Betamethasone-exposed preterm birth does not impair insulin action in adult sheep

Preterm birth is associated with increased risk of type 2 diabetes (T2D) in adulthood; however, the underlying mechanisms are poorly understood. We therefore investigated the effect of preterm birth at ~0.9 of term after antenatal maternal betamethasone on insulin sensitivity, secretion and key determinants in adulthood, in a clinically relevant animal model. Glucose tolerance and insulin secretion (intravenous glucose tolerance test) and whole-body insulin sensitivity (hyperinsulinaemic euglycaemic clamp) were measured and tissue collected in young adult sheep (14 months old) after epostane-induced preterm (9M, 7F) or term delivery (11M, 6F). Glucose tolerance and disposition, insulin secretion, β-cell mass and insulin sensitivity did not differ between term and preterm sheep. Hepatic PRKAG2 expression was greater in preterm than in term males (P = 0.028), but did not differ between preterm and term females. In skeletal muscle, SLC2A4 (P = 0.019), PRKAA2 (P = 0.021) and PRKAG2 (P = 0.049) expression was greater in preterm than in term overall and in males, while INSR (P = 0.047) and AKT2 (P = 0.043) expression was greater in preterm than in term males only. Hepatic PRKAG2 expression correlated positively with whole-body insulin sensitivity in males only. Thus, preterm birth at 0.9 of term after betamethasone does not impair insulin sensitivity or secretion in adult sheep, and has sex-specific effects on gene expression of the insulin signalling pathway. Hence, the increased risk of T2D in preterm humans may be due to factors that initiate preterm delivery or in early neonatal exposures, rather than preterm birth per se.

Erythropoietin ameliorates damage to the placenta and fetal liver induced by exposure to lipopolysaccharide

Intrauterine infection and inflammation have been causally linked to preterm birth and fetal brain injury. Using an ovine model of endotoxin-induced brain injury we have recently shown that recombinant human erythropoietin (rhEPO) reduces brain injury and protects against damage to myelination in major myelinated axon tracts. Our present objective was to determine whether rhEPO is also protective of the placenta and the fetal liver, organs which could influence fetal well-being. At 107 +/- 1 days of gestational age (DGA) chronically catheterized fetal sheep were randomly assigned to receive, on 3 consecutive days, either: 1) an i.v. bolus dose of lipopolysaccharide (LPS; approximately 0.9 microg/kg; n = 8); 2) i.v. bolus dose of LPS, followed at 1 h by 5000 IU/kg of rhEPO (LPS + rhEPO, n = 8); 3) rhEPO (n = 3). Seven untreated fetuses served as controls (n = 7). The placenta and fetal liver were examined histologically at 116 +/- 1 DGA; a placental injury index was formulated comprising measures of placental area, apoptosis, tissue injury and the size of the intervillous space. In LPS-exposed fetuses this index was greater than in control or rhEPO alone fetuses (p < 0.02). Treatment of LPS-exposed fetuses with rhEPO resulted in a reduction in the index (p < 0.05) and in the extent of liver necrosis. We conclude that rhEPO offers protection to the placenta and fetal liver in the presence of acute inflammation.

Right atrial stretch activates neurons in autonomic brain regions that project to the rostral ventrolateral medulla in the rat

Activation of the cardiac mechanoreceptors results in changes in sympathetic nerve activity and plays an important role in the responses elicited by elevated blood volume. Stimulation of the reflex influences several key autonomic regions, namely the paraventricular nucleus (PVN), the nucleus of the tractus solitarius (NTS) and the caudal ventrolateral medulla (CVLM). Neurons in these regions project directly to the rostral ventrolateral medulla (RVLM), a critical region in the generation of sympathetic vasomotor tone. The aim of the present experiments was to determine whether neurons in the PVN, NTS and CVLM that are activated by cardiac mechanoreceptor stimulation also project to the RVLM. Animals were prepared, under general anesthesia, by microinjection of a retrogradely transported tracer into the pressor region of the RVLM, and the placement of a balloon-tipped cannula at the junction of the right atrium and the superior vena cava. On the experimental day, in conscious rats, the balloon was inflated to stimulate cardiac mechanoreceptors (n = 9), or left uninflated (control, n = 8). Compared with controls, there was a significantly increased number of Fos-immunoreactive neurons (a marker of activation) in both the PVN (2.5-fold) and NTS (two-fold), but this was not seen in the CVLM. Compared with controls, a significant number of the neurons in the PVN (8%) and NTS (4.0%) that projected to the RVLM were activated. The data suggest that subgroups of RVLM-projecting neurons located in the PVN and NTS are involved in the central reflex pathway activated by cardiac mechanoreceptor stimulation.

