Myogenic vasoregulation overrides local metabolic control in resting rat skeletal muscle

Microvascular reactions to increases in intravascular pressure were studied in the cremaster muscle of the anesthetized rat by enclosing the animal in an airtight box with the muscle exteriorized for observation of the microcirculation. Since the cremaster was exposed to atmospheric pressure, increasing pressure within the box produced equal increases in arterial and venous pressures. Thus, intravascular pressure was altered without affecting the pressure gradient for blood flow. Raising box pressure had no effect on respiration or heart rate and did not change the systemic activity of the sympathetic system, angiotensin II, or vasopressin. Diameters and flows were measured for first (107 +/- 3 micron, mean +/- SEM), second (87 +/- 5), third (29 +/- 2), and fourth (15 +/- 2) order arterioles during increases in intravascular pressure of +10, +20, and +30 mm Hg. No significant changes in the diameters of first or second order arterioles were elicited when pressure was increased. However, when box pressure was increased to +10, +20, or +30 mm Hg, a sustained constriction occurred in third (29%, 45%, and 63%, respectively) and fourth (5%, 38%, and 57%, respectively) order arterioles. Blood flow was significantly reduced in all arterioles, and perivascular PO2 was decreased adjacent to third and fourth order arterioles. Furthermore, the third order arteriole constrictor response was not abolished by local alpha-receptor blockade (phentolamine), indicating that it was not mediated by a local sympathetic axon reflex. Collectively, these data indicate that a potent, non-neural, pressure-dependent mechanism for vasoregulation is present in small arterioles of the cremaster. The sustained constriction in the presence of reduced blood flow and reduced periarteriolar oxygen tension indicates that the vascular response is independent of and capable of overriding flow-dependent (i.e., metabolic) control in resting skeletal muscle. The observations are compatible with the operation of a powerful myogenic mechanism in small arterioles.     

Differential activation of alpha 1- and alpha 2-adrenoceptors on microvascular smooth muscle during sympathetic nerve stimulation

The relative contribution of postjunctional alpha 1- and alpha 2-adrenoceptors to constriction of microvessels was examined during sympathetic nerve stimulation and sympathetic escape (difference between peak and steady-state constriction). Large arterioles (120 +/- 4 microns control diameter) and venules (174 +/- 6 microns) and small arterioles (13 +/- 4 microns) were examined in rat cremaster skeletal muscle during stimulation of the cremaster efferent innervation (decentralized lumbar sympathetic chain, 0.5-16 Hz, 2-minute train). The muscle was suspended in a tissue bath, and diameter was measured with intravital microscopy. Frequency-response curves were obtained after vehicle (prazosin or rauwolscine) was added to the bath. In large arterioles, prazosin (10(-7) M) significantly attenuated constriction by 60-80%; a fivefold higher concentration had no additional effect. In contrast, rauwolscine (1 to 5 x 10(-7) M) had no effect. Venules evidenced minimal response to nerve stimulation. In small arterioles, rauwolscine (5 x 10(-7) M) significantly attenuated constriction by 50-60%, while prazosin (10(-7) M) had no effect. These data suggest that for large arterioles, which are known to possess both receptors, alpha 1-adrenoceptors are preferentially stimulated by nerve-released norepinephrine. In contrast, sympathetic constriction of small arterioles is mediated by alpha 2-adrenoceptors. Compared with large arterioles, small arterioles exhibited greater peak and steady-state constriction at all frequencies, with maximal responses achieved over the 0.5-4 Hz range. Large arterioles exhibit graded constriction over the entire frequency range. Sympathetic escape exhibited a small, negatively correlated frequency dependence for large arterioles, tended to be greater for small arterioles, and was more evident in large arterioles during alpha 2-adrenoceptor constriction at low-frequency stimulation. This distinct neural control of large resistance vessels by alpha 1-adrenoceptors and small terminal arterioles by alpha 2-adrenoceptors may allow neurogenic regulation of these vessel segments to be differentially susceptible to modulation by other extrinsic and intrinsic vasoactive controls that preferentially interact with alpha 1- and alpha 2-adrenergic contractile mechanisms.     

