The response of skeletal muscle arterioles to common carotid occlusion

Capillary communication: the role of capillaries in sensing the tissue environment, coordinating the microvascular, and controlling blood flow

Historically, capillaries have been viewed as the microvascular site for flux of nutrients to cells and removal of waste products. Capillaries are the most numerous blood vessel segment within the tissue, whose vascular wall consists of only a single layer of endothelial cells and are situated within microns of each cell of the tissue, all of which optimizes capillaries for the exchange of nutrients between the blood compartment and the interstitial space of tissues. There is, however, a growing body of evidence to support that capillaries play an important role in sensing the tissue environment, coordinating microvascular network responses, and controlling blood flow. Much of our growing understanding of capillaries stems from work in skeletal muscle and more recent work in the brain, where capillaries can be stimulated by products released from cells of the tissue during increased activity and are able to communicate with upstream and downstream vascular segments, enabling capillaries to sense the activity levels of the tissue and send signals to the microvascular network to coordinate the blood flow response. This review will focus on the emerging role that capillaries play in communication between cells of the tissue and the vascular network required to direct blood flow to active cells in skeletal muscle and the brain. We will also highlight the emerging central role that disruptions in capillary communication may play in blood flow dysregulation, pathophysiology, and disease.

Effect of endopeptidase-24.11 inhibitors and C-ANP receptor ligand on responses evoked in arterioles of rat cremaster muscle by atrial natriuretic peptide

1. The present study examined the effect of exogenous atrial natriuretric peptide (ANP), alone or in presence of inhibitors of the two major mechanisms for clearing ANP, metabolism by neutral endopeptidase-24.11 (NEP) and internalization by C-ANP receptors, on arteriolar responses using intravital microscopy on the rat cremaster muscle after intravenous or topical administration of the peptide. 2. Topical application of ANP (3 x 10(-10) to 3 x 10(-8) M) produced a gradual increase in arteriolar diameter. NEP inhibitors, thiorphan (30 mg kg-1, i.v.), kelatorphan (10 mg kg-1, i.v.) and retrothiorphan (25 mg kg-1, i.v.) alone, did not significantly affect vascular tone but caused significant potentiation of the arteriolar responses to topically applied ANP. 3. When given as an i.v. bolus, ANP dilates skeletal arterioles at a high dose (20 micrograms kg-1). At a lower dose (10 micrograms kg-2), ANP alone or with retrothiorphan or the C-ANP receptor ligand C-ANP (4-23) did not produce any arteriolar responses, while after the combined administration of the two inhibitors, an increase in arteriolar diameter was induced. 4. These results indicate that low doses of topically applied ANP dilate rat cremaster arterioles and that the vasodilator responses can be potentiated by NEP inhibition. When given as an i.v. bolus, a high dose of ANP can also dilate skeletal arterioles. However at a lower dose the rapid metabolism of the peptide prevents it from producing its action.

Reversible changes of skeletal muscular capillaries after application of a tourniquet

Morphological studies of skeletal muscular capillaries after a 60-min tourniquet application were performed in the rat extensor hallucis proprius muscle. On the basis of the period of reflow, the rats were divided into three groups (15-min group, 24-hr group, and 7-day group). FITC-albumin was injected intravenously to detect open capillaries. The percentage of interstitial space of the muscle was high in the 24-hr group, but had recovered in the 7-day group. Capillary density, calculated under a fluorescence microscope, was low in both the 15-min and 24-hr groups, but in the 7-day group, it had recovered. Intramuscular pressure was high in the 15-min group, whereas it was almost normal in the 7-day group. Electron microscopic studies showed stenosis or collapse of the capillaries in the 15-min group, whereas it was almost normal in the 7-day group. We consider that the changes in the microcirculatory system in the rat extensor hallucis proprius muscle after a 60-min tourniquet application are reversible.

Direct observations of the effects of baroreceptor stimulation on skeletal muscle circulation of the rat

1. In anaesthetized rats, supramaximal baroreceptor stimulation by carotid sinus inflation evoked a reflex fall in arterial pressure and an increase in vascular conductance and flow of muscles of the hindquarters. 2. Simultaneously, main arteries, primary and secondary arterioles (13-90 microns, internal diameter (i.d.)) and terminal arterioles (7-13 microns) of the spinotrapezius muscle all showed a diameter increase that reached a peak as arterial pressure neared its zenith; terminal arterioles then showed a diameter decrease to below control level. These responses were abolished by local application of phentolamine or guanethidine to the spinotrapezius, or by crushing the paravascular nerve supply. 3. It is suggested that the diameter increases were mediated by reflex inhibition of sympathetic tone, while the secondary diameter decrease in terminal arterioles was induced by a fall in local concentrations of vasodilator metabolites, caused by an increase in muscle blood flow. 4. But after sympathetic blockade 25% of all arterial vessels showed a diameter increase beginning as the fall in arterial pressure neared its zenith. These responses may be attributed to vasodilator metabolites accumulating as a consequence of a reduction in muscle blood flow secondary to the reflex reduction in perfusion pressure. 5. No venous vessels, from venules (9-18 microns) to main veins (65-130 microns) that drain the muscle, showed a diameter change in response to baroreceptor stimulation, in accord with evidence that they have no sympathetic supply. 6. These results accord with and can provide explanation for changes in blood flow, regional blood volume and capillary filtration evoked by baroreceptor stimulation in studies on whole-limb muscle. They support suggestions that active changes in capacity of venous vessels of muscle play a minor role in the baroreceptor reflex; both active and passive changes in vascular capacity may be due to large veins outside of muscle proper.

Microcirculatory responses to carotid sinus nerve stimulation at various ambient O2 tension in the rabbit tenuissimus muscle

To quantify the integrated effects of local and central control mechanisms through tissue metabolites and the autonomic nervous system on the peripheral vascular beds, microcirculatory responses to the carotid sinus nerve stimulation at various levels of ambient oxygen tension (PO2) were measured in the rabbit tenuissimus muscle suffused with oxygenated Tyrode solution, using a microscope-TV system. The statistical analysis of the experimental data exhibited that both capillary red cell velocity and perfused capillary density at the control state were significantly decreased as PO2 was elevated (P less than 0.01) and that the stimulation also significantly augmented their values (P less than 0.01) except for the peak velocity data. Regression analysis indicated that both the velocity and density responses to PO2 changes during stimulation were less sensitive than those at the control state. For instance, the vasodilating effect of stimulation on density at PO2 20 mm Hg was enhanced by about four-fold at 80 mm Hg, although the effect on velocity was increased only by 16% with the same PO2 change. From these results, it was concluded that the microcirculatory changes due to the arteriolar smooth muscle contraction evoked by unit sympathetic discharge was significantly influenced by the ambient PO2 level. Such synergistic interaction of the local and central control mechanisms like a series-coupled gain control system was suspected to play an important role in the overall regulation of the microcirculation.