Anti-calmodulin agents affect osmotic and angiotensin II-induced vasopressin release

V1a and V1b vasopressin receptors within the paraventricular nucleus contribute to hypertension in male rats exposed to chronic mild unpredictable stress

Depression is an independent nontraditional risk factor for cardiovascular disease and mortality. The chronic unpredictable mild stress (CMS) rat model is a validated model of depression. Within the paraventricular nucleus (PVN), vasopressin (VP) via V1aR and V1bR have been implicated in stress and neurocardiovascular dysregulation. We hypothesized that in conscious, unrestrained CMS rats versus control, unstressed rats, PVN VP results in elevated arterial pressure (MAP), heart rate, and renal sympathetic nerve activity (RSNA) via activation of V1aR and/or V1bR. Male rats underwent 4 wk of CMS or control conditions. They were then equipped with hemodynamic telemetry transmitters, PVN cannula, and left renal nerve electrode. V1aR or V1bR antagonism dose-dependently inhibited MAP after VP injection. V1aR or V1bR blockers at their ED50 doses did not alter baseline parameters in either control or CMS rats but attenuated the pressor response to VP microinjected into PVN by ∼50%. Combined V1aR and V1bR inhibition completely blocked the pressor response to PVN VP in control but not CMS rats. CMS rats required combined maximally inhibitory doses to block either endogenous VP within the PVN or responses to microinjected VP. Compared with unstressed control rats, CMS rats had higher plasma VP levels and greater abundance of V1aR and V1bR transcripts within PVN. Thus, the CMS rat model of depression results in higher resting MAP, heart rate, and RSNA, which can be mitigated by inhibiting vasopressinergic mechanisms involving both V1aR and V1bR within the PVN. Circulating VP may also play a role in the pressor response.

Haemodynamic and renal sympathetic responses to V1b vasopressin receptor activation within the paraventricular nucleus

NEW FINDINGS

What is the central question of this study? Does antagonism of V1b receptors prevent the haemodynamic and renal sympathetic nerve responses that occur with application of exogenous vasopressin into the paraventricular nucleus (PVN) of conscious, chronically instrumented rats? What is the main finding and its importance? Microinjection of vasopressin into the PVN increased mean arterial pressure, heart rate and renal sympathetic nerve activity, all of which were inhibited by pre-injection of the PVN with the V1b antagonist, nelivaptan. The administered vasopressin did not enter the peripheral circulation or increase plasma vasopressin. Ganglionic blockade prevented each of the responses, consistent with mediation by enhanced sympathetic output rather than an increase in circulating vasopressin. Vasopressin (VP) participates in regulation of haemodynamics and volume. Besides more classical actions as a circulating hormone, VP may act via release from axons and dendrites within the CNS. The paraventricular nucleus (PVN) possesses vasopressinergic neurons and a dense complement of VP receptors, including the V1b receptor, which has been implicated in several types of stress responses. We tested the hypothesis that antagonism of V1b receptors will prevent VP-induced increases in mean arterial pressure (MAP), heart rate (HR) and renal sympathetic nerve activity (RSNA). Studies were performed in conscious male Sprague-Dawley rats chronically instrumented with vascular catheters, renal nerve electrodes and a cannula stereotaxically directed into the PVN. Unilateral microinjection of VP into the PVN significantly increased MAP, HR and RSNA, peaking at 10 min. Pre-injection of the PVN with the selective V1b receptor antagonist, nelivaptan, did not alter baseline values but blocked the responses to VP. Ganglionic blockade with chlorisondamine decreased MAP and HR and abolished their increase in response to subsequent PVN application of VP. Injection of VP into the PVN did not alter plasma VP levels. Paraventricular nucleus injection with radiolabelled VP resulted in negligible radiolabelled VP in peripheral blood. These findings support the concept that, in basal conditions, PVN V1b receptor activation (rather than VP release into the periphery) may be implicated in the increases in MAP, HR and RSNA due to increased sympathetic outflow. While the role of V1a and oxytocin receptors cannot be excluded, these data suggest that further studies of the role of V1b receptor activation by endogenous VP during stress to effect neuroexcitation are warranted.

Echocardiographic assessment of hemodynamic changes produced by two methods of inducing fluid deficit in dogs

BACKGROUND

Hydration status is important to the cardiovascular system because of its effects on preload. Decreased preload can alter echocardiographic measurements of systolic and diastolic function, potentially confounding interpretation of results.