Differential sensitivity of arteriolar alpha 1- and alpha 2-adrenoceptor constriction to metabolic inhibition during rat skeletal muscle contraction.

Our previous studies in rat skeletal muscle have determined that neural constriction of large arterioles, which regulate blood flow and peripheral resistance, is mediated by alpha 1-adrenoceptors, whereas small arterioles, which determine effective capillary density, depend on alpha 2-receptors. During physical exercise, metabolic vasodilators from contracting skeletal muscle oppose neural vasoconstriction. By mechanisms that are not understood, adrenergic constriction of small arterioles is particularly sensitive to metabolic inhibition during imbalances in oxygen supply versus demand. This sensitivity may result from the reliance of small arterioles on alpha 2-receptors and a greater sensitivity of alpha 2 constriction to metabolic dilators. We previously demonstrated selective attenuation of arteriolar alpha 2 constriction during a reduction in the oxygen supply/demand ratio subsequent to decreased skeletal muscle perfusion. In the present study, intravital microscopy of rat cremaster skeletal muscle was used to examine the effect of increased oxygen demand on adrenergic constriction of arterioles. The effect of multiple frequencies of skeletal muscle contraction (via genitofemoral nerve stimulation) on alpha 1 (norepinephrine + rauwolscine) and alpha 2 (norepinephrine + prazosin) constriction was used to evaluate neural-metabolic interactions over a wide range of metabolic conditions. Low-frequency (less than or equal to 2 Hz) skeletal muscle contraction attenuated only alpha 2 constriction; contractions greater than or equal to 4 Hz attenuated alpha 1 constriction and further reduced alpha 2 constriction. Comparison of the frequency of contraction necessary to produce inhibition of 20% of maximal dilation indicated that alpha 2 constriction was approximately 10-fold more sensitive than alpha 1 constriction to "metabolic" inhibition. High-frequency, but not low-frequency, contraction also inhibited intrinsic tone. These data suggest that release of dilator substances during moderate exercise may preferentially attenuate alpha 2 constriction to produce small arteriolar dilation and increased capillary density. In contrast, metabolic signals associated with higher frequency muscle contraction may inhibit both intrinsic tone and large arteriolar alpha 1 tone so that blood flow and oxygen delivery increase to match the elevated oxygen demand of more heavily exercising muscle.     

Effect of reduced blood flow on alpha 1- and alpha 2-adrenoceptor constriction of rat skeletal muscle microvessels.

Adrenergic constriction of skeletal muscle arterioles, particularly small terminal arterioles, is opposed by decreased blood flow or increased metabolic rate. Our previous studies indicate that neural constriction of large arterioles, which have both postjunctional alpha 1- and alpha 2-adrenoceptors, is mediated by alpha 1-receptors; small arterioles depend on alpha 2-receptors. Also, alpha 2, but not alpha 1, constriction is reduced by acidosis. Differential sensitivity of alpha 1 versus alpha 2 constriction to metabolic signals such as H+ may underlie the sensitivity of arteriolar adrenergic constriction to metabolic inhibition. To examine this hypothesis, we studied the effect of reduced perfusion on alpha 1- versus alpha 2-mediated constriction of large arterioles and venules. Intravital microscopy of rat cremaster skeletal muscle was used to obtain concentration-response curves for phenylephrine (alpha 1-agonist) and UK-14,304 (alpha 2-agonist). Thirty percent reduction in cremasteric artery flow by venous outflow obstruction had no effect on baseline diameter, indicating no effect on "intrinsic tone." Reduced perfusion also had no effect on arteriolar or venular sensitivity to phenylephrine or venular sensitivity to UK-14,304 but significantly attenuated arteriolar response to UK-14,304. To examine a possible mechanism for the selective inhibition of alpha 2 constriction by acidosis, we determined the effect of acidosis on the partial alpha 1-agonist St587. Like alpha 2 constriction, St587-mediated constriction of arterioles was reduced during acidosis and was attenuated by nifedipine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Selective potentiation of angiotensin-induced constriction of skeletal muscle resistance arteries by chronic elevations in dietary salt intake.

