Hindlimb unweighting induces changes in the RhoA-Rho-kinase pathway of the rat abdominal aorta

BKCa compensates impaired coronary vasoreactivity through RhoA/ROCK pathway in hind-limb unweighted rats

Previous studies have demonstrated cardiac and vascular remodeling induced by microgravity exposure. Yet, as the most important branch of vasculatures circulating the heart, the coronary artery has been seldomly studied about its adaptations under microgravity conditions. Large-conductance Ca²⁺-activated potassium channel (BK_Ca) and the Ras homolog family member A (RhoA)/Rho kinase (ROCK) pathway play key roles in control of vascular tone and mediation of microgravity-induced vascular adjustments. Therefore, we investigated the adaptation of coronary vasoreactivity to simulated microgravity and the role of BK_Ca and the RhoA/ROCK pathway in it. Four-week-old hind-limb unweighted (HU) rats were adopted to simulate effects of microgravity. Right coronary artery (RCA) constriction was measured by isometric force recording. The activity and expression of BK_Ca and the RhoA/ROCK pathway were examined by Western blot, patch-clamp recordings, and immunoprecipitation. We found HU significantly decreased RCA vasoconstriction to KCl, serotonin, and U-46619, but increased protein expression and current densities of BK_Ca, inhibition of which by iberiotoxin (IBTX) further decreased RCA vasoconstriction (P < 0.05). Expression of RhoA and ROCK as well as active RhoA and phosphorylation of myosin light chain (MLC) at Ser¹⁹ and MLC phosphatase target-1 at Thr⁶⁹⁶ were significantly increased by HU, and ROCK inhibitor Y-27632 exerted greater suppressing effect on HU RCA vasoconstriction than that of control (P < 0.05). BK_Ca opener NS1619 increased HU RCA vasoconstriction, which was blocked by both RhoA and ROCK inhibitor, similar to the effect of IBTX. These results indicate that HU impairs coronary vasoconstriction but enhances BK_Ca activity acting as a protective mechanism avoiding excessive decrease of coronary vasoreactivity through activation of the RhoA/ROCK pathway.-Wu, Y., Yue, Z., Wang, Q., Lv, Q., Liu, H., Bai, Y., Li, S., Xie, M., Bao, J., Ma, J., Zhu, X., Wang, Z. BK_Ca compensates impaired coronary vasoreactivity through RhoA/ROCK pathway in hind-limb unweighted rats.

Fluid shift versus body size: changes of hematological parameters and body fluid volume in hindlimb-unloaded mice, rats and rabbits

The cardiovascular system is adapted to gravity, and reactions to the loss of gravity in space are presumably dependent on body size. The dependence of hematological parameters and body fluid volume on simulated microgravity have never been studied as an allometric function before. Thus, we estimated red blood cell (RBC), blood and extracellular fluid volume in hindlimb-unloaded (HLU) or control (attached) mice, rats and rabbits. RBC decrease was found to be size independent, and the allometric dependency for RBC loss in HLU and control animals shared a common power (-0.054±0.008) but a different Y₀ coefficient (8.66±0.40 and 10.73±0.49, respectively, P<0.05). Blood volume in HLU animals was unchanged compared with that of controls, disregarding body size. The allometric dependency of interstitial fluid volume in HLU and control mice shared Y₀ (1.02±0.09) but had different powers N (0.708±0.017 and 0.648±0.016, respectively, P<0.05), indicating that the interstitial fluid volume increase during hindlimb unloading is more pronounced in larger animals. Our data underscore the importance of size-independent mechanisms of cardiovascular adaptation to weightlessness. Despite the fact that the use of mice hampers application of a straightforward translational approach, this species is useful for gravitational biology as a tool to investigate size-independent mechanisms of mammalian adaptation to microgravity.

Prolonged bed rest impairs rapid CPI-17 phosphorylation and contraction in rat mesenteric resistance arteries to cause orthostatic hypotension

Prolonged bed rest (PBR) causes orthostatic hypotension (OH). Rapid constriction of splanchnic resistance arteries in response to a sudden increase in sympathetic tone contributes to the recovery of orthostatic arterial pressure upon standing. However, the molecular mechanism of PBR-induced dysfunction in arterial constriction is not fully understood. Previously, we showed that CPI-17, a regulatory protein for myosin phosphatase, mediates α_1A-adrenergic receptor-induced rapid contraction of small mesenteric arteries. Here, we tested whether PBR associated with OH affects the α₁-adrenergic receptor-induced CPI-17 signaling pathway in mesenteric arteries using rats treated by head-down tail-suspension hindlimb unloading (HDU), an experimental OH model. In normal anesthetized rats, mean arterial pressure (MAP) rapidly reduced upon 90° head-up tilt from supine position and then immediately recovered without change in heart rate, suggesting a rapid arterial constriction. On the other hand, after a 4-week HDU treatment, the fast orthostatic MAP recovery failed for 1 min. Alpha1A subtype-specific antagonist suppressed the orthostatic MAP recovery with a small decrease in basal blood pressure, whereas non-specific α₁-antagonist prazosin strongly reduced both basal MAP and orthostatic recovery. The HDU treatment resulted in 68% reduction in contraction in parallel with 83% reduction in CPI-17 phosphorylation in denuded mesenteric arteries 10 s after α₁-agonist stimulation. The treatment with either Ca²⁺-release channel opener or PKC inhibitor mimicked the deficiency in HDU arteries. These results suggest that an impairment of the rapid PKC/CPI-17 signaling pathway downstream of α_1A-adrenoceptors in peripheral arterial constriction, as an end organ of orthostatic blood pressure reflex, is associated with OH in prolonged bed rest patients.