HYPOTHESIS/OBJECTIVES

Mild fluid deficits are associated with measurable echocardiographic changes that are validated by physical and biochemical markers of decreased intravascular volume.

ANIMALS

Twenty-five healthy staff/student-owned dogs with no evidence of cardiac or renal disease.

METHODS

Prospective, interventional laboratory study. Dogs were randomly assigned to water deprivation (WD) alone for 8 hours (n = 13) or to furosemide treatment (FTx, 2.5mg/kg IV) followed by WD for 8 hours (n = 12). Echocardiograms, biochemical sampling, and physical parameters were measured at baseline, and after 4 and 8 hours.

RESULTS

Both protocols induced fluid deficit as indicated by significant (P < .00001) decreases in weight at 4 hours (WD, 1.1%; FTx, 3.7%) and 8 hours (WD, 2.7%; FTx, 4.5%). Furosemide significantly decreased left ventricular end-diastolic volume (54.3 +/- 19.3-42.1 +/- 17.3 mL, P < .0001), cardiac index (4.2 +/- 1.1-2.9 +/- 0.9 L/min/M2, P < .0001), and mitral valve E wave velocity (0.79 +/- 0.2-0.66 +/- 0.2 m/s, P = .0004). These changes were accompanied by significant increases in blood urea nitrogen concentration (13.8 +/- 2.6-14.8 +/- 2.7 mg/dL, P = .04), vasopressin concentration (1.4 +/- 1.2-3.3 +/- 1.9 pg/mL, P = .045), and PCV (49.8 +/- 4.5-53.2 +/- 6.5%, P = .006). Effects of water deprivation alone were similar, but less pronounced.

CONCLUSIONS AND CLINICAL IMPORTANCE

Mild fluid deficits have measurable hemodynamic effects in dogs. Hydration status should be considered when evaluating cardiac function by echocardiogram.

Vasopressin is a major vasoconstrictor involved in hindlimb vascular responses to stimulation of adenosine A(1) receptors in the nucleus of the solitary tract

Our previous study showed that stimulation of adenosine A(1) receptors located in the nucleus of the solitary tract (NTS) exerts counteracting effects on the iliac vascular bed: activation of the adrenal medulla and beta-adrenergic vasodilation versus vasoconstriction mediated by neural and unknown humoral factors. In the present study we investigated the relative contribution of three major potential humoral vasoconstrictors: vasopressin, angiotensin II, and norepinephrine in this response. In urethane-chloralose anesthetized rats we compared the integral changes in iliac vascular conductance evoked by microinjections into the NTS of the selective A(1) receptor agonist N(6)-cyclopentyladenosine (CPA; 330 pmol in 50 nl) in intact (Int) animals and following: V(1) vasopressin receptor blockade (VX), angiotensin II AT(1) receptor blockade (ATX), bilateral adrenalectomy + ganglionic blockade (ADX + GX; which eliminated the potential increases in circulating norepinephrine and epinephrine), ADX + GX + VX and ADX + GX + VX + ATX. In Int animals, stimulation of NTS A(1) adenosine receptors evoked typical variable responses with prevailing pressor and vasoconstrictor effects. VX reversed the responses to depressor ones. ATX did not significantly alter the responses. ADX + GX accentuated pressor and vasoconstrictor responses, whereas ADX + GX + VX and ADX + GX + VX + ATX virtually abolished the responses. Stimulation of NTS A(1) adenosine receptors increased circulating vasopressin over fourfold (26.4 + or - 10.4 vs. 117.0 + or - 19 pg/ml). These data strongly suggest that vasopressin is a major vasoconstrictor factor opposing beta-adrenergic vasodilation in iliac vascular responses triggered by stimulation of NTS A(1) adenosine receptors, whereas angiotensin II and norepinephrine do not contribute significantly to the vasoconstrictor responses.

