Up-regulation of the adrenomedullin system mediates hypotension and hypoaldosteronism induced by simulated microgravity

We recently demonstrated that prolonged simulated microgravity (SMG) induced hypotension and hypoaldosteronism in rats, and gathered preliminary evidence for an involvement of circulating adrenomedullin (AM). Thus, we aimed to investigate whether short-term SMG elicits the same effects, and whether up-regulation of adrenal AM system plays a relevant role. Rats were exposed for 8 days to SMG in the form of hindlimb unweighting, and then, along with control animals, were given an intraperitoneal injection of AM22-52 and/or angiotensin-II (Ang-II) (100 nmoles/kg) or the saline vehicle. Systolic blood pressure (SBP) was measured by tail-cuff sphygmomanometry. The adrenal expression of AM was assayed by semiquantitative RT-PCR. The plasma concentrations of aldosterone (PAC) and AM, and adrenal AM content were measured by RIA. Short-term SMG induced significant decreases in SBP and PAC. Conversely, both the plasma and adrenal levels of AM, and adrenal AM mRNA were enhanced in SMG-exposed animals. The SMG-induced hypotension and hypoaldosteronism were reversed by AM22-52, an AM-receptor antagonist, thereby demonstrating a causal link between these effects and the up-regulation of AM system. SMG hampered SBP and PAC responses to Ang-II; the co-administration of AM22-52 restored these responses. These findings accord well with the known ability of AM to counteract the effects of Ang-II on both blood vessels and adrenocortical cells. Taken together, our findings allow us to conclude that up-regulation of the adrenal AM system i) occurs early and takes part in the adaptative changes occurring during SMG conditions; and ii) may account for both hypotension and hypoaldosteronism on returning to the normogravitational environment.