[Problems and prospects for the drug correction of orthostatic tolerance in space medicine]

This paper reviews published data concerning physiological mechanisms of man's orthostatic tolerance and its decline under the influence of space flight factors. The principles, goals and potentialities of drug correction of orthostatic intolerance are presented. The applicability of selected drugs used separately or in combination with traditional countermeasures is discussed.

Atrial natriuretic peptide decreases circulatory capacitance in areflexic rats

The short-term hemodynamic response to atrial natriuretic peptide appears to be partly mediated by decreased venous return, which could result from increased circulatory capacitance or decreased blood volume. To determine if rat atrial natriuretic peptide 99-126 (0.5 microgram/kg/min IV for 30-70 minutes) dilated capacitance vessels or decreased blood volume, mean circulatory filling pressure (measured during brief circulatory arrest by inflating an intraatrial balloon) and blood volume (51Cr-erythrocytes) were measured in anesthetized rats. Mean circulatory filling pressure, central venous pressure, and blood volume decreased by 0.4 mm Hg, 0.5 mm Hg, and 3.4 ml/kg, respectively. To determine the total circulatory pressure-volume relationship without influence from autonomic reflexes, mean circulatory filling pressure and blood volume were measured in spinal-cord-transected rats before and immediately after infusing or withdrawing 5 ml blood. Atrial natriuretic peptide decreased mean circulatory filling pressure, central venous pressure, and blood volume by 0.9 mm Hg, 1.7 mm Hg, and 8.0 ml/kg, respectively, and displaced the pressure-volume relationship toward the pressure axis by decreasing extrapolated unstressed volume. Similar results were obtained in spinal-cord-transected rats that had initial vascular tone restored to a greater level by norepinephrine infusion. In anephric rats, atrial natriuretic peptide decreased central venous pressure by 0.3 mm Hg and blood volume by 1.6 ml/kg. The results indicate that short-term infusion of atrial natriuretic peptide reduced circulatory capacitance in rats and suggest that this reduction resulted from diminished blood volume due to urinary fluid loss followed by passive vascular recoil and active venoconstriction.(ABSTRACT TRUNCATED AT 250 WORDS)

Increase in plasma atrial natriuretic polypeptide (ANP) following sodium load in anesthetized rats

To investigate the releasing mechanisms of atrial natriuretic polypeptide (ANP), identical amounts of 5% glucose solution, isotonic (0.9%) or hypertonic (5%) saline were infused intravenously for 5 min (2 ml/min) in anesthetized rats. At the same time, plasma immunoreactive ANP (ir-ANP) was measured using a direct radioimmunoassay. Plasma ir-ANP increased after infusion of 5% glucose solution (P less than 0.01) and isotonic saline (P less than 0.05), and returned rapidly to the basal levels in the recovery period. Plasma ir-ANP increased to a greater degree in the group infused with hypertonic saline than in the other two groups. The major immunoreactive component of increased ir-ANP was identified as alpha-rat ANP, a 28 amino acid residue, by using reverse phase high-performance liquid chromatography. These results suggest that sodium ions may be a stimulating factor of ANP release as well as volume expansion.

Excessive sodium retention as a characteristic of salt-sensitive hypertension

To investigate the possibility that salt-sensitive hypertensives have deficient sodium excretion, the sodium retained by 20 hypertensive patients during a salt load (SL = 3.88 mEq/Kg/day) was calculated for 3 days immediately after 4 days of sodium deprivation (SD = 9 mEq/d). Patients were divided into two groups by arterial pressure responses to SD: responders (N = 10) whose pressures became normal rapidly and averaged less than 140/90 during SD and nonresponders (n = 10) whose pressures were not affected. Sodium retention (mEq/Kg) during SL was calculated as cumulative sodium intake minus cumulative urinary sodium divided by body weight. Responders retained more sodium during SL than nonresponders (3.71 +/- 0.96 [SD] vs. 2.52 +/- 1.05 mEq/Kg, p less than 0.02). This could not be explained by a measurable decrease in filtered sodium load since creatinine clearance was the same in each group. Neither was it associated with differences in plasma renin activity (PRA) or aldosterone excretion rates (AER). Whereas PRA was significantly lower in responders at the end of SD, AER was not different; with SL, group values were equally suppressed. Also sodium excretion was not correlated with arterial pressure except in non-responders on the last day of SL. These data indicate that salt-sensitive hypertensives handle sodium differently than nonsalt-sensitive hypertensives.

