Increased thirst and vasopressin secretion after myocardial infarction in rats

The neural basis of homeostatic and anticipatory thirst

Water intake is one of the most basic physiological responses and is essential to sustain life. The perception of thirst has a critical role in controlling body fluid homeostasis and if neglected or dysregulated can lead to life-threatening pathologies. Clear evidence suggests that the perception of thirst occurs in higher-order centres, such as the anterior cingulate cortex (ACC) and insular cortex (IC), which receive information from midline thalamic relay nuclei. Multiple brain regions, notably circumventricular organs such as the organum vasculosum lamina terminalis (OVLT) and subfornical organ (SFO), monitor changes in blood osmolality, solute load and hormone circulation and are thought to orchestrate appropriate responses to maintain extracellular fluid near ideal set points by engaging the medial thalamic-ACC/IC network. Thirst has long been thought of as a negative homeostatic feedback response to increases in blood solute concentration or decreases in blood volume. However, emerging evidence suggests a clear role for thirst as a feedforward adaptive anticipatory response that precedes physiological challenges. These anticipatory responses are promoted by rises in core body temperature, food intake (prandial) and signals from the circadian clock. Feedforward signals are also important mediators of satiety, inhibiting thirst well before the physiological state is restored by fluid ingestion. In this Review, we discuss the importance of thirst for body fluid balance and outline our current understanding of the neural mechanisms that underlie the various types of homeostatic and anticipatory thirst.

Prognostic impact of hyponatremia occurring at various time points during hospitalization on mortality in patients with acute myocardial infarction

We investigated the incidence and prognostic impact of hyponatremia occurring at various time points during hospitalization on long-term mortality in acute myocardial infarction (AMI) survivors. We retrospectively studied 1863 patients diagnosed with AMI. Baseline, nadir, and discharge sodium levels during hospitalization were recorded and analyzed. Hyponatremia was defined as a serum sodium level <135 mEq/L. On the basis of baseline, nadir, and discharge sodium levels during hospitalization, hyponatremia was diagnosed in 309 (16.6%), 518 (27.8%), and 147 (7.9%) patients, respectively. In a multivariate Cox-proportional regression analysis, discharge sodium level had the strongest significant relationship with long-term mortality (hazard ratio [HR] as continuous variable = 1.06, 95% confidence interval [CI]: 1.01-1.11, P = .026; HR as categorical variable = 1.71; 95% CI: 1.06-2.75; P = .028), but baseline and nadir sodium had no prognostic impact on long-term mortality after adjustment. The serum sodium level and incidence of hyponatremia varied at different time points during hospitalization. In addition, the association between sodium level and long-term mortality differed at these various time points. The discharge sodium level, among the various time points, seems the best predictor of long-term mortality in AMI survivors.

An intelligent flow control system for long term fluid restriction in small animals

Fluid retention is one of the most common symptoms in patients with chronic heart failure. Although fluid restriction may be a therapeutic strategy, the degree of fluid restriction necessary for the best therapeutic outcome remains unknown partly due to the lack of proper experimental method to restrict water consumption in small animals. The traditional methods that allow animals to access water only in a limited time window or within pre-determined daily volume can be stressful because the animals may become thirsty during the time of water deprivation. To provide a less stressful water restriction paradigm, we designed a feedback-control system of drinking flow to modulate the drinking behavior of small animals. This system consisted of an infrared droplet sensor for monitoring the drinking flow and a computer controlled electric valve to regulate the water availability. A light signal which synchronized with the command for opening the valve was set to establish a conditioned reflex. An animal test indicated that rats were adaptable to a precisely programmed water supply. This system may warrant investigation into the consequences of fluid restriction in chronic experimental animal study.

The cardioprotective effect of different doses of vasopressin (AVP) against ischemia-reperfusion injuries in the anesthetized rat heart

The aim of the present study was to investigate the protective effect of various doses of exogenous vasopressin (AVP) against ischemia-reperfusion injury in anesthetized rat heart. Anesthetized rats were randomly divided into seven groups (n=4-13) and all of them subjected to prolonged 30 min regional ischemia and 120 min reperfusion. Group I served as saline control with ischemia, in treatment groups II, III, IV and V, respectively different doses of AVP (0.015, 0.03, 0.06 and 1.2 μg/rat) were infused within 10 min prior to ischemia, in group VI, an AVP-selective V1 receptor antagonist (SR49059, 1mg/kg, i.v.) was administrated prior to effective dose of AVP injection and in group VII, SR49059 (1 mg/kg, i.v.) was only administrated prior to ischemia. Various doses of AVP significantly prevented the decrease in heart rate (HR) at the end of reperfusion compared to their baseline and decreased infarct size, biochemical parameters [LDH (lactate dehydrogenase), CK-MB (creatine kinase-MB) and MDA (malondialdehyde) plasma levels], severity and incidence of ventricular arrhythmia, episodes and duration of ventricular tachycardia (VT) as compared to control group. Blockade of V1 receptors by SR49059 attenuated the cardioprotective effect of AVP on ventricular arrhythmias and biochemical parameters, but partially returned infarct size to control. AVP 0.03 μg/rat was known as effective dose. Our results showed that AVP owns a cardioprotective effect probably via V1 receptors on cardiac myocyte against ischemia/reperfusion injury in rat heart in vivo.

