Mitsuru Saito

Human muscle sympathetic neural and haemodynamic responses to tilt following spaceflight

Orthostatic intolerance is common when astronauts return to Earth: after brief spaceflight, up to two‐thirds are unable to remain standing for 10 min. Previous research suggests that susceptible individuals are unable to increase their systemic vascular resistance and plasma noradrenaline concentrations above pre‐flight upright levels. In this study, we tested the hypothesis that adaptation to the microgravity of space impairs sympathetic neural responses to upright posture on Earth. We studied six astronauts ∼72 and 23 days before and on landing day after the 16 day Neurolab space shuttle mission. We measured heart rate, arterial pressure and cardiac output, and calculated stroke volume and total peripheral resistance, during supine rest and 10 min of 60 deg upright tilt. Muscle sympathetic nerve activity was recorded in five subjects, as a direct measure of sympathetic nervous system responses. As in previous studies, mean (±s.e.m.) stroke volume was lower (46 ± 5 vs. 76 ± 3 ml, P= 0.017) and heart rate was higher (93 ± 1 vs. 74 ± 4 beats min−1, P= 0.002) during tilt after spaceflight than before spaceflight. Total peripheral resistance during tilt post flight was higher in some, but not all astronauts (1674 ± 256 vs. 1372 ± 62 dynes s cm−5, P= 0.32). No crew member exhibited orthostatic hypotension or presyncopal symptoms during the 10 min of postflight tilting. Muscle sympathetic nerve activity was higher post flight in all subjects, in supine (27 ± 4 vs. 17 ± 2 bursts min−1, P= 0.04) and tilted (46 ± 4 vs. 38 ± 3 bursts min−1, P= 0.01) positions. A strong (r2= 0.91–1.00) linear correlation between left ventricular stroke volume and muscle sympathetic nerve activity suggested that sympathetic responses were appropriate for the haemodynamic challenge of upright tilt and were unaffected by spaceflight. We conclude that after 16 days of spaceflight, muscle sympathetic nerve responses to upright tilt are normal.

Cardiovascular and sympathetic neural responses to handgrip and cold pressor stimuli in humans before, during and after spaceflight

Astronauts returning to Earth have reduced orthostatic tolerance and exercise capacity. Alterations in autonomic nervous system and neuromuscular function after spaceflight might contribute to this problem. In this study, we tested the hypothesis that exposure to microgravity impairs autonomic neural control of sympathetic outflow in response to peripheral afferent stimulation produced by handgrip and a cold pressor test in humans. We studied five astronauts ≈72 and 23 days before, and on landing day after the 16 day Neurolab (STS‐90) space shuttle mission, and four of the astronauts during flight (day 12 or 13). Heart rate, arterial pressure and peroneal muscle sympathetic nerve activity (MSNA) were recorded before and during static handgrip sustained to fatigue at 40 % of maximum voluntary contraction, followed by 2 min of circulatory arrest pre‐, in‐ and post‐flight. The cold pressor test was applied only before (five astronauts) and during flight (day 12 or 13, four astronauts). Mean (±s.e.m.) baseline heart rates and arterial pressures were similar among pre‐, in‐ and post‐flight measurements. At the same relative fatiguing force, the peak systolic pressure and mean arterial pressure during static handgrip were not different before, during and after spaceflight. The peak diastolic pressure tended to be higher post‐ than pre‐flight (112 ± 6 vs. 99 ± 5 mmHg, P= 0.088). Contraction‐induced rises in heart rate were similar pre‐, in‐ and post‐flight. MSNA was higher post‐flight in all subjects before static handgrip (26 ± 4 post‐ vs. 15 ± 4 bursts min−1 pre‐flight, P= 0.017). Contraction‐evoked peak MSNA responses were not different before, during, and after spaceflight (41 ± 4, 38 ± 5 and 46 ± 6 bursts min−1, all P > 0.05). MSNA during post‐handgrip circulatory arrest was higher post‐ than pre‐ or in‐flight (41 ± 1 vs. 33 ± 3 and 30 ± 5 bursts min−1, P= 0.038 and 0.036). Similarly, responses of MSNA and blood pressure to the cold pressor test were well maintained in‐flight. We conclude that modulation of muscle sympathetic neural outflow by muscle metaboreceptors and skin nociceptors is preserved during short duration spaceflight.

