Cremaster Muscle Arterioles Research Articles

Active and passive mechanical properties of isolated arterioles from STZ-induced diabetic rats. Effect of aminoguanidine treatment.

Studies were performed to examine the effect of experimental diabetes (4-6 weeks duration) on both the passive elastic and active myogenic properties of isolated skeletal muscle arterioles. Studies were conducted on untreated streptozotocin (60 mg/kg)-induced diabetic rats and in similar rats treated daily with either amino-guanidine (25 mg/kg) or methylguanidine (25 mg/kg). First-order cremaster muscle arterioles were isolated, cannulated, and pressurized in the absence of intraluminal flow. Video microscopy was used to determine relationships between arteriolar diameter and intraluminal pressure both in the active and passive (o mmol/l Ca(2+)-2 mmol/l EGTA superfusated) tes. The measurements were used to calculate active myogenic responses, arteriolar distensibility, and stress-strain relationships. Under passive conditions, arterioles from untreated diabetic animals appeared to be stiffer and less distensible compared with similar arterioles from control animals. Under active conditions, i.e., in the presence of extracellular Ca2+, arterioles from the untreated diabetic group showed impaired myogenic reactivity as evidenced by a significant (P < 0.001) reduction in the negative slope of the pressure-diameter relationship over a physiological range of intraluminal pressures. Chronic treatment with aminoguanidine prevented the diabetes-induced changes in the active and passive properties of the isolated arterioles while treatment with methylguanidine appeared ineffective. Vasodilator responses to topically applied acetylcholine (10(-8) to 5 x 10(-6) mol/l) were significantly impaired in diabetic animals irrespective of treatment with aminoguanidine. The data indicate that experimental diabetes is associated with a decreased passive distensibility, or stiffening, of skeletal muscle arterioles that, in addition, may contribute to impaired active myogenic responses.

Role of endothelium-derived relaxing factors in arteriolar dilation during muscle contraction elicited by electrical field stimulation.

To determine the contribution of either endothelium-derived nitric oxide (EDNO) or prostaglandins in the functional vasodilation of first-order arterioles of the hamster cremaster muscle. First-order arterioles dilated from 72 +/- 3 microns to 93 +/- 4 microns in response to contraction of the cremaster muscle for 1 min (n = 7). After EDNO inhibition by topical application of 10 microM NW-nitro-L-arginine methyl ester (L-NAME), the resting diameter decreased to 66 +/- 3 microns and functional dilation was attenuated to 75 +/- 3 microns (P < 0.05). When the arteriolar diameter was returned to the control values by the addition of sodium nitroprusside, an NO donor, into the superfusion solution (n = 7), functional dilation was similar to that observed before EDNO inhibition (91 +/- 3 microns vs. 89 +/- 3 microns, P > 0.05). To evaluate whether the vasoconstrictor effect of L-NAME on functional dilation is same as other vasoconstrictors, norepinephrine was applied on the cremaster muscle to induce a vasoconstriction (72 +/- 2 to 66 +/- 1 micron, n = 7) equivalent to L-NAME. Norepinephrine treatment attenuated functional dilation to 77 +/- 3 microns which was to a level similar to L-NAME treatment (P > 0.05). Inhibition of prostaglandin synthesis by topical application of indomethacin (28 microM) resulted in no significant changes in the resting diameter but functional vasodilation was attenuated from 89 +/- 2 to 81 +/- 3 microns (n = 7, P < 0.05). These results suggest that EDNO is important for the resting tone of arterioles and that prostaglandins are important in modulating the functional dilation of the first-order arterioles in the hamster cremaster muscle.

Calcium entry and myogenic phenomena in skeletal muscle arterioles.

