Increase In Arteriolar Diameter Research Articles

Impairment of Flow-Induced Dilation of Skeletal Muscle Arterioles with Elevated Oxygen in Normotensive and Hypertensive Rats

The effects of elevated PO2 on flow-induced dilation of in situ skeletal muscle arterioles was assessed in cremaster muscle preparations from spontaneously hypertensive rats (SHR) and normotensive Wistar–Kyoto (WKY) rats. Blood flow increases in selected arterioles were initiated by occlusion of a parallel daughter branch from a parent arteriole. Changes in the diameter of the perfused arteriole were measured with a video micrometer and erythrocyte velocity was measured using optical Doppler velocimetry. Superfusate PO2 was controlled by changing the O2 concentration (0% O2 or 21% O2) of the equilibration gas mixture. The increase in arteriolar diameter during occlusion was reduced in SHR compared to WKY rats, resulting in an elevated wall shear rate in SHR. Elevated PO2 decreased flow-induced dilation in both groups and increased wall shear rate during parallel occlusion. An inhibitor of the formation of 20-HETE via cytochrome P450-4A enzymes (P450), dibromododecenyl methylsulfimide, minimized O2-induced constriction of arterioles and prevented the O2-induced decrease in flow-induced dilation and the increase in wall shear rate in both SHR and WKY rats. These results suggest that: (1) flow-induced dilation of in situ skeletal muscle arterioles is impaired in SHR compared to WKY, (2) elevated O2 compromises flow-induced dilation in both groups, (3) 20-HETE contributes to both the O2-induced increases in resting tone and the reduced flow-induced dilation of cremasteric arterioles with elevated PO2.

Potassium channels modulate cerebral autoregulation during acute hypertension.

We tested the hypothesis that constriction of cerebral arterioles during acute increases in blood pressure is attenuated by activation of potassium (K(+)) channels. We tested the effects of inhibitors of calcium-dependent K(+) channels [iberiotoxin (50 nM) and tetraethylammonium (TEA, 1 mM)] on changes in arteriolar diameter during acute hypertension. Diameter of cerebral arterioles (baseline diameter = 46 +/- 2 microm, mean +/- SE) was measured using a cranial window in anesthetized rats. Arterial pressure was increased from a control value of 96 +/- 1 mmHg to 130, 150, 170, and 200 mmHg by intravenous infusion of phenylephrine. Increases in arterial pressure from baseline to 130 and 150 mmHg decreased the diameter of cerebral arterioles by 5-10%. Greater increases in arterial pressure produced large increases in arteriolar diameter (i.e., "breakthrough of autoregulation"). Iberiotoxin or TEA inhibited increases in arteriolar diameter when arterial pressure was increased to 170 and 200 mmHg. The change in arteriolar diameter at 200 mmHg was 20 +/- 3% and -1 +/- 4% in the absence and presence of iberiotoxin, respectively. These findings suggest that calcium-dependent K(+) channels attenuate cerebral microvascular constriction during acute increases in arterial pressure, and that increases in arteriolar diameter at high levels of arterial pressure are not simply a passive phenomenon.

Indomethacin-Induced Disturbances in Villous Microcirculation in the Rat Ileum

Indomethacin is a widely used nonsteroidal antiphlogistic compound used—among others—for the treatment of rheumatoid arthritis in humans. Common side effects of indomethacin in the gastrointestinal tract include ulcerative lesions and petechial bleeding in the mucosa. In the rat, oral intake of indomethacin induces ulcerations in the mucosa of the stomach and, if administered systemically, edema, petechial bleeding, and ulcerations in the small intestine. In order to determine if systemic administration of indomethacin may induce changes in villous perfusion, we assessed the effect of indomethacin on erythrocyte velocity and the diameter of the main arteriole in the villi of the rat ileum. We found that indomethacin led to a significant decrease in mean arteriolar blood flow (6.3 ± 0.8 vs 5.0 ± 1.2 nl/min, means ± SD) 7 days after administration. Mean diameter of the main arteriole remained unchanged (control, 7.8 ± 0.8 vs indomethacin, 7.0 ± 0.9 μm). In conclusion, systemic application of indomethacin leads to a decrease in blood supply to the mucosa. We previously found an increase in villous perfusion when assessed 24 h after administration of indomethacin, accompanied by an increase in arteriolar diameter of the main arteriole. In summary, different regimens of application of indomethacin lead to varying observations in microcirculatory parameters in single villi. Further studies are required to identify the underlying mechanisms.

