Captopril blocks the cardiac actions of centrally administered angiotensin I in the trout Oncorhynchus mykiss

Central ventilatory and cardiovascular actions of angiotensin peptides in trout

In the brains of teleosts, angiotensin II (ANG II), one of the main effector peptides of the renin-angiotensin system, is implicated in various physiological functions notably body fluid and electrolyte homeostasis and cardiovascular regulation, but nothing is known regarding the potential action of ANG II and other angiotensin derivatives on ventilation. Consequently, the goal of the present study was to determine possible ventilatory and cardiovascular effects of intracerebroventricular injection of picomole doses (5-100 pmol) of trout [Asn(1)]-ANG II, [Asp(1)]-ANG II, ANG III, ANG IV, and ANG 1-7 into the third ventricle of unanesthetized trout. The central actions of these peptides were also compared with their ventilatory and cardiovascular actions when injected peripherally. Finally, we examined the presence of [Asn(1)]-ANG II, [Asp(1)]-ANG II, ANG III, and ANG IV in the brain and plasma using radioimmunoassay coupled with high-performance liquid chromatography. After intracerebroventricular injection, [Asn(1)]-ANG II and [Asp(1)]-ANG II two ANG IIs, elevated the total ventilation through a selective stimulatory action on the ventilation amplitude. However, the hyperventilatory effect of [Asn(1)]-ANG II was threefold higher than the effect of [Asp(1)]-ANG II at the 50-pmol dose. ANG III, ANG IV, and ANG 1-7 were without effect. In addition, ANG IIs and ANG III increased dorsal aortic blood pressure (P(DA)) and heart rate (HR). After intra-arterial injections, none of the ANG II peptides affected the ventilation but [Asn(1)]-ANG II, [Asp(1)]-ANG II, and ANG III elevated P(DA) (50 pmol: +80%, +58% and +48%, respectively) without significant decrease in HR. In brain tissue, comparable amounts of [Asn(1)]-ANG II and [Asp(1)]-ANG II were detected (ca. 40 fmol/mg brain tissue), but ANG III was not detected, and the amount of ANG IV was about eightfold lower than the content of the ANG IIs. In plasma, ANG IIs were also the major angiotensins (ca. 110 fmol/ml plasma), while significant but lower amounts of ANG III and ANG IV were present in plasma. In conclusion, our study suggests that the two ANG II isoforms produced within the brain may act as a neurotransmitter and/or neuromodulator to regulate the cardioventilatory functions in trout. In the periphery, two ANG IIs and their COOH-terminal peptides may act as a circulating hormone preferentially involved in cardiovascular regulations.

Brain neuropeptides in central ventilatory and cardiovascular regulation in trout

Many neuropeptides and their G-protein coupled receptors (GPCRs) are present within the brain area involved in ventilatory and cardiovascular regulation but only a few mammalian studies have focused on the integrative physiological actions of neuropeptides on these vital cardio-respiratory regulations. Because both the central neuroanatomical substrates that govern motor ventilatory and cardiovascular output and the primary sequence of regulatory peptides and their receptors have been mostly conserved through evolution, we have developed a trout model to study the central action of native neuropeptides on cardio-ventilatory regulation. In the present review, we summarize the most recent results obtained using this non-mammalian model with a focus on PACAP, VIP, tachykinins, CRF, urotensin-1, CGRP, angiotensin-related peptides, urotensin-II, NPY, and PYY. We propose hypotheses regarding the physiological relevance of the results obtained.

Central cardiovascular actions of angiotensin II in trout

In mammals, a large body of evidence supports the existence of a brain renin-angiotensin system (RAS) acting independently or synergistically with the endocrine RAS to maintain diverse physiological functions, notably cardiovascular homeostasis. The RAS is of ancient origin and although most components of the RAS are present within the brain of teleost fishes, little is known regarding the central physiological actions of the RAS in these vertebrates. The present review encompasses the most relevant functional data for a role of the brain RAS in cardiovascular regulations in our experimental animal model, the unanesthetized trout Oncorhynchus mykiss. This paper mainly focuses on the central effect of angiotensin II (ANG II) on heart rate, blood pressure, heart rate variability and cardiac baroreflex, after intracerebroventricular injection or local microinjection of the peptide within the dorsal vagal motor nucleus. The probable implications of the parasympathetic nervous system in ANG II-evoked changes in the cardiac responses are also discussed.

Hepatic conversion of angiotensin I and the portal hypertensive response to angiotensin II in normal and regenerating liver

BACKGROUND AND AIM

Angiotensin I (AI) and angiotensin II (AII) induce a portal hypertensive response (PHR) and the liver is able to convert AI into AII to trough the action of the angiotensin-converting enzyme (ACE). Our purpose was to characterize angiotensin I liver conversion.

METHODS

AI, AII or angiotensin (1-7) were used in monovascular or bivascular perfusions.

RESULTS

The maximum gain in portal pressure induced by AII took place significantly earlier (P = 0.031) than that occurring after an equimolar AI infusion. The AI-induced PHR was abolished both by captopril or losartan, whereas the AII-induced PHR was not affected by captopril, but was abolished by losartan. Angiotensin (1-7) has no hemodynamic effect in the perfused liver. After partial hepatectomy, the AII-PHR pattern changes from a rapid return to baseline values to a pattern where there was no return to baseline values (3-7 days ex-surgery). In the bivascular perfusion system when AII was infused in the arterial branch in the retrograde mode of perfusion (peptide available only to the periportal zone), the PHR was at least 50% of that obtained when the prograde mode was used (peptide available to the periportal and perivenous zones).

CONCLUSION

AI does not induce PHR; this effect is a result of its mandatory conversion into AII by the ACE and the sequential action of AII on the AII receptor type 1 located in the hepatic periportal zone. AII induced PHR pattern changes during liver regeneration.