Studies were conducted to examine Ca2+ entry in several vasomotor situations related to myogenic properties of arterioles (basal tone, vasomotion, and responsiveness to alterations in intravascular pressure). In vivo studies were performed on small cremaster muscle arterioles of anesthetized rats. Intravascular pressure was increased using the pressure-box technique. Voltage-operated Ca2+ channel (VOC) activity was inhibited by nifedipine or methoxyverapamil and was stimulated with BAY K 8644. To examine the effect of hyperpolarization, studies were performed in the presence of pinacidil. Nifedipine and methoxyverapamil exhibited a trend toward dose-dependent dilation; however, neither agent caused dilation comparable to adenosine (10(-4) M). BAY K 8644 produced a biphasic effect, constriction below 10(-8) M, and dilation at higher levels. These data indicate that basal tone can be modulated by agents acting on VOCs; however, as high concentrations of nifedipine do not abolish tone, other mechanisms contribute. At similar concentrations, the Ca2+ channel antagonist significantly inhibited vasomotion, abolishing vasomotion at concentrations above 5 x 10(-6) M. In contrast, BAY K 8644 caused a dose-dependent increase in vasomotion amplitude (e.g., 269 +/- 52% of basal at 10(-7) M). Thus vasomotion appears highly dependent on VOCs. Experiments performed in the presence of the antagonists/agonists indicated that VOCs are not the prime determinant of constrictor responses to acute increases in intravascular pressure. Exposure to pinacidil resulted in dose-dependent vasodilatation and inhibition of vasomotion while showing little effect on acute myogenic responses. Similar effects of pinacidil were observed in isolated, cannulated, cremaster arterioles.(ABSTRACT TRUNCATED AT 250 WORDS)

Nitric oxide-independent response to acetylcholine by terminal arterioles in rat cremaster muscle.

The aim of the present study was to establish whether, in terminal arterioles from the rat cremaster, acetylcholine (ACh) elicits nitric oxide (NO)-independent dilation corresponding to the transient ACh-induced endothelium-dependent hyperpolarization described in arteries. For this purpose, the responses of terminal arterioles [mean diam 15.0 +/- 0.4 (SE) microns] were studied by intravital microscopy in rat cremaster muscle. During 15 min of superfusion by 10(-5) M ACh, the response was characterized by an initial maximal dilation (peak time < 3 min) followed by a more sustained dilation that slightly decreased with time. Inhibition of NO synthesis by 2 x 10(-4) M N omega-nitro-L-arginine (L-NNA) significantly reduced, but did not eliminate, both the peak and sustained responses. Simultaneous administration of 2 x 10(-4) M L-NNA and 2 x 10(-5) M mefenamic acid, an inhibitor of prostaglandin synthesis, did not induce a significantly different response from that observed with L-NNA alone. Procaine (10(-3) M), which is known to inhibit completely ACh-induced hyperpolarization in carotid artery, drastically reduced the initial part of the ACh-induced dilation but not the sustained response. Simultaneous administration of procaine and L-NNA almost completely inhibited the peak response to ACh. Similar results were obtained when L-NNA was combined with a superfusion bath containing 20 mM KCl, a concentration known to reduce hyperpolarization in arteries.(ABSTRACT TRUNCATED AT 250 WORDS)

Skeletal muscle microcirculatory response to rat α-calcitonin gene-related peptide

We used in vivo video microscopy to determine the effect of increasing doses of rat α-calcitonin gene-related peptide (rCGRP) on rat cremaster muscle arterioles in the presence or absence of the nitric oxide synthase inhibitor N-ω-nitro-L-arginine (L-NNA). Male Sprague-Dawley rats (118–148 g) were anaesthetized with pentobarbital, and neurovascularly intact cremaster muscles were imaged. Changes in the diameter, erythrocyte velocity and volume flow in second- (A2), third- (A3), and fourth- (A4) order arterioles were determined. To produce uniform arteriolar tone, the cremaster preparation was challenged with norepinephrine (NE: 10 −7 M). L-NNA (10 −4 M), which was shown to inhibit acetylcholine- (ACh: 10 −6 M) induced arteriolar dilations, was added to 16 of the preparations. Preparations were then challenged by adding cumulative log concentrations of rCGRP (10 −12 – 10 −7 M; n = 16) or an equivalent volume of vehicle (n = 19) to the bath. Following rCGRP challenge, arterioles were maximally dilated with 10 −5 M nitroprusside (NP). rCGRP caused significant dose-dependent increases in erythrocyte velocity and volume flow in A2 arterioles, and in diameter, velocity, and volume flow in A3 and A4 arterioles, by 10 −8 M, when compared with vehicle-treated controls. L-NNA had no significant effect on rCGRP-induced responses. These data indicate that rCGRP causes dose-dependent dilation of skeletal muscle resistance arterioles at a concentration similar to that observed in larger vessels. This dilation does not appear to be dependent on the vascular production of nitric oxide from L-arginine.

