Yamasaki Masao

We investigated effect of microgravity environment during spaceflight on postnatal development of the rheological properties of the aorta in rats. The neonate rats were randomly divided at 7 days of age into the spaceflight, asynchronous ground control, and vivarium control groups (8 pups for one dam). The spaceflight group rats at 9 days of age were exposed to microgravity environment for 16 days. A longitudinal wall strip of the proximal descending thoracic aorta was subjected to stress-strain and stress-relaxation tests. Wall tensile force was significantly smaller in the spaceflight group than in the two control groups, whereas there were no significant differences in wall stress or incremental elastic modulus at each strain among the three groups. Wall thickness and number of smooth muscle fibers were significantly smaller in the spaceflight group than in the two control groups, but there were no significant differences in amounts of either the elastin or collagen fibers among the three groups. The decreased thickness was mainly caused by the decreased number of smooth muscle cells. Plastic deformation was observed only in the spaceflight group in the stress-strain test. A microgravity environment during spaceflight could affect postnatal development of the morphological and rheological properties of the aorta.

Although rats often show an upright standing behavior the cardiovascular response during the behavior has not yet been fully clarified. In this study we quantified the activity of upright standing behavior in rats using infrared beam detectors and measured cardiovascular variables during the behavior. Rats demonstrated a high level of upright standing activity as they showed the upright posture more than 500 times per day at 10 weeks of age. The average upright standing duration time was less than 10s. Arterial pressure slightly decreased while heart rate increased in response to the behavior and these responses were not affected by sino-aortic denervation. Our results indicate that other mechanisms such as the vestibulo-cardiovascular reflex may completely compensate the lack of the baroreceptor reflex to maintain cardiovascular homeostasis in response to acute positional changes in rats. Moreover rats demonstrate complex integrative mechanisms maintaining cardiovascular homeostasis against the upright standing behavior which frequently occurs in rats. (C) 2008 Elsevier Ireland Ltd. All rights reserved.

To investigate whether the rate of change in blood pressure affects the sensitivity of the aortic baroreceptor afferent response, the change in aortic nerve activity (ANA) to two different rates of ramp increase in mean blood pressure (MBP), elicited by phenylephrine administration, was determined in the rat under urethane (1.5 g kg(-1)) anesthesia. The sensitivity of the increase in ANA following a rapid (average ramp rate, 9.14 +/- 0.60 mmHg s(-1), n = 11) or gradual (1.78 +/- 0.24 mmHg s(-1), n = 11) increase in MBP was 2.03 +/- 0.14% and 1.81 +/- 0.20% of baseline mmHg(-1), respectively. These values were not significantly different from each other (P = 0.16). Furthermore, we found no correlation between the rate of ramp increase in MBP and the sensitivity of the increase in ANA (r = 0.24, P = 0.29, n = 22). These results suggest that, at least within the normal physiological range of MBP, the rate of the ramp change in blood pressure does not affect aortic baroreceptor afferent sensitivity in the anesthetized rat. (c) 2008 Elsevier Ireland Ltd. All rights reserved

To investigate postnatal developmental changes in functional characteristics of the afferent pathway of the aortic baroreceptor reflex, the responses of aortic nerve activity (ANA) to blood pressure (BP) changes elicited by phenylephrine and sodium nitroprusside administration were tested in 3-, 8- and 20-week-old male rats under chloralose (60 mg kg(-1) I.P.) and urethane (600 mg kg(-1) I.P.) anaesthesia. The function curve of ANA in response to BP changes showed a sigmoid shape that shifted to the right from 3 to 8 weeks of age. The maximal activity and maximal gain of the aortic nerve, which were calculated by a logistic function analysis, were significantly higher in 20-week-old rats (maximal activity, 532 +/- 47% of baseline; maximal gain, 7.9 +/- 0.8% of baseline mmHg(-1); n = 9) than in 3-week-old rats (maximal ANA, 268 +/- 25% of baseline, P < 0.001; maximal gain, 4.9 +/- 0.5% of baseline mmHg(-1), P < 0.01, n = 9) and 8-week-old rats (maximal ANA, 309 +/- 18% of baseline, P < 0.001; maximal gain, 4.9 +/- 0.3% of baseline mmHg(-1), P < 0.01, n = 11). These results suggest that the operating point of aortic baroreceptor afferents is reset to the higher pressure level during development from 3 to 8 weeks of age and, thereafter, the afferent gain increases from 8 to 20 weeks of age. This functional change may be an important factor to prevent an excess increase of BP, which would result in pathophysiological problems.