Small mouse cholangiocytes proliferate in response to H1 histamine receptor stimulation by activation of the IP3/CaMK I/CREB pathway

Cholangiopathies are characterized by the heterogeneous proliferation of different-sized cholangiocytes. Large cholangiocytes proliferate by a cAMP-dependent mechanism. The function of small cholangiocytes may depend on the activation of inositol trisphosphate (IP(3))/Ca(2+)-dependent signaling pathways; however, data supporting this speculation are lacking. Four histamine receptors exist (HRH1, HRH2, HRH3, and HRH4). In several cells: 1) activation of HRH1 increases intracellular Ca(2+) concentration levels; and 2) increased [Ca(2+)](i) levels are coupled with calmodulin-dependent stimulation of calmodulin-dependent protein kinase (CaMK) and activation of cAMP-response element binding protein (CREB). HRH1 agonists modulate small cholangiocyte proliferation by activation of IP(3)/Ca(2+)-dependent CaMK/CREB. We evaluated HRH1 expression in cholangiocytes. Small and large cholangiocytes were stimulated with histamine trifluoromethyl toluidide (HTMT dimaleate; HRH1 agonist) for 24-48 h with/without terfenadine, BAPTA/AM, or W7 before measuring proliferation. Expression of CaMK I, II, and IV was evaluated in small and large cholangiocytes. We measured IP(3), Ca(2+) and cAMP levels, phosphorylation of CaMK I, and activation of CREB (in the absence/presence of W7) in small cholangiocytes treated with HTMT dimaleate. CaMK I knockdown was performed in small cholangiocytes stimulated with HTMT dimaleate before measurement of proliferation and CREB activity. Small and large cholangiocytes express HRH1, CaMK I, and CaMK II. Small (but not large) cholangiocytes proliferate in response to HTMT dimaleate and are blocked by terfenadine (HRH1 antagonist), BAPTA/AM, and W7. In small cholangiocytes, HTMT dimaleate increased IP(3)/Ca(2+) levels, CaMK I phosphorylation, and CREB activity. Gene knockdown of CaMK I ablated the effects of HTMT dimaleate on small cholangiocyte proliferation and CREB activation. The IP(3)/Ca(2+)/CaMK I/CREB pathway is important in the regulation of small cholangiocyte function.

Nitric oxide modulation of ETB receptor-induced vasopressin release by rat and mouse hypothalamo-neurohypophyseal explants

Endothelin (ET) peptides stimulate vasopressin (AVP) secretion via ETB receptors at hypothalamic loci. Nitric oxide modulates the actions of ET in the cardiovascular system and also influences neurotransmission and specifically suppresses firing of magnocellular neurons. The purpose of these studies was to ascertain whether nitric oxide, generated in response to ETB receptor stimulation, buffers the stimulatory effect of ET and suppresses AVP release. Studies were performed using a pharmacological approach in hypothalamo-neurohypophyseal explants from rats, and an alternative strategy using explants from mice with an inactivating mutation of neuronal NOS (nNOS−/−) and their wild-type parent strain. Whole explants in standard culture or only the hypothalamus of compartmentalized explants was exposed to the ETB selective agonist, IRL 1620 (10−13 to 10−8 M). Rat and wild-type mouse explants displayed similar responses, although absolute basal release rates were higher from murine explants. Maximal AVP release at 0.1 nM IRL 1620 was 311 ± 63 (rat) and 422 ± 112% basal·explant−1·h−1 (mouse). Sodium nitroprusside (SNP; 0.1 mM) suppressed maximal AVP release to basal values. Nω-nitro-l-arginine methyl ester (l-NAME, 0.1 μM), which did not itself stimulate AVP secretion, more than doubled the response to 1 pM IRL 1620, from 136 ± 28 to 295 ± 49% basal·explant−1·h−1 ( P < 0.05) by rat explants. Explants from wild-type mice responded similarly. Explants from nNOS−/− mice had higher basal AVP secretory rate in response to 1 pM IRL 1620: 271 ± 48 compared with 150 ± 24% basal·explant−1·h−1 ( P < 0.05) from wild-type murine explants. In the nNOS−/−, SNP suppressed stimulated release, and l-NAME exerted no additional stimulatory effect: 243 ± 38% basal·explant−1·h−1. Thus nitric oxide inhibits the AVP secretory response induced by ETB receptor activation within the hypothalamo-neurohypophyseal system and is generated primarily by the nNOS isoform. The modulation of AVP secretion by ET and also nitric oxide can take place independently from their effects on cerebral blood flow, systemic hemodynamics, or the arterial baroreflex.

