Steven H. Platts

Vascular Smooth Muscle αvβ3 Integrin Mediates Arteriolar Vasodilation in Response to RGD Peptides

Arteriolar vasodilation and the resultant increase in blood flow are characteristic vascular responses to tissue injury. The dilatory mediators signaling these responses are incompletely understood. We show that integrin-binding peptides containing the Arg-Gly-Asp (RGD) tripeptide sequence cause immediate and, in some instances, sustained vasodilation when applied to isolated rat cremaster arterioles. The vasodilation is dependent on interaction of the soluble RGD sequence with the α v β 3 integrin expressed by smooth muscle cells in the arteriolar wall. Possible in vivo sources of soluble RGD sequences are fragments of extracellular matrix proteins that are generated after tissue injury. Indeed, protease-generated fragments of denatured collagen type I (a major source of RGD sequences) also cause cremaster arteriolar vasodilation through the α v β 3 integrin. Thus, extracellular matrix protein fragments containing the RGD sequence may act as vascular wound recognition signals to regulate blood flow to injured tissue.

Alteration of microtubule polymerization modulates arteriolar vasomotor tone.

Microtubules are important cytoskeletal elements that have been shown to play a major role in many cellular processes because of their mechanical properties and/or their participation in various cell signaling pathways. We tested the hypothesis that depolymerization of microtubules would alter vascular smooth muscle (VSM) tone and hence contractile function. In our studies, isolated cremaster arterioles exhibited significant vasoconstriction that developed over a 20- to 40-min period when they were treated with microtubule depolymerizing drugs colchicine (10 microM), nocodazole (10 microM), or demecolcine (10 microM). Immunofluorescent labeling of microtubules in cultured rat VSM revealed that both colchicine and nocodazole caused microtubule depolymerization over a similar time course. The vasoconstriction was maintained over a wide range of intraluminal pressures (30-170 cmH(2)O). The increased tone was not affected by endothelial denudation, suggesting that it was due to an effect on VSM. Microtubule depolymerization with demecolcine or colchicine had no effect on VSM intracellular Ca(2+) concentration ([Ca(2+)](i)). These data indicate that microtubules significantly interact with processes leading to the expression of vasomotor tone. The mechanism responsible for the effect of microtubules on vasomotor tone appears to be independent of both the endothelium and an increase in VSM [Ca(2+)](i).Microtubules are important cytoskeletal elements that have been shown to play a major role in many cellular processes because of their mechanical properties and/or their participation in various cell signaling pathways. We tested the hypothesis that depolymerization of microtubules would alter vascular smooth muscle (VSM) tone and hence contractile function. In our studies, isolated cremaster arterioles exhibited significant vasoconstriction that developed over a 20- to 40-min period when they were treated with microtubule depolymerizing drugs colchicine (10 μM), nocodazole (10 μM), or demecolcine (10 μM). Immunofluorescent labeling of microtubules in cultured rat VSM revealed that both colchicine and nocodazole caused microtubule depolymerization over a similar time course. The vasoconstriction was maintained over a wide range of intraluminal pressures (30-170 cmH2O). The increased tone was not affected by endothelial denudation, suggesting that it was due to an effect on VSM. Microtubule depolymerization with demecolcine or colchicine had no effect on VSM intracellular Ca2+ concentration ([Ca2+]i). These data indicate that microtubules significantly interact with processes leading to the expression of vasomotor tone. The mechanism responsible for the effect of microtubules on vasomotor tone appears to be independent of both the endothelium and an increase in VSM [Ca2+]i.

