Effect of nerve stimulation on percapillary sphincters, oxygen extraction, and hemodynamics in the intestines of cats

The diving paradox: new insights into the role of the dive response in air-breathing vertebrates

When aquatic reptiles, birds and mammals submerge, they typically exhibit a dive response in which breathing ceases, heart rate slows, and blood flow to peripheral tissues is reduced. The profound dive response that occurs during forced submergence sequesters blood oxygen for the brain and heart while allowing peripheral tissues to become anaerobic, thus protecting the animal from immediate asphyxiation. However, the decrease in peripheral blood flow is in direct conflict with the exercise response necessary for supporting muscle metabolism during submerged swimming. In free diving animals, a dive response still occurs, but it is less intense than during forced submergence, and whole-body metabolism remains aerobic. If blood oxygen is not sequestered for brain and heart metabolism during normal diving, then what is the purpose of the dive response? Here, we show that its primary role may be to regulate the degree of hypoxia in skeletal muscle so that blood and muscle oxygen stores can be efficiently used. Paradoxically, the muscles of diving vertebrates must become hypoxic to maximize aerobic dive duration. At the same time, morphological and enzymatic adaptations enhance intracellular oxygen diffusion at low partial pressures of oxygen. Optimizing the use of blood and muscle oxygen stores allows aquatic, air-breathing vertebrates to exercise for prolonged periods while holding their breath.

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.

Transit time heterogeneity in canine small intestine: significance for oxygen transport

We previously found that local O2 extraction efficacy in isolated pump-perfused intestine was enhanced when systemic reflex vasoconstriction was stimulated by hypovolemia (Samsel, R.W., and P.T. Schumacker. 1994. J. Appl. Physiol. 77: 2291-2298). The microvascular mechanism underlying this beneficial effect could involve a redistribution of flow between mucosa and serosa, or an adjustment in the heterogeneity of perfusion within those regions. We measured regional blood flows and distributions of flow and capillary erythrocyte transit times in two segments of small intestine in anesthetized dogs (n = 10). Each vascularly isolated segment of intestine was pump-perfused under high flow (O2 supply-independent VO2) and low flow (O2 supply-dependent) conditions. During the first gut segment, the animal was kept normovolemic using i.v. fluids to minimize reflex vasoconstriction. During the second, the animal was hemorrhaged to augment vasoconstriction (n = 7), or kept normovolemic to control for the effects of time (n = 3). Blood flow distributions were measured using 15 microm radiolabeled microspheres. Tissue blood volume was measured using 99mTc-labeled red blood cells. Capillary volume was determined as the product of tissue blood volume and the histologically derived fraction of vascular volume in the capillaries. Transit times were calculated as the ratio of capillary volume to flow. Each gut segment was fixed and sectioned into 350 approximately 100 mg tissue pieces for analysis. Data revealed significant spatial heterogeneity of blood flow and capillary transit times in both mucosa and muscularis, with relative dispersions (SD/Mean) ranging from 23 to 97%. Hypovolemia caused an increase in flow heterogeneity in muscularis at both high and low flow states, and in mucosa under high flow conditions. However, hypovolemia also elicited changes in capillary volume, such that transit time heterogeneity remained unchanged. Augmentation of vasoconstrictor tone caused a redistribution of flow toward mucosa (P < 0.003) under high and low flow conditions. This redistribution correlated with the improvements in O2 extraction ratio (P = 0.022). Thus, the improvement in gut O2 extraction efficacy seen with increased vasoconstriction may be explained mostly by an intramural redistribution of flow between mucosa and muscularis. Capillary transit time heterogeneity remained unchanged, suggesting that this variable is tightly regulated.

Effects of PAF antagonists on renal vascular escape and tachyphylaxis in perfused rabbit kidney

Renal vascular escape is a physiological phenomenon of adaptation that occurs in vascular smooth muscle. It has been described in many preparations subjected to electrical stimulation or treated with vasoactive agents, such as noreprinephrine, angiotensin and vasopressin. We have recently demonstrated that a naturally occurring ginkgolide (BN 52021), which is a PAF antagonist, was able to block norepinephrine-induced escape in perfused rabbit kidney. In the present work other PAF antagonists, such as the ginkgolides BN 52022 and BN 52024, and the synthetic compounds 48740 RP and WEB 2086, were tested. Their effects on renal vascular escape, perfusion pressure and tachyphylaxis were evaluated. They all were shown to block the escape. Among the ginkgolides, BN 52024 is generally recognized as one of the weaker PAF antagonists. However, in spite of this, BN 52024 was able to significantly and simultaneously block renal vascular escape and tachyphylaxis in perfused rabbit kidney infused with norepinephrine.

Is there an active mechanism limiting the influence of the sympathetic system on the cerebral vascular bed? Evidence for vasomotor escape from sympathetic stimulation in the rabbit

The influence of the cervical sympathetic chain on cerebral circulation in the rabbit was studied by means of 3 complementary techniques. Two dynamic techniques involving chronically implanted probes were used: blood flow in the caudate nucleus (CN) was measured by thermal clearance; tissue PO2 and PCO2 in the same structure were measured by mass spectrometry. Other variables measured continuously and simultaneously included arterial blood pressure (BP), PaO2 and PaCO2. The third technique was a tissue sampling method based on the Fick principle and using 14C1 ethanol as tracer. Blood flow in 7 regions was measured at stable BP, PaO2 and PaCO2. Stimulation of the sympathetic chain at 15 Hz induced mean maximal decreases in CN blood flow of 23.9% (thermal clearance) and 24.4% (ethanol technique). Mean decrease of PO2 in the CN at 15 Hz was 16.6%. Significant falls in blood flow were observed with the ethanol technique in all 7 structures measured. During prolonged stimulation (greater than 1 min) CN blood flow and PO2 were found to escape towards the baseline level, which was sometimes even exceeded during the stimulation (blood flow). Stimulation frequency had only a very moderate influence on the rate of escape, and no evidence of a metabolic mechanism was found, although injection of barbiturate decreased the escape. These results are discussed with respect to the conflicting evidence on the effects of sympathetic stimulation in the brain, and to possible mechanisms for the escape phenomenon.