Diuresis from left atrial receptors: effect of plasma on the secretion of the Malpighian tubules of Rhodnius prolixus

The peptide neuromedin U (NMU) is elevated in NYHA II and III heart failure

AIMS

Currently, there is no reliable biomarker to detect pre-heart failure in humans. An early risk signal is an elevated left atrial pressure (LAP) and preliminary results from animal studies strongly suggest the neuropeptide neuromedin U (NMU) is released in response to this increase in LAP. However, it is unknown whether NMU is elevated in patients with heart failure. Therefore, the aim of this study was to assess if NMU levels are elevated in human cases of heart failure.

METHODS AND RESULTS

Twenty-four serum samples were obtained from patients in stage II and III heart failure from the Royal Papworth Hospital in Cambridge UK and tested using a selective NMU-ELISA; the data were compared with serum obtained commercially from self-declared healthy donors. NMU concentrations in serum from heart failure patients were significantly higher (P = 0.0007; unpaired Student's t-test) than control, 8.48 ± 0.67 ng/mL (mean ± SEM) versus 5.43 ± 0.46 ng/mL. There was no significant difference between NYHA stage II and III patients (P = 0.85, unpaired Student's t-test), which were 8.33 ± 0.89 ng/mL (n = 9) and 8.6 ± 0.95 ng/mL (n = 15), respectively. Only mean right atrial pressure was found to have a significant correlation with serum NMU (R = 0.81, P < 0.00001; regression analysis).

CONCLUSIONS

NMU is elevated in serum from stage II and III heart failure patients, supporting data from our pre-heart failure animal model; however, further study is needed to determine whether NMU is a reliable biomarker for pre-heart failure.

Trypanosoma cruzi gene expression in response to gamma radiation

Trypanosoma cruzi is an organism highly resistant to ionizing radiation. Following a dose of 500 Gy of gamma radiation, the fragmented genomic DNA is gradually reconstructed and the pattern of chromosomal bands is restored in less than 48 hours. Cell growth arrests after irradiation but, while DNA is completely fragmented, RNA maintains its integrity. In this work we compared the transcriptional profiles of irradiated and non-irradiated epimastigotes at different time points after irradiation using microarray. In total, 273 genes were differentially expressed; from these, 160 were up-regulated and 113 down-regulated. We found that genes with predicted functions are the most prevalent in the down-regulated gene category. Translation and protein metabolic processes, as well as generation of precursor of metabolites and energy pathways were affected. In contrast, the up-regulated category was mainly composed of obsolete sequences (which included some genes of the kinetoplast DNA), genes coding for hypothetical proteins, and Retrotransposon Hot Spot genes. Finally, the tyrosyl-DNA phosphodiesterase 1, a gene involved in double-strand DNA break repair process, was up-regulated. Our study demonstrated the peculiar response to ionizing radiation, raising questions about how this organism changes its gene expression to manage such a harmful stress.

The effect of stimulation of the atrial receptors on plasma renin activity in the dog

1. To determine whether stimulation of atrial receptors reflexly results in a consistent change in plasma renin activity experiments were performed in dogs anaesthetized with alpha-chloralose. Either a large balloon in the lumen of the left atrium or small balloons positioned at the upper and middle pulmonary vein-atrial junctions and atrial appendage were distended. Plasma samples obtained from these dogs during control periods and during periods of stimulation were assayed for plasma renin activity. 2. In the first series of experiments, stimulation of left atrial receptors by either large or small balloon distension was shown to result in an increase in urine flow and a decrease in plasma renin activity. 3. In a second series of experiments, cooling of the cervical vagosympathetic nerve trunks abolished the response of an increase to balloon distension in urine flow and decrease in plasma renin activity. 4. It was shown that a decrease in plasma renin activity resulted in response to stimulation of atrial receptors, and that this decrease is reflex in nature and is likely to involve only atrial receptors discharging into myelinated vagal fibres.

Increase in atrial pressure releases atrial natriuretic peptide from isolated perfused rat hearts

To elucidate the mechanism involved in the release of atrial natriuretic peptide, we modified the isolated perfused rat heart preparation to permit a step-wise increase in right atrial tension. Perfusate was introduced into the right atrium through the superior vena cava and was collected via the pulmonary artery. Right atrial pressure was manipulated by changing the perfusion rate. Perfusate from the pulmonary artery was collected in 1-min-fractions, extracted, and assayed for atrial natriuretic peptide like immunoreactivity (ANP-li). The basal rate of ANP-li release at an atrial pressure of 1.41 +/- 0.31 mm Hg was 964 +/- 144 pg/min (n = 11). As right atrial pressure was increased (range 0.4-4.5 mm Hg), a linear correlation (r = 0.85, P less than 0.001) was observed between the change in ANP-li release and the change in atrial pressure. High pressure liquid chromatography revealed that the major fraction in the perfusate had the same elution time than alpha-rANP. This peak fraction, as well as synthetic atriopeptin III, caused a dose-dependent relaxation in rat aortic strips that had been subjected to contraction with norepinephrine. Further, it corresponded exactly to the material we previously identified in rat plasma. These results suggest that atrial distension is involved in the release of ANP. In addition, ANP is released per se, as the active peptide.

Release of atriopeptin in the rat by vasoconstrictors or water immersion correlates with changes in right atrial pressure

Vasopressin, its 1-deamino analog (dAVP), angiotensin II, and phenylephrine, administered intravenously, increased plasma atriopeptin immunoreactivity in chloral hydrate-anesthetized rats. A continuous one hour infusion of either dAVP or phenylephrine caused a sustained elevation in: a) systemic blood pressure; b) right atrial pressure; c) left ventricular end diastolic pressure; and d) plasma atriopeptin immunoreactivity. While continuous infusion of angiotensin II also produced a sustained elevation in left ventricular end diastolic pressure, the changes in right atrial pressure and plasma atriopeptin were only transient. These data suggest that plasma atriopeptin most closely correlates with right atrial pressure. Consistent with this hypothesis, we found that the release of atriopeptin directly correlated with changes in right atrial pressure in anesthetized, water-immersed rats.

Atrial reflexes and renal function

Distension of small balloons in the venous-atrial junctions results in an increase in heart rate, urinary flow and sodium excretion. Two types of atrial receptors are described: one type, histologically known, discharging into myelinated fibers, and a second type, discharging into nonmyelinated ("C") fibers. These responses are mediated by the myelinated fibers. Experiments have shown that simulation of receptors discharging into the large myelinated vagal fibers is responsible for a reflex increase in heart rate mediated only by sympathetic nerves and for an increase in urinary flow. The efferent pathway of the diuretic response is shown to be nervous and hormonal. Stimulation of atrial receptors causes (1) a reduction of activity in nerves to the kidney, causing an increase in both urinary volume and sodium excretion, and (2) the release of a blood-borne agent, possibly diuretic, that increases urinary volume but does not affect sodium excretion.