Effects of High-LET Radiation Exposure and Hindlimb Unloading on Skeletal Muscle Resistance Artery Vasomotor Properties and Cancellous Bone Microarchitecture in Mice

Weightlessness during spaceflight leads to functional changes in resistance arteries and loss of cancellous bone, which may be potentiated by radiation exposure. The purpose of this study was to assess the effects of hindlimb unloading (HU) and total-body irradiation (TBI) on the vasomotor responses of skeletal muscle arteries. Male C57BL/6 mice were assigned to control, HU (13-16 days), TBI (1 Gy (56)Fe, 600 MeV, 10 cGy/min) and HU-TBI groups. Gastrocnemius muscle feed arteries were isolated for in vitro study. Endothelium-dependent (acetylcholine) and -independent (Dea-NONOate) vasodilator and vasoconstrictor (KCl, phenylephrine and myogenic) responses were evaluated. Arterial endothelial nitric oxide synthase (eNOS), superoxide dismutase-1 (SOD-1) and xanthine oxidase (XO) protein content and tibial cancellous bone microarchitecture were quantified. Endothelium-dependent and -independent vasodilator responses were impaired in all groups relative to control, and acetylcholine-induced vasodilation was lower in the HU-TBI group relative to that in the HU and TBI groups. Reductions in endothelium-dependent vasodilation correlated with a lower cancellous bone volume fraction. Nitric oxide synthase inhibition abolished all group differences in endothelium-dependent vasodilation. HU and HU-TBI resulted in decreases in eNOS protein levels, while TBI and HU-TBI produced lower SOD-1 and higher XO protein content. Vasoconstrictor responses were not altered. Reductions in NO bioavailability (eNOS), lower anti-oxidant capacity (SOD-1) and higher pro-oxidant capacity (XO) may contribute to the deficits in NOS signaling in skeletal muscle resistance arteries. These findings suggest that the combination of insults experienced in spaceflight leads to impairment of vasodilator function in resistance arteries that is mediated through deficits in NOS signaling.

Differential constrictor responses of cephalic and caudal vasculature to α-adrenoceptor agonist after hind limb unloading

We examined the effects of hind limb unloading (HLU, 14 days) on constriction of carotid and iliac arterial beds in vivo in thiobutabarbital-anaesthetized rats and isolated carotid and iliac arteries in vitro. Both control and HLU rats had similar arterial pressure and carotid and iliac arterial flows. The HLU rats had increased carotid arterial but reduced iliac arterial constriction in response to methoxamine (α1-adrenoceptor agonist) in vivo. In contrast, constriction in response to methoxamine was reduced in the isolated carotid and unchanged in the iliac artery of HLU rats relative to control rats. Thus, HLU is associated with increased constriction of carotid arterial bed but reduced constriction of the isolated carotid artery, and reduced constriction of iliac arterial bed but unchanged constriction of the isolated iliac artery. These results show differential influence of HLU on constriction of cephalic and caudal arterial beds, and differential effect on constrictions of arterial beds relative to conduit arteries.

Hindlimb unweighting induces changes in the p38MAPK contractile pathway of the rat abdominal aorta

Hindlimb unweighting (HLU) of rats is a model used to mimic the cephalic fluid shift potentially involved in the orthostatic intolerance experienced by astronauts. Certain arteries in these rats exhibit a decreased contractile response to adrenergic agonists. It was shown previously that this may be caused by changes in thick filament regulation (Summers et al., Vascul Pharmacol 48: 208-214, 2008). In the present study, it was hypothesized that HLU also modifies thin filament regulation by effects on p38(MAPK) and ERK. Abdominal aorta rings from 20-day HLU rats and untreated controls were subjected to phenylephrine and phorbol 12,13-dibutyrate (PDBU) concentration response curves in the presence and absence of two inhibitors: the p38(MAPK) inhibitor SB-203580 and the MEK inhibitor U-0126. SB-203580 decreased control sensitivity to both agonists, but HLU sensitivity was not significantly affected. U-0126, which blocks enzymes immediately upstream of ERK, affected sensitivity to both agonists equally between control and HLU. Western blot analysis revealed no change in total levels of p38(MAPK) and its downstream target heat shock protein 27 but did reveal a decrease in phosphorylated levels of both after stimulation with PDBU and phenylephrine after HLU treatment. Neither total ERK nor phosphorylated levels after stimulation were affected by HLU. Total levels of caldesmon, a molecule downstream of both pathways, were decreased, but phosphorylated levels after stimulation were decreased by roughly twice as much. The results of this study demonstrate that HLU downregulates p38(MAPK), but not ERK, signaling. In turn, this may decrease actin availability for contraction.