Sprague-Dawley rats were fed either a high-salt (HS, 4.0% NaCl) or a low-salt (LS, 0.4% NaCl) diet for 3 days (short-term) or 4-8 weeks (chronic). Vasoconstrictor responses to angiotensin II and norepinephrine were determined in isolated skeletal muscle resistance arteries and in distal arterioles of the in situ cremaster muscle. Myogenic responses to increases in transmural pressure were also assessed in skeletal muscle resistance arteries of animals on high- or low-salt diets. Chronic (but not short-term) HS diet selectively potentiated angiotensin II-induced constriction of skeletal muscle resistance arteries relative to vessels from LS controls. Myogenic responses and norepinephrine-induced constriction of resistance arteries were unaffected by either chronic or short-term HS diet. Constriction of cremasteric arterioles in response to angiotensin II and norepinephrine was unaffected by chronic or short-term elevations in dietary salt intake. These data suggest that chronic elevations in dietary salt intake lead to a selective increase in the constriction of skeletal muscle resistance arteries to angiotensin II that may allow these vessels to continue to regulate their tone in response to this peptide, despite the suppression of angiotensin II that occurs with high-salt diet.     

Decreased influence of nitric oxide on deoxycorticosterone acetate (DOCA)–salt hypertension

The response to exogenous administration of acetylcholine and the effects of nitric oxide (NO) synthesis blockade were assessed for small arterioles in the cremaster muscle of deoxycorticosterone acetate (DOCA)–salt hypertensive and normotensive rats using intravital microscopy. The NO synthesis inhibitor Nω-L-nitro-arginine methyl ester (L-NAME) produced significantly less constriction in the arterioles of DOCA–salt hypertensive rats. Exposure to increasing concentrations of acetylcholine (10⁻¹⁰ to 10⁻⁵ mol/L) or sodium nitroprusside (10⁻¹⁰ to 10⁻⁵ mol/L) produced similar arteriolar dilation in both groups. These results suggest that attenuated basal release of NO by arterioles may play a role in the development of increased peripheral resistance observed in DOCA–salt hypertension.     

Effect of acidosis on contraction of microvascular smooth muscle by alpha 1- and alpha 2-adrenoceptors. Implications for neural and metabolic regulation

Our previous studies have identified that adrenergic regulation of large arterioles and venules in skeletal muscle uses both postjunctional alpha 1- and alpha 2-adrenoceptors, whereas terminal arterioles appear to be subserved primarily by alpha 2-receptors. Adrenergic constriction of terminal arterioles is known to be particularly susceptible to inhibition by increased tissue metabolic rate. The purpose of this study was to examine the influence of tissue acidosis on alpha 1- and alpha 2-adrenoceptor constriction of skeletal muscle microvessels to determine if this differential receptor distribution might have significance in neural-metabolic interactions. Intravital microscopy of rat cremaster skeletal muscle was used to obtain concentration-response curves (diameter changes) of large distributing arterioles (mean diameter, 100 microns), small precapillary arterioles (20 microns), and capacitance venules (150 microns) for addition to the tissue bath of alpha-adrenergic agonists during normal pH (7.4) and during tissue bath acidosis (pH 7.1) produced by increasing bath PCO2. The following alpha-agonists were used: phenylephrine (alpha 1), B-HT 933 (alpha 2), and norepinephrine (mixed alpha 1/alpha 2). Acidosis had no effect on baseline diameter of the three vessel types, indicating a lack of effect on "intrinsic tone." Acidosis also had no effect on large microvessel sensitivity to phenylephrine but markedly reduced responses to B-HT 933. Acidosis had no effect on large arteriolar and venular sensitivity to norepinephrine but markedly decreased (x300) small precapillary arteriolar sensitivity. These data suggest that 1) alpha 2- but not alpha 1-adrenoceptor-mediated constriction of microvessels may be selectively sensitive to modest reductions in tissue pH, and 2) the prevalence of alpha 2-receptors on terminal arterioles and the marked sensitivity of alpha 2 constriction to tissue acidosis may contribute to the particular susceptibility of neural constriction at this level of the microcirculation to metabolic inhibition.     