Drug therapy in the management of acute renal failure

Pharmacologic intervention aimed at altering the natural history of acute renal failure is a routine practice without scientific support of efficacy. Oliguria has become a separate disease entity with an apparent disregard for the underlying condition that caused it. Volume expansion is clearly beneficial in preventing many volume depleted patients from progressing to acute renal failure. While mannitol and furosemide have been used to "convert" oliguric acute renal failure to the more easy to manage non-oliguric acute renal failure, published reports suggest that responders were not as ill as non-responders. The use of dopamine to increase urine flow in patients with established acute renal failure is the current fashion, but there is little evidence that this drug raises the glomerular filtration rate or shortens the course of acute renal failure. These pharmacologic therapies increase the complexity and cost of care with little tangible evidence of benefit to the patient or the physician caring for the patient.

Hemodynamic effects of atrial natriuretic hormone

The atrial natriuretic hormone (ANH) alters cardiovascular function independent of changes in body fluid volume. Most investigators agree that ANH decreases mean arterial pressure (MAP). However, although some investigators have observed a decrease in total peripheral resistance in association with the decrease in MAP, a more frequent observation has been decreased cardiac output (CO). The mechanism whereby ANH decreases CO is unknown, but does not appear to be the result of direct myocardial depression, reductions in intravascular or cardiopulmonary volumes, or venodilation. Alterations in skeletal muscle and splanchnic blood flow have been reported by some but not all investigators. Although increases in renal blood flow have been reported, they are transitory and have not been consistently observed by all researchers. The cardiovascular effects of ANH appear to be influenced not only by the dose, but also by the cardiovascular control mechanisms that operate at the time of ANH administration. Non-renin-dependent hypertensive models exhibit a decrease in MAP associated with decreased CO, whereas in renin-dependent animals this hypotension is associated with a decrease in total peripheral resistance.

[The effect of low dose dopamine on CBF in patients with subarachnoid hemorrhage]

Cerebral blood flow was measured in 10 patients with subarachnoid hemorrhage using the 133Xe inhalation methods. Regional cerebral blood flow (r-CBF) values were calculated with initial slope index, their values were compared with the blood flow response to the administration of low dose dopamine (DA). CBF values of just after and continuous administration of DA were compared with control values. The measurement of r-CBF was performed immediately after the infusion of 5 micrograms/kg/min DA for 30 min. The focal ischemic lesions decreased, bi-hemispheric mean CBF value (7 cases without A-Com patients) increased by 10.3% significantly (paired T test), affected hemispheric mean CBF value increased by 8.7% significantly and non-affected hemispheric mean CBF value increased by 16.6% significantly. The PaCO2 value didn't change but the mean systemic blood pressure increased by 5.1% significantly. The CBF values of continuous administration of DA for 10 days to 2 weeks were measured. The CBF values increased during the administration of DA. The mean CBF values increased without the steal phenomenon and the mean arterial blood pressure increased slightly after the administration of DA. These phenomena were observed more significantly in non affected side than affected side. We suggest that the administration of DA increase the CBF values in patients with subarachnoid hemorrhage due to the stimulation of DA receptor.

Acute effects of dihydralazine mesylate, furosemide, and metoprolol on maternal hemodynamics in pregnancy-induced hypertension

The intervillous blood flow and other maternal hemodynamic parameters were assessed in 40 mothers with pregnancy-induced hypertension in late pregnancy before and after hypotensive treatment. Furosemide caused a significant decrease in the intervillous blood flow, whereas the effects of dihydralazine and metoprolol were both negative and positive, that is, more individual and diverse. These changes in the intervillous blood flow did not have any correlation with the changes in other maternal hemodynamic parameters. The importance of placental hemodynamic studies in the evaluation of hypotensive drugs is stressed.

