Resistance training affects GLUT-4 content in skeletal muscle of humans after 19 days of head-down bed rest.
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This study assessed the effects of inactivity on GLUT-4 content of human skeletal muscle and evaluated resistance training as a countermeasure to inactivity-related changes in GLUT-4 content in skeletal muscle. Nine young men participated in the study. For 19 days, four control subjects remained in a -6 degrees head-down tilt at all times throughout bed rest, except for showering every other day. Five training group subjects also remained at bed rest, except during resistance training once in the morning. The resistance training consisted of 30 isometric maximal voluntary contractions for 3 s each; leg-press exercise was used to recruit the extensor muscles of the ankle, knee, and hip. Pauses (3 s) were allowed between bouts of maximal contraction. Muscle biopsy samples were obtained from the lateral aspect of vastus lateralis (VL) muscle before and after the bed rest. GLUT-4 content in VL muscle of the control group was significantly decreased after bed rest (473 +/- 48 vs. 398 +/- 66 counts. min-1. microgram membrane protein-1, before and after bed rest, respectively), whereas GLUT-4 significantly increased in the training group with bed rest (510 +/- 158 vs. 663 +/- 189 counts. min-1. microgram membrane protein-1, before and after bed rest, respectively). The present study demonstrated that GLUT-4 in VL muscle decreased by approximately 16% after 19 days of bed rest, and isometric resistance training during bed rest induced a 30% increase above the value of GLUT-4 before bed rest. (+info)
Incidence of bradycardia during recovery from spinal anaesthesia: influence of patient position.
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We administered 0.5% plain bupivacaine 4 ml intrathecally (L2-3 or L3-4) in three groups of 20 patients, according to the position in which they were nursed in the post-anaesthesia care unit (PACU): supine horizontal, 30 degrees Trendelenburg or hammock position (trunk and legs 30 degrees elevated). Patients were observed until anaesthesia descended to less than S1. The incidence of severe bradycardia (heart rate < 50 beat min-1) in the PACU was significantly higher in patients in the Trendelenburg position (60%) than in the horizontal (20%, P < 0.01) or hammock (10%, P < 0.005) position. After 90 min, following admission to the PACU, only patients in the hammock position did not have severe bradycardia. In this late phase, the incidence of severe bradycardia in the Trendelenburg group was 35% (P < 0.005) and 10% in patients in the supine horizontal position. In four patients, severe bradycardia first occurred later than 90 min after admission to the PACU. The latest occurrence of severe bradycardia was recorded 320 min after admission to the PACU. We conclude that for recovery from spinal anaesthesia, the Trendelenburg position should not be used and the hammock position is preferred. (+info)
Sympathetic responses to head-down rotations in humans.
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Muscle sympathetic nerve activity (MSNA) increases with head-down neck flexion (HDNF). The present study had three aims: 1) to examine sympathetic and vascular responses to two different magnitudes of HDNF; 2) to examine these same responses during prolonged HDNF; and 3) to determine the influence of nonspecific pressure receptors in the head on MSNA. The first experiment tested responses to two static head positions in the vertical axis [HDNF and intermediate HDNF (I-HDNF; approximately 50% of HDNF)]. MSNA increased above baseline during both I-HDNF and HDNF (from 219 +/- 36 to 301 +/- 47 and from 238 +/- 42 to 356 +/- 59 units/min, respectively; P < 0.01). Calf blood flow (CBF) decreased and calf vascular resistance increased during both I-HDNF and HDNF (P < 0.01). Both the increase in MSNA and the decrease in CBF were linearly related to the magnitude of the downward head rotations (P < 0.01). The second experiment tested responses during prolonged HDNF. MSNA increased (from 223 +/- 63 to 315 +/- 79 units/min; P < 0.01) and CBF decreased (from 3.2 +/- 0.4 to 2.6 +/- 0.04 ml. 100 ml-1. min-1; P < 0.01) at the onset of HDNF. These responses were maintained throughout the 30-min period. Mean arterial blood pressure gradually increased during the 30 min of HDNF (from 94 +/- 4 to 105 +/- 3 mmHg; P < 0.01). In a third experiment, head-down neck extension was performed with subjects in the supine position. Unlike HDNF, head-down neck extension did not affect MSNA. The results from these studies demonstrate that MSNA: 1) increases in magnitude as the degree of HDNF increases; 2) remains elevated above baseline during prolonged HDNF; and 3) responses during HDNF are not associated with nonspecific receptors in the head activated by increases in cerebral pressure. (+info)
Contributions of MSNA and stroke volume to orthostatic intolerance following bed rest.