Attenuation of blood flow-induced dilation in arterioles after muscle contraction

The response of third-order arterioles (n = 15) in rat cremaster muscle to increased luminal flow was studied after brief (20-30 s) occlusion of a neighboring arteriole in pentobarbital-anesthetized rats. Red cell velocity increased almost fivefold (485 +/- 54% of control) during occlusion, and vessel diameter increased 63 +/- 11%. Initially, the calculated wall shear rate increased to 430 +/- 40% of control during occlusion but then decreased to 308 +/- 35% of control as a consequence of arteriolar dilation. The muscle was subsequently stimulated to contract for 1 min, and the occlusion procedure was repeated after arteriolar diameter and red cell velocity had returned to control levels. In this instance the vessel dilation was 34 +/- 10% or about one-half of that seen during the previous occlusion, although velocity and shear rate rose to a similar degree (474 +/- 54 and 397 +/- 35%, respectively). Dilation during a third occlusion 2-7 min after the vessel recovered from the second occlusion was as great as during the first occlusion (77 +/- 20%). The results indicate that flow-induced dilation in arterioles of rat cremaster muscle is transiently attenuated after muscle contraction.

Endothelial dysfunction augments myogenic arteriolar constriction in hypertension.

To elucidate the underlying reason or reasons for the increased peripheral resistance in hypertension, we investigated the pressure-diameter relation--the myogenic response--of isolated, cannulated arterioles (approximately 50 microns) of cremaster muscle of 12-week-old Wistar-Kyoto (WKY) rats, spontaneously hypertensive rats (SHR), and normal Wistar (NW) rats. All arterioles constricted in response to step increases in perfusion pressure from 20 to 160 mm Hg. This constriction was, however, significantly enhanced from 60 to 160 mm Hg in arterioles of SHR compared with NW or WKY rats. For example, at 80 and 140 mm Hg, respectively, the normalized diameter (expressed as a percentage of the corresponding passive diameter of arterioles of SHR) was 11.8% and 27.6% (P < .05) less compared with those of WKY rats. Endothelium removal eliminated the enhanced pressure-induced tone in SHR. Similarly, indomethacin (10(-5) mol/L, sufficient to block prostaglandin synthesis) or SQ 29,548 (10(-6) mol/L), a thromboxane A2-prostaglandin H2 receptor blocker that inhibited vasoconstriction to the thromboxane agonist U46619, attenuated the enhanced pressure-diameter curve and reversed the blunted dilation to arachidonic acid in SHR. In contrast, the thromboxane A2 synthesis inhibitor CGS 13,080 (5 x 10(-6) mol/L) did not affect the increased pressure-induced tone or the reduced dilation to arachidonic acid in SHR. Thus, the present findings suggest that in early hypertension pressure-induced arteriolar constriction is increased. This seems to be due to an enhanced production of endothelium-derived constrictor factors, primarily prostaglandin H2.

Role of shear stress and endothelial prostaglandins in flow- and viscosity-induced dilation of arterioles in vitro.