Influence of microgravity on astronauts' sympathetic and vagal responses to Valsalva's manoeuvre

When astronauts return to Earth and stand, their heart rates may speed inordinately, their blood pressures may fall, and some may experience frank syncope. We studied brief autonomic and haemodynamic transients provoked by graded Valsalva manoeuvres in astronauts on Earth and in space, and tested the hypothesis that exposure to microgravity impairs sympathetic as well as vagal baroreflex responses. We recorded the electrocardiogram, finger photoplethysmographic arterial pressure, respiration and peroneal nerve muscle sympathetic activity in four healthy male astronauts (aged 38–44 years) before, during and after the 16 day Neurolab space shuttle mission. Astronauts performed two 15 s Valsalva manoeuvres at each pressure, 15 and 30 mmHg, in random order. Although no astronaut experienced presyncope after the mission, microgravity provoked major changes. For example, the average systolic pressure reduction during 30 mmHg straining was 27 mmHg pre‐flight and 49 mmHg in flight. Increases in muscle sympathetic nerve activity during straining were also much greater in space than on Earth. For example, mean normalized sympathetic activity increased 445 % during 30 mmHg straining on earth and 792 % in space. However, sympathetic baroreflex gain, taken as the integrated sympathetic response divided by the maximum diastolic pressure reduction during straining, was the same in space and on Earth. In contrast, vagal baroreflex gain, particularly during arterial pressure reductions, was diminished in space. This and earlier research suggest that exposure of healthy humans to microgravity augments arterial pressure and sympathetic responses to Valsalva straining and differentially reduces vagal, but not sympathetic baroreflex gain.

Responses in muscle sympathetic nerve activity to sustained hand-grips of different tensions in humans

SummaryTo clarify whether sympathetic nerve activity increases in relation to the tension of a sustained muscle contraction, muscle sympathetic nerve activity (MSA) was recorded directly from the peroneal nerve fascicle at the popliteal fossa by means of tungsten microelectrodes in five healthy male subjects. A sustained muscle contraction was performed by handgrip for two minutes in a supine position at tensions of 10, 30 and 45% of maximal grip strength (MGS). MSA, electrocardiogram (ECG) using bipolar electrodes from the chest and surface electromyogram (EMG) from the extensor pollicis longus were recorded simultaneously before and during the sustained handgrip. Arterial blood pressure was measured at the resting upper arm by auscultation. During handgrip with tensions of 10, 30 and 45% MGS, average MSA burst rate (bursts · min−1) increased to 122, 152 and 230% of the resting value, respectively. During the same experimental procedures with tensions of 10, 30 and 45% MGS, average heart rate increased to 105, 110 and 111% of the resting value. These results confirm that sympathetic outflow to a resting muscle is increased with elevation of tension in an active muscle. This process would promote perfusion pressure in the active muscle.

Identification of the pregnancy responsible for gestational trophoblastic disease by DNA analysis

In three cases of choriocarcinoma, genetic loci including a variable number of tandem repeat regions were amplified by the polymerase chain reaction method on DNA from three established cell lines and from lymphocytes of patients and their husbands to identify the responsible pregnancy. Case 1, from whom NaUCC-3 was derived, had only one full-term fetal death. Case 2, from whom NaUCC-4 was derived, had one normal delivery followed by one complete molar delivery and one normal delivery. Case 3, from whom NaUCC-2 was derived, had one normal delivery followed by one complete molar delivery. In case 1, NaUCC-3 was found to be of parental origin and derived from the pregnancy with full-term fetal death. In cases 2 and 3, NaUCC-4 and NaUCC-2 were of probable androgenetic origin and were derived from the pregnancy with complete hydatidiform mole. We also conducted the restriction fragment length polymorphism method using case 1 samples, and it confirmed the results based on the polymerase chain reaction method product patterns. All nine cases of hydatidiform mole and three cases of invasive mole were of androgenetic origin. The polymerase chain reaction method thus makes it possible to identify easily the pregnancy responsible for choriocarcinoma using only a few specimens without isotopes.

Neural and humoral controlling mechanisms of cardiovascular functions in man under weightlessness simulated by water immersion.

To clarify how neural and humoral mechanisms operate to control cardiovascular functions in man under weightlessness, the response of sympathetic nerve activity was observed in healthy human subjects by means of microneurographic technique with the changes of several hemodynamic parameters and hormonal responses during thermoneural head-out water immersion. Muscle sympathetic nerve activity was markedly suppressed by head-out immersion, concomitantly with a reduction of the leg volume, an increase of the stroke volume and a reduction of total peripheral resistance. At the same time, plasma level of norepinephrine, vasopressive and antidiuretic hormones (ADH, aldosterone, renin activity, angiotensin I-II) were reduced, while vasodepressive and diuretic hormone (ANP) was markedly increased. The systemic blood pressure was maintained almost unchanged during head-out water immersion. The suppressive response of sympathetic nerve activity seemed to be age-dependent. This response was less prominent in the elderly than in young subjects. It is concluded that the suppressive response of muscle sympathetic activity plays an important role to maintain hemodynamic homeostasis under weightlessness to compensate for the cephalad fluid shift and the resultant increase of the stroke volume in cooperation with the hormonal responses.

