Biomedical subjects

V A Convertino

Publications and source records attributed to V A Convertino.

At least 19 recordsLinked to original sources

PubMed1996

Exercise as a countermeasure for physiological adaptation to prolonged spaceflight.

Exercise represents the primary countermeasure used during spaceflight to maintain or restore maximal aerobic capacity (VO2max), musculoskeletal structure, and orthostatic function. However, no single exercise or combination of prescriptions has proven entirely effective in restoring cardiovascular and musculoskeletal functions to preflight levels following prolonged spaceflight. As human spaceflight exposures increase in duration, assessment and development of various effective exercise-based protective procedures become paramount. This must involve improvement in specific countermeasure prescription as well as development of additional approaches that will allow space travelers greater flexibility and medical safety during long flights. Effective exercise prescription will be based on identification of basic physiological stimuli that maintain normal function in terrestrial gravity and understanding of how specific combinations of exercise characteristics e.g., duration, frequency, intensity, mode) can mimic these stimuli and affect the overall process of adaptation to microgravity. This can be accomplished only with greater emphasis of research on ground-based experiments. Future attention must be directed to improving exercise compliance while minimizing both crew time and the impact of the exercise on life-support resources.

Adaptation, Physiological↗

PubMed1996

Clinical aspects of the control of plasma volume at microgravity and during return to one gravity.

Plasma volume is reduced by 10-20% within 24-48 h of exposure to simulated or actual microgravity. The clinical importance of microgravity induced hypovolemia is manifested by its relationship with orthostatic intolerance and reduced maximal oxygen uptake (VO2max) after return to one gravity (1G). Since there is no evidence to suggest that plasma volume reduction during microgravity is associated with thirst or renal dysfunctions, a diuresis induced by an immediate blood volume shift to the central circulation appears responsible for microgravity-induced hypovolemia. Since most astronauts choose to restrict their fluid intake before a space mission, absence of increased urine output during actual space flight may be explained by low central venous pressure (CVP) which accompanies dehydration. Compelling evidence suggests that prolonged reduction in CVP during exposure to microgravity reflects a "resetting" to a lower operating point, which acts to limit plasma volume expansion during attempts to increase fluid intake. In ground based and space flight experiments, successful restoration and maintenance of plasma volume prior to returning to an upright posture may depend upon development of treatments that can return CVP to its baseline IG operating point. Fluid-loading and lower body negative pressure (LBNP) have not proved completely effective in restoring plasma volume, suggesting that they may not provide the stimulus to elevate the CVP operating point. On the other hand, exercise, which can chronically increase CVP, has been effective in expanding plasma volume when combined with adequate dietary intake of fluid and electrolytes. The success of designing experiments to understand the physiological mechanisms of and development of effective counter measures for the control of plasma volume in microgravity and during return to IG will depend upon testing that can be conducted under standardized controlled baseline conditions during both ground-based and space flight investigations.

Adaptation, Physiological↗

PubMed1996

Restoration of plasma volume after 16 days of head-down tilt induced by a single bout of maximal exercise.

Seven healthy men performed maximal exercise 24 h before the end of 16 days exposure to 6 degrees head-down tilt (HDT) to test the hypothesis that such an exercise technique could restore plasma volume (PV) at the end of a simulated space mission. Exercise consisted of supine cycling with graded work rates increasing by 16 W/min to volitional fatigue and required an average of 16 min. The experimental protocol was a standard cross-over design in which the order of treatment (exercise or control) was counterbalanced across all seven subjects. PV, fluid intake (ad libitum), urine output, renal function, and hormones associated with fluid homeostasis were measured before HDT, 24 h before the end of HDT just prior to exercise, and at the end of HDT 24 h after exercise. HDT reduced PV by 16% in both control and exercise conditions. Maximal exercise completely restored plasma volume within 24 h to 3.9 +/- 3.2% of pre-HDT levels despite continued HDT. Compared with control, exercise induced a 660-ml larger positive fluid balance because of greater fluid intake and reduced urine volume during the 24 h after exercise. These results suggest that one bout of maximal leg exercise before return from 16 days of spaceflight may be completely effective in stimulating thirst and restoring plasma volume to preflight levels.