Elevated Oxygen Tension Inhibits Flow-Induced Dilation of Skeletal Muscle Arterioles

The purpose of this study was to determine if elevated oxygen tension affects flow-induced dilation of in situ skeletal muscle arterioles. Cremaster muscle preparations from Sprague-Dawley rats were superfused with physiological salt solution (PSS) and viewed via television microscopy. A video micrometer was used to measure changes in arteriolar diameter in response to the increase in flow produced by occlusion of a parallel branch from the parent arteriole. Erythrocyte velocity was measured with an optical Doppler velocimeter. The superfusate PO2 was altered by changing the oxygen concentration of the equilibration gas between 0 and 21% O2. Elevation of superfusate PO2 to 10% O2 and 21% O2 significantly decreased arteriolar diameter compared to the control diameter during 0% O2 superfusion. Increases in arteriolar diameter during parallel arteriolar occlusion were reduced as superfusate PO2 was elevated. However, elevated PO2 had no significant effect on erythrocyte velocity through the perfused daughter vessel, either prior to or during parallel occlusion. As a result, blood flow through the dilating vessel was reduced with elevated PO2. As a result of the reduced arteriolar diameter, wall shear rate in the perfused daughter vessel was increased with elevated PO2, both prior to and during parallel occlusion. These observations demonstrate that elevated PO2 can override flow-induced vasodilation in the skeletal muscle microcirculation, leading to the persistence of an elevated wall shear rate.

C-peptide induces a concentration-dependent dilation of skeletal muscle arterioles only in presence of insulin.

In this study we tested the hypothesis that C-peptide alone or in conjunction with insulin may cause a dilation of skeletal muscle arterioles. First-order arterioles (88 microm) isolated from rat cremaster muscles were pressurized (65 mmHg), equilibrated in a Krebs bicarbonate-buffered solution (pH 7.4), gassed with 10% O2 (balance 5% CO2, 85% N2), and studied in a no-flow state. C-peptide administered at concentrations of 0.3, 1, 3, 10, 100, 300, and 1,000 ng/ml evoked arteriolar dilation that was not concentration dependent. In contrast, the administration of the four lower physiological concentrations of C-peptide to arterioles exposed to a nondilating concentration of insulin evoked a significant concentration-dependent increase in arteriolar diameter from 8.6 to 42.3% above control. The arteriolar dilation to C-peptide in the presence of insulin was completely inhibited by administration of NG-nitro-L-arginine (10(-4) M). Responses to ACh and adenosine were not enhanced when these drugs were administered in the presence of insulin. These results indicate that C-peptide has the capacity to evoke arteriolar dilation in skeletal muscle via a nitric oxide-mediated mechanism that appears to be enhanced by an interaction with insulin. Furthermore, the effects of insulin appear to be specific for C-peptide and are not the result of a general enhancement of endothelium-dependent or endothelium-independent dilation.

Role of soluble guanylate cyclase in dilator responses of the cerebral microcirculation

Responses of cerebral blood vessels to nitric oxide (NO) are mediated by soluble guanylate cyclase (sGC)-dependent and potentially by sGC-independent mechanisms. One sGC-independent mechanism by which NO may produce vasodilatation is inhibition of formation of a vasoconstrictor metabolite produced through the cytochrome P450 pathway. In these experiments, we examined the hypothesis that dilatation of cerebral microvessels in response to NO is dependent on activation of sGC. Diameters of cerebral arterioles (baseline diameter=94±5 μm, mean±S.E.) were measured using a closed cranial window in anesthetized rabbits. Under control conditions, YC-1 [3-(5′-hydroxymethyl-2′-furyl)-1-benzyl indazole], an NO-independent activator of sGC, produced vasodilation that was blocked by ODQ (1H-[1,2,4]oxadiazolo[4,3,- a]quinoxalin-1-one)(10 μM), an inhibitor of sGC. These findings indicate that sGC is functionally important in cerebral arterioles. In addition, acetylcholine (which stimulates endogenous production of NO by endothelium) produced dilatation of cerebral arterioles that was inhibited by ODQ. For example, 1 μM acetylcholine dilated cerebral arterioles by 34±7 and 5±1% in the absence and presence of ODQ (10 μM), respectively. Increases in arteriolar diameter in response to sodium nitroprusside (1 μM, an NO donor) were inhibited by approximately 80% by ODQ, but were not affected by 17-ODYA (10 μM) or clotrimazole (10 μM), inhibitors of the cytochrome P450 pathway. Thus, dilatation of the cerebral microcirculation in response to exogenously applied and endogenously produced NO is dependent, in large part, on activation of sGC.