We have studied the effect of changes in shear stress on diameter of isolated arterioles of rat cremaster muscle. The steady-state active diameter of arterioles at a constant perfusion pressure (60 mm Hg) was 80 +/- 1.2 microns. The vessels' passive diameter (Ca(2+)-free solution) was 156 +/- 1.8 microns. Changes in shear stress were induced either by an increase in flow (velocity) or by an increase in viscosity of the perfusion solution. At a constant perfusion pressure, the stepwise increase in perfusion flow (0-80 microliters/min in 10-microliters/min steps) elicited, with a delay of approximately 20 seconds, a gradual increase in diameter up to 46%. At a constant 20-microliters/min flow rate, increases in viscosity of the perfusate (2%, 4%, and 6% dextran [molecular weight, 77,800]) caused a gradual vasodilation up to 22%. Varying flow and viscosity of the perfusate simultaneously resulted in an upward shift of the flow-diameter curve. Both flow- and viscosity-induced dilations were eliminated by the removal of the endothelium of arterioles (by air) or were inhibited by indomethacin (10(-5) M). The efficacy and specificity of these inhibitory treatments were assessed with vasoactive agents whose action, with regard to endothelial mediation, has been determined previously. The arteriolar dilation maintained calculated wall shear stress close to control values during increases in flow and/or viscosity of the perfusate, but when the dilation was inhibited by removal of the endothelium or by indomethacin, wall shear stress increased significantly in a cumulative manner.(ABSTRACT TRUNCATED AT 250 WORDS)

L-arginine analogues blunt prostaglandin-related dilation of arterioles.

The effects of arginine analogues, inhibitors of endothelium-derived nitric oxide synthesis, on dilation of arterioles in response to various vasoactive substances were studied. At 65 mmHg intravascular pressure, isolated arterioles of rat cremaster muscle developed tone spontaneously and achieved control diameters similar to those observed in vivo (84.1 +/- 2.0 microns vs. passive diameter: 161.3 +/- 3.4 microns). Acetylcholine (ACh, 5 x 10(-8) M), sodium nitroprusside (SNP, 5 x 10(-8) M), arachidonic acid (AA, 10(-7) M), prostaglandin E2 (PGE2, 10(-9) M), and adenosine (ADO, 10(-6) M) were added to the Krebs bicarbonate buffer solution, suffusing the vessels. The peak vasodilator effects of all agents were studied before and after the administration of various doses of N omega-nitro-L-arginine (L-NNA; 10(-5), 10(-4), and 10(-3) M), which significantly reduced, in a dose-dependent manner, the basal diameter of arterioles by 3.6, 15.2, and 18.9%, respectively. The lowest concentration of L-NNA significantly inhibited arteriolar dilations to ACh by approximately 26%. Higher concentrations of L-NNA and N omega-monomethyl-L-arginine (L-NMMA; 10(-4) M) caused a further significant reduction in the dilation to ACh (to approximately 47%) and also significantly reduced dilator responses to AA and PGE2. In the presence of the highest concentration of L-NNA (10(-3) M), dilation to SNP and ADO were also significantly reduced. Removal of endothelium abolished dilation to ACh and AA but did not alter that to SNP, PGE2, or ADO.(ABSTRACT TRUNCATED AT 250 WORDS)

Nitric oxide synthase inhibition reverses arteriolar hyporesponsiveness to catecholamines in septic rats

Induction of nitric oxide synthase by cytokines has been hypothesized as a mechanism of the hyporesponsiveness to catecholamines that occurs in clinical septic shock. We measured responses of resistance arterioles in rat cremaster muscle to topically suffused norepinephrine in vivo with the use of image-shearing videomicroscopy. Rats made septic by cecal ligation and puncture were compared with controls that underwent sham ligation. The norepinephrine concentration-response curve was shifted to the right in septic rats [50% effective concentration (EC50) 9.1 +/- 5.4 vs. 0.10 +/- 0.02 microM, P < 0.05]. Contractions at doses of 10(-9), 10(-8), and 10(-7) M norepinephrine were 26, 41, and 38%, respectively, of sham controls. Superfusion of the muscle with the nitric oxide synthase inhibitor NG-monomethyl-L-arginine at 100 microM restored the arteriolar responsiveness of the septic rats (EC50 0.14 +/- 0.07 vs. 6.8 +/- 3.1 microM, P < 0.05). This effect was reversed with superfusion of excess (1 mM) L-arginine. These experiments demonstrate impaired vasoconstriction in response to norepinephrine in resistance arterioles of septic rats in vivo. NG-monomethyl-L-arginine reversed this hyporesponsiveness, implying that nitric oxide synthase may mediate the decreased catecholamine responsiveness associated with sepsis.