Expression of cytochrome P450-4A isoforms in the rat cremaster muscle microcirculation

OBJECTIVE: This study sought to identify any specific cytochrome P450 (CYP450) -4A enzyme isoforms expressed in arterioles and/or the surrounding parenchymal tissue of the rat cremaster muscle. METHODS: RT-PCR was used to detect the presence of specific CYP450-4A isoforms in isolated muscle fibers and arterioles from the cremaster muscle of Sprague-Dawley rats; CYP450-4A protein expression was determined by Western blotting. RESULTS: CYP450-4A3 mRNA was expressed in isolated muscle fibers and in cremasteric arterioles, while CYP450-4A8 mRNA was expressed only in cremasteric arterioles. CYP450-4A1 and CYP450-4A2 mRNA were not expressed in arterioles and skeletal muscle cells, although all four isoforms were strongly expressed in the liver. CYP450-4A protein was detected in both the isolated muscle fibers and in the isolated arterioles. CONCLUSIONS: The present study identifies the specific pattern of cytochrome P450-4A isoform expression in arterioles and parenchymal cells of the skeletal muscle microcirculation, and supports the hypothesis that the cytochrome P-450 enzymes may play a role in the regulation of microvascular function in the skeletal muscle microcirculation.     

Microvascular effects of atrial natriuretic factor: interaction with alpha 1- and alpha 2-adrenoceptors

The cremaster skeletal muscle of anesthetized rats was denervated and extended with intact circulation into a tissue bath. Intravital microscopy was used to measure microvessel diameter at three different anatomical levels within the microcirculation: large distributing arterioles (x control diameter = 100 +/- 7 micron), large capacitance venules (147 +/- 8 micron), and small terminal arterioles (17 +/- 1 micron). Norepinephrine (NE) was added to the cremaster bath to produce intermediate reductions in diameter of large arterioles and venules (55% and 38% of maximum constriction, respectively). In the presence of NE tone, bath-added atrial natriuretic factor (ANF) produced concentration-dependent dilation of both arterioles and venules. Arteriolar IC25 = 18 pmol and IC50 = 1.2 X 10(-10) M; venules exhibited similar sensitivity. However, the highest ANF concentration examined (10(-7) M) only reversed NE-induced tone by 70%. In a second large vessel group ANF completely reversed constriction induced by the alpha 1-adrenoceptor agonist, phenylephrine, in the presence of 5 X 10(-7) M yohimbine. However, vessels constricted with the alpha 2-receptor agonist UK-14,304 (in the presence of 10(-8) M prazosin) were insensitive to ANF. A third group of terminal arterioles, which possess considerable spontaneous "intrinsic" tone, were studied in the absence of alpha-receptor agonists. Significant dilation occurred at greater than 10(-7) M, and the maximal response was only 25% of complete dilation with adenosine. These data indicate that ANF exhibits a high potency and selectivity for reversal of alpha 1-adrenoceptor-mediated constriction of large arterioles and venules. Constriction produced by alpha 2-adrenoceptor occupation or by nonadrenergic "intrinsic" mechanisms appears to be insensitive to ANF. We propose that the ability of ANF to reduce microvascular resistance depends on the relative contribution of alpha 1-, alpha 2-, and intrinsic vasoconstrictor components to the prevailing level of smooth muscle tone. Differences in these components among regional circulations and between arterial and venous smooth muscle may contribute to the systemic hemodynamic pattern produced by ANF.     

Preservation of venular but not arteriolar smooth muscle alpha-adrenoceptor sensitivity during reduced blood flow.