Adult↗

PubMed1996

Application of acute maximal exercise to protect orthostatic tolerance after simulated microgravity.

We tested the hypothesis that one bout of maximal exercise performed at the conclusion of prolonged simulated microgravity would improve blood pressure stability during an orthostatic challenge. Heart rate (HR), mean arterial blood pressure (MAP), norepinephrine (NE), epinephrine (E), arginine vasopressin (AVP), plasma renin activity (PRA), atrial natriuretic peptide (ANP), cardiac output (Q), forearm vascular resistance (FVR), and changes in leg volume were measured during lower body negative pressure (LBNP) to presyncope in seven subjects immediately prior to reambulation from 16 days of 6 degrees head-down tilt (HDT) under two experimental conditions: 1) after maximal supine cycle ergometry performed 24 h before returning to the upright posture (exercise) and 2) without exercise (control). After HDT, the reduction of LBNP tolerance time from pre-HDT levels was greater (P = 0.041) in the control condition (-2.0 +/- 0.2 min) compared with the exercise condition (-0.4 +/- 0.2 min). At presyncope after HDT, FVR and NE were higher (P < 0.05) after exercise compared with control, whereas MAP, HR, E, AVP, PRA, ANP, and leg volume were similar in both conditions. Plasma volume (PV) and carotid-cardiac baroreflex sensitivity were reduced after control HDT, but were restored by the exercise treatment. Maintenance of orthostatic tolerance by application of acute intense exercise after 16 days of simulated microgravity was associated with greater circulating levels of NE, vasoconstriction, Q, baroreflex sensitivity, and PV.

Adult↗

PubMed1996

Catecholamine response to maximal exercise following 16 days of simulated microgravity.

INTRODUCTION: We tested the hypothesis that the elevated peak heart rate (HR) response to maximal exercise following microgravity exposure is associated with increased plasma levels of catecholamines. METHODS: To do this, plasma norepinephrine (NE) and epinephrine (E) were measured in venous blood samples obtained from 7 subjects before and immediately after graded supine cycle exercise to volitional fatigue performed prior to and at the conclusion of 16 d of 6 degrees head-down tilt (HDT). RESULTS: Resting HR was increased (p = 0.014) following HDT (56 +/- 3 to 63 +/- 2 bpm), but pre-exercise NE concentration (217 +/- 18 pg.ml-1) was not different (p = 0.224) from that measured before HDT (183 +/- 23 pg.ml-1). Peak exercise HR was greater (p = 0.001) after simulated microgravity exposure compared to pre-HDT (181 +/- 3 bpm vs. 172 +/- 4 bpm). NE measured at volitional fatigue was 64% higher (p = 0.004) after HDT compared to that measured prior to HDT (1337 +/- 181 vs. 2191 +/- 189 pg.ml-1 for pre- and post-HDT, respectively). Plasma E concentration at exhaustion was unchanged by microgravity exposure. CONCLUSIONS: Our observation of greater levels of plasma NE and peak exercise HR suggests that sympathetic influences on cardiac chronotropic control may be altered by exposure to prolonged microgravity.

Adult↗

PubMed1996

Fluid homeostasis after heart transplantation: the role of cardiac denervation.