Skeletal muscle arteriolar constriction to ANG II: evaluation of a myogenic component.

This study addressed the hypothesis that an increase in blood pressure contributes to the overall constrictive response of skeletal muscle arterioles to angiotensin II (ANG II). Diameters of second-order arterioles (2A) and third-order arterioles (3A) in the rat cremaster muscle were quantitated after intravenous administration of ANG II. Hindquarter blood pressure was either allowed to increase or was maintained at normal levels. Constriction of 3A to bolus injection of ANG II was the same whether hindquarter pressure increased or not. However, the total vascular constrictive response of the cremaster muscle (based on 2A blood flow) and of the entire hindquarter (based on iliac arterial blood flow) to bolus ANG II was greater when hindquarter pressure was held constant. During slow infusion of ANG II, 3A constriction was unaffected by an abrupt decrease or increase in hindquarter pressure. However, an abrupt reduction of hindquarter pressure caused a significant decline in hindquarter vascular resistance. Thus an increase in blood pressure, whether rapid or gradual, does not influence 3A constriction to ANG II. However, in the entire hindquarter, a rapid rise in blood pressure opposes constriction to ANG II, whereas a gradual pressure rise evokes a mechanism that enhances constrictive response to the peptide.

Regulation of arteriolar tone and responses via L-arginine pathway in skeletal muscle.

With in vivo television microscopy, changes in arteriolar diameter to topical administration of various vasoactive agents were examined in the absence or in the presence of NG-monomethyl-L-arginine (L-NMMA, topical 100 microM) or NG-nitro-L-arginine (L-NNA, 2.5 microM, 20 microliters/min ia), specific inhibitors of endothelium-derived relaxing factor (EDRF) biosynthesis. In cremaster muscle arterioles (15-22 microns) of rats (n = 6-11), dilations to acetylcholine (1-100 ng) were significantly inhibited (60-70%) by either of the arginine analogues. This inhibition was reversed by subsequent administration of 1 mM L-arginine. Dose-dependent constriction to norepinephrine was enhanced by L-NMMA. Indomethacin treatment reduced arteriolar dilation to bradykinin (BK, 1-100 ng), which was significantly inhibited by additional administration of L-NNA. Application of L-NNA first, followed by additional indomethacin, elicited similar results. Dilations to sodium nitroprusside and adenosine were not reduced in the presence of the inhibitors. L-NMMA or L-NNA caused no change in systemic blood pressure but elicited a significant reduction in arteriolar diameter; this effect was not reversed by 1 mM L-arginine. These data demonstrate the presence of an L-arginine pathway to produce EDRF (nitric oxide) in skeletal muscle microcirculation that mediates and/or modulates arteriolar responses to vasoactive agents and could contribute to the regulation of basal vascular tone.

Endothelium-dependent dilation to L-arginine in isolated rat skeletal muscle arterioles.

