Influence of vasoconstrictor systems on leg vasodilation during heating of dehydrated baboons

Abstract

This study, carried out in two parts, sought to determine the importance of vasopressin (VP), the renin-angiotensin system (RAS), and the sympathetic nervous system in the dehydration-produced attenuation of hindlimb (cutaneous) vasodilation during environmental heating (EH). Baboons, chronically instrumented for blood sampling and for measurement of mean iliac blood flow (MIBF), arterial pressure, and core temperature (Tc), were subjected to EH while in euhydrated and dehydrated (64-72 h of water deprivation) states. EH consisted of exposure to an elevated ambient temperature (40-42 degrees C) until Tc reached 39.5 degrees C. In part I, indexes of the above vasoconstrictor systems were measured. Base-line plasma renin activity (PRA) and VP and norepinephrine concentrations were all significantly elevated by dehydration. In addition, the increase in PRA during EH was accentuated by dehydration. In part II, the effects of blockades of the RAS, the pressor action of VP, and the innervation of the hindlimb on hindlimb vasodilation during EH were assessed. None of these blockades, singly or together, reversed the dehydration-produced attenuation of the increase in MIBF during EH. Thus we conclude that other mechanisms are responsible for the dehydration-produced attenuation of cutaneous vasodilation in baboons during EH.

Acute inhibition of the hypothalamic paraventricular nucleus decreases renal sympathetic nerve activity and arterial blood pressure in water-deprived rats

“…Several lines of evidence suggest that water deprivation activates the sympathetic nervous system (14,40,49), but the neural circuits mediating this effect have not been examined previously. Because the PVN is one site in the central nervous system that plays an important role in regulating sympathetic activity and ABP during changes in P osmol , circulating ANG II levels, and blood volume (13,17,30,50), we hypothesized that acute inhibition of the PVN with bilateral microinjection of the GABA A receptor agonist muscimol would decrease RSNA and ABP in water-deprived rats.…”

“…Despite the contraction of blood volume, arterial blood pressure (ABP) is maintained within a normal range by both hormonal and neural factors. Indeed, studies have reported increased activity of the peripheral renin-angiotensin system (31, 43, 49), increased circulating levels of vasopressin (5, 46, 49), and increased sympathoadrenal activity (14,40,41,49) in waterdeprived rats. The latter is reflected by the increased levels of circulating catecholamines (49), tachycardia (14,40,41), and elevated sympathetic nerve activity when expressed as a percentage of the baroreflex maximum (40).…”

“…Indeed, studies have reported increased activity of the peripheral renin-angiotensin system (31,43,49), increased circulating levels of vasopressin (5,46,49), and increased sympathoadrenal activity (14,40,41,49) in waterdeprived rats. The latter is reflected by the increased levels of circulating catecholamines (49), tachycardia (14,40,41), and elevated sympathetic nerve activity when expressed as a percentage of the baroreflex maximum (40). Collectively, these observations suggest that sympathetic outflow is elevated in water-deprived rats; however, the neural circuits that underlie this response are not known.…”

Water deprivation activates a glutamatergic projection from the hypothalamic paraventricular nucleus to the rostral ventrolateral medulla

“…Activation of the sympathoadrenal system by water deprivation contributes to the maintenance of arterial blood pressure and is reflected by increased levels of circulating catecholamines (Thornton and Proppe, 1988;Kiss et al, 1994), elevated heart rate (Scrogin et al, 1999;Scrogin et al, 2002), increased lumbar sympathetic nerve activity (Scrogin et al, 1999), and an exaggerated depressor response to ganglionic blockade (Stocker et al, 2004b;. The elevated sympathetic tone caused by water deprivation depends upon neurons located within the hypothalamic paraventricular nucleus (PVH) (Stocker et al, 2004a, b;.…”

Water deprivation increases Fos immunoreactivity in PVN autonomic neurons with projections to the spinal cord and rostral ventrolateral medulla

“…In turn, this increases plasma vasopressin and ANG II levels and activates the sympathoadrenal system, thereby maintaining arterial blood pressure (ABP). The sympathoadrenal activation is reflected by increases in circulating catecholamines (50), elevated heart rate (HR) (17,40,41,44), and an apparent increase in lumbar sympathetic nerve activity (SNA) (40). Moreover, our laboratory recently reported that ganglionic blockade with hexamethonium produced a significantly greater fall in ABP of waterdeprived rats than control rats (44).…”