BACKGROUND: Orthotopic heart transplantation may interrupt key neural and humoral homeostatic mechanisms that normally adjust Na+ and fluid excretion to changes in intake. Such an interruption could lead to plasma volume expansion. METHODS: We measured plasma volume and fluid regulatory hormones under standardized conditions in 11 heart transplant recipients (58 +/- 7 years old; mean +/- standard deviation) 21 +/- 4 months after transplantation, in 6 liver transplant recipients (51 +/- 6 years old) 13 +/- 8 months after transplantation (cyclosporine control group), and in 7 normal healthy control subjects (61 +/- 9 years old). Administration of all diuretics and antihypertensive drugs was discontinued before the study. After 3 days during which subjects ate a constant diet containing 87 mEq of Na+ per 24 hours, plasma volume was measured by a modified Evans blue dye (T-1824) dilution technique. Renal creatinine clearance was measured and blood samples were drawn for determination of plasma levels of vasopressin, angiotensin II, aldosterone, atrial natriuretic peptide, and plasma renin activity. RESULTS: Supine resting plasma renin activity, angiotensin II, and aldosterone (renin-angiotensin-aldosterone axis) and vasopressin levels were not different among the control, heart transplant, and liver transplant groups. However, there was a trend toward elevated angiotensin II (p < or = 0.08) and aldosterone (p < or = 0.08) levels in the heart transplant recipients. Atrial natriuretic peptide levels were significantly elevated two to threefold in the heart transplant recipients when compared with those in the two control groups. Blood volume, normalized for body weight (milliliters per kilogram), was significantly greater (14%) in the heart transplant recipients when compared with that in liver transplant recipients and normal healthy control subjects. Blood volume values did not differ (p > or = 0.05) between the two control groups. CONCLUSIONS: Extracellular fluid volume expansion (+14%) occurs in clinically stable heart transplant recipients who become hypertensive. Although hyperactivity of the renin-angiotensin-aldosterone axis is not apparent during supine resting conditions, our data suggest that the renin-angiotensin-aldosterone system is not responsive to a hypervolemic stimulus and this is likely a consequence of chronic cardiac deafferentation. Thus, poor adaptation of the renin-angiotensin-aldosterone system to fluid retention may be partly responsible for the incidence and severity of posttransplantation hypertension in some heart transplant recipients.

Aged↗

PubMed1996

Effects of hypovolemia and posture on responses to the Valsalva maneuver.

INTRODUCTION: We tested the hypotheses that hypovolemia would result in attenuated elevation in blood pressure, greater baroreflex-mediated tachycardia, and reduced capacity for vasoconstriction during a Valsalva maneuver (VM). METHODS: Heart rate (HR) and mean arterial pressure (MAP) were measured beat-by-beat before strain, during a 15-s VM strain at 30 mmHg expiratory pressure, and post-strain. Eight subjects performed three VM trials in each of three postures (supine, sitting, and standing) under two experimental conditions (normovolemic and hypovolemic). Hypovolemia was acutely induced by a bolus injection of 30 mg furosemide. Each experimental condition was conducted on a different day, separated by one week. delta MAP was used in analyses of phase I, late phase II (an indicator of vasoconstriction) and phase III of VM. The ratio delta HR/delta MAP, an index of nonspecific baroreflex control of HR, was used in analysis of early phase II and phase IV of the VM. RESULTS: Compared to normovolemia, hypovolemia resulted in 12% lower plasma volume (p = 0.0001). delta MAP for both phase I and phase III of the VM differed between postures (p = 0.0132 and p = 0.0003, respectively) and was lower in the hypovolemic condition than in the normovolemic condition for phase I in the standing posture (-5 mmHg, p = 0.0385). CONCLUSIONS: HR and blood pressure responses to alterations in intrathoracic pressure are affected by fluid redistribution (posture change), but not by circulating blood volume. Therefore, our results did not support our hypothesis that hypovolemia would result in attenuated elevation in blood pressure, greater baroreflex-mediated tachycardia, and reduced capacity for vasoconstriction during a Valsalva maneuver. However, moderate hypovolemia can be specifically predicted by the phase I response to a VM performed in the standing posture.

Adult↗

PubMed1996

Application of USAF G-suit technology for clinical orthostatic hypotension: a case study.

