ALBERT

Description: Infusions of artificial cerebrospinal fluid (CSF) into the cerebroventricles of conscious rats can raise CSF pressure (CSFp). This response can be modified by some neuropeptides. One of these, angiotensin, facilitates the rise in CSFp. We measured CSFp in conscious rats with a computerized system and evaluated resistance to CSF outflow during infusion of artificial CSF, with or without angiotensin, from the decay kinetics of superimposed bolus injections. Angiotensin (10 ng/min) raised CSFp (P less than 0.05) compared with solvent, but the resistance to CSF outflow of the two groups was similar (P greater than 0.05). Because CSFp was increased by angiotensin without an increase in the outflow resistance, a change in some volume compartment is likely. Angiotensin may raise CSFp by increasing CSF synthesis; this possibility is supported, since the choroid plexuses contain an intrinsic isorenin-angiotensin system. Alternatively, angiotensin may dilate pial arteries, leading to an increased intracranial blood volume.

Description: Endocrine regulation of fluids and electrolytes during 7 days of -6 degrees head-down bed rest (HDBR) was compared in male (n = 8) and, for the first time, female (n = 8) volunteers. The subjects' responses to quiet standing for 2 h before and after HDBR were also tested. In both sexes, diuresis and natriuresis were evident during the first 2-3 days of HDBR, resulting in a marked increase in the urinary Na(+)-to-K+ ratio and significant Na+ retention on re-ambulation. After the 1st day of HDBR, plasma renin activity (PRA) was increased relative to aldosterone (Aldo), plasma volume was decreased, and the renal response to Aldo appeared to be appropriate. Circulating levels of arginine vasopressin, cortisol, and ACTH were unchanged during HDBR. Plasma testosterone decreased slightly on day 2 of HDBR in males. The ratio of early morning ACTH to cortisol was lower in females than in males because ACTH was lower in females. Urinary cortisol increased and remained elevated throughout the HDBR in males only. There were no gender differences in the responses to 7 days of HDBR, except those in the pituitary-adrenal system; those differences appeared unrelated to the postural change. The provocative cardiovascular test of quiet standing before and after HDBR revealed both sex differences and effects of HDBR. There were significant sex differences in cardiovascular responses to standing before and after HDBR. Females had greater PRA and Aldo responses to standing before HDBR and larger Aldo responses to standing after HDBR than males.(ABSTRACT TRUNCATED AT 250 WORDS).

Description: The effects of atrial natriuretic peptide (ANP), vasopressin (AVP) and angiotensin (ANG) on blood and intraocular pressures of pentobarbital anesthetized rats were evaluated following intravenous, intracerebroventricular or anterior chamber routes of administration. Central injections did not affect intraocular pressure. Equipressor intravenous infusions of ANG raised, whereas AVP decreased, intraocular pressure. Direct infusions of a balanced salt solution (0.175 microliter/min) raised intraocular pressure between 30 and 60 min. Adding ANG or ANP slightly reduced this solvent effect but AVP was markedly inhibitory. An AVP-V1 receptor antagonist reversed the blunting of the solvent-induced rise by the peptide, indicating receptor specificity. Acetazolamide pretreatment lowered intraocular pressure, but the solvent-induced rise in intraocular pressure and inhibition by AVP still occurred without altering the temporal pattern. Thus, these effects appear unrelated to aqueous humor synthesis rate. The data support the possibility of intraocular pressure regulation by peptides acting from the blood and aqueous humor.

Description: Continuous intracameral infusions of a balanced salt solution (0.175 microliter min-1) have been reported to raise intraocular pressure (IOP) in anesthetized rats. Palm et al. (1995) previously reported that this effect was attenuated significantly by inclusion of arginine-vasopressin (AVP, 10 ng 0.175 microliter-1) in the infusate. This study used experimental and computer simulation methods to investigate factors underlying these changes in IOP. First, constant intracameral infusions of artificial cerebrospinal fluid (aCSF) at different fixed rates (0.049-0.35 microliter min-1) were used to estimate the outflow resistance. Secondly, IOP responses were measured during an 2 hr intracameral infusion of either aCSF or AVP that was the sum of a small constant component (0.05 microliter min-1) and a larger periodic component (0.25 microliter min-1, cycling for 4 min on, then 4 min off); the mean infusion rate was 0.175 microliter min-1. As shown previously for 0.175 microliter min-1 constant infusions, the periodic aCSF infusion induced a significant rise in IOP that was attenuated by AVP administration. Complex demodulation analysis and the estimated gain parameter of a second order transfer function fit to the periodic responses indicated that outflow resistance increased significantly during the infusions in both aCSF and AVP groups, but that the indices of resistance did not differ significantly between aCSF and AVP infused eyes. This finding implies that changes in outflow resistance do not explain the difference in IOP responses to intracameral aCSF and AVP. The two responses differed significantly, though, in damping factors, such that the aCSF responses were considerably more underdamped than the AVP responses. It is hypothesized that aCSF-induced increase in IOP reflects both (1) a small component reflecting increased outflow resistance and (2) a larger non-resistive component. Since the non-resistive component is insensitive to pretreatment with acetazolamide, it is suggested that the aCSF-induced elevation in IOP reflects primarily vascular perfusion changes that are reduced by local vasoconstrictor actions of AVP. The latter mechanism likely maintains vascular perfusion of the globe when intraocular hypertension develops.