Role of K+ channels in arteriolar vasodilation mediated by integrin interaction with RGD-containing peptide

Integrins are transmembrane adhesion receptors found on most cells, including vascular smooth muscle cells. Several integrins bind to the conserved amino acid sequence Arg-Gly-Asp (RGD), and synthetic RGD-containing peptides can cause endothelium-independent arteriolar vasodilation by interacting with the αvβ3-integrin expressed by vascular smooth muscle. We hypothesized that RGD peptide-induced vasodilation involves K+ channels. Rat cremaster arterioles were treated with cRGD (GPenGRGDSPCA) in the presence or absence of the nonselective K+channel inhibitor tetraethylammonium (TEA, 20 mM). TEA caused arterioles to constrict by 19 ± 5% and inhibited cRGD-induced vasodilation ( n = 7, P < 0.05). Vessels preconstricted with phenylephrine (5 × 10-7 M) showed no significant inhibition of the dilatory response to cRGD, indicating that inhibition by TEA was not related to increased vasomotor tone. Further evidence for the involvement of K+ channels was obtained by addition of 100 mM KCl ( n = 5), which inhibited vasodilation caused by cRGD. Inhibition of large and small conductance, Ca2+-activated K+ channels with iberiotoxin (100 nM) or apamin (25 nM), respectively, had no effect on cRGD-induced vasodilation. Partial inhibition of vasodilation was observed with inhibitors of voltage-gated (4-aminopyridine, 1 mM), ATP-sensitive (glibenclamide, 1 μM), and inward rectifying (barium, 50 μM) K+ channels. These data support the hypothesis that integrin-signaling pathways leading to arteriolar vasodilation may involve modulation of K+ channel function.

Compression Garments as Countermeasures to Orthostatic Intolerance

INTRODUCTION All astronauts experience some degree of orthostatic intolerance following spaceflight, ranging from tachycardia to orthostatic hypotension and syncope. The purpose of this study was to evaluate the ability of two compression garments, the National Aeronautics and Space Administrations inflatable antigravity suit (AGS) and the Russian Federal Space Agencys non-inflatable compression garment (Kentavr), to prevent hypovolemia-related orthostatic intolerance. METHODS To mimic the plasma volume loss experienced by astronauts during spaceflight 19 healthy subjects received an intravenous dose of a diuretic, furosemide (0.5 mg x kg(-1)), and then consumed a low-salt diet for 36 h. Thereafter, subjects participated in a 15-min 80 degrees head-up tilt test wearing either the AGS (N = 9) or Kentavr (N = 10). Compression garments were used in the fashion recommended by the respective agencies, delivering approximately 78 mmHg and approximately 30 mmHg of compression in the AGS and Kentavr, respectively. Incidence of presyncope and hemodynamic responses during upright tilt were compared to a separate group of hypovolemic control subjects (N = 16). RESULTS Subjects wearing the AGS or Kentavr completed the full 15 min of upright tilt without incidence of orthostatic hypotension or presyncope. In contrast, only 9 control subjects (56%) were able to complete the tilt test. In addition, both types of compression garments maintained systolic blood pressure and significantly reduced tilt-induced tachycardia and reductions in stroke volume. CONCLUSIONS Although both garments successfully countered hypovolemia-induced orthostatic intolerance, the Kentavr provided protection by using lower levels of compression. Determining the optimal compression level required for protection of intolerance may improve crewmember comfort and decrease restrictions on physical activities after spaceflight.

Aerobic Exercise Deconditioning and Countermeasures During Bed Rest

Bed rest is a well-accepted model for spaceflight in which the physiologic adaptations, particularly in the cardiovascular system, are studied and potential countermeasures can be tested. Bed rest without countermeasures results in reduced aerobic capacity and altered submaximal exercise responses. Aerobic endurance and factors which may impact prolonged exercise, however, have not been well studied. The initial loss of aerobic capacity is rapid, occurring in parallel with the loss of plasma volume. Thereafter, the reduction in maximal aerobic capacity proceeds more slowly and is influenced by central and peripheral adaptation. Exercise capacity can be maintained during bed rest and may be improved during recovery with appropriate countermeasures. Plasma volume restoration, resistive exercise, orthostatic stress, aerobic exercise, and aerobic exercise plus orthostatic stress all have been tested with varying levels of success. However, the optimal combination of elements-exercise modality, intensity, duration, muscle groups exercised and frequency of aerobic exercise, orthostatic stress, and supplementary resistive or anaerobic exercise training-has not been systematically evaluated. Currently, frequent (at least 3 days per week) bouts of intense exercise (interval-style and near maximal) with orthostatic stress appears to be the most efficacious method to protect aerobic capacity during bed rest. Further refinement of protocols and countermeasure hardware may be necessary to insure the success of countermeasures in the unique environment of space.