To compare arteriolar versus venular smooth muscle sensitivity to myogenic and metabolic inhibition during reduced blood pressure and flow, we measured the diameter of first-order venules (diameter, 230 microns) and arterioles (diameter, 156 microns) of the denervated, blood-perfused rat cremaster skeletal muscle that was suspended in a tissue bath. Sensitivity was determined for bath-added norepinephrine in the presence of yohimbine or prazosin to produce alpha 1- and alpha 2-adrenoceptor constriction, respectively, and for KCl to examine non-receptor-mediated sensitivity. To reduce venular pressure and flow, vasopressin, which constricts cremaster arterioles but not venules, was applied locally at a maximally effective concentration. This arteriolar constriction had no effect on venular sensitivity to alpha 1-adrenoceptor and KCl-mediated constriction. Venular sensitivity (-log M EC50) to alpha 1 and to KCl activation was 6.20 +/- 0.10 and 1.20 +/- 0.04 in the absence and 6.34 +/- 0.09 and 1.30 +/- 0.03 in the presence of arteriolar constriction, respectively. Venular sensitivity to alpha 2 activation was actually greater during arteriolar constriction (6.25 +/- 0.11 in the absence of constriction versus 7.06 +/- 0.13 in the presence of constriction, p less than 0.001). In a second series, the effect of reduced cremaster perfusion pressure and flow on both arteriolar and venular sensitivity was examined by mechanically lowering cremaster inflow. Reduction of first-order arteriolar and venular flow by 82-85% attenuated arteriolar alpha 1 and abolished alpha 2 sensitivity but had no effect on venular adrenergic sensitivity; KCl sensitivity was increased. These data indicate that, in contrast to arteriolar smooth muscle, venular smooth muscle alpha-adrenoceptor sensitivity is preserved during reduced pressure and flow and, thus, is little affected by metabolic and myogenic regulation. The selective depressant effect on arteriolar adrenergic but not KCl constriction suggests that myogenic/metabolic inhibition of arterioles is receptor specific.     

Endothelin-induced vasoconstriction of small resistance vessels in the microcirculation of the rat cremaster muscle

Recently, a peptide (endothelin) which has been shown to have potent vasoconstrictor properties has been isolated from the vascular endothelium. In the present study, we assessed the responsiveness of small arterioles and venules in the rat cremaster muscle to topical application of endothelin using closed-circuit television microscopy. Exposure to increasing concentrations of endothelin (10(-15)-10(-7) M) produced a dose-dependent constriction in large (90 +/- 8 microns), intermediate (50 +/- 6 microns), and small (21 +/- 4) arterioles. Large (144 +/- 17 microns) and intermediate (79 +/- 18 microns) venules also constricted to the peptide, but the responses were inconsistent and smaller in magnitude. The constriction to endothelin was long lasting and resistant to washout. Arteriolar reactivity to endothelin was similar for all vessel levels with ED50 values ranging from 10(-9) to 10-s;1(0) M. Exposure to the calcium entry blocker, verapamil, attenuated the endothelin-induced constriction in 3A arterioles, suggesting that the constriction in skeletal muscle arterioles is at least partially due to the increased entry of extracellular calcium.     

5-HT7 receptors mediate dilation of rat cremaster muscle arterioles in vivo

Objective

Serotonin (5-HT) infusion in vivo causes hypotension and a fall in total peripheral resistance. However, the vascular segment and the receptors that mediate this response remain in question. We hypothesized that 5-HT₇ receptors mediate arteriolar dilation to 5-HT in skeletal muscle microcirculation.

Methods

Cremaster muscles of isoflurane-anesthetized male Sprague-Dawley rats were prepared for in vivo microscopy of third- and fourth-order arterioles and superfused with physiological salt solution at 34°C. Quantitative real-time PCR (RT-PCR) was applied to pooled samples of first- to third-order cremaster arterioles (2–4 rats/sample) to evaluate 5-HT₇ receptor expression.

