Response of Pituitary-Thyroid Axis to a Short-Term Shift in Deuterium Content in the Body

We studied functional changes in rat pituitary-thyroid axis after a short-term shift in deuterium body content. Male Wistar rats consumed deuterium-enriched (500,000 ppm) or deuterium-depleted water (10 ppm) for 24 h. Rats of both experimental groups demonstrated elevated concentration of bound with transport proteins thyroxine and reduced level of thyroid-stimulating hormone in serum. No changes in the rate of thyroxine conversion to triiodothyronine were found. Thus, both the increase and reduction of deuterium body content produced similar changes in the function of the pituitary-thyroid axis with primary affection of the thyroid gland, indicative of its higher sensitivity to shift in deuterium levels.

Mechanisms for hemodynamic instability related to renal replacement therapy: a narrative review

Hemodynamic instability related to renal replacement therapy (HIRRT) is a frequent complication of all renal replacement therapy (RRT) modalities commonly used in the intensive care unit. HIRRT is associated with increased mortality and may impair kidney recovery. Our current understanding of the physiologic basis for HIRRT comes primarily from studies of end-stage kidney disease patients on maintenance hemodialysis in whom HIRRT is referred to as ‘intradialytic hypotension’. Nonetheless, there are many studies that provide additional insights into the underlying mechanisms for HIRRT specifically in critically ill patients. In particular, recent evidence challenges the notion that HIRRT is almost entirely related to excessive ultrafiltration. Although excessive ultrafiltration is a key mechanism, multiple other RRT-related mechanisms may precipitate HIRRT and this could have implications for how HIRRT should be managed (e.g., the appropriate response might not always be to reduce ultrafiltration, particularly in the context of significant fluid overload). This review briefly summarizes the incidence and adverse effects of HIRRT and reviews what is currently known regarding the mechanisms underpinning it. This includes consideration of the evidence that exists for various RRT-related interventions to prevent or limit HIRRT. An enhanced understanding of the mechanisms that underlie HIRRT, beyond just excessive ultrafiltration, may lead to more effective RRT-related interventions to mitigate its occurrence and consequences.

Different Effects on Fluid Distribution between Tolvaptan and Furosemide in a Liver Cirrhosis Patient with Chronic Kidney Disease

A 73-year-old man with liver cirrhosis and advanced chronic kidney disease was admitted to our hospital due to bilateral lower leg edema and appetite loss. Furosemide to treat fluid retention markedly decreased extracellular water compared with intracellular water, but the addition of tolvaptan equally decreased both with a greater diuretic response than furosemide alone. Furthermore, tolvaptan administration increased the plasma colloid osmotic pressure, which might facilitate the shift of fluid from the extravascular space to the intravascular space. This is the first case showing different effects on the fluid distribution between furosemide and additional tolvaptan in the same patient.

Factors predisposing to worsening of sleep apnea in response to fluid overload in men

OBJECTIVES

Obstructive sleep apnea (OSA) is highly prevalent in patients with fluid-retaining conditions. Using bioimpedance measurements, previous studies have shown that the greater the amount of fluid redistributed from the legs to the neck overnight, the greater the severity of OSA. Our objective was to investigate factors that predispose the development or worsening of OSA in response to experimental fluid overload.

METHODS

Fifteen normotensive and non-obese adult men with and without OSA underwent polysomnography (PSG) during which normal saline was infused intravenously at a minimal rate to keep the vein open (control) or as a bolus of 22 ml/kg body weight (approximately 2 L) in a random order and crossed over after a week.

RESULTS AND CONCLUSIONS

Before and after sleep, neck circumference and bioimpedance were measured to calculate neck resistance, reactance, phase angle, and fluid volume. Subjects who experienced more than a twofold increase in apnea-hypopnea index (AHI) or obstructive AHI from control to intervention and had an AHI>10 during intervention were considered susceptible to the development or worsening of OSA. Baseline neck circumference and phase angle before saline infusion were independently associated with increased susceptibility to developing or worsening OSA in response to saline infusion. In non-obese men, a larger neck circumference and bioimpedance phase angle of the neck, which may be associated with larger pharyngeal tissue content, is associated with increased susceptibility for worsening of OSA in response to fluid overloading.