Gradient compression garments as a countermeasure to post-spaceflight orthostatic intolerance.

INTRODUCTION Post-spaceflight orthostatic intolerance affects approximately 30% of short-duration and 80% of long-duration crewmembers. While the current NASA antigravity suit is effective during Space Shuttle re-entry, it is not designed to be worn postflight and has several drawbacks. The purpose of this study was to evaluate the use of commercially available, thigh-high, gradient compression garments to prevent post-spaceflight orthostatic intolerance. METHODS Before spaceflight, five male Shuttle astronauts were fitted for compression garments. Postflight stand time, blood pressure, heart rate, stroke volume, cardiac output, and peripheral resistance during 10-min, 80 degrees head-up tilt test within 4 h of landing in these astronauts were retrospectively compared to a group of nine male astronauts not wearing the compression garments. RESULTS On landing day, three of nine non-countermeasure astronauts developed presyncopal symptoms and could not complete the test, while no countermeasure subjects became presyncopal. Compared to the non-countermeasure subjects, the countermeasure subjects had higher systolic blood pressure (116 +/- 3 vs. 134 +/- 2 mmHg), stroke volume (42 +/- 5 vs. 57 +/- 6 ml), and cardiac output (3.1 +/- 0.3 vs. 4.6 +/- 0.4 L). Heart rate was not different between groups. CONCLUSIONS In this small pilot study, the rate of presyncope in the non-countermeasure group was similar to that reported previously in subjects without a compression garment. In contrast, thigh-high graded compression garments mitigated the symptoms of orthostatic intolerance by improving stroke volume, cardiac output, and systolic blood pressure responses to standing.

Orthostatic Intolerance After ISS and Space Shuttle Missions.

INTRODUCTION Cardiovascular deconditioning apparently progresses with flight duration, resulting in a greater incidence of orthostatic intolerance following long-duration missions. Therefore, we anticipated that the proportion of astronauts who could not complete an orthostatic tilt test (OTT) would be higher on landing day and the number of days to recover greater after International Space Station (ISS) than after Space Shuttle missions. METHODS There were 20 ISS and 65 Shuttle astronauts who participated in 10-min 80° head-up tilt tests 10 d before launch, on landing day (R+0), and 3 d after landing (R+3). Fishers Exact Test was used to compare the ability of ISS and Shuttle astronauts to complete the OTT. Cox regression was used to identify cardiovascular parameters associated with OTT completion and mixed model analysis was used to compare the change and recovery rates between groups. RESULTS The proportion of astronauts who completed the OTT on R+0 (2 of 6) was less in ISS than in Shuttle astronauts (52 of 65). On R+3, 13 of 15 and 19 of 19 of the ISS and Shuttle astronauts, respectively, completed the OTT. An index comprised of stroke volume and diastolic blood pressure provided a good prediction of OTT completion and was altered by spaceflight similarly for both astronaut groups, but recovery was slower in ISS than in Shuttle astronauts. CONCLUSIONS The proportion of ISS astronauts who could not complete the OTT on R+0 was greater and the recovery rate slower after ISS compared to Shuttle missions. Thus, mission planners and crew surgeons should anticipate the need to tailor scheduled activities and level of medical support to accommodate protracted recovery after long-duration microgravity exposures.