Results

Topical 5-HT (1–10 nmols) or the 5-HT_1/7 receptor agonist, 5-carboxamidotryptamine (10–30 nM), dilated third- and fourth-order arterioles, responses that were abolished by 1 μM SB269970, a selective 5-HT₇ receptor antagonist. In contrast, dilation induced by the muscarinic agonist, methacholine (100 nmols) was not inhibited by SB269970. Serotonin (10 nmols) failed to dilate cremaster arterioles in 5-HT₇ receptor knockout rats whereas arterioles in wild-type litter mates dilated to 1 nmol 5-HT, a response blocked by 1 μM SB269970. Quantitative RT-PCR revealed that cremaster arterioles expressed mRNA for 5-HT₇ receptors.

Conclusions

5-HT₇ receptors mediate dilation of small arterioles in skeletal muscle and likely contribute to 5-HT-induced hypotension, in vivo.     

Effects of hemorrhagic shock on the microvasculature of skeletal muscle

The relationship between arteriolar and venular dimensions and the progressive failure of the homeostatic mechanisms leading to irreversibility in hemorrhagic shock was evaluated in mammalian skeletal muscle (rat cremaster). The small distribution arterioles (diameter = 17 μm) were observed to lose their tone and vasomotion at irreversibility although at 15 min after hemorrhage they exhibited enhanced vascular activity. Slowing of flow was seen to occur in the large venules (100 μm) and late in shock in smaller venules (25 μm). Venular dilatation was adjudged to be the vascular defect associated with the onset of irreversibility. Muscle surface pH and PO₂ followed a course similar to that seen in unanesthetized subjects. The red cell aggregation seen in the venules during the low flow state was generally reversed after reinfusion of the shed blood and restoration of arterial pressure.     

Role of Endothelium-Derived Relaxing Factors in Arteriolar Dilation During Muscle Contraction Elicited by Electrical Field Stimulation

Objective: 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. Methods: First-order arterioles dilated from 72 ± 3 μm to 93 ± 4 μm in response to contraction of the cremaster muscle for 1 min (n = 7). After EDNO inhibition by topical application of 10 μM N^ω-nitro-l-arginine methyl ester (L-NAME), the resting diameter decreased to 66 ± 3 μm and functional dilation was attenuated to 75 ± 3 μm (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 μm vs. 89 ± 3 μm, 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 μm, n = 7) equivalent to L-NAME. Results: Norepinephrine treatment attenuated functional dilation to 77 ± 3 μm which was to a level similar to L-NAME treatment (P > 0.05). Inhibition of prostaglandin synthesis by topical application of indomethacin (28 μM) resulted in no significant changes in the resting diameter but functional vasodilation was attenuated from 89 ± 2 to 81 ± 3 μm (n = 7, P < 0.05). Conclusions: 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.     

Microvascular responses to E. coli endotoxin with altered adrenergic activity

The cremaster muscle microcirculation of pentobarbital-anesthetized Wistar rats was studied using videomicroscopy. The left cremaster muscle was spread over an optical port in a bath filled with modified Krebs solution (pH 7.4, 34 degrees C). The right femoral artery was cannulated for determination of mean arterial pressure (Pm). Following control measurements of Pm and arteriolar and venular dimensions, dose-response curves of arteriolar and venular dimensions to topical norepinephrine (10(-10) M to 10(-3) M) was obtained. The rats were then administered E. coli endotoxin (6 mg/kg, iv, LD100) over a 1-hr period. The dose response curves were than repeated at intervals of 30 min. Before endotoxin the threshold dose for norepinephrine was consistently 10(-9) M or 0(-8) M. Pm decreased progressively with time postendotoxin. After endotoxin infusion, there was a gradual and progressive constriction of both arterioles and venules. The threshold dosage for norepinephrine to produce constriction of both arterioles and venules increased progressively with time. At 3 hr postendotoxin the threshold dose had increased to 10(-6) M to 10(-4) M. This is the dose that produces maximum constriction of arterioles in the preendotoxin control period. The study was terminated when the animal died or the field was obscured by petechiae. The microvessel sensitivity to norepinephrine is markedly reduced during endotoxin shock possibly due to increase in the active state of the vascular smooth muscle or to change in length of the muscle fibers or to changes in sympathetic alpha-adrenergic activity. The response was not prevented by H1 and H2 receptor blockade, but was prevented by alpha-adrenergic blockade with phentolamine.     