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We examined whether the altered orthostatic tolerance following 14 days of head-down tilt bed rest (HDBR) was related to inadequate sympathetic outflow or to excessive reductions in cardiac output during a 10- to 15-min head-up tilt (HUT) test. Heart rate, blood pressure (BP, Finapres), muscle sympathetic nerve activity (MSNA, microneurography), and stroke volume blood velocity (SVV, Doppler ultrasound) were assessed during supine 30 degrees (5 min) and 60 degrees (5-10 min) HUT positions in 15 individuals who successfully completed the pre-HDBR test without evidence of orthostatic intolerance. Subjects were classified as being orthostatically tolerant (OT, n = 9) or intolerant (OI, n = 6) following the post-HDBR test. MSNA, BP, and SVV during supine and HUT postures were not altered in the OT group. Hypotension during 60 degrees HUT in the post-bed rest test for the OI group (P < 0.05) was associated with a blunted increase in MSNA (P < 0.05). SVV was reduced following HDBR in the OI group (main effect of HDBR, P < 0.02). The data support the hypothesis that bed rest-induced orthostatic intolerance is related to an inadequate increase in sympathetic discharge that cannot compensate for a greater postural reduction in stroke volume. (+info)
Increase in epinephrine-induced responsiveness during microgravity simulated by head-down bed rest in humans.
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The epinephrine (Epi)-induced effects on the sympathetic nervous system (SNS) and metabolic functions were studied in men before and during a decrease in SNS activity achieved through simulated microgravity. Epi was infused at 3 graded rates (0.01, 0.02, and 0. 03 microg. kg(-1). min(-1) for 40 min each) before and on the fifth day of head-down bed rest (HDBR). The effects of Epi on the SNS (assessed by plasma norepinephrine levels and spectral analysis of systolic blood pressure and heart rate variability), on plasma levels of glycerol, nonesterified fatty acids (NEFA), glucose and insulin, and on energy expenditure were evaluated. HDBR decreased urinary norepinephrine excretion (28.1 +/- 4.2 vs. 51.5 +/- 9.1 microg/24 h) and spectral variability of systolic blood pressure in the midfrequency range (16.3 +/- 1.9 vs. 24.5 +/- 0.9 normalized units). Epi increased norepinephrine plasma levels (P < 0.01) and spectral variability of systolic blood pressure (P < 0.009) during, but not before, HDBR. No modification of Epi-induced changes in heart rate and systolic and diastolic blood pressures were observed during HDBR. Epi increased plasma glucose, insulin, and NEFA levels before and during HDBR. During HDBR, the Epi-induced increase in plasma glycerol and lactate levels was more pronounced than before HDBR (P < 0.005 and P < 0.001, respectively). Epi-induced energy expenditure was higher during HDBR (P < 0.02). Our data suggest that the increased effects of Epi during simulated microgravity could be related to both the increased SNS response to Epi infusion and/or to the beta-adrenergic receptor sensitization of end organs, particularly in adipose tissue and skeletal muscle. (+info)
Effects of a three-day head-down tilt on renal and hormonal responses to acute volume expansion.