Direct coronary vasodilator action of adrenomedullin is mediated by nitric oxide

Increased circulating levels of adrenomedullin (ADM) cause peripheral vasodilatation and hypotension, accompanied by cardiac actions including tachycardia and increases in cardiac contractility, cardiac output, coronary conductance (CC) and coronary blood flow (CBF). It is unclear to what extent these cardiac effects are direct actions of ADM or secondary to the hypotension and altered cardiac loading. The direct cardiac actions of ADM were examined in conscious sheep previously implanted with aortic and coronary flow probes, and an indwelling left coronary artery cannula. Responses to infusion of ADM (0.5 microg kg(-1) h(-1) for 1 h) into the left coronary artery or jugular vein were compared (n=6). The effect of blockade of nitric oxide (NO) synthase with intracoronary (i.c.) N(omega)-nitro-l-arginine (l-NNA; 1.5 mg kg(-1) h(-1), infused for 2 h before and during ADM infusion, was assessed to determine whether the responses to ADM were mediated by NO (n=5). I.c. ADM caused large and sustained increases in CC (0.35+/-0.07-0.55+/-0.13 ml min(-1) mmHg-1, P<0.05) and CBF (28+/-6-42+/-9 ml min(-1), P<0.05), but had no effect on arterial pressure or indices of cardiac contractility (first differential of the upstroke of systole and peak aortic flow rate). Intravenous infusion of ADM had no effects. I.c. l-NNA, at a dose that abolished the coronary vasodilator action of acetylcholine, blocked ADM-induced coronary vasodilatation. In conclusion, ADM had a direct coronary vasodilator action that was mediated by release of endogenous NO and resulted in increased CBF. There was no evidence for a direct inotropic action of ADM.

Role of brain angiotensin II in renal nerve inhibition elicited by volume expansion in the conscious rabbit

Losartan (10 microg/25 microl) or vehicle was injected into the fourth brain ventricle prior to volume expansion (VE) with Haemaccel (2 ml/min for 30 min). RSNA was reduced by a maximum of 45% in response to the VE following vehicle and by 33% following losartan. There was no significant difference between the treatments in RSNA, nor in the blood pressure and heart rate responses. We conclude that endogenous angiotensin II does not make a major contribution to the reflex reduction in RSNA initiated by VE.

Glucocorticoid-induced renal vasodilatation is mediated by a direct renal action involving nitric oxide

Glucocorticoids increase renal blood flow (RBF) and glomerular filtration rate, but the mechanisms are unclear. We investigated whether the cortisol-induced increment in RBF is a direct renal action or secondary to its systemic effects and whether nitric oxide (NO) plays a role in this response. In conscious sheep, cortisol infused intravenously (5.0 mg/h) or into the renal artery (1.3 mg/h) for 5 h increased RBF by 66 +/- 8 and 53 +/- 11 ml/min, respectively. Plasma glucose was increased by intravenous cortisol (0.4 +/- 0.1 mmol/l) but not by intrarenal cortisol. Renal vein plasma cortisol levels were similar at the end of each infusion (193 +/- 31 intravenously; 151 +/- 25 nmol/l intrarenal), but systemic levels were different (277 +/- 31 intravenous; 69 +/- 10 nmol/l intrarenal). Inhibition of NO synthesis by N omega-nitro-L-arginine infused intravenously (10 mg/kg followed by 5 mg.kg-1.h-1) or intrarenally (2 mg.kg-1.h-1) significantly reduced the cortisol-induced renal vasodilatation. In contrast, constriction of the renal vasculature with intrarenal angiotensin (0.3 microgram/h) did not prevent the cortisol-induced renal vasodilatation. These findings demonstrate that cortisol acts directly on the kidney to cause renal vasodilatation and to increase RBF and suggest that this response involves the endothelium-derived relaxing factor NO.

Investigation of the mineralocorticoid and hypertensinogenic activity of 18-hydroxycortisol in conscious sheep

The increased urinary excretion of 18-hydroxycortisol (18-OHF) in patients with primary aldosteronism has raised the possibility that 18-OHF is involved in the maintenance and/or pathogenesis of the associated hypertension. This study has investigated the mineralocorticoid, glucocorticoid, and hypertensinogenic activities of 18-OHF in the conscious sheep. Infusion of 18-OHF (400 micrograms/h i.v. 5 days; n = 5) alone had no effect on blood pressure or on fluid and electrolyte balance. Infusion of a combination of five adrenal steroids (aldosterone 3 micrograms/h, cortisol 5 mg/h, corticosterone 0.5 mg/h, 11-deoxycortisol 1 mg/h and deoxycorticosterone 25 micrograms/h, i.v. 5 days; n = 5) increased blood pressure by 14 +/- 1 mmHg (p < .001), but when 18-OHF was infused together with the five adrenal steroids, no additional increase in blood pressure was observed. In another group of sheep (n = 4) 18-OHF was infused at a range of doses (5, 50, 100, 200, 500, and 1000 micrograms/h i.v.), each for 2 h, into sodium-replete and sodium-deplete, adrenalectomized sheep. 18-OHF had no effect on the urinary sodium or potassium excretion or on the salivary Na/K ratio in either group as compared with vehicle infusion. To examine the renal effects of 18-OHF, a range of doses of 18-OHF (5, 50, 100, 200, 500, and 1000 micrograms/h) were infused directly into the renal artery of conscious sheep (n = 4). 18-OHF did not affect the renal blood flow nor the urinary sodium or potassium excretion compared with vehicle infusion. In summary, we could not demonstrate any mineralocorticoid, glucocorticoid, or hypertensiongenic effects of 18-OHF in conscious sheep at a dose of 400 micrograms/h. Thus, a cautious approach to interpreting the role that 18-OHF plays in the clinical manifestations of primary aldosteronism, is necessary.