The effect of fluid overload by saline infusion on heart rate variability in men during sleep

Fluid shift from the legs and into the neck during sleep has been shown to increase tissue pressure around the upper airway and increase sleep apnea severity. Recently, acute fluid overload via saline infusion during sleep was shown to increase sleep apnea severity in an older men (≥ 40 years of age) but not the younger men (<;40 years of age). The purpose of this study was to estimate the changes in the autonomic nervous system response to saline infusion in the younger and older men using well studied time- and frequency domain heart rate variability metrics (HRV). Eighteen healthy men (10 in the younger group and 8 in the older group) slept for up to 3 hours during the day while ≈ 2L of saline was intravenously injected during sleep. An electrocardiogram was collected continuously during the sleep period. Two five minute segments of stage 1 or 2 sleep were selected for analysis of HRV: one before saline infusion and one after completion of saline infusion. Time- and frequency domain metrics of HRV were used to characterize autonomic nervous system response. Results generally showed increased HRV as measured by time-domain statistical measures in the younger men after saline infusion. In the frequency domain, the change in high frequency power from pre- to post-saline infusion was significantly greater in the younger compared to the older men. In addition, there was a borderline significant trend showing an increase in HF power from pre- to post-saline infusion in the younger, but not the older men. These results suggest that only in the younger men and not the older men, parasympathetic nervous activity increased in response to acute fluid overload. This might be one of the mechanisms that increased the severity of OSA in older and not the younger men.

Fluid distribution kinetics during cardiopulmonary bypass

OBJECTIVE

The purpose of this study was to examine the isovolumetric distribution kinetics of crystalloid fluid during cardiopulmonary bypass.

METHODS

Ten patients undergoing coronary artery bypass grafting participated in this prospective observational study. The blood hemoglobin and the serum albumin and sodium concentrations were measured repeatedly during the distribution of priming solution (Ringer's acetate 1470 ml and mannitol 15% 200 ml) and initial cardioplegia. The rate of crystalloid fluid distribution was calculated based on 3-min Hb changes. The preoperative blood volume was extrapolated from the marked hemodilution occurring during the onset of cardiopulmonary bypass. Clinicaltrials.gov: NCT01115166.

RESULTS

The distribution half-time of Ringer's acetate averaged 8 minutes, corresponding to a transcapillary escape rate of 0.38 ml/kg/min. The intravascular albumin mass increased by 5.4% according to mass balance calculations. The preoperative blood volume, as extrapolated from the drop in hemoglobin concentration by 32% (mean) at the beginning of cardiopulmonary bypass, was 0.6-1.2 L less than that estimated by anthropometric methods (p<0.02). The mass balance of sodium indicated a translocation from the intracellular to the extracellular fluid space in 8 of the 10 patients, with a median volume of 236 ml.

CONCLUSIONS

The distribution half-time of Ringer's solution during isovolumetric cardiopulmonary bypass was 8 minutes, which is the same as for crystalloid fluid infusions in healthy subjects. The intravascular albumin mass increased. Most patients were hypovolemic prior to the start of anesthesia. Intracellular edema did not occur.

Effects of insulin detemir and NPH insulin on renal handling of sodium, fluid retention and weight in type 2 diabetic patients

AIMS/HYPOTHESIS

In type 2 diabetic patients, insulin detemir (B29Lys(ε-tetradecanoyl),desB30 human insulin) induces less weight gain than NPH insulin. Due to the proposed reduction of tubular action by insulin detemir, type 2 diabetic patients should have increased urinary sodium excretion, thereby reducing extracellular volume and body weight when changed from NPH insulin to insulin detemir.

METHODS

In a randomised, open-labelled, two-way crossover study of 24 patients with type 2 diabetes, patients were first treated with NPH insulin or insulin detemir for 8 weeks. Thereafter, they were changed to the other insulin for 8 weeks. In a third 1 week period, they were changed back to the first insulin.

RESULTS

At the end of 8 weeks, body weight was reduced by 0.8 ± 0.2 kg (mean ± SEM) on insulin detemir compared with NPH insulin (p < 0.01). After insulin detemir treatment, we also observed a significant reduction of lean body mass (0.8 ± 0.2 kg, p < 0.05) and a non-significant reduction of extracellular volume (0.8 ± 0.5 l/1.73 m², p = 0.14). The weight loss occurred after as early as 1 week (0.8 ± 0.2 kg, p < 0.001), with a simultaneous and transient increase of urinary sodium excretion (p = 0.07).

