in the rat HNS. Moreover, the equipotence of Ang II and Ang-(1-7) indicates that Ang-(1-7) may participate in the control of AVP release.

The reflex control of arginine-vasopressin release in essential hypertension.

1. The reflex control of arginine-vasopressin release was studied in 12 essential established hypertensive patients and in 12 age-matched normal subjects by comparing the effects of head-up tilt and a variable-pressure neck-chamber. 2. After 45 min 85 degrees head-up tilt, normal subjects showed an increase in plasma arginine-vasopressin and in plasma renin activity, while plasma volume decreased. In hypertensive patients, plasma arginine-vasopressin showed changes after tilt opposite to those of controls, while the changes in plasma renin activity and plasma volume were similar to those observed in the normal group. In both groups the changes in systolic and diastolic blood pressure were not statistically significant. A reduction in carotid sinus transmural pressure obtained by increasing neck-tissue pressure (+50 mmHg) by means of a neck-chamber, evoked different responses in mean blood pressure in the two groups, but failed to induce any significant change in plasma arginine-vasopressin concentration both in the normal and in the hypertensive subjects. 3. These results seem to suggest that carotid sinus baroreceptors, though active in blood pressure control, do not play a direct role in arginine-vasopressin release and, therefore, the opposite response of arginine-vasopressin observed after tilt in the two groups of subjects should be ascribed to more complex mechanisms.