Central actions of angiotensin II on spontaneous baroreflex sensitivity in the trout Onc orhynchus mykiss

The goal of the present study was to investigate the central action of native angiotensin II (ANG II) on the spontaneous baroreflex sensitivity (BRS) in unanesthetized trout. The animals were equipped with two subcutaneous electrocardiographic (ECG) electrodes, a dorsal aorta catheter and an intracerebroventricular (ICV) cannula which was inserted within the third ventricle of the brain. The ECG and the systolic blood pressure (SBP) signals were recorded during a pre-injection period of 5 min and during five post-injection periods of 5 min. All injections were made at the fifth minute of the test. The time-series were processed with a sequence technique in order to detect the sequences of three or more consecutive increases in the SBP pulse, or three or more decreases in the SBP pulse correlated respectively with one delay beat increase of the RR interval of the ECG signal or shortening of this interval. The slope of the average regression line between the SBP and the RR intervals for each type of sequence was taken as a measure of the spontaneous BRS. Compared with pre-injection values, the ICV injection of vehicle (0.5 microl) had no effect on heart rate (HR), SBP, the total number of positive or negative sequences or on the spontaneous BRS during the post-injection periods. By contrast, ANG II at doses of 5 and 50 pmol increased HR but only 50 pmol ANG II elevated SBP. For all doses, ANG II depressed the spontaneous BRS, but the peptide had no effect upon the number of each baroreflex sequences. Intra-arterial injections of atropine dramatically reduced the number of positive and negative baroreflex sequences and decreased the sensitivity of the few remaining sequences, suggesting that the autonomic control of the cardiac BRS was solely due to vagal parasympathetic control. In atropinized trout the ICV injection of 5 pmol ANG II had no effect upon HR, SBP and the baroreflex parameters. This study determines for the first time the spontaneous BRS in a non-mammalian species and demonstrates an inhibitory action of ICV injection of ANG II upon this variable through a probable control of the vagal parasympathetic activity.

Positive feedback of hepatic angiotensinogen expression in silver sea bream (Sparus sarba)

The renin-angiotensin system (RAS) is involved in the maintenance of fluid homeostasis in vertebrates. Production of the precursor protein, angiotensinogen, is regulated by other components within the RAS. Angiotensin II (Ang II) stimulates the production and secretion of angiotensinogen in many mammalian models. However, the existence of a similar positive feedback mechanism for angiotensinogen has not been demonstrated for any non-mammalian species. In the present study, we have cloned the angiotensinogen for silver sea bream (Sparus sarba) and investigated the role of Ang II on angiotensinogen expression. The nucleotide sequence of angiotensinogen for S. sarba only exhibits a fair resemblance to other fish angiotensinogens and shows 76.6% similarity to that of Takifugu rubripes and 57.2% similarity to that of Danio rerio. Angiotensinogen transcripts have been identified in the brain, liver, kidney, and various parts of the intestine of sea bream, an observation, which probably implies the presence of a local RAS at the tissue level. The liver is probably the major source of angiotensinogen, as it exhibits the highest angiotensinogen transcript abundance among different tissues. Differential angiotensinogen expression was found among different regions of the intestine where the pyloric caeca exhibits the highest expression. Putative Ang I is identified at the N-terminal of the deduced protein with a novel sequence [Asn1, Ile5, His9]-Ang I. Hepatic angiotensinogen expression in sea bream adapted to different salinities remained constant and this is probably due to desensitization of the angiotensin receptors by angiotensin. A positive feedback mechanism of angiotensinogen by Ang II has been demonstrated as exogenous Ang II increased the amount of angiotensinogen transcript in isolated hepatocytes in vitro. Blockade of endogenous RAS by the angiotensin converting enzyme (ACE) inhibitor, captopril, significantly lowered the hepatic expression of angiotensinogen in vivo. The effect of Ang II stimulation on angiotensinogen expression is more potent in fish than that in mammals. These data suggest that the positive feedback mechanism of angiotensinogen by Ang II has already evolved in teleosts and such mechanism may be involved in the maintenance of angiotensinogen secretion under resting and hypertensive conditions.

A new method in applying power spectral statistics to examine cardio-respiratory interactions in fish

Power spectral analysis (PSA) provides a powerful tool for determining frequency oscillations in time signals, and it is accepted that mammals can show distinct components in the heart rate (fH) spectrum that are synchronous with ventilatory frequency (fV). Using similar signal processing techniques, these fundamental components at fV are not apparent in the spectrum calculated from fish fH. Here we compare conventional PSA on the R-R interval tachogram generated from ECG traces recorded in rats and fish, with PSA on the raw ECG waveform. The rat R-R tachogram showed a defined sigmoidal component, whereas the fish R-R tachogram was a more chaotic waveform. In agreement with the literature, PSA of these respective waveforms produced a component at the same frequency as ventilation in the rat, but of lower frequency than ventilation for the fish. Applying PSA to the rat ECG produced a spectrum with a fundamental component of similar frequency to that observed in the R-R tachogram spectrum, indicating that the latter adequately contained heart rate variability (HRV) oscillations. However, PSA of the ECG in fish contrasted with that from the R-R tachogram, with components observed in the latter spectrum being absent from the former. This suggests that the frequency components determined by PSA on the fish R-R tachogram were not true components, but were aliased (or folded-back) from higher up in the spectrum. Using established aliasing equations, recalculation of these peaks showed that their true frequency was similar to that of the ventilatory frequency for individual fish. The extent of cardio-respiratory interaction, resulting in fV < f(H/2) in rats but fV > f(H/2) in fish, is suggested to be the origin of the differences observed.