INTRODUCTION: The purpose of this study was to determine the effectiveness of a USAF anti-gravity suit (G-suit) on the stability of a patient with chronic orthostatic hypotension. METHODS: A 37-yr-old female with a history of insulin-dependent diabetes mellitus (IDDM) and symptomatic orthostasis was evaluated and the results were compared with those of non-diabetic controls, matched for age, height, and weight. Cardiac vagal tone was assessed by determination of standard deviation of 100 R-R intervals (R-R SD). We assessed the carotid-cardiac baroreflex response by plotting R-R intervals (ms) at each of eight neck pressure steps with their respective carotid distending pressures (mm Hg). Heart rate and blood pressure were recorded in response to the Valsalva maneuver (VM) performed at an expiratory pressure of 30 mmHg to assess integrated baroreflex responses. Blood pressures and heart rate were measured during three 5-min stand tests to assess orthostatic responses: a) without G-suit; b) with noninflated G-suit; and c) with inflated G-suit (50 mm Hg). RESULTS: The IDDM patient had minimal baseline cardiac vagal tone (R-R SD = 8.5 ms) compared with the average response of a control group of 24 subjects with orthostatic stability (R-R SD = 67.2 +/- 7.1 ms). Carotid-cardiac baroreflex response was virtually non-existent in the IDDM patient (Gain = 0.06 ms.mm Hg-1) compared to the control subjects (4.4 +/- 0.8 ms.mm Hg-1). VM responses corroborated the lack of cardiac baroreflex response in the IDDM patient, while blood pressure changes during the VM were similar to those of the controls. Upon standing, the IDDM patient demonstrated severe orthostatic hypotension (90 mm Hg SBP) and tachycardia without the G-suit. The G-suit, with and without pressure, reduced hypotension and tachycardia during standing. CONCLUSION: These results demonstrate successful application of Air Force technology as a useful alternative to pharmacologic intervention in the treatment of a patient with autonomic dysfunction leading to supine hypertension and orthostatic hypotension.

Adult↗

PubMed1995

Hormonal responses in elders experiencing pre-syncopal symptoms during head-up tilt before and after exercise training.

BACKGROUND: Hormonal responses of elderly individuals experiencing pre-syncopal symptoms during head-up tilt testing (HUT) were compared with responses of nonsymptomatic subjects both before (T1) and after (T2) 6 months of endurance training. METHODS: Based on responses to HUT at T1, 35 men and women (ages 61-79 years) were placed into symptomatic and nonsymptomatic groups for analysis. Symptomatic subjects (n = 5) experienced lightheadedness, nausea, sweating, or syncope during T1 HUT but completed 15 minutes of HUT at T2. Training consisted of treadmill walking or stairclimbing 3 x/wk, 30-45 min/day, at 75-85% of maximal heart rate reserve. Adrenocorticotropic hormone (ACTH), vasopressin, aldosterone, norepinephrine, epinephrine, hemoglobin, and hematocrit were measured during supine rest prior to HUT, and either at the end of the 15-minute HUT or at symptom onset. Plasma volume (PV) was measured at supine rest; tilt-induced changes in PV were calculated from changes in hemoglobin and hematocrit. RESULTS: During T1 HUT, symptomatic subjects had greater increases in vasopressin and a greater rate of PV loss (p < .05). Increases in ACTH and aldosterone were greater in symptomatic subjects at T1 and T2, while increases in norepinephrine were greater at T2 (p < .05). Reductions in tilt-induced vasopressin concentration and a decreased rate of PV loss were seen at T2 in symptomatic subjects. CONCLUSIONS: T1 results from symptomatic subjects are consistent with greater stimulation of volume-sensitive receptors induced by a greater rate of fall in PV. Exercise training resulted in increased tilt tolerance for symptomatic subjects associated with reductions in vasopressin concentration and rate of PV loss during tilt.

Aged↗

PubMed1995

Effect of 6 months of exercise training on cardiovascular responses to head-up tilt in the elderly.

The purpose of this investigation was to evaluate the effect of 6 months of exercise training on cardiovascular responses to 70 degrees head-up tilt (HUT) in the elderly. Forty-four elderly men and women (ages 60-82 years) were assigned to endurance training alone (n = 18), endurance training in combination with selected resistance exercises (n = 17), or to a non-exercising control group (n = 9). Head-up tilt testing at the start (T1) and end (T2) of 26 weeks of training consisted of 30 min of supine rest, 15 min of 70 degrees HUT, and 15 min of supine recovery. Endurance training consisted of uphill treadmill walking or stair climbing exercise 3 times per week, 30-45 min/day, at 75-85% of maximal heart rate reserve. In addition, the endurance/resistance group completed one set of 8-15 repetitions of biceps curl (BC), triceps extension (TE), and leg press, 3 times per week. After 26 weeks, increases in VO2max averaged 16.2% and 12.3% for endurance and endurance/resistance groups, respectively. In addition, the endurance/resistance group increased BC and TE strength by 25.3% and 26.1%, respectively. Results from the HUT test indicated that only the endurance group increased supine resting stroke volume (SV) and cardiac output (Q) from T1 to T2; however, SV and Q during HUT were not augmented as a result of training. Training did not affect heart rate, blood pressure, or peripheral resistance responses at rest or during HUT in any of the groups. These results suggest that 6 months of endurance training, alone or in combination with selected resistance exercises, is not detrimental to blood pressure controlling mechanisms to head-up tilt in the elderly.