The vascular actions of L-arginine (L-Arg) were studied in isolated, pressurized first-order rat cremaster muscle arterioles (93 +/- 2.9 microns) bathed in a Krebs bicarbonate-buffered solution, pH 7.4, equilibrated with 21% O2-5% CO2. Arterioles were studied before and after either the administration of NG-nitro-L-arginine (L-NNA, 10(-3) M), an inhibitor of the synthesis of endothelium-derived relaxing factor (EDRF), or the removal of the endothelium. Acetylcholine (ACh, 10(-8) and 10(-6) M), sodium nitroprusside (SNP, 10(-8) M) and phenylephrine (PE, 10(-7) M) evoked dilation and constriction, respectively. L-Arg, (10(-5)-10(-3) M) the precursor of EDRF, evoked dose-dependent arteriolar dilation; whereas D-arginine (D-Arg, 10(-5)-10(-3) M) was without any significant effect. Administration of L-NNA significantly reduced basal diameters and significantly inhibited the arteriolar dilations to both ACh and L-Arg but had no effect on the dilation to SNP. Removal of the arteriolar endothelium with air inhibited dilations to both ACh and L-Arg, had no effect on responses to SNP, and potentiated vasoconstrictor responses to PE. These findings suggest that in skeletal muscle arterioles the dilations to ACh and L-Arg are endothelium dependent and that microvascular endothelium modulates constrictor responses to PE. Thus EDRF may play an important role in the local regulation of arteriolar resistance and blood flow.

Evidence for protein kinase C involvement in arteriolar myogenic reactivity

Little information exists as to the cellular events that couple the myogenic contractile response of an arteriole to an acute rise in intravascular pressure. The aim of this study was to examine whether protein kinase C (PKC), which has been implicated in the contractile response to agonists, contributes to myogenic vasoconstriction of cremaster muscle arterioles. Studies were performed on anesthetized rats, enclosed in an airtight Plexiglas box, with the cremaster exteriorized into a bath containing Kreb's solution. Pressure in the box was increased to elevate intravascular pressure by 20 mmHg. To examine PKC involvement, studies were performed in the absence or presence of inhibitors of PKC: H 7 (10(-9)-10(-5) M) or staurosporine (10(-10)-10(-7) M). Inhibitors were added to the tissue bath and produced no observable systemic effects or alterations in arteriolar diameter. Third-order arteriole (16 +/- 1 microns diam) responses to these agents and alterations in intravascular pressure were monitored by in vivo microscopy and vessel diameter was measured with a video caliper. H 7 produced a concentration-dependent inhibition of myogenic vasoconstriction, inhibiting the extent of constriction by 75% at 10(-5) M. Similarly, staurosporine caused a concentration-dependent inhibition of pressure-induced constriction. At 10(-7) M staurosporine the myogenic response was inhibited by 82%. Further support for a role for PKC in the myogenic response was provided by the observation that indolactam (10(-6) M), a stimulator of PKC activity, induced myogenic reactivity in first-order arterioles, which under basal conditions show passive distension to increased intravascular pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelial impairment inhibits prostaglandin and EDRF-mediated arteriolar dilation in vivo

The role of endothelium in the vasodilation of third order arterioles of cremaster muscle to a variety of vasoactive agents was investigated in pentobarbital-anesthetized rats. Changes in diameter to topical administration of agents were measured with image shearing, before and after mercury light/sodium fluorescein (light/dye) treatment of a 50- to 100-microns segment of the arteriole under study and were recorded with video microscopy. Before light/dye treatment, arachidonic acid (10(-5) M), prostaglandin E2 (5 x 10(-6) M), A23187 (2 x 10(-6) M), acetylcholine (10(-5) M), and adenosine (10(-4) M) elicited dilation between 75 and 106% of basal diameter. After light/dye treatment, dilations to arachidonic acid, A23187, and acetylcholine were completely eliminated; however, the responses to prostaglandin E2 and adenosine were not altered. These results indicate that light/dye treatment interferes with the production of or response to prostaglandins as well as other endothelial mediators, like endothelium-derived relaxing factor (EDRF). In a second series of experiments, bradykinin, in concentrations of 10(-9), 10(-8), and 10(-7) M elicited dose-dependent dilations, which were partially inhibited by indomethacin and completely abolished after additional light/dye treatment. Dilation of arterioles to adenosine was maintained throughout these experiments. These data suggest that vasodilation to bradykinin is mediated partly via prostaglandin production and partly via other endothelium-derived factor(s).