Description: Effective and practical preventive procedures for postflight orthostatic intolerance are highly desirable. The current practice of attempts to expand plasma volume by ingestion of salt and fluids before reentry has proven benefits. This study evaluated alternative options using fludrocortisone (F) to expand plasma volume (PV), dextroamphetamine (Dex) to enhance norepinephrine (NE) release, and atropine (A) to reduce the effects of vagal stimulation. Seven subjects with proven post-bedrest orthostatic intolerance returned for a 7-day 6-deg head-down bedrest study. F (0.2 mg) was given at 8:00 AM and 8:00 PM the day before and 8:00 AM the day the subjects got out of bed (2 hours before standing). PV was measured before and 1 hour after the last dose of F. Dex (5 mg) and A (0.8 mg) were then taken orally 1 hour before the stand test. F expanded PV by 16 percent and caused sodium retention. Four of the 7 subjects stood for 1 hour post-bedrest and heart rate, plasma NE and plasma renin responses to standing were greatly enhanced and sustained. Although there was a narrowing of pulse pressure, the ability to overcome orthostatic intolerance with these countermeasures was largely due to vasoconstriction and sustained high heart rate.

Description: The effect of hydration status on early endocrine responses and on osmotic and intravascular volume changes during immersion was determined in humans undergoing successive periods of dehydration, immersion, rehydration, and immersion. Immersion caused an isotonic expansion of plasma volume, as well as suppression of plasma renin activity and aldosterone, which all occurred independently of hydration status. On the other hand, the concentration of plasma vasopressin (PVP) was found to decrease during dehydrated immersion, but not during rehydrated immersion. It is concluded that plasma tonicity is not a factor influencing PVP suppression during water immersion.

Description: The effects of lower body and abdominal pressure, produced by antigravity suit inflation, on blood pressure, pulse rate, fluid and electrolyte shift, plasma vasopressin and plasma renin activity in humans in upright postures were studied. Five men and two women stood upright for 3 hr with the suit being either inflated or uninflated. In the control tests, the suit was inflated only during the latter part of the trials. Monitoring was carried out with a sphygnomanometer, with sensors for pulse rates, and using a photometer and osmometer to measure blood serum characteristics. The tests confirmed earlier findings that the anti-g suit eliminates increases in plasma renin activity. Also, the headward redistribution of blood obtained in the tests commends the anti-g suit as an alternative to water immersion or bed rest for initial weightlessness studies.

Description: If plasma vasopressin (PVP), plasma renin (PRA), and plasma aldosterone (PA) responses to change in posture are mediated only by alterations in intrathoracic baroreceptor activity hydration status should have minimal influence on these responses. To test this hypothesis, six male subjects underwent 45 min of 70 deg head-up tilt (HUT) following 26 h dehydration, and again, 105 min later, following rehydration. Compared with preceding supine hydrated control values, PVP, PRA, and PA increased (p less than 0.001) during dehydrated HUT, but only PVP and PRA increased during rehydrated HUT (p less than 0.001). The dissociation during rehydrated HUT of PRA and PA may have been related more to the reduction (p less than 0.001) in plasma potassium concentration than to the accompanying decrease (p less than 0.001) in plasma osmolality and sodium concentration. Although increases in PVP and PRA during HUT were attenuated (p less than 0.01) following rehydration, this attenuation was associated with the absence of symptoms of overt hypotension following rehydration. However, since rehydration did not abolish the increases in PVP and PRA induced by HUT, it is concluded that the present observations support the concept of intrathoracic baroreceptor involvement in the regulation of vasopressin secretion and renin release.

Description: In an effort to determine the influence of the parathyroids on bone growth and composition, 28-day-old male Sprague-Dawley rats were sacrificed 28, 56, and 84 days after parathyroidectomy or sham parathyroidectomy. Body growth as well as femur growth were retarded following parathyroidectomy. Hypocalcemia and hyperphosphatemia occurred in all parathyroidectomized rats; no alterations in plasma magnesium levels were noted. Femur magnesium was increased by 22-30% in the parathyroidectomized rats whereas femur calcium remained unchanged. Bone phosphorus was increased 56 and 84 days following parathyroidectomy. Results of this study indicate that parathyroidectomy retards growth while increasing bone magnesium and phosphorus content.

Description: There is a shift of blood from the lower parts of the body to the thoracic circulation during bed rest, water immersion, and presumably during weightlessness. On earth, this central fluid shift is associated with a profound diuresis. However, the mechanism involved is not yet well understood. The present investigation is concerned with measurements regarding the plasma vasopressin, fluid, electrolyte, and plasma renin activity (PRA) responses in subjects with normal preimmersion plasma vasopressin (PVP) concentration. In the conducted experiments, PRA was suppressed significantly at 30 min of immersion and had declined by 74 percent by the end of the experiment. On the basis of previously obtained results, it appears that sodium excretion during immersion may be independent of aldosterone action. Experimental results indicate that PVP is not suppressed by water immersion in normally hydrated subjects and that other factors may be responsible for the diuresis.