Conducted Vascular Responses: Communication across the Capillary Bed

Conducted vasomotor responses are important for the effective distribution of blood flow, although the mechanism by which these responses are initiated is not well understood. ATP, a substance which is released from circulating red blood cells in response to lowPO2and low pH, two conditions which are associated with decreased supply relative to demand, has been shown to initiate conducted vasodilation following its intraluminal application in first and second order arterioles. Since such lowPO2and low pH conditions would most likely occur on the venous side of the vasculature, we evaluated the response of the arteriolar and capillary networks to application of ATP into venules in the Saran-covered hamster cheek pouch retractor muscle usingin vivovideo microscopy. Intraluminal application of 40 and 400 pl of 10−6M ATP resulted in dose-dependent increases in arteriolar diameter >450 μm upstream from the site of application. These changes in arteriolar diameter were accompanied by significant increases in red blood cell flux. In capillaries, red blood cell flux doubled in response to ATP administration. Since NO was previously determined to be involved in the vascular response to intraluminal ATP in arterioles, we evaluated its role in these responses. We found that systemic administration ofl-NAME prior to ATP application eliminated any conducted response and this effect ofl-NAME was reversed by the systemic administration ofl-arginine. These data suggest that ATP, which is released from red blood cells in response to lowPO2and low pH, conditions which would be found in the venular microvasculature, may serve a role in distributing perfusion in response to alterations in supply.

Endothelial dysfunction in cerebral microcirculation during hypothermic cardiopulmonary bypass in newborn lambs

Objectives: Inflammatory stimuli or mechanical stresses associated with hypothermic cardiopulmonary bypass could potentially impair cerebrovascular function, resulting in inadequate cerebral perfusion. We hypothesize that hypothermic cardiopulmonary bypass is associated with endothelial or vascular smooth muscle dysfunction and associated cerebral hypoperfusion. Therefore we studied the cerebrovascular response to endothelium-dependent vasodilator, acetylcholine, endothelium-independent nitric oxide donor, sodium nitroprusside, and vasoactive amine, serotonin, in newborn lambs undergoing hypothermic cardiopulmonary bypass (nasopharygeal temperature = 18° C). Methods: Studies were performed on 13 newborn lambs equipped with a closed cranial window, allowing for direct visualization of surface pial arterioles. Six animals were studied while undergoing hypothermic cardiopulmonary bypass, whereas seven served as nonbypass, warm (37° C) controls. Pial arteriolar caliber (range = 111 to 316 μm diameter) was monitored using video microscopy. Results: Topical application of acetylcholine caused a dose-dependent increase in arteriolar diameter in the control group that was absent in animals undergoing hypothermic cardiopulmonary bypass. Hypothermic cardiopulmonary bypass did not alter the vasodilation in response to sodium nitroprusside. Furthermore, the contractile response to serotonin was fully expressed during hypothermic cardiopulmonary bypass. Conclusions: The specific loss of acetylcholine-induced vasodilation suggests endothelial cell dysfunction rather than impaired ability of vascular smooth muscle to respond to nitric oxide. It is speculated that loss of endothelium-dependent regulatory factors in the cerebral microcirculation during hypothermic cardiopulmonary bypass may enhance vasoconstriction, and impaired cerebrovascular function may be a basis for associated neurologic injury during or after hypothermic cardiopulmonary bypass. (J thorac Cardiovasc Surg 1998;15:1047-54)

Endothelial Dysfunction in Cerebral Microcirculation During Hypothermic Cardiopulmonary Bypass in Newborn Lambs † 1908