CONCLUSIONS/INTERPRETATION

Insulin detemir induces significant and sustained weight loss, which is first observed at 1 week after changing from NPH insulin. The initial weight loss seems to be related to changes in fluid volume and may reflect changed insulin action in the kidneys.

Fluid overload during cardiopulmonary bypass is effectively reduced by a continuous infusion of hypertonic saline/dextran (HSD)

OBJECTIVE

Cardiopulmonary bypass (CPB) is associated with fluid overload. We examined how a continuous infusion of hypertonic saline/dextran (HSD) influenced fluid shifts during CPB.

MATERIALS AND METHODS

Fourteen animals were randomized to a control-group (CT-group) or a hypertonic saline/dextran-group (HSD-group). Ringer's solution was used as CPB-prime and as maintenance fluid at a rate of 5 ml/kg/h. In the HSD group, 1 ml/kg/h of the maintenance fluid was substituted with HSD. After 60 min of normothermic CPB, hypothermic CPB was initiated and continued for 90 min. Fluid was added to the CPB-circuit as needed to maintain a constant level in the venous reservoir. Fluid balance, plasma volume, total tissue water (TTW), intracranial pressure (ICP) and fluid extravasation rates (FER) were measured/calculated.

RESULTS

In the HSD-group the fluid need was reduced with 60% during CPB compared with the CT-group. FER was 0.38(0.06) ml/kg/min in the HSD-group and 0.74 (0.16) ml/kg/min in the CT-group. TTW was significantly lower in the heart and some of the visceral organs in the HSD-group. In this group ICP remained stable during CPB, whereas an increase was observed in the CT-group (p<0.01).

CONCLUSIONS

A continuous infusion of HSD reduced the fluid extravasation rate and total fluid gain during CPB. TTW was reduced in the heart and some visceral organs. During CPB ICP remained normal in the HSD-group, whereas an increase was present in the CT-group. No adverse effects were observed.

[Effect of hypocapnia/alkalosis on the fluid filtration rate in isolated and perfused rabbit lungs]

Hypocapnia/alkalosis is a consequence of several lung and metabolic pathologies. The aim of this study was to determine whether the increase of fluid filtration rate (FFR) that occurs during Hypocapnia/alkalosis circumstances is determined by hypocapnia, alkalosis or both. 7 groups were formed (N=36) using isolated rabbit lungs. Group 1: Control (PCO2 6%, pH: 7.35-7.45); Group 2 (n=6): Hypocapnia/Alkalosis (CO2 1%, pH: 7.9); Group 3 (n=6): Hypocapnia/Normo-pH (CO2 1% pH 7.35-7.45), Group 4 (n=6) Normocapnia/Alcalosis (CO2 6%, pH: 7.9). Fenoterol, papaverine and hydrocortisone were added to Groups 5, 6 and 7 (n=4) respectively, all under Normocapnia/Alkalosis. FFR and Pulmonary Arterial Pressure (Pap) were considerably higher in group 2 than in control (FFR: 1.92g/min +/- 0.6 vs 0.0 g/min +/- 0.006). A strong influence exerted by pH was observed when Group 3 and group 4 were compared (FFR: 0.02 g/min +/- 0.009 vs 2.3 g/min +/- 0.9) and (Pap: 13.5 cmH2O +/- 1.4 vs 90 cmH2O +/- 15). A reduced effect was observed in groups 5 and 6 (papaverine and hydrocorisone) and a totally abolished effect was observed in group 7 (fenoterol) (FFR: 0.001 +/- 0.0003 mL/min and Pap: 14 +/- 0.8 cmH2O). Pulmonary edema induced by Hypocapnia/alkalosis is a consequence of alkalosis and not of hypocapnia. This effect could be due to inflammatory damage in the lung parenchyma and alkalosis-mediated vasoconstriction.

Estimation of acute fluid shifts using bioelectrical impedance analysis in horses

BACKGROUND

Multi-frequency bioelectrical impedance analysis (MF-BIA) has been used to evaluate extracellular fluid volume (ECFV), but not fluid fluxes associated with fluid or furosemide administration in horses. If able to detect acute changes in ECFV, MF-BIA would be useful in monitoring fluid therapy in horses.

HYPOTHESIS

The purpose of this study was to evaluate the ability of MF-BIA to detect acute fluid compartment changes in horses. We hypothesized that MF-BIA would detect clinically relevant (10-20%) changes in ECFV.

