Philip T. Nowicki

Age‐Dependent Changes in the Postnatal Intestinal Microcirculation

Significant changes occur in intestinal hemodynamics during the transition from fetal to newborn life and then again during the first postnatal month. Most importantly, basal vascular resistance substantially decreases following birth. It then decreases further between postnatal days 1 and 3, plateaus, and then begins a slow, progressive increase between postnatal days 12 and 30. The basal rate of intestinal blood flow mirrors the changes in vascular resistance in an inverse manner. The postnatal changes in vascular resistance appear to be mediated, in large part, by an increase in the constitutive and stimulated production of nitric oxide. Most importantly, the diameter of terminal mesenteric arteries (150–300u2003µm diameter) in newborn (i.e., 1 day old) swine is determined by three intrinsic vascular control systems: endothelial production of nitric oxide and endothelin, and the inherent myogenic response of vascular smooth muscle. In contrast, these vessels in older subjects (i.e., 35 days old) are primarily passive in nature and fail to demonstrate significant diameter change in response to blockade of endogenous nitric oxide production or endothelin receptors, or applied perturbations of pressure or flow rate. The circulatory physiology of the perinatal and newborn intestine is exceptional when compared to the adult condition inasmuch as several hemodynamic variables change quite dramatically between fetal and neonatal life and during the first postnatal month. The unique hemodynamic conditions that characterize the perinatal and newborn intestine appear to be part of the overall physiological transition that occurs as the fetus, once born, replaces the placenta with his gastrointestinal tract to obtain nutrition. The goal of this review is to describe the circulatory physiology of the perinatal and newborn intestine, with a particular emphasis on those portions of the intestinal microcirculation that have thus far been studied. First, however, it is important to discuss the age‐dependent changes that occur within the intestinal circulation during perinatal and early newborn life.

The effect of phenotype on mechanical stretch-induced vascular smooth muscle cell apoptosis

The present study evaluated mechanical stretch-induced apoptosis in swine vascular smooth muscle cells (VSMC) of different phenotypes. We demonstrated that differentiated VSMC express a greater level of Bcl-2-associated death factor (BAD) and have a significant cell loss when exposed to mechanical stretch (10% elongation, 1 Hz) for 24 h. We further demonstrated that apoptosis was significantly increased only in differentiated VSMC exposed to mechanical stretch. To test the hypothesis that the intracellular level of BAD in VSMC determines its response to mechanical stretch-induced apoptosis, we examined whether BAD expression was upregulated by mechanical stretch-induced apoptosis and was associated with the increase in the apoptosis level of differentiated VSMC. When exposed to mechanical stretch, the expression of BAD in differentiated VSMC was elevated at 1 h and remained at higher levels during the application of stretch (24 h). In contrast, Bcl-2 expression was suppressed during the application of stretch. Moreover, the proapoptotic function of BAD was inhibited by overexpression of Bcl-2 through transient transfection of VSMC with pCEP4-Bcl-2 or incubation of VSMC with vascular epithelial growth factor. These results suggest that mechanical stretch-induced VSMC apoptosis is phenotype dependent. The higher levels of apoptosis of differentiated VSMC upon mechanical stretch were, at least in part, dependent on their intrinsic level of BAD.