Long-acting α1-adrenoceptive sympathomimetic agent suppresses sympathetic outflow to muscles in humans

To clarify the effect of a long-acting selective alpha 1-adrenoceptive sympathomimetic agent on sympathetic outflow to muscles (muscle sympathetic activity, MSA) in humans, 5 mg of midodrine hydrochloride was injected intravenously in ten healthy male subjects aged 19-25. Spontaneous MSA was significantly suppressed as well as plasma norepinephrine level without changing heart rate, or systemic blood pressure. The suppression continued for more than 90 min, and this is supposed to be due to negative feedback action on the baroreflex loop. The absence of significant changes in heart rate or systemic blood pressure in healthy subjects suggests that hemodynamic homeostasis is maintained by reducing the MSA in response to a long-acting sympathomimetic agent, like midodrine hydrochloride, which constricts peripheral vessels directly.

Further Studies of the Polysaccharide of Klebsiella pneumoniae Possessing Strong Adjuvanticity: III. Augmentation of the Antibody Response to Subcutaneously Injected Sheep Red Blood Cells by the Adjuvant Polysaccharide

The adjuvant action of the O3 antigen of Klebsiella (KO3) on the antibody response to sheep red blood cells (SRBC) was elucidated by injecting both KO3 and SRBC subcutaneously at the right inguinal region of SMA mice. We demonstrated that KO3 exhibits a novel ability to augment anti‐SRBC plaque‐forming cell responses in both the local lymph node and the spleen at a relatively late stage of immunization. Escherichia coli lipopolysaccharide, dextran sulfate and concanavalin A showed such an action only minimally. In parallel with the development of the adjuvant action, KO3 definitely activated B cells in the local lymph node polyclonally for either IgM or IgG synthesis, suggesting that the mechanism of the adjuvant action includes direct stimulation of B cells by KO3 at the late stage. Neither increase in trapping of lymphocytes in the local lymph node nor change in tissue distribution of antigen was shown to be primarily involved in the mechanism of the adjuvant action.

Methotrexate-induced resistance to dactinomycin in choriocarcinoma

NaUCC‐2, a choriocarcinoma cell line, was derived from a patient who had a very poor clinical response to combination chemotherapy. Methotrexate (MTX) might have inhibited the antitumor effect of dactinomycin. To investigate this point, in vitro studies were performed to determine the sensitivity and uptake of MTX and dactinomycin (administered individually and in combination) to NaUCC‐2 and three other choriocarcinoma cell lines. Dihydrofolate reductase (DHFR) concentrations were studied as well. Although NaUCC‐2 showed sensitivity to MTX and dactinomycin, which were comparable to the other cell lines when they were given separately, NaUCC‐2 was unique in that the combination of MTX and dactinomycin was less lethal than dactinomycin given by itself. The uptake of MTX in NaUCC‐2 was significantly higher than that in the other cell lines, and MTX also induced an increase in dactinomycin uptake in NaUCC‐2. There was no significant difference in DHFR activity. Although additional studies are necessary to determine the mechanism responsible for this effect, these findings suggest that a mechanism other than drug uptake or DHFR activity must play a role in the drug resistance for choriocarcinoma. These findings also suggest that the most commonly used combination chemotherapy for choriocarcinoma, dactinomycin and MTX, may not always be the best method.

Effect of L-threo-3,4-dihydroxyphenylserine on muscle sympathetic nerve activity in humans.

To clarify the effect of L-threo-3,4-dihydroxyphenylserine (L-threo-DOPS), a precursor of norepinephrine (NE), the effect of this drug on microneurographically recorded muscle sympathetic nerve activity (MSA) from the tibial nerve was analyzed in ten healthy male volunteers. A single dose of 600 mg of L-threo-DOPS was orally administered and the effect of this norepinephrine precursor on MSA at resting and at upright standing positions, as well as the MSA responsiveness to head-up tilting was examined by comparing the data obtained after administration of the drug with those obtained under control conditions. The plasma NE levels were determined in two subjects. The results were as follows: (1) resting MSA increased significantly 80 min after administration of L-threo-DOPS and was accompanied by an increase in plasma NE levels; (2) standing MSA when treated with the drug was not significantly different from values obtained under control conditions; and (3) MSA responsiveness to orthostasis was reduced after L-threo-DOPS. We conclude from the activation of MSA by L-threo-DOPS that this drug raised blood pressure not only through an increase in metabolized plasma NE levels, but also through the enhancement of MSA by activation of descending noradrenergic or adrenergic pathways proximal to the recording site of the sympathetic discharge.