Effects of endothelial impairment on arteriolar dilator responses in vivo.

In pentobarbital-anesthetized rats we investigated the role of the endothelium in the dilation of third-order arterioles of cremaster muscle to acetylcholine, adenosine, and sodium nitroprusside in vivo. Responses to the topical administration of these agents were measured with image shearing and recorded with video microscopy before and after light-dye (L-D) treatment of a 50- to 100-microns segment of the arteriole under study. L-D treatment consisted of intravascular administration of sodium fluorescein and the illumination of a discrete area of the arteriole under study with its excitation light from a mercury lamp. Before L-D treatment, acetylcholine (10(-7) to 10(-5) M) and adenosine (10(-6) to 10(-4) M) produced dose-dependent increases in arteriolar diameter (vasodilation). After L-D treatment of the arteriolar segment, administration of 10(-7) M acetylcholine evoked a vasoconstriction (-19% from control), and the dilator responses to 10(-6) and 10(-5) M were inhibited by 91 and 77%, respectively. In contrast, the arteriolar dilator responses to all doses of adenosine and sodium nitroprusside (2 x 10(-7) M) were not altered by this treatment. In addition, the dilator responses to acetylcholine were not changed in the nonilluminated, distal, and proximal segments of the arterioles. These findings suggest that L-D treatment selectively alters the function of the endothelium resulting in the loss of vasodilation to acetylcholine, whereas arteriolar smooth muscle function does not appear to be affected.

Altered microvascular reactivity in streptozotocin-induced diabetes in rats

This study examined the effect of short-term streptozotocin-induced diabetes in rats on the response of cremaster muscle arterioles to angiotensin II (ANG II) and vasodilatory prostaglandins. Topically applied ANG II (10(-10) to 10(-6) M) caused significantly greater vasoconstriction of third-order arterioles in diabetic animals in comparison with controls. For example, in response to 10(-6) M ANG II arterioles of the diabetic animals constricted to 43 +/- 10% of basal diameter compared with controls' 67 +/- 6% (P less than 0.05). Furthermore, the magnitude of the secondary vasodilatation after ANG II-induced constriction was decreased in diabetic animals (108 +/- 4 and 131 +/- 9%, P less than 0.025). Cyclooxygenase inhibition resulted in marked arteriolar constriction, with this effect being less evident in diabetic animals. In response to indomethacin (2.8 x 10(-5) M), arterioles of the diabetic animals constricted to 84 +/- 7% of basal diameter compared with 56 +/- 4% in controls (P less than 0.01). Arterioles of the diabetic animals were less responsive to exogenous prostaglandin I2 (PGI2) and PGE2 (10(-12) to 10(-6) M) despite evidence of increased in vitro PGI2 production. The data demonstrate potentiation of the vasoconstrictor response and a diminution of the secondary vasodilator response to ANG II in experimental diabetes. These alterations may be due, in part, to decreased responsiveness of skeletal muscle arterioles to vasodilatory prostaglandins.

Alterations in distribution of cardiac output in experimental diabetes in rats.