Inflammatory stimuli and/or mechanical stresses associated with hypothermic cardiopulmonary bypass (HCPB) could potentially impair cerebrovascular function resulting in inadequate cerebral perfusion. We hypothesize that HCPB is associated with endothelial and/or vascular smooth muscle dysfunction and associated cerebral hypoperfusion. Therefore, we studied the cerebrovascular response to endothelium-dependent vasodilator, acetylcholine (ACh), endothelium-independent nitric oxide donor, sodium nitroprusside (SNP), and vasoactive amine, serotonin, in newborn lambs undergoing HCPB (nasopharygeal temperature = 18°C). Studies were performed on thirteen newborn lambs equipped with a closed cranial window allowing for direct visualization of surface pial arterioles. Six animals were studied while undergoing HCPB while seven served as nonbypass, warm (37°C) controls. Pial arteriolar caliber(range = 111 to 316 μm diameter) was monitored using video microscopy. Topical application of ACh caused dose-dependent increase in arteriolar diameter in the control group (18±6%; 163±22 μm to 190±18 μm) that was absent in animals undergoing HCPB. HCPB did not alter the vasodilation in response to SNP (19±7%; 173±26 μm to 202±21 μm). Furthermore, the contractile response to serotonin(26±4%; 172±14 μm to 124±9 μm) was fully expressed during HCPB. The specific loss of ACh-induced vasodilation suggests endothelial cell dysfunction rather than impaired ability of vascular smooth muscle to respond to nitric oxide. It is speculated that loss of endothelium-dependent regulatory factors in the cerebral microcirculation during HCPB may enhance vasoconstriction and impaired cerebrovascular function may be a basis for associated neurological injury during or following HCPB.

Effect of L-arginine on reactivity of hamster cheek pouch arterioles during diabetes mellitus.

The goal of this study was to determine whether exogenous application of L-arginine could restore impaired agonist-induced increases in arteriolar diameter during diabetes mellitus. We used intravital microscopy to examine reactivity of cheek pouch arterioles (50 microns in diameter) in nondiabetic and diabetic (2 weeks after injection of streptozotocin) hamsters in response to histamine and substance P. In nondiabetic hamsters histamine (1.0 and 5.0 microM) dilated cheek pouch arterioles by 15 +/- 1 and 22 +/- 1%, respectively, and substance P (50 and 100 nM) dilated arterioles by 14 +/- 3 and 21 +/- 4%, respectively. In addition, dilatation of arterioles in response to histamine and substance P in nondiabetic hamsters was abolished by application of an enzymatic inhibitor of nitric oxide synthase (L-NMMA). In contrast, histamine- and substance P-induced increases in arteriolar diameter were markedly reduced in diabetic hamsters. Histamine (1.0 and 5.0 microM) dilated arterioles by only 5 +/- 1 and 4 +/- 2%, respectively, and substance P (50 and 100 nM) dilated arterioles by only 6 +/- 2 and 5 +/- 3%, respectively (p < 0.05 vs. nondiabetic hamsters). Nitroglycerin produced similar vasodilatation in nondiabetic and diabetic hamsters. Next, we examined whether exogenous application of L-arginine (100 microM) could restore impaired histamine- and substance P-induced increases in arteriolar diameter in diabetic hamsters. We found that L-arginine did not restore altered nitric oxide synthase-dependent vasodilatation in diabetic hamsters. These findings suggest that short-term diabetes mellitus alters agonist-induced increases in arteriolar diameter. In addition, the mechanism of altered arteriolar reactivity during diabetes mellitus does not appear to be related to an impaired availability of L-arginine.

Role of Nitric Oxide, Adenosine, and ATP-Sensitive Potassium Channels in Insulin-Induced Vasodilation

The resistance of various tissues to the vasodilator and metabolic effects of insulin may be an important risk factor in the genesis of hypertension observed in several pathological states. Because of this, it is important to understand the mechanisms by which insulin causes vasodilation. Because insulin is known to raise metabolism, one mechanism by which insulin causes vasodilation could be through metabolic vasodilation. Recently, however, it has been suggested that the insulin-induced vasodilation is mediated by the release of endothelium-derived nitric oxide. Using a model of muscle microcirculation (hamster cremaster), we examined the interactions between insulin, nitric oxide, and tissue metabolism to understand the potential mechanisms by which insulin causes vasodilation. Topical application of insulin (200 microU/mL) to the cremaster resulted in significant increases in arteriolar diameter. Second-order arteriolar diameter increased from 69.6 +/- 6 to 79.8 +/- 5 microns and fourth-order arteriolar diameter from 11.3 +/- 1 to 15.1 +/- 2 microns (n = 8). During nitric oxide synthase inhibition, topical application of insulin caused significant vasodilation in both second- and fourth-order arterioles. In contrast, both adenosine receptor antagonism and blockade of ATP-sensitive potassium channels prevented insulin-induced increases in arteriolar diameter. Our findings suggest a role for increased tissue metabolism, particularly the metabolite adenosine, in mediating insulin-induced vasodilation.