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To clarify whether exposure to 6 degrees head-down tilt (HDT) leads to alterations in body fluid volumes and responses to a saline load similar to those observed during space flight we investigated eight healthy subjects during a 4-day, 6 degrees HDT and during a time-control ambulatory period with cross-over. Compared with the ambulatory period, HDT was associated with greater urinary excretion of water and sodium (UV, U(Na)V) from 0 to 12 h (cumulated UV 1,781 +/- 154 vs. 1,383 +/- 170 ml, P < 0.05; cumulated U(Na)V 156 +/- 14 vs. 117 +/- 9 mmol, P < 0.05), and with higher plasma atrial natriuretic factor (ANF) at 4 h. Hemoglobin and hematocrit increased over the first 24 h, and blood and plasma volumes were decreased after 48 h of HDT (P < 0.05). Plasma renin activity (PRA) and aldosterone did not differ between the two groups. With prolongation of HDT, UV and U(Na)V returned close to baseline values. On the fourth HDT day, a 30-min infusion of 20 ml/kg isotonic saline was performed, while a large oral water load maintained a high urine output. The ambulatory period experiment was done with the subjects in the acute supine posture. Sodium excreted within 4 h of loading was 123 +/- 8 mmol during HDT vs. 168 +/- 16 mmol during the ambulatory period (P < 0.05). The increase in plasma ANF and decrease in PRA were greater during HDT than during the ambulatory period (ANF 30 +/- 5 vs. 13 +/- 4 pg/ml, P < 0.05; PRA -1.4 +/- 0.4 vs. -0.5 +/- 0.2 ng. ml(-1). h(-1), P < 0.05). Our data suggest that after a 3-day HDT period, thoracic volume receptor loading returns to the level seen in the upright position, leading to blunted responses to volume expansion, compared with acute supine control. (+info)
Influences of thigh cuffs on the cardiovascular system during 7-day head-down bed rest.
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Thigh cuffs, presently named "bracelets," consist of two straps fixed to the upper part of each thigh, applying a pressure of 30 mmHg. The objective was to evaluate the cardiac, arterial, and venous changes in a group of subjects in head-down tilt (HDT) for 7 days by using thigh cuffs during the daytime, and in a control group not using cuffs. The cardiovascular parameters were measured by echography and Doppler. Seven days in HDT reduced stroke volume in both groups (-10%; P < 0.05). Lower limb vascular resistance decreased more in the cuff group than in the control group (-29 vs. -4%; P < 0.05). Cerebral resistance increased in the control group only (+6%; P < 0.05). The jugular vein increased (+45%; P < 0.05) and femoral and popliteal veins decreased in cross-sectional area in both groups (-45 and -8%, respectively; P < 0.05). Carotid diameter tended to decrease (-5%; not significant) in both groups. Heart rate, blood pressure, cardiac output, and total resistance did not change significantly. After 8 h with thigh cuffs, the cardiac and arterial parameters had recovered their pre-HDT level except for blood pressure (+6%; P < 0.05). Jugular vein size decreased from the pre-HDT level (-21%; P < 0.05), and femoral and popliteal vein size increased (+110 and +136%, respectively; P < 0.05). The thigh cuffs had no effect on the development of orthostatic intolerance during the 7 days in HDT. (+info)
Noninvasive cardiac output measurement in orthostasis: pulse contour analysis compared with acetylene rebreathing.
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We tested the reliability of noninvasive cardiac output (CO) measurement in different body positions by pulse contour analysis (CO(pc)) by using a transmission line model (K. H. Wesseling, B. De Wit, J. A. P. Weber, and N. T. Smith. Adv. Cardiol. Phys. 5, Suppl. II: 16-52, 1983). Acetylene rebreathing (CO(rebr)) was used as a reference method. Twelve subjects (age 21-34 yr) were studied: 1) six in whom CO(rebr) and CO(pc) were measured in the standing and 6 degrees head-down tilt (HDT) postures and 2) six in whom CO was measured in the 30 degrees HDT, supine, 30 degrees head up-tilt (HUT), and 70 degrees HUT postures on a tilt table. The CO(rebr)-to-CO(pc) ratio in (near) the supine position during rebreathing was used as the calibration factor for CO(pc) measurements. Calibrated CO(pc) (CO(cal sup)) consistently overestimated CO in the upright posture. The drop in CO with upright posture was underestimated by approximately 50%. CO(cal sup) and CO(rebr) values did not differ in the 30 degrees HDT position. Changes in the CO(rebr)-to-CO(pc) ratio are highly variable among subjects in response to a change in posture. Therefore, CO(pc) must be recalibrated for each subject in each posture. (+info)