Effects of sustained flow reduction on postnatal intestinal circulation

Studies were conducted to determine the effect of mechanically induced sustained flow reduction on intestinal hemodynamics and oxygenation in 3- and 35-day-old swine. In vitro gut loops were perfused under controlled-pressure conditions from an oxygenated blood reservoir at age-appropriate perfusion pressures; pressure was rapidly reduced to a level that lowered flow rate to approximately 50% of its baseline value, and pressure was then kept at that level for 2 h. In 3-day-old intestine, vascular resistance (Ri) increased by 20% immediately after pressure and flow reduction but then stabilized for 3-4 min; thereafter, flow began to decrease despite maintenance of perfusion pressure, so that Ri increased an additional 15% by 30 min after flow reduction. Flow continued to diminish over the next 90 min, though at much slower rate. Intestine from 35-day-old swine demonstrated an immediate increase in Ri after pressure and flow reduction, but thereafter Ri increased very little. The protocol was repeated within in vitro gut loops perfused under controlled-flow conditions, and within autoperfused, innervated gut loops developed in vivo and similar observations were made in both preparations. In 3-day-old intestine, pretreatment with the L-arginine analog Nomega-monomethyl-L-arginine (10(-4) M) had no effect on the immediate rise in resistance occurring in the first 1 min but substantially attenuated the subsequent slow, progressive rise noted thereafter. Pretreatment with the angiotensin 1A receptor antagonist losartan (2 x 10(-6) M) had no effect on hemodynamic changes during the first 60 min after mechanical perfusion pressure reduction but attenuated the very slight increase in resistance noted during the final 60 min of the protocol. The postnatal intestinal circulation demonstrates progressive vasoconstriction when its flow rate is mechanically reduced in a sustained manner, and this effect is age specific, occurring in 3- but not 35-day-old swine. These changes in gut vascular resistance may be consequent to loss of nitric oxide production and/or local production of angiotensin.Studies were conducted to determine the effect of mechanically induced sustained flow reduction on intestinal hemodynamics and oxygenation in 3- and 35-day-old swine. In vitro gut loops were perfused under controlled-pressure conditions from an oxygenated blood reservoir at age-appropriate perfusion pressures; pressure was rapidly reduced to a level that lowered flow rate to ∼50% of its baseline value, and pressure was then kept at that level for 2 h. In 3-day-old intestine, vascular resistance ( R i) increased by 20% immediately after pressure and flow reduction but then stabilized for 3-4 min; thereafter, flow began to decrease despite maintenance of perfusion pressure, so that R i increased an additional 15% by 30 min after flow reduction. Flow continued to diminish over the next 90 min, though at much slower rate. Intestine from 35-day-old swine demonstrated an immediate increase in R i after pressure and flow reduction, but thereafter R i increased very little. The protocol was repeated within in vitro gut loops perfused under controlled-flow conditions, and within autoperfused, innervated gut loops developed in vivo and similar observations were made in both preparations. In 3-day-old intestine, pretreatment with thel-arginine analog N ω-monomethyl-l-arginine (10-4 M) had no effect on the immediate rise in resistance occurring in the first 1 min but substantially attenuated the subsequent slow, progressive rise noted thereafter. Pretreatment with the angiotensin 1A receptor antagonist losartan (2 × 10-6 M) had no effect on hemodynamic changes during the first 60 min after mechanical perfusion pressure reduction but attenuated the very slight increase in resistance noted during the final 60 min of the protocol. The postnatal intestinal circulation demonstrates progressive vasoconstriction when its flow rate is mechanically reduced in a sustained manner, and this effect is age specific, occurring in 3- but not 35-day-old swine. These changes in gut vascular resistance may be consequent to loss of nitric oxide production and/or local production of angiotensin.

Postnatal changes in gut hemodynamics: a possible role for substance P.

Studies were conducted in young postnatal swine to determine if substance P (SP) participates in the regulation of postnatal intestinal hemodynamics and oxygenation. SP was present in homogenates of whole intestine from postnatal swine in an age-dependent manner as follows: 1 day old and never fed, 126 ± 35; 3 days old and fasted, 148 ± 30; and 14 days old, 51 ± 10 pg/mg protein ( P < 0.01, 14- vs. 1- or 3-day-olds). Phenylephrine-precontracted rings of mesenteric artery from 3-day-old subjects mounted for tension recording within buffer-filled myographs demonstrated brisk relaxation in response to SP (EC50, 2 × 10-10 M). This relaxation was eliminated by mechanical removal of the endothelium or blockade with the l-arginine analog N G-monomethyl-l-arginine (l-NMMA) and was significantly attenuated by pretreatment with N-acyl-l-Trp-3,5-bis-(trifluoromethyl) benzyl ester (NATB), a highly selective NK-1 receptor antagonist (pA2 5 × 10-10 M). Infusion of exogenous SP into the mesenteric artery of innervated in vivo gut loops reduced intestinal vascular resistance 35% and increased tissue oxygen uptake 40% in both 3- and 14-day-old subjects. By contrast, blockade of the NK-1 receptor for SP with NATB increased intestinal vascular resistance 19% in 3-day-old subjects but only 5% in 14-day-old subjects ( P < 0.01). SP-induced changes in gut vascular resistance were significantly attenuated by prior coinfusion of NATB orl-NMMA, indicating that the peptide exerted this vascular effect via the NK-1 receptor, which is linked to endothelial cell nitric oxide synthase. Both NATB andl-NMMA attenuated flow-induced dilation within pump-perfused in vitro gut loops from 3-day-old subjects. SP appears to participate in the regulation of the newborn intestinal circulation, especially during the first days after birth.