Kappa opiate agonist RU 51599 inhibits vasopressin gene expression and osmotically-induced vasopressin secretion in vitro

Kappa (kappa) opioid agonists induce a water diuresis and inhibit vasopressin (AVP) secretion. Hypothalamic and neurohypophysial sites have both been implicated in the response. The present study was designed to ascertain if kappa-agonist inhibition of osmotically-stimulated AVP secretion is associated with parallel changes in AVP gene expression. Experiments were performed using the selective kappa-agonist RU 51599 (RU) in compartmentalized hypothalamo-neurohypophysial explants. When added to either the hypothalamus or the neural lobe, RU dose dependently inhibited osmotically-induced AVP secretion that was reversed by the highly selective kappa-antagonist nor-binaltorphimine (nor-BNI) only at the hypothalamic, not the neurohypophysial level. AVP mRNA content paralleled the changes in AVP secretory rate induced by hypothalamic kappa-agonism. AVP mRNA levels were unaltered when RU was applied to the neural lobe. Neurohypophysial AVP content did not change. These data indicate that hypothalamic kappa-agonism inhibits osmotically induced AVP secretion and that a non-kappa1 opiate receptor mediates posterior pituitary opioid inhibition of AVP release. Neural or receptor inputs to the hypothalamus or magnocellular cell body may downwardly modulate AVP mRNA content by altering AVP gene transcription and/or message stability. Inhibition of AVP release directly at the neurohypophysis can be uncoupled from the cellular mechanisms that generate changes in AVP mRNA content.

Heart failure alters the strength and mechanisms of the muscle metaboreflex

We hypothesized that excessive sympathoactivation observed during strenuous exercise in subjects with heart failure (HF) may result from tonic activation of the muscle metaboreflex (MMR) via hypoperfusion of active skeletal muscle. We studied MMR responses in dogs during treadmill exercise by graded reduction of terminal aortic blood flow (TAQ) before and after induction of HF by rapid ventricular pacing. At a low workload, in both control and HF experiments, large decreases in TAQ were required to elicit the MMR pressor response. During control experiments, this pressor response resulted from increased cardiac output (CO), whereas in HF CO did not increase; thus the pressor response was solely due to peripheral vasoconstriction. In HF, MMR activation also induced higher plasma levels of vasopressin, norepinephrine (NE), and renin. At a higher workload, in control experiments any reduction of TAQ elicited MMR pressor responses. In HF, before any vascular occlusion, TAQ was already below MMR control threshold levels and reductions in TAQ again did not result in higher CO; thus SAP increased via peripheral vasoconstriction. NE rose markedly, indicating intense sympathetic activation. We conclude that in HF, the MMR is likely tonically active at moderate workloads and contributes to the tonic sympathoactivation.

Effect of endothelin-3 on vasopressin release in vitro and water excretion in vivo in Long-Evans rats

1. The endothlins (ETs) are a family of homologous peptides originally isolated and purified from cultured porcine endothelial cells. Although initial studies focused on the ETs as potent vasoconstrictor substances, recent findings support a role for ET in vasopressin (AVP) secretion and action. We used cultured explants of the hypothalamo-neurohypophysial complex (HNC) to examine the effects of ET-3 on AVP release. 2. ET-3 produced a significant and concentration-dependent rise in AVP release from the explants of Long-Evans rats at 48 h, independent of medium osmolality. AVP release during two sequential control periods did not differ, and osmotically stimulated AVP release was comparable to that exhibited by HNC explants from Sprague-Dawley rats. ET-3 (1 nM) induced a 3-fold rise in AVP that was completely blocked by rabbit antiET serum, which did not alter basal AVP secretion. 3. Clearance experiments were performed in anaesthetized water-loaded rats given a non-pressor dose of ET-3 (0.40 nmol (kg body weight)-1 intravenously). The means of body weight, arterial blood pressure, plasma Na+ concentrations, and plasma osmolalities did not differ between the first and second periods nor among the groups. Despite no changes in renal plasma flow and inulin clearance, free water clearance significantly increased in the ET-treated group. This response could not be attributed to changes in osmolal clearances or Na+ reabsorption. The concurrent administration of antiET serum not only blocked this effect, but was associated with the free water clearance falling significantly below the pretreatment level. 4. Taken together, our in vivo and in vitro findings support the hypothesis that ET-3 stimulates AVP secretion. Furthermore, our data are consistent with the site of action of ET-3 residing within the blood-brain barrier, though an independent effect by a higher concentration of ET at neural loci outside the barrier cannot be totally excluded. Finally, the subpressor dose of ET-3 amplifies free water excretion independent of systemic and renal haemodynamics, Na+ excretion, osmolal clearance, or circulating AVP levels.