Angiotensin II infusion during beta-adrenergic stimulation by isoproterenol. Effects on hepatic, splenic and cardiac blood volumes and on the magnitude and distribution of cardiac output in the dog

The cardiac and peripheral vascular adjustments to angiotensin II (0.1-0.2 microgram kg-1 min-1 i.v.) during high beta-adrenergic activity by a continuous isoproterenol infusion (0.2-0.3 microgram kg-1 min-1 i.v.) were examined in anaesthetized, atropinized dogs. Hepatic, splenic and left ventricular (LV) volume changes were estimated by an ultrasonic technique, and the blood flow distribution was measured by injecting radioactive microspheres and by electromagnetic flowmetry on the caval veins, the hepatic artery and the portal vein. During isoproterenol infusion, angiotensin II increased the systolic LV pressure by 45 +/- 3 mmHg and the stroke volume by 17 +/- 6%. Concomitantly, the hepatic and splenic blood volumes declined by 29 +/- 4 and 14 +/- 6 ml, respectively, and the LV end-diastolic segment length increased by 3 +/- 1%. The flow through the inferior caval vein increased by 39 +/- 9%, whereas the superior vena caval flow remained unchanged. The hepatic arterial flow more than doubled. Thus, at high inotropy by isoproterenol infusion, angiotensin II relocates blood from the liver and the spleen towards the heart. By activating the Frank-Starling mechanism, cardiac output is increased and conducted through the lower body, especially through the hepatic artery, because of the poor autoregulation of flow through this vessel.

Body fluid volumes in rats with mestranol-induced hypertension

Because estrogens have been reported to produce sodium retention, this study investigated the possibility that hypertension in rats resulting from the ingestion of an estrogen used as an oral contraceptive could be due to increases in body fluid volumes. Female rats were given feed containing mestranol for 1, 3, and 6 mo; control rats were given the feed without mestranol. The mestranol-treated rats had higher arterial pressures than the controls only after 6 mo of treatment. Plasma volume, extracellular fluid volume, and total body water were measured in each rat by the distribution volumes of radioiodinated serum albumin, 35SO4, and tritiated water, respectively. Values for blood volume, interstitial fluid volume, and intracellular fluid volume were derived from these measurements. These body fluid volumes, expressed per 100 g of body weight, were not different between the mestranol-treated rats and their controls at any of the three treatment times. Due to differences in body weight and lean body mass between the mestranol-treated and the control rats, these volumes also were expressed per 100 g of lean body mass. Again, no differences were observed between the mestranol-treated rats and the control rats for any of these body fluid compartments at any of the treatment times. These studies, therefore, were unable to provide evidence that increases in body fluid volumes contributed to the elevated arterial pressure in this rat model of oral contraceptive hypertension.

Role of arginine vasopressin in fetal renal response to hypertonicity

We studied the effects of hyperosmolality on fetal renal function and the role of arginine vasopressin (AVP) in these responses. NaCl (9%) was injected intravenously into chronically catheterized ewes and their fetuses, followed by a continuous infusion of 9% NaCl into the ewes. The fetuses were either normal, infused with AVP, or infused with an AVP antagonist. In normal fetuses NaCl injection caused fetal and maternal blood osmolalities to be elevated by 10-15 mosmol/kg for 4 h with no change in fetal blood volume; fetal plasma AVP rose 42%. Fetal arterial pressures rose transiently by 2-10 mmHg. Fetal urine flow rose transiently by 73% after NaCl injection and then averaged 27% below control after 1 h, whereas fetal urine osmolality increased from 188 +/- 31 to 282 +/- 33 mosmol/kg. In a second group of fetuses AVP infusion alone caused fetal urine osmolality to increase by 123 +/- 39 mosmol/kg and urine flow to fall 31%, whereas in a third group the antagonist alone had no effect on urine flow or osmolality. After hypertonic injection into fetuses infused with AVP or the antagonist, the transient changes were similar to those in normal fetuses. However, the sustained increase in urine osmolality and decrease in flow after hypertonic injection were abolished in AVP-infused and antagonist-infused fetuses. Thus it appears that the transient changes in fetal renal function after hypertonic injection are not AVP-induced and may be due to transient increases in arterial pressure, whereas the prolonged changes in urine flow and osmolality appear to be mediated by increases in fetal plasma AVP levels.