Microcirculatory effects of arachidonic acid and a prostaglandin endoperoxide (PGH2)

We have examined the vascular effects of arachidonic acid and a prostaglandin endoperoxide, precursors of prostaglandins (PG), in the rat cremaster (skeletal muscle) microcirculation. Five-week-old male, Wistar-strain rats were anesthetized with pentobarbital (30 mg/kg) and the cremaster muscle was prepared for direct in vivo observation and quantitation of changes in vascular diameters in response to the topical application of arachidonic acid (AA), PGH₂, or PGE₂. All three agents elicited dose-dependent arteriolar dilator responses. However, threshold doses were different; for AA they were between 0.5 × 10^?7 and 0.5 × 10^?6M, for PGH₂ between 0.5 × 10^?8 and 0.5 × 10^?7M, and for PGE₂ between 0.5 × 10^?9 and 0.5 × 10^?8M. At the high dose studied (0.5 × 10^?3M) both AA and PGH₂ required approximately 60 sec for the development of a maximum dilator response, whereas PGE₂ required only about 30 sec. Indomethacin, an inhibitor of prostaglandin synthesis, did not significantly alter control arteriolar diameters but inhibited completely the responses to AA. We conclude that the rat cremaster skeletal muscle microcirculatory compartment does possess the necessary enzymatic machinery for the conversion of AA and PGH₂ into vasodilator prostaglandin metabolites.     

Effect of angiotensin II on blood pressure and on microvascular beds in mesentery,skin, and skeletal muscle of the rat

Registration of the systemic blood pressure and serial photomicrography of the vascular beds in mesentery, skin, and skeletal muscle were performed in normotensive rats following intravenous administration of a standard angiotensin bolus. The microvascular response was expressed as the relative change in diameter of individual vessels, divided into three size groups of arterioles and venules. The fast component of the blood pressure increase, with the mean maximum in the 8th sec, is paralleled by a constriction of skin arterioles with diameters of 100 to 200 μm, of skeletal muscle arterioles with diameters of 50 to 200 μm, and of mesenteric venules with diameters up to 100 μm. The second pressor phase, culminating in the 29th sec, coincides with the constriction of several segments in the skeletal muscle and skin arterioles, and with the narrowing of mesenteric and skeletal muscle venules. Although the data obtained cannot completely explain the hemodynamic reaction, they reveal the participation of the individual peripheral microvascular beds in monitoring the systemic blood pressure level.     

Flow-Dependent Dilation and Myogenic Constriction Interact to Establish the Resistance of Skeletal Muscle Arterioles

Objective: To test the hypothesis that the diameter of skeletal muscle arterioles is determined by the interaction of responses elicited by intravascular pressure and flow. Methods: Experiments were conducted on isolated, cannulated, first-order arterioles of cremaster muscle of male Wistar rats. The diameter of arterioles was followed by videomicroscopy. Perfusion pressures and flows were controlled. Results: In the absence of perfusate flow, increases in perfusion pressure (from 0 to 120 mm Hg), after initial dilation, elicited endothelium independent constrictions of arterioles. At 60 mm Hg of perfusion pressure, the active diameter of vessels was 84.9 ± 1.9 μm. The passive diameter of arterioles (Ca²⁺-free solution) was 150.6 ± 2.4 μm. Increases in perfusate flow resulted in a significant upward shift in the pressure—diameter curves; in the presence of perfusate flows of 20, 40, and 60 μL/min, the constriction of the vessels at a pressure of 60 mm Hg was attenuated by 25.1 ± 3.9%, 35.2 ± 3.0%, and 46.8 ± 4.4%, respectively. In contrast, the corresponding diameter of arterioles at perfusate flows of 10 to 60 μL/min was significantly reduced when perfusion pressure was increased from 60 to 80 and 100 mm Hg (at a flow of 60 μL/min) by 12.0 ± 4.3% and 37.1 ± 2.8%, respectively. Hence, both flow- and shear stress—diameter curves were significantly shifted downward when perfusion pressure increased from 60 to 100 mm Hg. Conclusion: These results demonstrate that an interplay between pressure and flow-sensitive mechanisms is an important determinant of the arteriolar resistance in skeletal muscle.     