Role of nitric oxide and prostaglandins in the regulation of diaphragmatic arteriolar tone in the rat.

We evaluated by intravital microscopy in rats the relative importance of nitric oxide (NO) and prostaglandins in 1) the maintenance of basal diaphragmatic arteriolar tone and 2) the response of diaphragmatic arterioles to the endothelium-dependent vasodilator acetylcholine (ACh). One hundred two mechanically ventilated rats were studied. Separate applications of N omega-nitro-L-arginine (L-NNA) and mefenamic acid (MA), which are specific inhibitors of NO and prostaglandin synthesis, respectively, elicited a significant reduction in basal diaphragmatic arteriolar diameter. A dramatic potentiation of the effect of each inhibitor was observed when both agents were applied simultaneously. ACh application induced a significant and dose-dependent increase in arteriolar diameter that was not significantly modified by the separate application of L-NNA or MA. Conversely, the simultaneous administration of L-NNA and MA almost completely prevented ACh-induced arteriolar dilatation. Dilatation in response to sodium nitroprusside was not significantly modified in the presence of both inhibitors. These results suggest that NO and prostaglandins act in concert to regulate basal diaphragmatic arteriolar tone and to mediate diaphragmatic arteriolar response to ACh.

Cerebral vasodilation during hypercapnia. Role of glibenclamide-sensitive potassium channels and nitric oxide.

The purpose of these experiments was to examine mechanisms by which hypercapnia produces vasodilatation in brain. We examined the hypothesis that dilatation of cerebral arterioles during hypercapnia is dependent on activation of ATP-sensitive potassium channels and formation of nitric oxide. Diameters of cerebral arterioles were measured using a closed cranial window in anesthetized rabbits. Changes in diameter of arterioles were measured in response to topical application of acetylcholine and sodium nitroprusside and during two levels of systemic hypercapnia. Increasing arterial PCO2 from 32 +/- 1 mm Hg (mean +/- SE) to 54 +/- 1 and 66 +/- 1 mm Hg dilated cerebral arterioles by 25 +/- 3% and 38 +/- 5%, respectively, from a control diameter of 93 +/- 3 microns. The response to the low level of hypercapnia was attenuated (25 +/- 3% versus 16 +/- 4%, P < .05) by glibenclamide (1 mumol/L), an inhibitor of ATP-sensitive potassium channels. Vasodilatation in response to the high level of hypercapnia was not affected by glibenclamide. Increases in arteriolar diameter in response to sodium nitroprusside were not inhibited by glibenclamide. NG-nitro-L-arginine (300 mumol/L), an inhibitor of nitric oxide synthase, completely inhibited dilatation of cerebral arterioles in response to the low level of hypercapnia and inhibited vasodilatation during the high level of hypercapnia by 66%. Thus, activation of glibenclamide-sensitive potassium channels may contribute to dilatation of cerebral arterioles during hypercapnia. Cerebral vasodilatation during hypercapnia is dependent in large part on production of nitric oxide.

Role of EDRFs in the control of arteriolar diameter during increased metabolism of striated muscle

This experiment was designed to determine the role that the release of endothelium-derived relaxing factors (EDRFs), endothelium-derived nitric oxide (EDNO), or prostaglandins have in the control of arteriolar vasodilation during an increased metabolic rate in striated muscle. A silicone stopcock grease dam was placed across the distal portion of the cremaster muscle of pentobarbital-anesthetized hamsters to localize the application of the metabolic stimulator 2,4-dinitrophenol (DNP). Application of DNP (10 mM) to the distal region resulted in significant increases in red cell velocity (from 6 +/- 1 to 10 +/- 2 mm/s) and arteriolar diameter (from 75 +/- 3 to 91 +/- 5 microns) (P < 0.05; n = 6) in the first-order arterioles located approximately 11 mm upstream from the silicone dam. Administration of N omega-nitro-L-arginine methyl ester (L-NAME; 2 mg iv) resulted in significant vasoconstriction of the first-order arterioles and a significant decrease in the vasodilator response to acetylcholine (1 microM). Addition of sodium nitroprusside (380 microM) to the superfusion solution during L-NAME treatment resulted in a return of arteriolar diameter to control levels. DNP treatment during L-NAME and sodium nitroprusside treatment did not inhibit the arteriolar vasodilation [75 +/- 3 to 87 +/- 4 microns (P > 0.05)] after a significant increase in red cell velocity from 7 +/- 1 to 11 +/- 1 mm/s. Before indomethacin treatment, DNP treatment resulted in an increase in arteriolar diameter from 72 +/- 3 to 90 +/- 3 microns, preceded by an increase in red cell velocity from 6 +/- 1 to 10 +/- 1 mm/s.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of muscle flap denervation on flow hemodynamics: A new model for chronic in vivo studies