Alterations in blood flow distribution in streptozotocin-induced diabetes in the rat were examined. Blood flow between tissues was estimated by the distribution of radiolabeled microspheres, and a second series of experiments examined skeletal muscle microcirculation by in vivo microscopy. Studies were performed in anesthetized rats 1-8 wk after induction of diabetes. Cardiac index was transiently increased in diabetic animals (29.6 +/- 1.0 ml.min-1.100 g-1) in comparison with control animals (23.0 +/- 1.4 ml.min-1.100 g-1) at 2-wk duration. Cardiac index was similar in both groups of animals at all other time points studied. The increased cardiac index coincided with transiently increased blood flow to diaphragm and abdominal wall and a significant vasodilatation of small cremaster muscle arterioles. Blood flow to skin and some skeletal muscles was thereafter significantly decreased in the diabetic animals. Blood flow (ml.min-1.100 g-1) to the brain and main thoracic and abdominal organs was similar in nonfasting control and diabetic animals throughout the period of study. As a result of hyperplasia, blood flow to the small intestine (%cardiac output) was increased in the diabetic animals (at 4 wk of diabetes, 34.5 +/- 2.1 vs. 17.5 +/- 0.8%, P less than 0.001). Despite reduction in blood flow to the intestine, by dietary restriction, flow to skin and skeletal muscle remained significantly decreased. Insulin treatment, at a dose aimed at preventing body weight loss but maintaining hyperglycemia, led to renal hyperperfusion in comparison with untreated diabetic and control animals. The alterations in regional blood flow appear progressive in nature and are not simply related to each other but may reflect tissue adaptation to the metabolic disorder.

Endothelium-associated vasodilators in rat skeletal muscle microcirculation

Pharmacological probes were used to assess the possible roles of guanosine 3',5'-cyclic monophosphate (cGMP)-associated endothelium-derived relaxing factor (EDRF) in mediating microvascular responses to endogenous and exogenous agents in vivo. Pentobarbital-anesthetized rats (Wistar, 6 wk old) were prepared for in vivo microscopic observation and quantification of changes in diameter of third-order arterioles (15-25 microns) in the cremaster muscle to topical application of all agents. In indomethacin-pretreated preparations, cremasteric arteriolar dilator responses to acetylcholine, bradykinin, or ATP, but not to adenosine, histamine, or prostaglandin E2, were inhibited by hydroquinone (50 microM). Vasodilation to acetylcholine was also inhibited by methylene blue (5 microM), a blocker of guanylate cyclase activation. Constrictor responses to norepinephrine were not affected by hydroquinone or methylene blue. The inhibition of acetylcholine-induced vasodilation by hydroquinone and methylene blue was reversed by superoxide dismutase, suggesting that superoxide anion antagonized the response. On the other hand, basal arteriolar diameters or responses to acetylcholine were not affected by oxygen metabolite scavengers. Unlike in isolated arteries, vasodilator responses to the calcium ionophore A23187 or arachidonic acid were completely antagonized by cyclooxygenase inhibition. These data suggest that EDRF could be involved in the control of microvascular tone; however, significant differences exist in the stimuli that elicit dilation through this mediator in small and large blood vessels.

Videomicroscopic demonstration of defective cholinergic arteriolar vasodilation in atherosclerotic rabbit.

In atherosclerotic rabbits (SCLER), decreases in vascular resistance in response to acetylcholine (ACH), an endothelium-dependent agent, are suppressed, whereas those to nitroprusside (NP), an endothelium-independent vasodilator, are preserved. To determine whether defective vasodilation in SCLER is related to altered reactivity of resistance vessels, we visualized arterioles of rabbit cremaster muscle by videomicroscopy. Arteriolar diameter was monitored during topical (superfusional) delivery of ACh and NO, interventions that did not affect systemic hemodynamics. Diameter changes in response to NP (0.01-100.0 microM) did not differ between SCLER and controls; maximal dilations amounted to 110 +/- 10% (mean +/- SE). In contrast, responses to ACH (0.001-100 microM) differed; maximal dilations averaged 54 +/- 4% in SCLER and 124 +/- 9% in controls (P less than 0.001). These differences persisted after blockade with phentolamine, propranolol, and indomethacin. Phenidone and hydroquinone blockers of endothelium-dependent vasodilation, inhibited arteriolar dilation to ACH without affecting that to NP. Microvascular responses to intra-arterial drug were similar to those elicited by topical drug. Thus, hypercholesterolemia and atherosclerosis in the rabbit appear to produce a microvascular defect characterized by an impaired endothelium-dependent dilation and a preserved endothelium-independent dilation. This defect could play a role in limiting vasodilator reserve in atherosclerosis.