Fluid balance and arterial blood pressure during intracarotid infusions of atrial natriuretic peptide (ANP) in water-deprived goats

Changes in blood pressure and muscle sympathetic nerve activity during water drinking in humans

To investigate the possible involvement of the sympathetic nervous system in pressor response during water drinking, muscle sympathetic nerve activity (MSNA), blood pressure (BP), and heart rate (HR) were continuously measured in healthy young volunteers throughout the experiments of a 5-min control, 2 min of drinking 500 ml water, and a 28-min recovery. To avoid the effects of water passing through the oropharyngeal and esophageal regions and/or effects of swallowing, an equal amount of water was directly infused to the stomach through a stomach tube for 2 min. Water drinking caused a transient increase in mean arterial pressure (MAP) and HR immediately after drinking (DeltaMAP, 12.6 +/- 2.1 mmHg; DeltaHR, +19.9 +/- 1.7 beats/min at the peak). An abrupt decrease of MSNA was observed directly during water drinking (Deltaburst rate, -6.9 +/- 1.3 bursts/min; Deltatotal activity, -2,606 +/- 491 U/min), and it increased to the baseline level thereafter. Gastric infusion had little or no effect on MAP, HR, and MSNA. The present study demonstrated that a pressor response during water drinking was associated with the attenuation of MSNA and not generated by gastric infusion of water at the same rate as in this drinking manner. In conclusion, the rapid rise in BP might be caused through stimulations from the oropharyngeal region, swallowing-induced factors, and/or a feedforward mechanism by a central descending signal from the higher brain centers.

Mechanisms involved in the water intake-related pressor response in the rat

OBJECTIVE

In this study we aimed to characterize and clarify the mechanisms involved in the acute blood pressure increase observed concomitantly with water intake in moderately dehydrated rats.

DESIGN

Short-term water deprivation was employed as a model to induce controlled water intake to study concomitant cardiovascular responses in the rat.

METHODS

Male Wistar rats were deprived of water for 18-24 h before the experiments and were allowed to drink for 20 s periods during the experimental session. During these periods water intake was accompanied by steady arterial pressure increases. This pressor response was unaffected by topical anesthesia of the oral cavity. Direct administration of water into the stomach did not cause pressor responses. The pressor response was not affected by bilateral adrenal demedullation or by pretreatment with diazepam, homatropine methyl bromide, d(CH2)5 Tyr(Me)AVP, losartan or RX821002. The pressor response was significantly reduced by ganglionic blockade with mecamylamine or pretreatment with the alpha1-adrenoceptor antagonist, prazosin.

CONCLUSIONS

Our results show that: (1) short-term dehydration can be used as a model to study cardiovascular responses associated with water intake in rats; and (2) the sympathetic nervous system and vascular smooth muscle alpha1-adrenoceptors are involved in the pressor response to water intake by dehydrated rats.

Electrical remodeling due to atrial fibrillation in chronically instrumented conscious goats: roles of neurohumoral changes, ischemia, atrial stretch, and high rate of electrical activation

BACKGROUND

Recently, we developed a goat model of chronic atrial fibrillation (AF). Due to AF, the atrial effective refractory period (AERP) shortened and its physiological rate adaptation inversed, whereas the rate and stability of AF increased. The goal of the present study was to evaluate the role of (1) the autonomic nervous system, (2) ischemia, (3) stretch, (4) atrial natriuretic factor (ANF), and (5) rapid atrial pacing in this process of electrical remodeling.

METHODS AND RESULTS

Twenty-five goats were chronically instrumented with multiple epicardial atrial electrodes. Infusion of atropine (1.0 mg/kg; n=6) or propranolol (0.6 mg/kg; n=6) did not abolish the AF-induced shortening of AERP or interval (AFI). Blockade of K+(ATP) channels by glibenclamide (10 micromol/kg; n=6) slightly increased the AFI from 95+/-4 to 101+/-5 ms, but AFI remained considerably shorter than during acute AF (145 ms). Glibenclamide had no significant effect on AERP after electrical cardioversion of AF (69+/-14 versus 75+/-15 ms). Volume loading by 0.5 to 1.0 L of Hemaccel (n=12) did not shorten AERP. The median plasma level of ANF increased from 42 to 99 pg/mL after 1 to 4 weeks of AF (n=6), but ANF infusion (0.1 to 3.1 microg/min, n=4) did not shorten AERP. Rapid atrial pacing (24 to 48 hours; n=10) progressively shortened AERP from 134+/-10 to 105+/-6 ms and inversed its physiological rate adaptation.