Theoretical analysis of the mechanisms of a gender differentiation in the propensity for orthostatic intolerance after spaceflight

BackgroundA tendency to develop reentry orthostasis after a prolonged exposure to microgravity is a common problem among astronauts. The problem is 5 times more prevalent in female astronauts as compared to their male counterparts. The mechanisms responsible for this gender differentiation are poorly understood despite many detailed and complex investigations directed toward an analysis of the physiologic control systems involved.MethodsIn this study, a series of computer simulation studies using a mathematical model of cardiovascular functioning were performed to examine the proposed hypothesis that this phenomenon could be explained by basic physical forces acting through the simple common anatomic differences between men and women. In the computer simulations, the circulatory components and hydrostatic gradients of the model were allowed to adapt to the physical constraints of microgravity. After a simulated period of one month, the model was returned to the conditions of earths gravity and the standard postflight tilt test protocol was performed while the model output depicting the typical vital signs was monitored.ConclusionsThe analysis demonstrated that a 15% lowering of the longitudinal center of gravity in the anatomic structure of the model was all that was necessary to prevent the physiologic compensatory mechanisms from overcoming the propensity for reentry orthostasis leading to syncope.

Left ventricular remodeling during and after 60 days of sedentary head-down bed rest.

Short periods of weightlessness are associated with reduced stroke volume and left ventricular (LV) mass that appear rapidly and are thought to be largely dependent on plasma volume. The magnitude of these cardiac adaptations are even greater after prolonged periods of simulated weightlessness, but the time course during and the recovery from bed rest has not been previously described. We collected serial measures of plasma volume (PV, carbon monoxide rebreathing) and LV structure and function [tissue Doppler imaging, three-dimensional (3-D) and 2-D echocardiography] before, during, and up to 2 wk after 60 days of 6° head down tilt bed rest (HDTBR) in seven healthy subjects (four men, three women). By 60 days of HDTBR, PV was markedly reduced (2.7 ± 0.3 vs. 2.3 ± 0.3 liters,P< 0.001). Resting measures of LV volume and mass were ∼15% (P< 0.001) and ∼14% lower (P< 0.001), respectively, compared with pre-HDTBR values. After 3 days of reambulation, both PV and LV volumes were not different than pre-HDTBR values. However, LV mass did not recover with normalization of PV and remained 12 ± 4% lower than pre-bed rest values (P< 0.001). As previously reported, decreased PV and LV volume precede and likely contribute to cardiac atrophy during prolonged LV unloading. Although PV and LV volume recover rapidly after HDTBR, there is no concomitant normalization of LV mass. These results demonstrate that reduced LV mass in response to prolonged simulated weightlessness is not a simple effect of tissue dehydration, but rather true LV muscle atrophy that persists well into recovery.

Sex differences in blood pressure control during 6° head-down tilt bed rest

Spaceflight-induced orthostatic intolerance has been studied for decades. Although ∼22% of the astronaut corps are women, most mechanistic studies use mostly male subjects, despite known sex differences in autonomic control and postflight orthostatic intolerance. We studied adrenergic, baroreflex, and autonomic indexes during continuous infusions of vasoactive drugs in men and women during a 60-day head-down bed rest. Volunteers were tested before bed rest (20 men and 10 women) and around day 30 (20 men and 10 women) and day 60 (16 men and 8 women) of bed rest. Three increasing doses of phenylephrine (PE) and sodium nitroprusside were infused for 10 min after an infusion of normal saline. A 20-min rest period separated the phenylephrine and sodium nitroprusside infusions. Autonomic activity was approximated by spectral indexes of heart rate and blood pressure variability, and baroreflex sensitivity was measured by the spontaneous baroreflex slope. Parasympathetic modulation and baroreflex sensitivity decreased with bed rest, with women experiencing a larger decrease in baroreflex sensitivity by day 30 than men. The sympathetic activation of men and parasympathetic responsiveness of women in blood pressure control during physiological stress were preserved throughout bed rest. During PE infusions, women experienced saturation of the R-R interval at high frequency, whereas men did not, revealing a sex difference in the parabolic relationship between high-frequency R-R interval, a measurement of respiratory sinus arrhythmia, and R-R interval. These sex differences in blood pressure control during simulated microgravity reveal the need to study sex differences in long-duration spaceflight to ensure the health and safety of the entire astronaut corps.