Thoracic duct lymph flow in pregnant sheep and response to blood volume expansion

Left thoracic duct lymph flow rate averaged 0.077 +/- 0.003 (SD) and 0.078 +/- 0.003 ml X min-1 X kg-1 in near-term pregnant and nonpregnant sheep (P greater than 0.5). Lymph and plasma protein concentrations were unaltered in the pregnant compared with the nonpregnant animals. The thoracic duct lymph flow responses to three serial intravenous infusions of lactated Ringer solution were essentially the same in the pregnant and nonpregnant animals. Blood volume and vascular pressure changes during and after volume loading were essentially the same in both groups. In addition, terbutaline administration after volume loading caused no change in thoracic duct lymph flow rates. Thus the present study suggests that basal lymph flow rates, lymphatic function, and vascular as well as interstitial compliances are largely unaltered late in pregnancy in the sheep. In addition, beta-mimetic stimulation with terbutaline does not appear to suppress lymph flow rate.

Alpha 1- and alpha 2-adrenoceptor stimulation: changes in venous capacitance in intact dogs

The peripheral circulatory effects of alpha 1-adrenoceptor stimulation with methoxamine hydrochloride were compared with those of alpha 2-stimulation with UK 14304-18 in 12 intact dogs. Doses of each agent were infused to increase systemic vascular resistance and arterial pressure 50 and then 100% above control. Heart rate was controlled with atropine. At the higher dose, methoxamine increased mean aortic pressure (PAo) from a control of 77.3 +/- 1.6 to 152.9 +/- 3.2 mmHg, mean circulatory filling pressure (MCFP) from 8.0 +/- 0.4 to 13.3 +/- 1.3 mmHg, and central blood volume (CBV) from 21.3 +/- 1.1 to 25.9 +/- 1.5 ml X kg-1, whereas cardiac output did not change. UK 14304-18 increased PAo from 78.1 +/- 2.6 to 148.9 +/- 2.7 mmHg, MCFP from 7.9 +/- 0.4 to 10.6 +/- 0.4 mmHg, and CBV from 21.0 +/- 1.1 to 24.1 +/- 1.5 ml X kg-1, whereas cardiac output decreased from 151.7 +/- 9.4 to 126.3 +/- 5.8 ml X kg-1 X min-1. Mean circulatory filling pressure and CBV were higher with methoxamine than with UK 14304-18. Effective vascular compliance, determined by serial measurements of MCFP during ganglionic blockade after rapid changes in blood volume, decreased from a control value of 1.9 +/- 0.1 to 1.3 +/- 0.3 ml X mmHg-1 X kg-1 with methoxamine, but did not change with UK 14304-18 (1.9 +/- 0.1 ml X mmHg-1 X kg-1). At any given change in blood volume, there was a higher MCFP with alpha 1-stimulation compared with alpha 2-stimulation. Both agents decreased unstressed vascular volume.(ABSTRACT TRUNCATED AT 250 WORDS)

Attempts at prevention or treatment of intracranial arterial spasm: an update

Numerous approaches to the prevention and treatment of intracranial arterial spasm have been made during the past three decades. In the present article, the author has assembled data about such attempts that were not included in a similar compilation published in 1980. Since 1980, there has been some progress toward the satisfactory management of this detrimental condition, but much remains to be done.