ANIMALS

Six healthy mares were used in the study.

METHODS

This is an original experimental study. Mares were studied in 3 experiments: (1) crystalloid expansion of normally hydrated subjects, (2) furosemide-induced dehydration followed by crystalloid administration, and (3) acute blood loss followed by readministration of lost blood. MF-BIA measurements were made before, during, and after each fluid shift and compared to known changes in volume calculated based on the intravenous fluids that were administered in addition to urinary fluid losses. Mean errors between MF-BIA estimated change and known volume change were compared using nonparametric analysis of variance. Estimated ECFV pre- and post-fluid administration similarly were compared. The level of statistical significance was set at P < .05.

RESULTS

Results of the study revealed a statistically significant change in ECFV and total body water during crystalloid expansion and dehydration. Statistically significant changes were not observed during blood loss and administration. Mean errors between MF-BIA results and measured net changes were small.

CONCLUSIONS AND CLINICAL IMPORTANCE

MF-BIA represents a practical and accurate means of assessing acute fluid changes during dehydration and expansion of ECFV using isotonic crystalloids with potential clinical applications in equine critical care.

Dexamethasone increases fluid absorption via Na+/H+ exchanger (NHE) 3 activation in normal human middle ear epithelial cells

The proper homeostasis of the liquid lining the surface of the middle ear cavity is vitally important for maintaining a fluid-free middle ear cavity. Disruption of this homeostasis leads to fluid collection in the middle ear cavity and results in otitis media with effusion. We demonstrated the molecular and functional expression of the Na+/H+ exchanger (NHE)s in normal human middle ear epithelial (NHMEE) cells. We also evaluated the role of NHEs in fluid absorption and the effect of dexamethasone on NHE function and NHE-dependent fluid absorption in NHMEE cells. Western blot analysis was performed for NHE1, -2, and -3 in NHMEE cells. The fluid absorption rate was measured after liquid application on the luminal surface of the cells. Intracellular pH (pHi) was measured using the pH-sensitive fluorescent probe bis-(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF)-AM. NHE activity was determined as Na+-induced pHi recovery from an acid load achieved by luminal exposure to 40 mmol/l NH4Cl. NHE1, -2 and -3 were all expressed in the NHMEE cells. The pHi recovery rate was suppressed by inhibition of NHE2 and -3 with HOE694 at concentrations greater than 50 microM. Inhibition of NHE3 with 650 microM of HOE694 or S3226 significantly decreased the fluid absorption rate. Dexamethasone increased the Na+-induced pHi recovery rate which was reversed by the inhibition of NHE3 with 650 microM of HOE694. Dexamethasone treatment up-regulated NHE3 expression in a dose-dependent manner. The fluid absorption rate was increased by treatment with dexamethasone (10(-7) M) and reversed by the inhibition of NHE3. In summary, we have shown that NHE3 are involved in the regulation of both pHi and fluid absorption on the luminal surface of NHMEE cells. Dexamethasone stimulates NHE3 expression and NHE3-dependent fluid absorption in NHMEE cells. These findings provide a new insight into mechanisms that regulate periciliary fluid and the therapeutic mechanisms behind steroid treatment of otitis media with effusion.

In vivo demonstration of the absorptive function of the middle ear epithelium

The present study investigated in vivo fluid and ion transport across the middle ear epithelium. The tympanic membrane of rats was punctured under general anesthesia. A capillary tube was fitted to the external auditory canal and the bulla filled with various solutions. Middle ear (ME) fluid volume variations were then measured at constant pressure. When saline was used, a linear decrease of fluid volume was apparent. Replacement of sodium with a non-permeable cation (N-methyl-D-glucamin) reduced the absorption rate from 0.065+/-0.008 to 0.019+/-0.003 microl/min (P<0.05, n=6). Similarly, amiloride (10(-3)M), a sodium channel antagonist, reduced the absorption rate to 0.027+/-0.006 microl/min (P<0.05, n=6). Net absorption was abolished when chloride was substituted with gluconate: -0.008+/-0.004 microl/min (P<0.02, n=6), which might have been related (i) to the role of chloride as a diffusible anion through the paracellular pathway, or (ii) to the secretion of chloride through apical channels. However in this condition, 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, a chloride channel blocker, did not affect the rate of fluid exchange -0.008+/-0.007 microl/min (P=0.75, n=6). This model provides the first in vivo evidence for the absorptive function of the ME. Fluid introduced into the ME cavity disappears due to active transport through the mucosa. This process is sodium-dependent and can be hindered by high concentration of amiloride. The rate of absorption is high enough to allow total clearance of fluid from the cavity of the middle ear within 13 h. This process might play a role in the maintaining a fluid-free and gas-filled middle ear cavity.