Effects of systemic hypotension on postnatal intestinal circulation: role of angiotensin

Systemic hypotension causes a greater degree of vasoconstriction in intestine from 3- than from 35-day-old postnatal swine. To determine the basis for this age-dependent difference, systemic hypotension (pressure reduction to approximately 50% of baseline) was induced by creating pericardial tamponade in postnatal swine instrumented to allow measurement of intestinal hemodynamics and oxygenation in vivo. Hypotension caused gut vascular resistance to increase 77 +/- 6% in 3-day-old subjects but only 18 +/- 3% in 35-day-old subjects. Prior blockade of alpha1-receptors with phentolamine, vasopressin receptors with [d(CH2)5,D-Phe2,Ile4,Ala9-NH2]AVP, or surgical denervation of the gut loop had no effect on hypotension-induced gut vasoconstriction. Losartan, which blocks angiotensin AT1 receptors, significantly attenuated hypotension-induced gut vasoconstriction in both age groups. BQ-610, which blocks endothelin ETA receptors, also limited the magnitude of vasoconstriction but only in younger subjects. This effect may have been consequent to an interaction between endothelin and angiotensin, inasmuch as a subpressor concentration of endothelin increased the contractile response to angiotensin in mesenteric artery rings. The substantial rise in 3-day-old gut vascular resistance was partly consequent to a locally mediated vasoconstriction that occurred in response to pressure and/or flow reduction during hypotension, as evidenced by the significant attenuation of this constriction when blood flow was held constant by controlled-flow perfusion to the gut loop during hypotension. Intestinal O2 uptake was compromised to a significantly greater degree in 3- than in 35-day-old subjects during hypotension. This difference was primarily due to the inability of younger intestine to increase O2 extraction in the face of reduced blood flow and may be mediated, in part, by an effect of angiotensin II on intestinal capillary perfusion.Systemic hypotension causes a greater degree of vasoconstriction in intestine from 3- than from 35-day-old postnatal swine. To determine the basis for this age-dependent difference, systemic hypotension (pressure reduction to ∼50% of baseline) was induced by creating pericardial tamponade in postnatal swine instrumented to allow measurement of intestinal hemodynamics and oxygenation in vivo. Hypotension caused gut vascular resistance to increase 77 ± 6% in 3-day-old subjects but only 18 ± 3% in 35-day-old subjects. Prior blockade of α1-receptors with phentolamine, vasopressin receptors with [d(CH2)5,d-Phe2,Ile4,Ala9-NH2]AVP, or surgical denervation of the gut loop had no effect on hypotension-induced gut vasoconstriction. Losartan, which blocks angiotensin AT1 receptors, significantly attenuated hypotension-induced gut vasoconstriction in both age groups. BQ-610, which blocks endothelin ETA receptors, also limited the magnitude of vasoconstriction but only in younger subjects. This effect may have been consequent to an interaction between endothelin and angiotensin, inasmuch as a subpressor concentration of endothelin increased the contractile response to angiotensin in mesenteric artery rings. The substantial rise in 3-day-old gut vascular resistance was partly consequent to a locally mediated vasoconstriction that occurred in response to pressure and/or flow reduction during hypotension, as evidenced by the significant attenuation of this constriction when blood flow was held constant by controlled-flow perfusion to the gut loop during hypotension. Intestinal O2 uptake was compromised to a significantly greater degree in 3- than in 35-day-old subjects during hypotension. This difference was primarily due to the inability of younger intestine to increase O2extraction in the face of reduced blood flow and may be mediated, in part, by an effect of angiotensin II on intestinal capillary perfusion.