Vasoactive Agents and Blood Pressure Regulation in Sequential Ultrafiltration and Hemodialysis

Hypotension is a common and sometimes dangerous side effect of hemodialysis. Its etiology is multifactorial and largely unknown. Earlier studies on the role of endogenous blood pressure regulating agents such as catecholamines and renin have rendered conflicting results. We studied the influence of ultrafiltration and isovolemic hemodialysis separately on the plasma concentrations of the following blood pressure regulating agents: adrenaline, noradrenaline, dopamine, neuropeptide Y, calcitonin gene-related peptide (CGRP), renin (PRA), angiotensin II, vasopressin, aldosterone and Cortisol. During isolated ultrafiltration, plasma levels of two strong vasoconstrictors (noradrenaline and angiotensin II) and one strong vasodilator (calcitonin gene-related peptide, CGRP) increased significantly (noradrenaline 3.24 ± 0.60 nM to 4.31 ± 0.55 nM; p = 0.032, angiotensin II 19.74 ± 3.46 pmol/l to 28.49 ± 7.24 pmol/l; p= 0.047) No symptomatic hypotension occurred. At the end of isovolemic hemodialysis, plasma levels of all the vasoconstricting agents had decreased to pretreatment values, but those of CGRP had continued to rise (from 85.3 ± 17.6 pmol/l to 114.5 ± 25.3 pmol/l, p=0.031). During isovolemic hemodialysis, blood pressure fell to symptomatic levels, but was restored at the end of treatment. The study shows that hemodialysis patients respond to fluid removal by ultrafiltration with an increase in plasma levels of CGRP, noradrenaline and angiotensin II. The net effect is an appropriate vasoconstriction and adequate blood pressure is maintained during isolated ultrafiltration. During hemodialysis, the patients’ blood volumes remained unchanged as evident by a stable hematocrit, the vasoconstrictor hormones returned to normal levels, but in several patients there was a continuous release of vasodilating CGRP. Hypotension, then occurred in these patients due to the imbalance between vasoconstricting and vasodilating factors.

Atrial natriuretic peptide and circadian blood pressure regulation: clues from a chronobiological approach

A critical review of the data available in the literature today permits a better understanding of the multiple actions of atrial natriuretic peptide (ANP) on the cardiovascular system. Moreover, the results of chronobiological studies suggest a role for this peptide in the determination of the circadian rhythm of blood pressure (BP). ANP can affect BP by several mechanisms, including modification of renal function and vascular tone, counteraction of the renin-angiotensin-aldosterone system, and action on brain regulatory sites. A series of interrelated events may follow from very small changes in the plasma levels of ANP. The endpoints are blood volume and BP reduction, but they are rapidly offset (mainly by reactive sympathetic activation) as soon as blood volume or pressure is threatened. The circadian rhythms of BP and ANP are antiphasic under normal conditions and in essential hypertension. The loss in the nocturnal decrease of BP is accompanied by a comparable loss in the nocturnal surge of ANP in hypertensive renal failure and hypotensive heart failure. In the latter condition, BP and ANP variabilities correlate significantly both before and after therapy-induced functional recovery, independently of the mean BP levels. Autonomic function modulates the secretion of ANP, which seems more apt to determine only transient changes in BP levels, as suggested by the short half-life of the peptide and the buffering role of its clearance receptors. There is now sufficient evidence that ANP contributes to short-term control over BP and electrolyte balance, in contrast and in opposition to the renin-angiotensin-aldosterone system, which is involved primarily in long-term BP control. By interfering with other well-established neurohormonal factors, ANP appears to be an additional modulator of the circadian rhythm of BP.