Gender differences in organ density in a rat simulated microgravity model

Research investigating the physiological effects of microgravity on the human body has demonstrated a shift of body fluids in actual spaceflight and in simulated Earth-based microgravity models in both males and females, possibly causing many deleterious physiological effects. Twenty-five anatomically normal female (NF) and 20 ovariectomized (OE) Fischer 344 rats were randomly selected to be in an experimental (1 h of 45 degrees head-down tilt, 45HDT) or control (1 h of prone position) group. At the end of the hour experimental period, the density of the brain, lungs, heart, liver, and left and right kidneys were measured using spiral computed tomography (SCT) while the rats remained in their experimental positions. A sub-group of OE rats (N=6) was administered estrogen replacement therapy on a daily basis (5 micrograms/kg body weight, s.c.) for 4 days and then underwent 1 h of 45HDT and SCT analysis at one day, 2 days, and 5 days to determine if estrogen replacement therapy would alter organ densities. Our data demonstrate that 1 h of 45HDT produced significant increases (p<0.05) in the organ densities of the brain, liver, left kidney, and lung of the OE female group compared to their prone controls. However, only the brain density was significantly increased in the NF group. Estrogen replacement therapy caused a significant decrease in brain organ density at the 5 day time point compared to the 24 h time point. We conclude that estrogen plays a role in fluid distribution in a rat 45HDT model.

Effects of hypertonic saline (7.5%) on extracellular fluid volumes in healthy volunteers

This study evaluated the effects of 7.5% saline on plasma and other extracellular fluid volumes. After baseline measurements, eight healthy postmenopausal female volunteers received 4 ml.kg-1 of hypertonic saline over 30 min. After the fluid infusion, the volunteers were studied for 60 min. Plasma volume was measured using a dilution of 125-iodine-labelled human albumin. Extracellular water and cardiac output were measured by whole body impedence cardiography. The infused volume was 4 ml.kg-1 (average 260 ml). Plasma volume increased rapidly during the infusion (mean +/- standard deviation, 442 +/- 167 ml). At the end of the 1-h follow-up period, plasma volume had increased by on average 465 ml (SD 83). The increase of extracellular water at the end of infusion and at the end of study was 650 ml (SD 93) and 637 ml (SD 192), respectively. The highest serum sodium recorded in the volunteers was 158 mmol.l-1. The effect of 7.5% saline on plasma volume was rapid and lasted for at least 1 h. Plasma volume remained elevated by more than the infused volume at the end of the study. The increase in plasma and extracellular fluid volumes was partly achieved by mobilizing intracellular water to extracellular compartment.

Effect of a venotonic agent on the main arteries and veins during a 5 day HDT

OBJECTIVE

To evaluate the cardiac, arterial and venous effect of a venotonic drug (Cirkan "Ck") administrated orally daily to 6 subjects in HDT position during 5 days. These subjects underwent a second 5d HDT without Ck treatment one month later. Pre and post HDT the subjects were submitted to a stand test.

METHOD

The cardiovascular parameters were measured by echography and Doppler.

RESULTS AND DISCUSSION

The Cirkan treatment contributed to reduce the increase in cerebral resistance, and to maintain the lower limb resistance at a higher level than on controls. It reduces the vein section at the extremities (Jugular, femoral) and in the central vein system connected to the right heart (sub hepatics). On the other hand it increases the portal vein section which means that the blood stagnation at the splanchnic level is increased. Despite these arterial and venous significant modifications the clinical and ECG and Blood pressure response to the Stand test was similar in both groups.