The purpose of this study was to evaluate microcirculatory dynamics following muscle flap denervation. A modification of standard cremaster muscle flap was used. Fifty male Sprague-Dawley rats were studied in two experimental groups of 25 rats each, as follows: Group I (control), after flap isolation, the neurovascular pedicle was left intact; group II (denervation), following muscle isolation at 1 cm segment of genitofemoral nerve was excised and denervated muscle was preserved in the medial border of rat hind limb. For chronic evaluation the cremaster was withdrawn from the leg and prepared for in vivo observations after days 1, 3, 7, and 14. The following measurements were taken: vessel diameters, red blood cell velocities, and number of perfused capillaries. In group II a 15% increase in arteriolar diameter was observed. Throughout the entire 14 day period denervated flaps presented 27% more perfused capillaries. Flap denervation proved to increase capillary perfusion significantly (P < 0.05). The cremaster muscle tube-flap model introduced in this study allows for chronic observation of the microcirculation.

Peptidases modulate bradykinin-induced arteriolar dilation in the hamster cheek pouch.

The purpose of this study was to investigate whether angiotensin-converting enzyme (ACE; EC 3.4.15.1) and neutral endopeptidase (NEP; EC 3.4.24.11), two membrane-bound metalloenzymes that are widely distributed in the peripheral microcirculation and degrade kinins very effectively, modulate bradykinin-induced arteriolar dilation in vivo. Using intravital microscopy, we measured diameter of second-order arterioles in the hamster cheek pouch during suffusion of bradykinin (0.1-10.0 microM) before and after topical application of captopril (10.0 microM) and phosphoramidon (10.0 nM). We found that each inhibitor significantly potentiated bradykinin-induced increase in arteriolar diameter (P < 0.05). Suffusion of other proteinase inhibitors (excluding ACE and NEP inhibitors) had no significant effect on bradykinin-induced responses. Captopril and phosphoramidon did not potentiate isoproterenol (0.1 microM)-induced arteriolar dilation in the cheek pouch. Collectively, these data indicate that ACE and NEP each plays an important role in regulating bradykinin-induced vasorelaxation in the peripheral microcirculation in vivo.

Importance of venular flow in control of arteriolar diameter in hamster cremaster muscle

These experiments tested the hypothesis that an intact venular flow is important for the control of upstream arteriolar diameter during an increase in metabolism. A silicone stopcock grease dam was placed across the distal portion of the cremaster muscle to localize treatment of the metabolic stimulator 2,4-dinitrophenol (DNP). Thus only the distal area would have an increased metabolic rate, with the proximal area of the cremaster having a normal metabolic rate. During DNP treatment, the first-order venule, approximately 5 mm proximal to the Silastic dam, was occluded to prevent the transport of metabolites. DNP treatment (10 mM) resulted in a significant increase in the arteriolar diameter from 75 +/- 3 to 90 +/- 4 microns (n = 7, P < 0.05), 12.1 +/- 0.3 mm upstream from the distal region. After venular occlusion, arteriolar diameter decreased to 78 +/- 3 microns (P < 0.05). As an additional test of our hypothesis we altered the experimental sequence. DNP treatment during venular occlusion did not affect arteriolar diameter, but after release of the occlusion there was a significant increase in arteriolar diameter from 78 +/- 3 to 91 +/- 4 microns (P < 0.05). These results suggest that an intact venular flow is necessary for control of arteriolar diameter during an increased metabolic rate caused by DNP treatment, providing evidence for the significance of the venular-arteriolar diffusion of vasoactive metabolites.

Nitric oxide mediates vasodilatation in response to activation of N-methyl-D-aspartate receptors in brain.

