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Diminished mesenteric vaso- and venoconstriction and elevated plasma ANP and BNP with simulated microgravity

¹Division of Exercise Physiology, Department of Physiology and Pharmacology, and the Center for Interdisciplinary Research in Cardiovascular Sciences, West Virginia University School of Medicine, Morgantown, West Virginia; ²Department of Applied Physiology and Kinesiology and Center for Exercise Science, University of Florida, Gainesville, Florida; ³Department of Medical Physiology, College of Medicine, Cardiovascular Research Division of Lymphatic Biology, Texas A&M University System Health Science Center, College Station, Texas; and ⁴Departments of Medicine and Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Hershey, Pennsylvania

Submitted 29 August 2006 ; accepted in final form 22 January 2008

Diminished constriction of arteries and veins following exposure to microgravity or bed rest is associated with a reduced ability to augment peripheral vascular resistance (PVR) and stroke volume during orthostasis. We tested the hypothesis that small mesenteric arteries and veins, which are not exposed to large pressure shifts during simulated microgravity via head-down tail suspension (HDT), will exhibit decrements in adrenergic constriction after HDT in rats. Small mesenteric arteries and veins from control (Con; n = 41) and HDT (n = 35) male Sprague-Dawley rats were studied in vitro. Vasoactive responsiveness to norepinephrine (NE) in arteries (10^–9 to 10^–4 M) and veins (pressure-diameter responses from 2 to 12 cmH₂O after incubation in 10^–6 or 10^–4 M NE) were evaluated. Plasma concentrations of atrial (ANP) and NH₂-terminal prohormone brain (NT-proBNP) natriuretic peptides were also measured. In mesenteric arteries, sensitivity and maximal responsiveness to NE were reduced with HDT. In mesenteric veins there was a diminished venoconstriction to NE at any given pressure in HDT. Plasma concentrations of both ANP and NT-proBNP were increased with HDT, and maximal arterial and venous constrictor responses to NE after incubation with 10^–7 M ANP or brain natriuretic peptide (BNP) were diminished. These data demonstrate that, in a vascular bed not subjected to large hydrodynamic differences with HDT, both small arteries and veins have a reduced responsiveness to adrenergic stimulation. Elevated levels of circulating ANP or NT-proBNP could adversely affect the ability of these vascular beds to constrict in vivo and conceivably could alter the intrinsic constrictor properties of these vessels with long-term exposure.

arteries; veins; hindlimb unloading

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