High salt diet sensitizes cardiopulmonary baroreflexes in Dahl salt-resistant rats

Compared with Dahl salt-resistant (R) rats, Dahl salt-sensitive (S) rats on a low salt diet have impaired cardiopulmonary baroreflex control of sympathetic nerve activity. The purpose of this study was to examine the sensitivity of cardiopulmonary baroreflex function in both strains of Dahl rats when they are challenged with a high salt diet. We studied Dahl R and S rats after 6 weeks of low and high salt diets. To assess cardiopulmonary baroreflex function, we measured decreases in splanchnic sympathetic nerve activity produced by increases in left ventricular end-diastolic pressure during graded volume expansion (dextran 75) after bilateral sinoaortic denervation. A given amount of infused volume produced comparable increases in left ventricular end-diastolic pressure in Dahl R rats on low and high salt diets but significantly greater decreases in sympathetic nerve activity in the high salt group than in the low salt group. Thus, the high salt diet augmented the sympathoinhibitory response to volume expansion in the Dahl R rats. In contrast, among the Dahl S rats equivalent increases in left ventricular end-diastolic pressure during volume expansion produced smaller sympathoinhibitory responses in the high salt group than in the low salt group. The gain of the cardiopulmonary baroreflex, expressed as the percentage decrease in sympathetic nerve activity per mm Hg increase in left ventricular end-diastolic pressure, was significantly increased by a high salt diet in Dahl R rats but tended to be decreased by a high salt diet in Dahl S rats.(ABSTRACT TRUNCATED AT 250 WORDS)

The endocrine control of blood volume, blood pressure and sodium balance: atrial hormone and renin system interactions

The atrial peptide hormone exhibits remarkable vasorelaxant, natriuretic and hypovolaemic actions, suggesting that it is part of a cardiovascular control mechanism which operates to regulate blood pressure, blood volume and sodium balance. In this endocrine control, the renin axis provides the primary defence against sodium volume depletion and hypotension while atrial hormone plays an increasingly active counter-role for coping with situations that involve a sodium-volume surfeit or rising blood volume or blood pressure levels. For this assignment, the atrial hormone acts in four different ways to counter or oppose the renin-angiotensin-aldosterone axis. Thus, it promptly reduces renin secretion, it relaxes angiotensin-constricted vessels, it blocks angiotensin-induced aldosterone synthesis, and its natriuresis opposes aldosterone-induced sodium retention. Another special action of atrial hormone seems to be rapidly adjust central blood volumes by promoting the rapid transfer of fluid to the extracellular spaces. This hypovolaemic action probably reflects an action on capillary fluid exchange. It is reflected by a prompt and sustained rise in haematocrit levels. A similar action on the glomerular capillaries may be involved in causing natriuresis. These effects serve to rapidly decompress blood volumes and thereby reduce cardiac work. More work is needed to define the afferent stimuli to the right and left heart which elicit atrial hormone release. The possible role(s) of this new hormonal system in hypertensive and oedematous cardiovascular disorders also remains to be defined.

Haemodynamic changes and left ventricular performance during high-dose oestrogen administration to male transsexuals

Serial haemodynamic and systolic time interval measurements in 17 male transsexuals during high-dose oestrogen administration were compared with control values obtained after oestrogen withdrawal. During oestrogen treatment, cardiac output, stroke volume, plasma volume and total blood volume increased while total peripheral resistance, diastolic blood pressure and red cell mass decreased significantly. Changes in heart rate and systolic blood pressure were not significant. Despite a much higher dose of oestrogen, the haemodynamic changes observed in this study are similar in magnitude to those occurring in women taking oral contraceptives. Significant changes in systolic time intervals during oestrogen treatment were a shortened pre-ejection period (PEP) index, a prolonged left ventricular ejection time (LVET) index and a decreased PEP/LVET ratio indicating that left ventricular performance was augmented.

