Enteral Feeding Reduces Endothelial Nitric Oxide Synthase in the Caudal Intestinal Microvasculature of Preterm Piglets

Abstract

ABSTRACT:The initiation of enteral feeding represents a challenge to the neonatal intestinal microcirculation, especially in preterms where it predisposes to necrotizing enterocolitis (NEC). We hypothesized that a structural microvascular deficiency may occur when enteral feeding is initiated in preterm piglets susceptible to NEC. Stereologic volume densities of a pan-endothelial marker (vWF), and the main vasodilator endothelial nitric oxide synthase (eNOS), were determined along the small intestine of 1) unfed preterm piglets, 2) piglets receiving total parenteral nutrition (TPN) for 2-3 d, and 3) piglets fed 2 d sow's colostrum (TPNϩSOW) or milk formula (TPNϩFOR) following TPN. In the mucosa, vWF-density decreased in a cranio-caudal direction. A corresponding mucosal eNOS gradient appeared only after initiating enteral feeding. In TPNϩSOW, eNOS induction may lag behind the mucosal growth of the caudal region. In TPNϩFOR, formula-related factors (i.e. bacteria, cytokines) may suppress mucosal eNOS, indicated by increased stresssensitive nuclear HIF1␣ staining. The low mucosal endothelial eNOS density was related to the presence of NEC lesions, maybe via increased hypoxia-sensitivity, especially in the caudal region as indicated by nuclear HIF1␣-staining. Our results suggest an insufficient structural adaptation of the microvasculature to enteral feeding, especially of mucosal eNOS, which may lead to NEC. T he birth process converts the relatively dormant fetal intestine into an organ of intense metabolic activity and associated abundant perfusion, as it becomes the sole site for nutrient absorption. This profound transition is particularly problematic at premature birth, where it may result in NEC, a severe inflammatory bowel disease associated with the initiation of enteral feeding, especially formula (1-3). Therefore, a period of total parenteral nutrition (TPN) in preterm infants is often required before enteral feeding is tolerated (4). The etiology of NEC is unknown, but a dysfunctional intestinal microcirculation could contribute to the etiopathogenesis (5,6).The newborn intestinal circulation shows some unique features, up to now intensely studied from a functional point of view (5,7). Nitric oxide (NO), a gaseous free radical produced mainly by endothelial nitric oxide synthase (eNOS), is the most important vasodilator during the perinatal period in sheep (8) and pig (9). Although eNOS is constitutively expressed in the endothelium, both eNOS mRNA and protein expression can be modulated (10), including by nutrient intake (7). Furthermore, intestinal blood flow shows a cranial-tocaudal decrease along the small intestine in neonatal and adult dogs (11), pigs (12,13), and humans (14). It is, however, unknown whether this functional gradient in the small intestine is associated with a similar gradient in microvessel distribution and/or eNOS protein expression. Histologic analyses of the regional morphometrics of the intestinal endothelium and eNOS expression during the first days of life are therefore inte...

Transition from parenteral to enteral nutrition induces immediate diet-dependent gut histological and immunological responses in preterm neonates

“…There is evidence indicating that NO suppresses IL-18 processing by inhibiting caspase-1 activity (16). In this study we showed that endothelial NOS (eNOS) mRNA was downregulated as an early response to formula feeding, consistent with the earlier indication that low mucosal eNOS was related to NEC lesions in preterm pigs (32). Likely an early feeding-and ischemiainduced decrease in NO levels results in activation of caspase-1 and further secretion of IL-18.…”

Inflammatory signaling in NEC: Role of NF-κB, cytokines and other inflammatory mediators

“…Animal studies also have shown that iNOS is upregulated in a neonatal rat NEC model 57 and that iNOS mediates LPS-induced increase in intestinal permeability 111 . In the intestine of neonatal rats fed with formula eNOS and nNOS were downregulated while iNOS was upregulated when compared with dam fed controls 112 and in piglets, formula feeding downregulated eNOS 113 . When L-NAME, a NO synthase inhibitor, was administered intravenously in a neonatal piglet model of NEC, it worsened the intestinal injury while L-arginine, a NO synthase substrate, attenuates intestinal injury 114 .…”

Altered intestinal microcirculation is the critical event in the development of necrotizing enterocolitis

“…In these baseline studies, NEC is produced in vivo; and serum levels or tissue levels of various mediators are then determined after killing of the animal or removal of the operative specimen [16][17][18][19][20][21][22][23]. Alternately, groups have explored microvascular responses in NEC with different vasoactive mediators by explanting intestinal blood vessels and evaluating their activity ex vivo under a variety of conditions [24].…”