Neurons release nitric oxide (NO) in response to activation of receptors for the excitatory amino acid N-methyl-D-aspartate (NMDA). We examined the hypothesis that activation of receptors for NMDA produces dilatation of the cerebral microcirculation that is mediated by NO. Diameters of cerebral arterioles were measured using a closed cranial window in anesthetized rabbits. Under control conditions, topical NMDA produced concentration-related dilatation of pial arterioles. Dilatation in response to NMDA was inhibited selectively by MK-801 (an NMDA receptor antagonist) and tetrodotoxin, suggesting that responses to NMDA were receptor mediated and dependent on neuronal activation. Increases in arteriolar diameter in response to NMDA were not affected by L-arginine but were inhibited by NG-nitro-L-arginine, suggesting that the vasodilatation was mediated by NO. Dilatation of cerebral arterioles in response to NMDA was not inhibited by indomethacin, suggesting that cyclooxygenase products do not mediate the response. Using isolated cerebral arteries, we also examined whether NMDA elicited direct cerebral vascular effects. In intact arteries studied in vitro, NMDA had no effect on vascular tone, suggesting that cerebral arteries lack receptors for NMDA. These findings suggest that NO generated in response to activation of receptors for NMDA in vivo is neuronally derived and not due to a direct vascular effect. Thus, NO may mediate increases in local blood flow during increases in neuronal activity in response to excitatory amino acids.

Cerebrovascular reactivity to adenosine analogues in 0.6-0.7 gestation and near-term fetal sheep.

During acute hypoxia, fetal sheep less than 0.7 gestation increase cerebral blood flow (CBF) relatively less than fetal sheep near term. We hypothesized that cerebrovascular reactivity to a hypoxic vasodilator metabolite such as adenosine might be diminished in immature fetuses. This study examined cerebral vasoreactivity to adenosine analogues in nine sheep fetuses less than 0.7 gestation (90-103 days) and nine near term (129-143 days). Fetuses were equipped in utero with a closed cranial window, and pial arterioles were studied by intravital microscopy. 5'-(N-ethylcarboxamido)-adenosine (NECA; 10(-9)-10(-5) M) and N6-cyclopentyladenosine (CPA; 10(-9)-10(-4) M) each caused a dose-dependent increase in arteriolar diameter that was attenuated in the presence of the adenosine receptor antagonist 8-phenyltheophylline (8-PT; 5 x 10(-6) M). Dose-response curves to the agonists were similar for both age groups. NECA was a more potent vasodilator than CPA, in keeping with their affinity for the A2 receptor. Suffusion of 8-PT alone at less than 10(-5) M had no effect on arteriolar diameter. We conclude that adenosine is able to dilate fetal cerebral arterioles as young as 0.6 gestation by acting at an A2 receptor, although resting tone is not influenced by adenosine. The immature fetal sheep CBF response to hypoxia is not attributable to undeveloped vasoreactivity to adenosine.

EDRF released from microvascular endothelial cells dilates arterioles in vivo

Microvascular endothelial cells (MECs) from rat epididymal fat pad were isolated and cultured in vitro on Cytodex 3 microcarrier beads. In Krebs-suffused cremaster muscle of pentobarbital-anesthetized rats arteriolar diameters (mean control diam 20.9 +/- 0.9 micron) were measured using image shearing video microscopy. Two lines of suffusate (1.5 ml/min each) were established; one contained a column of microcarrier beads only (no cells in line; NC) the other contained a 1-ml column of MECs grown on beads (through cells; TC). The muscle preparation and the MECs were first treated with indomethacin (Indo; 28 microM). Indo treatment blocked arteriolar dilation to A23187 (1 microM) and arachidonic acid (AA; 0.25 microM) administered into the NC line. A 4.0 +/- 0.6 micron increase in arteriolar diameter was observed, however, when A23187 (but not AA) was infused through the TC line containing Indotreated MECs on beads. The A23187-elicited dilation was abolished by the introduction of NG-monomethyl-L-arginine (L-NMMA; 200 microM) into the TC line. Administration of atropine (2 microM) onto the cremaster muscle via the NC line inhibited the dilations in response to acetylcholine (ACh; 2.7 microM) given through the NC line. Infusion of ACh through the TC line onto the atropine-treated cremaster muscle, however, elicited a 5.8 +/- 1.3 micron increase in arteriolar diameter, a response that was blocked by prior administration of L-NMMA into the TC line. Arteriolar dilation induced by adenosine (0.5 microM) or sodium nitroprusside (0.5 microM) applied via the NC or TC line was unaffected by L-NMMA.(ABSTRACT TRUNCATED AT 250 WORDS)