[Effect of hyperoncotic volume administration on capillary protein filtration and lymph flow in the thoracic duct]

The effect of an intravenous injection (2 ml/kg) of hyperoncotic (30 g%) Dextran (MW 60000) or Albumin on lymph formation and thoracic duct lymph flow was studied in 16 dogs. As a result of the injection, plasma colloid osmotic pressure increased, plasma volume increased due to fluid reabsorption from the extravascular compartment, and thus cardiac output became enhanced. Total body lymph formation was not affected as shown by an unchanged transcapillary disappearance rate of macromolecules. Thoracic duct lymph flow, however, was increased. The concomitant augmentation of lymph protein concentration and the elevated hepatic blood flow following the injection of hyperoncotic Dextran show that the increased thoracic duct lymph flow was probably brought about by an increase in hepatic lymph flow.

Acute haemodynamic effects of the atrial natriuretic hormone in rats

Haemodynamic effects of atriopeptin II (AP II) were determined in conscious and anaesthetized rats chronically instrumented with a Doppler flow probe on the ascending aorta. Intravenous injection of AP II (7 micrograms/kg) produced a decrease in mean arterial pressure within 5 min; however, a biphasic change occurred in total peripheral resistance (TPR). At 1 min in the anaesthetized rats, TPR decreased while cardiac output tended to increase, and similar changes occurred in the conscious rats. By 5-8 min the haemodynamic profile had reversed: cardiac output had decreased in both anaesthetized and conscious rats and TPR had increased in anaesthetized rats. Mean circulatory filling pressure (MCFP) was measured during brief circulatory arrest by inflating an intracardiac balloon. Blood volume was measured with 51Cr-erythrocytes and organ blood volume by whole-animal freezing in liquid nitrogen. There were no changes in MCFP in the conscious and the anaesthetized rats, nor in the blood volume or cardiopulmonary blood volume in the anaesthetized rats, at 5-7 min after AP II. Atriopeptin III (10 micrograms/kg intravenously) had no effects on MCFP and the blood volume in the conscious rats at 5 and 15 min after injection. These results suggest multiple mechanisms of action for the acute haemodynamic effects of atrial peptides.

[Comparison of volume substitutes 5 percent human albumin and 6 percent hydroxyethyl starch (40,000/0.5) in pediatric anesthesia]

Human albumin 5% (HA), frequently used in paediatric anaesthesia as a human plasma substitute, could be replaced by hydroxyethyl starch 6% (HES) 40,000/0.5 provided its use would not entail any disadvantages but rather advantages instead. This problem was studied by examining 30 children (mean body weight 32.5 kg) in general anaesthesia. During about 3 hours of surgery the patients lost up to 15 per cent (approx. 400 ml) of blood volume. In a randomised study the blood loss was compensated either via HA or HES with 14 ml/kg body weight each, respectively. Blood pressure and pulse rate remained within the normal range. Haemodilution was represented by a drop in the erythrocyte count to about 3 million/microliter, haematocrit (HCT) to about 30% and haemoglobin to approx. 10 g/100 ml. Slight metabolic acidosis (Bodansky unit = -4 mmol/l) was found to be statistically not significant (= n.s.) with both methods, as was the difference in serum albumin and during serum electrophoresis. Serum [Na+] was reduced in those children who had been treated with HES, to 137.33 +/- 33.30 mmol/l; however, in those children who received HA with low sodium content (statistically significant difference = s.s.) the corresponding level was 134.15 +/- 2.36 mmol/l. Serum creatinine rose in each case from 60 to 80 mmol/l (s.s.), renal function being slightly impaired probably due to the anaesthesia and surgery. The value according to Quick's test and the partial thromboplastin time (PTT) remained in the normal range both with HA and HES treatment (in each case over 70% and below 25s, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

[Volume replacement with a new hydroxyethyl starch preparation (3 percent HES 200/0.5) in heart surgery]

In a randomized study including 55 patients undergoing elective aorto-coronary bypass surgery efficacy of a low concentrated hydroxyethylstarch (3% HES 200/0.5) was tested after extracorporeal circulation (ECC). All patients received 1,000 ml autologous washed erythrocytes (cell saver) and 400 ml fresh frozen plasma (FFP). Thereafter patients were randomized into 4 groups. Hemodynamic parameters, laboratory parameters as well as extravascular lung water (EVLW - double indicator dilution technique) were measured before ECC as well as before and after infusion therapy. With regard to hemodynamic efficiency and the change in laboratory data no relevant differences between group I, II and III could be seen. EVLW-measurement demonstrated the lowest increase in lung water after infusion of HES; simultaneously pulmonary gas exchange was less compromised in comparison to the other infusion groups. 3% HES 200/0.5 solution can be considered as an effective volume substitute with short intravascular retention time, which seems to be of advantage in patients coming off extracorporeal circulation.