Aged↗

PubMed1995

Resistance exercise-induced fluid shifts: change in active muscle size and plasma volume.

The purpose of this study was to test the hypothesis that the reduction in plasma volume (PV) induced by resistance exercise reflects fluid loss to the extravascular space and subsequently selective increase in cross-sectional area (CSA) of active but not inactive skeletal muscle. We compared changes in active and inactive muscle CSA and PV after barbell squat exercise. Magnetic resonance imaging (MRI) was used to quantify muscle involvement in exercise and to determine CSA of muscle groups or individual muscles [vasti (VS), adductor (Add), hamstring (Ham), and rectus femoris (RF)]. Muscle involvement in exercise was determined using exercise-induced contrast shift in spin-spin relaxation time (T2)-weighted MR images immediately postexercise. Alterations in muscle size were based on the mean CSA of individual slices. Hematocrit, hemoglobin, and Evans blue dye were used to estimate changes in PV. Muscle CSA and PV data were obtained preexercise and immediately postexercise and 15 and 45 min thereafter. A hierarchy of muscle involvement in exercise was found such that VS > Add > Ham > RF, with the Ham and RF showing essentially no involvement. CSA of the VS and Add muscle groups were increased 10 and 5%, respectively, immediately after exercise in each thigh with no changes in Ham and RF CSA. PV was decreased 22% immediately following exercise. The absolute loss of PV was correlated (r2 = 0.75) with absolute increase in muscle CSA immediately postexercise, supporting the notion that increased muscle size after resistance exercise reflects primarily fluid movement from the vascular space into active but not inactive muscle.

Adult↗

PubMed1995

A single bout of exhaustive exercise affects integrated baroreflex function after 16 days of head-down tilt.

We tested the hypothesis that one bout of maximal exercise performed 24 h before reambulation from 16 days of 6 degrees head-down tilt (HDT) could increase integrated baroreflex sensitivity. Isolated carotid-cardiac and integrated baroreflex function was assessed in seven subjects before and after two periods of HDT separated by 11 mo. On the last day of one HDT period, subjects performed a single bout of maximal cycle ergometry (exercise). Subjects did not exercise after the other HDT period (control). Carotid-cardiac baroreflex sensitivity was evaluated using a neck collar device. Integrated baroreflex function was assessed by recording heart rate (HR) and blood pressure (MAP) during a 15-s Valsalva maneuver (VM) at a controlled expiratory pressure of 30 mmHg. The ratio of change in HR to change in MAP (delta HR/ delta MAP) during phases II and IV of the VM was used as an index of cardiac baroreflex sensitivity. Baroreflex-mediated vasoconstriction was assessed by measuring the late phase II rise in MAP. Following HDT, carotid-cardiac baroreflex sensitivity was reduced (2.8 to 2.0 ms/mmHg; P = 0.05) as was delta HR/ delta MAP during phase II (-1.5 to -0.8 beats/mmHg; P = 0.002). After exercise, isolated carotid baroreflex activity and phase II delta HR/ delta MAP returned to pre-HDT levels but remained attenuated in the control condition. Phase IV delta HR/ delta MAP was not altered by HDT or exercise. The late phase II increase of MAP was 71% greater after exercise compared with control (7 vs. 2 mmHg; P = 0.041).(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1995

Catecholaminergic effects of prolonged head-down bed rest.