Effects of feeding frequency and voluntary salt intake on fluid and electrolyte regulation in athletic horses

The effect of feeding frequency and voluntary sodium intake (VSI) on fluid shifts and plasma aldosterone concentration (PAC) were studied at rest and after exercise in six athletic horses. The horses were fed twice a day (2TD) and six times a day (6TD) for 25 days for each protocol, according to a changeover design. VSI was measured by weighing each horse's salt block daily. Feeding 2TD or 6TD caused no major alterations in fluid shifts, but in the 2TD treatment there was a postprandial increase in plasma protein concentration and osmolality that lasted <1 h. PAC and VSI were not affected by feeding frequency. VSI ranged from 0 to 62 mg x kg body weight-1 x day-1 and caused significant alterations in PAC. At VSI <26 mg x kg body weight-1 x day-1, a diurnal rhythm for PAC was noted. Water intake, fecal concentrations of sodium and potassium, and packed cell volume during exercise were influenced by VSI. The response to exercise did not differ between treatments. In conclusion, VSI, but not feeding frequency, has significant effects on fluid and electrolyte regulation in athletic horses.

University of Miami Division of Clinical Pharmacology therapeutic rounds: drug-induced hyperkalemia

Drug-induced hyperkalemia is an important but often overlooked problem encountered commonly in clinical practice. It may occur in the ambulatory as well as the impatient setting. Every evaluation of a hyperkalemic patient should include a careful review of medications to determine if a drug capable of causing or aggravating hyperkalemia is present. Medications generally produce hyperkalemia either by causing redistribution of potassium (beta2 -adrenergic blockers, succinylcholine, digitalis overdose, hypertonic mannitol) or by impairing renal potassium excretion. Drugs cause impaired renal potassium excretion by (1) interfering with the production and/or secretion of aldosterone (nonsterodial anti-inflammatory drugs, angiotensin-converting enzyme inhibitors, angiotensin-II receptor antagonists, heparin, cyclosporine, and FK 506) or (2) blocking the kaliuretic effects of aldosterone (potassium-sparing diuretics, trimethoprim, pentamidine, and nefamostat mesilate). Because severe renal insufficeiency is generally required to cause hyperkalemia, an elevated serum potassium concentration in a patient with mild-to-moderate renal failure should not be ascribed to renal failure alone. A careful search for "hidden" potassium loads and for causes of impaired tubular secretion of potassium (including drugs) is necessary. Finally, it is important to recognize that the causes of hyperkalemia may be additive. Patients may have more than one cause of hyperkalemia at the same time. Therefore, all potential causes of hyperkalemia, including drugs, should be systematically evaluated in every hyperkalemic patient.

Infusions with molsidomine and isosorbide-5-mononitrate in congestive heart failure: mechanisms underlying attenuation of effects

The use of nitrates for treatment of heart failure is encumbered by tolerance, caused by whatever mechanism, which has been reported only in a few instances with sydnonimines. Accordingly, we compared molsidomine (6 mg/h) and isosorbide-5-mononitrate (3.75 mg/h) with respect to maximal hemodynamic effects, rapidity and extent of attenuation, and underlying mechanisms by means of constant infusions over 24 h each in 15 patients with chronic congestive heart failure (NYHA II-III) with a placebo-controlled, double-blind, randomized, crossover protocol. Hemodynamic measurements and determinations of neurohormones were performed at baseline and at 2, 8, and 24 h after the beginning of infusions. With molsidomine, reductions of diastolic pulmonary artery pressure by 29% (p < 0.001), by 24% (p < 0.01), and by 24% (p < 0.01) versus placebo were found at 2, 8, and 24 h, which amounted to 19% (p < 0.01), 10% (NS), and 14% (NS) with the nitrate. Cardiac output was meaningfully affected only with molsidomine (+5%, NS, at 2 h; +9%, p < 0.05, at 8 h; and +15%, p < 0.05, at 24 h), as was systemic vascular resistance (-13%, p < 0.05; -9%, NS; and -18%, p < 0.01) at the corresponding times. Increases in renin activity amounted to 130% (p < 0.001), 117% (p < 0.001), and 112% (p < 0.001) with molsidomine, and to 14, 16%, and 0 (each NS) with the nitrate at the corresponding times. Hematocrit was reduced by 5% (p < 0.001), 7% (p < 0.001), and 12% (p < 0.01) with molsidomine and by 5% (NS), 5% (p < 0.05), and 5% (NS) with the nitrate. We conclude that neurohumoral counterregulation or fluid shift, which is even more pronounced with molsidomine despite longer-lasting effects, has no essential role in nitrate-tolerance development. With molsidomine, such a role cannot be ruled out, although alternatively, a fluid shift from arterial to the low-pressure arm of circulation during the later course of infusion would be even more likely.