Cortical activation and lamina terminalis functional connectivity during thirst and drinking in humans

The pattern of regional brain activation in humans during thirst associated with dehydration, increased blood osmolality, and decreased blood volume is not known. Furthermore, there is little information available about associations between activation in osmoreceptive brain regions such as the organum vasculosum of the lamina terminalis and the brain regions implicated in thirst and its satiation in humans. With the objective of investigating the neuroanatomical correlates of dehydration and activation in the ventral lamina terminalis, this study involved exercise-induced sweating in 15 people and measures of regional cerebral blood flow (rCBF) using a functional magnetic resonance imaging technique called pulsed arterial spin labeling. Regional brain activations during dehydration, thirst, and postdrinking were consistent with the network previously identified during systemic hypertonic infusions, thus providing further evidence that the network is involved in monitoring body fluid and the experience of thirst. rCBF measurements in the ventral lamina terminalis were correlated with whole brain rCBF measures to identify regions that correlated with the osmoreceptive region. Regions implicated in the experience of thirst were identified including cingulate cortex, prefrontal cortex, striatum, parahippocampus, and cerebellum. Furthermore, the correlation of rCBF between the ventral lamina terminalis and the cingulate cortex and insula was different for the states of thirst and recent drinking, suggesting that functional connectivity of the ventral lamina terminalis is a dynamic process influenced by hydration status and ingestive behavior.

Osmosensation in vasopressin neurons: changing actin density to optimize function

The proportional relation between circulating vasopressin concentration and plasma osmolality is fundamental for body fluid homeostasis. Although changes in the sensitivity of this relation are associated with pathophysiological conditions, central mechanisms modulating osmoregulatory gain are unknown. Here, we review recent data that sheds important light on this process. The cell autonomous osmosensitivity of vasopressin neurons depends on cation channels comprising a variant of the transient receptor potential vanilloid 1 (TRPV1) channel. Hyperosmotic activation is mediated by a mechanical process where sensitivity increases in proportion with actin filament density. Moreover, angiotensin II amplifies osmotic activation by a rapid stimulation of actin polymerization, suggesting that neurotransmitter-induced changes in cytoskeletal organization in osmosensory neurons can mediate central changes in osmoregulatory gain.

Actin filaments mediate mechanical gating during osmosensory transduction in rat supraoptic nucleus neurons

Osmosensory transduction is a bidirectional process displayed by neurons involved in the control of thirst and antidiuretic hormone release, and is therefore crucial for body fluid homeostasis. Although this mechanism is known to involve the activation of nonselective cation channels during hypertonicity-evoked shrinking, and the inhibition of these channels during hypotonicity-evoked swelling, the basis for this regulation is unknown. Here, we investigated this process using whole-cell patch-clamp recordings from neurons acutely isolated from the supraoptic nucleus of adult rats. The mechanosensitivity index, defined as the ratio of conductance change to normalized volume change, was quantitatively equivalent whether cell volume was increased or decreased by changes in extracellular fluid osmolality, or by changes in pipette pressure. Moreover, responses induced by hyperosmotic or hypo-osmotic media could be reversed by increasing or decreasing pipette pressure, respectively. The mechanosensitivity index was significantly reduced in neurons treated with cytochalasin-D, a compound that promotes the depolymerization of actin filaments. Conversely, cells treated with jasplakinolide, a compound that promotes actin polymerization, showed a significant increase in mechanosensitivity index. Finally, the depolarizing and excitatory effects of hypertonic stimuli were significantly enhanced by jasplakinolide and reduced by cytochalasin-D. We conclude that osmosensory transduction in these neurons is a reversible mechanical process that depends on an intact actin cytoskeleton, and the sensitivity of the transducer appears to vary in proportion with the density of actin filaments.

Chronic vagal stimulation decreased vasopressin secretion and sodium ingestion in heart failure rats after myocardial infarction

Chronic vagal stimulation (VS) markedly improved long-term survival in the heart failure rats. We examined the effects of VS on arginine vasopressin (AVP) secretion and salt ingestion in heart failure rats after myocardial infarction (MI). Surviving rats after MI were randomly assigned to two groups. One group was treated with sham stimulation (SS), and the other group was treated with VS. All rats could access water and 1.8% NaCl solution ad libitum. Treatment started at 2 weeks after MI, and continued for 6 weeks. We monitored drinking behavior during treatment. At the end of treatment, we measured hemodynamics and plasma levels of AVP and brain natriuretic peptide (BNP). The plasma AVP and BNP levels were significantly lower in the VS group than the SS group. VS significantly inhibited the ingestion of 1.8% NaCl solution. The normalized biventricular weight of the VS group was significantly lower than that of the SS group. The VS group had significantly lower left ventricular end-diastolic pressure, and higher cardiac index than the SS group. In conclusion, these results suggest that chronic VS regulates the water balance by suppression of plasma AVP level and salt ingestion in the heart failure rats after MI.

Enhanced reactivity to vasopressin in rat basilar arteries during vasospasm after subarachnoid hemorrhage

Subarachnoid hemorrhage increases the plasma level of vasopressin, a well-known vasoconstrictor. We examined the sensitivity to vasopressin in rat basilar artery after subarachnoid hemorrhage using a rat subarachnoid hemorrhage model. Vasospasm was observed 1-2 days after subarachnoid hemorrhage induction, and the contractile response to vasopressin in rat basilar arteries was assessed. The concentration-response curve for vasopressin in subarachnoid hemorrhage (1 day) rats shifted leftward compared with that of control rats. The concentration-response curve for vasopressin V(1) receptor agonist also shifted leftward and upward compared with that of control rats. The concentration-response curve for vasopressin was inhibited not by vasopressin V(2) receptor antagonist but by vasopressin V(1) receptor antagonist. Thus, it was demonstrated that the vasoconstricting effect of vasopressin was significantly enhanced in the vasospasm phase after subarachnoid hemorrhage.