Microvascular responses of intact and adrenal medullectomized rats to hemorrhagic shock

Evidence indicates that during the later stages of hemorrhagic shock there appears to be a loss of response to the control systems that would normally maintain an adequate peripheral resistance. Therefore, the reactivity of the cremaster muscle microcirculation of pentobarbital-anesthetized Wistar rats, intact and adrenal medullectomized, was studied using videomicroscopy. The left cremaster muscle was spread over an optical port in a bath filled with modified Krebs solution (pH 7.4, 34 degrees C). The right femoral artery was cannulated for determination of mean arterial pressure (Pm) and for hemorrhage of the rat. Following control measurements of Pm and microvessel diameters, cumulative dose-response curves of arteriolar and venular diameters to topical norepinephrine (NE) (10(-9) - 10(-4) M) were obtained. The protocols for intact and medullectomized groups were: 1) hypovolemic shock (shed blood not reinfused)--hemorrhage of 3.2 ml/100 g, compensation allowed, and NE dose-response curves repeated and obtained again during late shock as determined by Pm declining below 60 mmHg; and 2) normovolemic shock (condition after reinfusion of shed blood)--hemorrhage into a reservoir to Pm of 40 mmHg, maintenance at this level until 25% of the bled volume had been taken back (irreversible shock), and then reinfusion of the remainder of the blood. After blood reinfusion, the NE dose-response curves were repeated and obtained again during late shock, as determined by Pm below 60 mmHg. In all of the bled animals, the A1 arterioles were constricted posthemorrhage. The A2 arterioles were constricted only in the hypovolemic intact group. The A3 arterioles of all groups were not significantly changed from control. The constricted arterioles remained so. However, the other arterioles in all groups were unchanged during the several hours until death. The threshold concentration of NE for constriction of arterioles (10% or greater) was significantly increased (decreased sensitivity) during shock in all four groups. The response of the medullectomized rats to normovolemic shock was similar to that of the intact group, indicating that the circulating catecholamines were not essential. The response of medullectomized rats to hypovolemic shock was more severe and indicated the need for circulating catecholamines to compensate for the blood volume loss.     

Multiple serotonin receptors on large arterioles in striated muscle.

In rat striated muscle, serotonin (5-hydroxytryptamine; 5-HT) constricts large arterioles (first-order; A1) via 5-HT2 receptor activation but dilates smaller arterioles via a 5-HT1-like receptor. In this preparation, A1 arterioles possess little basal tone. The purpose of this study was to determine if 5-HT would elicit A1 dilation if arteriolar tone was first induced. In anesthetized rats, A1 diameters of the cremaster muscle were measured via in vivo videomicroscopy. Topical application of angiotensin II caused a 26 +/- 3% constriction of the vessels. 5-HT dilated the preconstricted A1s by 36 +/- 7% while constricting normal tone A1s by 33 +/- 5%. This dilation was enhanced by blocking 5-HT2 receptors with LY53857, but abolished with methysergide, a 5-HT1 and 5-HT2 receptor antagonist indicating that the dilation was mediated by a 5-HT1-like receptor. Thus, A1 arterioles possess both 5-HT2 and 5-HT1-like receptors. The net result of 5-HT application in striated muscle, dilation or constriction, will depend on the initial tone of the vessels.