Changes in blood volume distribution between legs and trunk during halothane anaesthesia

To assess the influence of halothane anaesthesia on the distribution of blood volume in supine humans, we used albumin labelled with 99mTc to measure blood volume distribution along the craniocaudal axis. We studied 6 volunteers in the supine position before, during and after anaesthesia with 1% halothane and 66% nitrous oxide. Using collimated detectors above and below the subject, counts were obtained from the legs, pelvis, abdomen, rib cage and head, with the arms excluded. During anaesthesia, the proportion of counts detected in the legs increased, but failed to achieve significance (P = 0.059). On recovery from anaesthesia, leg counts decreased significantly. Counts in the abdomen and rib cage decreased significantly during anaesthesia and the abdomen counts increased again on recovery (P = 0.036 for all changes). These results confirm other studies of the vascular effects of halothane, and do not support the hypothesis that blood volume redistributes from the legs to within the chest wall during anaesthesia.

During acute hypercapnia vasopressin inhibits an angiotensin drive to ventilation in conscious dogs

Intravenous infusion of arginine vasopressin (AVP) depresses the slope of the ventilatory response to CO2 during acute hypercapnia. We therefore tested the hypothesis that AVP V1-receptor blockade would increase the slope of the ventilatory response to CO2. After a 20-min control period, an AVP V1-receptor antagonist (d(CH2)5[Tyr(Me)2]AVP) was injected into six conscious resting dogs. Thirty minutes after AVP V1-receptor blockade, dogs were exposed to sequential 20-min periods of 5 and 6.5% inspired CO2 in air. A second protocol (no AVP V1-receptor blockade) was conducted as a control. As predicted, AVP V1-receptor blockade enhanced ventilation during inhalation of 6.5% CO2 in association with an increased metabolic rate and increased plasma angiotensin II (ANG II). In eupneic dogs, stimulation of respiration by AVP V1-receptor blockade is mediated by ANG II. A third protocol with ANG II-receptor blockade (intravenous infusion of saralasin) combined with AVP V1-receptor blockade indicated that ANG II mediated the increase in metabolism and the augmented ventilation during inhalation of 6.5% CO2. We conclude that during acute hypercapnia of sufficient magnitude, and perhaps duration, AVP inhibits an ANG II-mediated stimulation of metabolism and respiration.

Diuretic as a means for rapid adaptation to weightlessness

In a number of flights, cosmonauts and astronauts have experienced aggravation of their health status and general condition in the initial hours and days in a weightless environment. One of the trigger mechanisms for the onset of these unfavourable conditions at the start of space flight is a redistribution of body fluids and a blood shift towards the head. To ensure controlled hypohydration as a countermeasure to the deleterious effects of 0-g and to investigate the feasibility to control adaptation, six cosmonauts were administered lasix once a day during the first 3 days of a mission. All data of the experiment (correction test, questionnaire, hematocrit) were recorded on a special form in the logbook and transmitted to the control centre for processing. Results showed that the diuretic weakened the sensation of discomfort and improved the cosmonauts' general condition. Objective indices of the correction test indicate an increased work ability of cosmonauts. After hypohydration, circulating plasma volumes in the group were reduced by 6.8 + 1.0% on average.

Changes in the loco-regional Cerebral Blood Flow (r.C.B.F.) during a simulation of weightlessness

Experiments of prolonged bedrest in antiorthostatic position are conducted in order to simulated cardio-circulatory modifications observed in weightlessness. Until now, no studies of r.C.B.F. have been effected in these conditions. Six young, healthy volunteers (average age 23.8) were placed in strict bedrest and in antiorthostatic position -4 degrees for 7 days. The r.C.B.F. measurements were studied by 133Xe inhalation method using a 32 detectors system. Studies were made first in basal conditions, then between the 6th and 12th hr, and finally between the 72nd and the 78th hr after the beginning of the experiment. Three of the subjects received 0.450 mg of Clonidine daily during the experiment. In the subjects having taken no Clonidine, we observed a constant increase in r.C.B.F. (12, 17 and 16% respectively) in the first 12 hr; at the 72nd hour, all values had returned to basal state. This findings agrees with the well known notion of a rapid correction of hemodynamic disturbances observed in the first days of weightlessness. In the subjects treated with Clonidine, the increase of r.C.B.F. did not occur. Several mechanisms of action are possible; the Clonidine affecting either the heart by inhibiting volemic atrial receptors or the brain by direct vasoconstriction.