Prolonged head-down bed rest (HDBR) provides a model for examining responses to chronic weightlessness in humans. Eight healthy volunteers underwent HDBR for 2 wk. Antecubital venous blood was sampled for plasma levels of catechols [norepinephrine (NE), epinephrine, dopamine, dihydroxyphenylalanine, dihydroxyphenylglycol, and dihydroxyphenylacetic acid] after supine rest on a control (C) day and after 4 h and 7 and 14 days of HDBR. Urine was collected after 2 h of supine rest during day C, 2 h before HDBR, and during the intervals 1-4, 4-24, 144-168 (day 7), and 312-336 h (day 14) of HDBR. All subjects had decreased plasma and blood volumes (mean 16%), atriopeptin levels (31%), and peripheral venous pressure (26%) after HDBR. NE excretion on day 14 of HDBR was decreased by 35% from that on day C, without further trends as HDBR continued, whereas plasma levels were only variably and nonsignificantly decreased. Excretion rates of dihydroxyphenylglycol and dihydroxyphenylalanine decreased slightly during HDBR; excretion rates of epinephrine, dopamine, and dihydroxyphenylacetic acid and plasma levels of catechols were unchanged. The results suggest that HDBR produces sustained inhibition of sympathoneural release, turnover, and synthesis of NE without affecting adrenomedullary secretion or renal dopamine production. Concurrent hypovolemia probably interferes with detection of sympathoinhibition by plasma levels of NE and other catechols in this setting. Sympathoinhibition, despite decreased blood volume, may help to explain orthostatic intolerance in astronauts returning from spaceflights.

Adult↗

PubMed1995

The ventriculo-arterial coupling ratio during transient Gz events.

It has been shown by Westerhof et al. that the ratio tau/T (where tau = Rp*SAC and Rp is total peripheral resistance, SAC is systemic arterial compliance, and T is heart period) is approximately a constant in all mammals under resting conditions such that diastolic pressure is sufficiently high to assure adequate coronary perfusion. The aim of this study is to determine if the ratio tau/T is constant under the transient condition of a rapid onset rate +Gz acceleration. We hypothesize that the ratio is held constant by the cardiovascular control system. Four male baboons were subjected to 10 second rapid onset +Gz profiles and aortic pressure and flow were recorded. Rp and C were calculated using a 2-element windkessel model and T was determined by the inverse of heart rate. All parameters were calculated on a beat-to-beat basis. It was found that the ratio increased very little at +2 Gz and increased dramatically at +3 and +5 Gz, disproving our hypothesis. This increase was due to large increases in peripheral resistance and changes in heart rate over the course of a Gz run. The increases are presumed to represent baroreceptor reflex response to the drop in aortic pressure experienced during +Gz stress. The ratio tau/T returned to its initial resting value shortly after the +Gz stressor was removed.

Animals↗

PubMed1995

Back pain during 6 degrees head-down tilt approximates that during actual microgravity.

Astronauts often experience back pain during spaceflight. Retrospectively, Wing et al. (14) found that during spaceflight, 14 of 19 Shuttle crewmembers experienced back pain, which they described as dull (62%), localized to the lower back (50%), and with an intensity of 2 on a 5-point scale. Further, the spine lengthens 4-7 cm in microgravity. Our objective was to compare back pain and spinal lengthening (body height increase) during simulated microgravity (6 degrees head-down tilt, HDT) with the same parameters during actual microgravity. Eight male subjects completed a modified McGill pain questionnaire with intensity graded from zero (no pain) to five (intense and incapacitating pain) each day at 7:00 pm during 2 d pre-HDT control, 16 d HDT, and 1 d post-HDT recovery periods. Also, the subjects' heights were measured each day while supine (control and recovery) and during HDT. Back pain increased from zero (pre-tilt control period) to 2.3 +/- 0.4 at days 1 to 3 of HDT, and was categorized as dull and/or burning pain in subjects' lower backs. Only 2 subjects reported any pain after day 9 of HDT and during recovery. Heights increased 2.1 +/- 0.5 cm by day 3 of HDT and remained at that level until the end of the HDT period. Although spinal lengthening in space is greater than that during HDT, the HDT model approximates the level, type, distribution, and time course of back pain associated with actual microgravity. In the HDT model, pain subsides in intensity when spinal lengthening stops.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1995

Effect of training on blood volume and plasma hormone concentrations in the elderly.

