Mechanisms involved in angiotensin II induced increases in cardiac output in pithed rats

Chronotropic and vasoactive properties of the gut bacterial short-chain fatty acids in larval zebrafish

Short-chain fatty acids (SCFAs) produced by the gut bacteria have been associated with cardiovascular dysfunction in humans and rodents. However, studies exploring effects of SCFAs on cardiovascular parameters in the zebrafish, an increasingly popular model in cardiovascular research, remain limited. Here, we performed fecal bacterial 16S sequencing and gas chromatography/mass spectrometry (GC-MS) to determine the composition and abundance of gut microbiota and SCFAs in adult zebrafish. Following this, the acute effects of major SCFAs on heart rate and vascular tone were measured in anesthetized zebrafish larvae using fecal concentrations of butyrate, acetate, and propionate. Finally, we investigated if coincubation with butyrate may lessen the effects of angiotensin II (ANG II) and phenylephrine (PE) on vascular tone in anesthetized zebrafish larvae. We found that the abundance in Proteobacteria, Firmicutes, and Fusobacteria phyla in the adult zebrafish resembled those reported in rodents and humans. SCFA levels with highest concentration of acetate (27.43 µM), followed by butyrate (2.19 µM) and propionate (1.65 µM) were observed in the fecal samples of adult zebrafish. Immersion in butyrate and acetate produced a ∼20% decrease in heart rate (HR), respectively, with no observed effects of propionate. Butyrate alone also produced an ∼25% decrease in the cross-sectional width of the dorsal aorta (DA) at 60 min (*P < 0.05), suggesting compensatory vasoconstriction, with no effects of either acetate or propionate. In addition, butyrate significantly alleviated the decrease in DA cross-sectional width produced by both ANG II and PE. We demonstrate the potential for zebrafish in investigation of host-microbiota interactions in cardiovascular health.NEW & NOTEWORTHY We highlight the presence of a core gut microbiota and demonstrate in vivo short-chain fatty acid production in adult zebrafish. In addition, we show cardio-beneficial vasoactive and chronotropic properties of butyrate, and chronotropic properties of acetate in anesthetized zebrafish larvae.

Effect of angiotensin II on the left ventricular function in a near-term fetal sheep with metabolic acidemia

We tested the hypothesis that, in acute metabolic acidemia, the fetal left ventricle (LV) has the capacity to increase its contractility in response to angiotensin II infusion. Eleven ewes and their fetuses were instrumented at 127–138/145 days of gestation. The effect of angiotensin II on fetal LV function was assessed using intraventricular pressure catheter and tissue Doppler imaging (TDI). Angiotensin II increased fetal arterial blood pressure, whereas pH and pO₂ decreased. The heart rate and systemic venous pressure were not affected significantly. The LV end-diastolic and end-systolic pressures, as well as dP/dt_max, increased. The TDI-derived LV longitudinal myocardial isovolumic contraction velocity and its acceleration and velocity during early filling were higher than those at baseline. The incidence of absent isovolumic relaxation velocity was greater during angiotensin II infusion. In summary, during acute metabolic acidemia, the fetal left ventricle could increase its contractility in response to inotropic stimulus even in the presence of increased afterload. The diastolic LV function parameters were altered by angiotensin II.

Differential responses to systemic and local angiotensin II infusions in conscious postnatal sheep

Angiotensin II (ANG II) increases blood pressure (MAP) via specific ANG II receptors (AT) and is considered important in regulating MAP after birth. In adult animals, AT(1) receptors predominate in vascular smooth muscle (VSM) and mediate vasoconstriction. In newborn sheep, AT(2) receptors, which do not mediate vasoconstriction, predominate in vascular smooth muscle until 2 wk postnatal when they are replaced by AT(1). Thus, the mechanisms whereby ANG II increases MAP after birth are unclear. We examined the effects of ANG II on femoral vascular resistance (FmVR) and blood flow (FmBF) in serial studies of newborn sheep (n = 7) at 7-14 d, 15-21 d, and 22-35 d. Animals had femoral catheters implanted for systemic ANG II infusions and cardiovascular monitoring, and a flow probe was implanted on the contralateral artery proximal to the superficial saphenous artery, which contained a catheter for intra-arterial ANG II infusions. Studies were performed using a range of systemic and intra-arterial ANG II doses. Systemic ANG II increased MAP dose-dependently at all ages (p < 0.001); however, responses were not age dependent. FmBF rose dose dependently at 7-14 d (p < 0.001) and was unchanged at older ages. FmVR was unaffected at 7-14 d, but values increased dose dependently at 15-21 d and 22-3 5d (p < 0.001), although never exceeded relative increases in MAP. Local ANG II did not alter MAP, FmBF, or FmVR at any age. Although systemic ANG II increases MAP and FmVR dose dependently after birth, ANG II-induced vasoconstriction is attenuated. Furthermore, intra-arterial ANG II does not alter FmVR in the absence of systemic responses, suggesting incomplete vascular smooth muscle AT(1) expression, stimulation of local ANG II antagonists, or ANG II-mediated release of another vasoconstrictor.

Effects of angiotensin II on myocardial contractility during short-term pressor responses to angiotensin II

BACKGROUND

Cardiac angiotensin AT1 receptors have been found in several animal species. In-vitro studies performed on cardiac preparations have shown that angiotensin II (ANG II) exerts a positive inotropic effect; however, in-vivo results have allowed no definitive conclusion to be drawn. The reasons behind these controversial results remain unknown, and could originate both from different experimental conditions and from the techniques used to assess myocardial contractility.

OBJECTIVE

To investigate, by means of echocardiographic measurements, whether ANG II, administered to intact and to sinoaortic denervated isoflurane-anesthetized rabbits, was able to directly increase myocardial contractility.

METHODS

The effect of ANG II on cardiac contractility was assessed with the use of simultaneous pressure measurements and Doppler-echocardiographic recordings. Specifically, we used both the relationship between the left ventricular end-systolic wall stress and the velocity of heart-rate-corrected circumferential fiber shortening (VCFC) and the maximum rate of rise of the ventricular pressure as indices of changes in myocardial contractility. Cardiac contractility was evaluated both in intact and in chronically sinoaortic denervated isoflurane-anesthetized rabbits under basal conditions and after ANG II infusion (50 ng/kg per min).

RESULTS

After ANG II infusion, increases in mean arterial blood pressure, left ventricular end-diastolic diameter and pressure were observed both in intact and in chronically sinoaortic denervated rabbits. The left ventricular end-systolic wall stress (a function of the mean arterial pressure and chamber size) and the maximum rate of rise of the ventricular pressure rose markedly in rabbits of both groups, whereas the VCFC decreased significantly. However, when compared the VCFC under ANG II infusion with that calculated at the same level of left ventricular afterload under basal conditions, we observed that ANG II infusion induced no significant change in VCFC either in intact of in chronically sinoaortic denervated rabbits.

CONCLUSION

Our results indicate that administration of ANG II to isoflurane-anesthetized rabbits induces a marked rise in ventricular pre- and after-loads and exerts no significant effect on the cardiac contractility. In light of this, it is reasonable to assume that the short-term increase in arterial blood pressure can be ascribed mainly to the increase in peripheral arterial resistance.