Dabertrand F, Porte Y, Macrez N, Morel JL. Spaceflight regulates ryanodine receptor subtype 1 in portal vein myocytes in the opposite way of hypertension. Gravity has a structural role for living systems. Tissue development, architecture, and organization are modified when the gravity vector is changed. In particular, microgravity induces a redistribution of blood volume and thus pressure in the astronaut body, abolishing an upright blood pressure gradient, inducing orthostatic hypotension. The

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... present study was designed to investigate whether isolated vascular smooth muscle cells are directly sensitive to altered gravitational forces and, second, whether sustained blood pressure changes act on the same molecular target. Exposure to microgravity during 8 days in the International Space Station induced the decrease of ryanodine receptor subtype 1 expression in primary cultured myocytes from rat hepatic portal vein. Identical results were found in portal vein from mice exposed to microgravity during an 8-day shuttle spaceflight. To evaluate the functional consequences of this physiological adaptation, we have compared evoked calcium signals obtained in myocytes from hindlimb unloaded rats, in which the shift of blood pressure mimics the one produced by the microgravity, with those obtained in myocytes from rats injected with antisense oligonucleotide directed against ryanodine receptor subtype 1. In both conditions, calcium signals implicating calcium-induced calcium release were significantly decreased. In contrast, in spontaneous hypertensive rat, an increase in ryanodine receptor subtype 1 expression was observed as well as the calcium-induced calcium release mechanism. Taken together, our results shown that myocytes were directly sensitive to gravity level and that they adapt their calcium signaling pathways to pressure by the regulation of the ryanodine receptor subtype 1 expression. ryanodine receptor; calcium signaling; microgravity; hindlimb suspension; hypertension DURING SPACEFLIGHT, THE MICROGRAVITY (G) induces blood volume redistribution and concomitant modification of the upright blood pressure gradient in the astronaut body from 70 mmHg in the head and 200 mmHg in the lowest resistance arteries to a constant pressure near 100 mmHg in the whole body (23). Thus a decrease in blood pressure below the heart seems to be one of the first consequences of G, which can precede and trigger the adaptation of the vascular system (56). Many factors known to regulate blood pressure likely coexist in the vascular adaptation to G: 1) the control of fluid volume and associated decrease in plasma volume (45), 2) the cardiac hypofunction and the alteration of baro-and cardiopulmonary reflexes (24), and 3) the adaptation of vascular responses at endothelial and vascular smooth muscle cell (VSMC) levels. Indeed, simulated G, using the hindlimb unloaded (HU) rat model, can modify the vascular reactivity in infero-posterior vessels like abdominal aorta, mesenteric, femoral arteries, and portal vein (8, 20, 40, 47) . In the animal model of hypertension, a dysregulation of vasoreactivity encoded by Ca 2ϩ signals has been reported. Indeed, the contraction induced by depolarization is greater in spontaneously hypertensive rats (SHR) than in control rats, and a change in Ca 2ϩ signaling pathways implicating intracellular Ca 2ϩ stores has been evidenced (1, 29, 30). More recently, a global adaptation of Ca 2ϩ signaling pathways was suggested in this hypertensive model (51) . The vascular adaptation to changes in blood pressure is due to smooth muscle contractile activity and its regulation by endothelium, hormones, and neurotransmitters (13). In fact, the myogenic response can be considered as the equilibrium between contraction and relaxation pathways. Both pathways involve Ca 2ϩ signaling. In VSMC, ryanodine receptors (RyRs) are reticulum Ca 2ϩ channels likely implicated in both relaxation and contraction processes (5). Indeed, RyRs can be activated via the Ca 2ϩ -induced Ca 2ϩ mechanism (CICR) that amplifies InsP3R and Ca 2ϩ entry primary signals and thus participates to the cytoplasmic Ca 2ϩ increase that can trigger contraction (22, 27) . However, RyRs produce localized Ca 2ϩ events (Ca 2ϩ sparks), which activate Ca 2ϩ -activated K ϩ currents leading to membrane hyperpolarization and thus relaxation through inhibition of L-type channels (28). Three RyR genes have been described, and, although their expression pattern in VSMC varies between species and studies, the common expression of the three subtypes (RyR1-3) has been reported in rat aorta, superior and small mesenteric arteries, and hepatic portal vein (9, 44). Each RyR subtype had a specific function in Ca 2ϩ signals. In smooth muscle, it has been shown that both RyR1 and RyR2 are able to generate Ca 2ϩ sparks and Ca 2ϩ waves (9). RyR expression is modulated in physiopathological conditions like muscular dystrophy and after blood redistribution following a chronic position change as hindlimb unloading (8, 10, 39, 40) . We designed the present study to investigate the effect of real G on RyR expression in portal veins from mice and on rat primary cultured VSMC exposed to G. To evaluate the functional effect of G exposure, we have measured Ca 2ϩ signals observed in HU rat model. Second, we have evaluated whether the regulation observed in HU (hypotension model) could be the opposite of those observed in SHR (hypertension model). In portal vein, the expression pattern of RyR1 was decreased by exposure to G in both cultured VSMC and mouse portal vein as well as in portal vein from HU rat. We mimicked altered Ca 2ϩ signals measured in portal vein from HU rats by in vivo injections of antisense oligonucleotide Address for reprint requests and other correspondence: J. L.