Guanabenz: a centrally acting, natriuretic antihypertensive drug

Guanabenz is acutely natriuretic and diuretic in saline expanded animals. In man, guanabenz has not resulted in sodium retention as seen with other comparable antihypertensives. To directly define the action of guanabenz on sodium and water excretion in man, we performed clearance studies during water diuresis on eight hypertensive subjects under metabolic balance conditions. Each subject underwent three studies: 1) baseline study: no drug, a water diuresis study; this was followed by a saline load (= 2% BW); 2) acute study (24 hr after baseline): 16 mg guanabenz PO; and 3) chronic study: after one week of guanabenz 8 mg PO BID. In the acute guanabenz studies there were: 1) no changes in GFR or ERPF; 2) an increase in both sodium excretion and fractional sodium excretion; 3) a rise in free H2O clearance (CH2O) and (CH2O/GFR) X 100%. These findings were not sustained in the chronic guanabenz studies. We conclude that in man (preconditioned with prior saline loading) guanabenz is acutely natriuretic and water diuretic. These effects are due to decreased tubular sodium and water reabsorption, probably related to inhibition of alpha adrenergic activity. The data are consistent with selectively reduced renal sympathetic activity affecting sodium transport and provide a basis for the absence of edema and sodium retention associated with guanabenz therapy.

Effects of nifedipine on hepatic blood volume in cats: indirect venoconstriction and absence of inhibition of postsynaptic alpha 2-adrenoceptor responses

Intravenous administration of hypotensive doses (30-200 micrograms/kg) of nifedipine to cats anesthetized with pentobarbital caused an increase in cardiac output accompanied by hepatic venoconstriction. The hepatic venoconstriction and the increase in cardiac output were abolished in animals in which the hepatic sympathetic nerves were cut, the adrenal glands were excluded, and the kidneys were removed. This contrasts with the indirect hepatic venoconstrictor action of isoproterenol which was shown previously not to be abolished by these procedures. Further experiments showed that the hepatic venoconstrictor effect of nifedipine was blocked by removal of the kidneys, but not by removal of the hepatic sympathetic nerves and adrenals. These results support the hypothesis that venoconstriction plays an important role when drugs produce increased cardiac output. In nephrectomized animals, nifedipine had no direct effects on hepatic blood volume and it did not alter the effects of infusions of norepinephrine on hepatic blood volume, which have previously been shown to be mediated through alpha 2-adrenoceptors. However, it did reduce the hepatic venous responses to hepatic sympathetic nerve stimulation by 30%.

Effects of bromocryptine on hepatic blood volume responses to hepatic nerve stimulation in cats

Arterial pressures, portal pressures, and hepatic blood volumes were recorded after hepatic denervation in cats anesthetized with pentobarbital. Bromocryptine (50 micrograms/kg) lowered arterial pressure but did not significantly change portal pressure or hepatic blood volume. However, both portal pressure and hepatic blood volume responses to hepatic nerve stimulation were significantly depressed after bromocryptine especially at low frequencies of stimulation. Responses to intraportal infusions of norepinephrine were significantly impaired only at the highest dose. The inhibitory effect of bromocryptine on the neural responses may, therefore, involve a presynaptic inhibition of norepinephrine release, but the mechanism requires further study. These data provide further support for the hypothesis that drugs which impair hepatic venous responses to sympathetic stimuli cause significant impairment of postural reflexes and orthostatic hypotension during clinical use.