The purpose of this investigation was to determine the effects of 6 months of endurance training on resting plasma (PV) and blood volume (BV), and resting hormone and electrolyte concentrations in the elderly. Thirty-eight elderly men and women (ages 60-82 yr) were assigned to endurance exercise training (N = 29) or to control (N = 9) groups. Resting plasma levels of adrenocorticotropic hormone, vasopressin, aldosterone, norepinephrine, epinephrine, sodium, potassium, and protein were measured at the start (T1) and end (T2) of 26 wk of training. PV measurement was performed using the Evan's blue dye technique. Endurance training consisted of uphill treadmill walking or stairclimbing exercise 3 times.wk-1, 30-45 min.d-1, at 75-84% of maximal heart rate reserve. The exercise group increased VO2max by 11.2% (P < or = 0.05) and increased resting PV and BV by 11.2% and 12.7% (P < or = 0.05), respectively. Hormone and electrolyte levels in the exercise group remained unchanged; all variables were unchanged in the control group. These results are similar to findings in younger individuals. Because plasma hormone concentrations were maintained despite a chronically elevated BV, endurance training in healthy, elderly subjects may be associated with a resetting of volume receptors.

Adrenocorticotropic Hormone↗

PubMed1994

The sympathetic nervous system and the physiologic consequences of spaceflight: a hypothesis.

Many of the physiologic consequences of weightlessness and the cardiovascular abnormalities on return from space could be due, at least in part, to alterations in the regulation of the autonomic nervous system. In this article, the authors review the rationale and evidence for an autonomic mediation of diverse changes that occur with spaceflight, including the anemia and hypovolemia of weightlessness and the tachycardia and orthostatic intolerance on return from space. This hypothesis is supported by studies of two groups of persons known to have low catecholamine levels: persons subjected to prolonged bedrest and persons with syndromes characterized by low circulating catecholamines (Bradbury-Eggleston syndrome and dopamine beta-hydroxylase deficiency). Both groups exhibit the symptoms mentioned. The increasing evidence that autonomic mechanisms underlie many of the physiologic consequences of weightlessness suggests that new pharmacologic approaches (such as administration of beta-blockers and/or sympathomimetic amines) based on these findings may attenuate these unwanted effects.

Anemia↗

PubMed1994

Autonomic functions and orthostatic responses 24 h after acute intense exercise in paraplegic subjects.

We tested the hypothesis that a bout of graded exercise designed to elicit maximal effort would increase the sensitivity of autonomically mediated baroreflexes and enhance blood pressure (BP) stability in subjects prone to postural hypotension. Therefore, we measured heart rate (HR), BP, forearm vascular resistance (FVR), and vasoactive hormone responses before and during 15 min of 70 degrees head-up tilt (HUT) in 10 paraplegic subjects (21-65 yr) on two occasions: 1) 24 h after maximal arm-crank exercise (postexercise) and 2) without exercise (control). During HUT, HR increased 30 beats/min in both postexercise and control, but the reduction in systolic BP (SBP) during control (-12.0 +/- 4.6 mmHg) was larger (P = 0.017) than that during HUT after exercise (-0.3 +/- 4.3 mmHg). The postexercise increase in FVR from supine to HUT of 17.0 +/- 2.4 to 24.8 +/- 3.2 peripheral resistance units (PRU) was greater (P = 0.042) than the increase observed during control (18.3 +/- 3.7 to 19.5 +/- 3.1 PRU). The gain of the carotid-cardiac baroreflex was also increased (P = 0.049) after exercise. Responses in norepinephrine, vasopressin, and plasma renin-angiotensin induced by HUT were similar for control and postexercise, and there was no difference in either leg compliance or plasma volume between the two conditions. Additionally, HR and SBP responses to phases II and IV of the Valsalva maneuver, indexes of integrated baroreflex sensitivity, were increased (P < 0.05) after maximal exercise compared with control. Thus acute intense exercise eliminated orthostatic hypotension in paraplegics, was associated with increased FVR and baroreflex sensitivity, and was independent of blood volume changes.

Adult↗