CONCLUSIONS

Electrical remodeling by AF is not mediated by changes in autonomic tone, ischemia, stretch, or ANF. The high rate of electrical activation itself provides the stimulus for the AF-induced changes in AERP.

Effects of head-down tilt on atrial natriuretic peptide and the renin system in pregnancy

We studied the effects of head-down tilt to 10 degrees for 30 minutes on plasma atrial natriuretic peptide and the renin-aldosterone system in 8 preeclamptic pregnant women, 8 healthy pregnant women, and 11 nonpregnant women of fertile age. Mean arterial blood pressure did not change in the pregnant groups but increased significantly in the nonpregnant control subjects. Heart rate decreased significantly in preeclamptic women but remained unchanged in both control groups. Baseline atrial natriuretic peptide concentration was significantly higher in both preeclamptic (66 +/- 4 pmol/L) and pregnant (54 +/- 6 pmol/L) control subjects compared with nonpregnant subjects (40 +/- 2 pmol/L), but the difference between the pregnant groups was not significant. Head-down tilting induced a significant increase in atrial natriuretic peptide only in healthy pregnant women. Baseline plasma renin activity and aldosterone concentrations were significantly higher in pregnant control subjects compared with both the preeclamptic and nonpregnant groups. The differences between the preeclamptic and nonpregnant control groups were nonsignificant. After head-down tilting, plasma renin activity decreased significantly only in nonpregnant control subjects, whereas aldosterone decreased significantly in preeclamptic and nonpregnant control subjects. In preeclampsia, atrial natriuretic peptide release followed blood pressure and not changes in cardiac output. When all 27 women were studied, a correlation between atrial natriuretic peptide and mean arterial pressure was found in the left lateral supine position. The results suggest that pregnant women developing preeclampsia lose their usual hemodynamic control and show reactions resembling the nonpregnant state when subjected to head-down tilt.

Intraruminal fluid administration to goats: effects of handling and fluid temperature

Physiological stress response to intraruminal fluid administration was studied in 5 female goats. The fluid was given through a stomach tube. The water was cold (10 degrees C) or warm (38 degrees C) and in an amount of 7.5-10% of body weight. Plasma concentrations of adrenaline, noradrenaline and cortisol increased significantly after both treatments. Adrenaline and noradrenaline responses were greater and more longlasting after cold water administration, but there was no difference in cortisol response between cold and warm water. Haematocrit and plasma protein concentrations increased also and the effects of cold water lasted longer. Blood pressure showed a sharp rise of short duration and was independent of water temperature. The immediate tachycardiac response was similar with both treatments, but cold water caused an additional peak 15 min later. The cooling did not increase plasma histamine level. The results suggest that stomach intubation and administration of water into the rumen leads to strong activation of hormonal and cardiovascular stress parameters even in goats well adapted to handling. Low temperature of the fluid further heightens the effect. Warming of fluids to near body temperature before their administration is thus recommended.

Centrally administered atriopeptin III reduces water intake and vasopressin secretion in dehydrated sheep

Two experiments were carried out to investigate the responses of sheep (N = 6) to intracerebroventricular (ICV) injections of atriopeptin III [atrial natriuretic factor (5-28), AP III]. In Experiment, 1, 24-h dehydrated animals were given 0 (saline vehicle control), 10 or 30 micrograms AP III directly before the presentation of water. The highest dose of AP III significantly (p less than 0.02) reduced the amount of water drunk in the subsequent 20 min. In Experiment 2, blood samples were taken at various intervals before and after ICV injection of 0 or 30 micrograms AP III when the sheep were water replete or 24-h dehydrated. Plasma concentrations of vasopressin (AVP) and cortisol were measured and estimates were made of plasma osmolality. In the dehydration condition, AVP levels were somewhat reduced (p less than 0.059) after AP III administration but no decrease was observed when the animals were euhydrated. No significant changes in plasma osmolality or cortisol concentrations were observed in response to AP III or the saline vehicle. Because a large dose of AP III (30 micrograms ICV) was required to produce the comparatively small behavioral and endocrine effects observed in this study, the results are suggestive of a pharmacological, rather than a physiological, action of the peptide in this species.