Arteriolar oxygen reactivity: where is the sensor and what is the mechanism of action?

Abstract

Arterioles in the peripheral microcirculation are exquisitely sensitive to changes in PO2 in their environment: increases in PO2 cause vasoconstriction while decreases in PO2 result in vasodilatation. However, the cell type that senses O2 (the O2 sensor) and the signalling pathway that couples changes in PO2 to changes in arteriolar tone (the mechanism of action) remain unclear. Many (but not all) ex vivo studies of isolated cannulated resistance arteries and large, first-order arterioles support the hypothesis that these vessels are intrinsically sensitive to PO2 with the smooth muscle, endothelial cells, or red blood cells serving as the O2 sensor. However, in situ studies testing these hypotheses in downstream arterioles have failed to find evidence of intrinsic O2 sensitivity, and instead have supported the idea that extravascular cells sense O2 . Similarly, ex vivo studies of isolated, cannulated resistance arteries and large first-order arterioles support the hypotheses that O2 -dependent inhibition of production of vasodilator cyclooxygenase products or O2 -dependent destruction of nitric oxide mediates O2 reactivity of these upstream vessels. In contrast, most in vivo studies of downstream arterioles have disproved these hypotheses and instead have provided evidence supporting the idea that O2 -dependent production of vasoconstrictors mediates arteriolar O2 reactivity, with significant regional heterogeneity in the specific vasoconstrictor involved. Oxygen-induced vasoconstriction may serve as a protective mechanism to reduce the oxidative burden to which a tissue is exposed, a process that is superimposed on top of the local mechanisms which regulate tissue blood flow to meet a tissue's metabolic demand.

Keywords: arterioles; microcirculation; oxygen; oxygen sensing; vasoconstriction; vasodilatation.

Figure 1. Arteriolar O₂ reactivity in superfused microvascular preparations

A, data from Klitzman et al. (1982). Data are mean ± SEM diameters of arterioles in hamster cremaster muscles when exposed to solutions equilibrated with gases containing different $P_{{\mathrm{O}}_2}$values and 5% CO₂ (solution $P_{{\mathrm{O}}_2}$ values, upper x‐axis labels). The $P_{{\mathrm{O}}_2}$ values shown were measured with O₂ microelectrodes in the free solution flowing over the preparations. The tissue $P_{{\mathrm{O}}_2}$ values shown on the lower x‐axis were measured at the midpoint between the venous ends of two capillaries. The arteriolar $P_{{\mathrm{O}}_2}$ values (top x‐axis labels) were estimated from haemoglobin oxygen saturation measurements in hamster cheek pouch (Jackson & Duling, 1983). B, typical O₂‐induced vasoconstriction in a hamster cheek pouch preparation exposed to superfusion solutions with approximate $P_{{\mathrm{O}}_2}$values as indicated using methods as described (Jackson, 1987). C, same vessel as in B after exposure to ruthenium red (0.001%) to label mast cells (Shepherd & Duling, 1995). Scale bar in B, 25 μm.

Figure 2. Schematic diagram of O₂ signalling in the microcirculation

Oxygen in the environment of arterioles can act directly on the arteriolar wall or on cells in the lumen to produce a vasomotor effect (dilatation in the case of reduced $P_{{\mathrm{O}}_2}$, or constriction in the case of elevated $P_{{\mathrm{O}}_2}$). Alternatively, changes in $P_{{\mathrm{O}}_2}$ may be sensed by extravascular cells (parenchymal cells, mast cells, nerves, etc.), a mediator produced, which then acts on the vessel wall to produce the appropriate vasomotor effect. The $P_{{\mathrm{O}}_2}$ values shown below the cross section of the arterioles refer to tissue $P_{{\mathrm{O}}_2}$ in a superfused, intravital preparation measured at the midpoint between the venous ends of two capillaries as reference values, only.

Figure 3. Local increases in PO2 have no effect on arteriolar tone

Data shown are modified from Jackson (1987). A, schematic diagram of a segment (1–8 mm) of an arteriole in a hamster cheek pouch from which the parenchyma has been surgically removed (aparenchymal arteriole) to obviate effects of local $P_{{\mathrm{O}}_2}$ changes on parenchymal and other extravascular cells. A Whalen‐type recessed tip O₂ microelectrode was inserted through the wall of the vessel into the lumen as shown to monitor luminal $P_{{\mathrm{O}}_2}$. A temperature‐controlled micropipette filled with PSS equilibrated with varied $P_{{\mathrm{O}}_2}$ (fluid‐filled micropipette) was positioned opposite the O₂ microelectrode. Pressurization of the fluid‐filled micropipette ejected the O₂‐equilibrated solution onto the surface of the arteriole to produce a local change in $P_{{\mathrm{O}}_2}$ _. The entire cheek pouch preparation was superfused with PSS, the $P_{{\mathrm{O}}_2}$ of which could be varied to produce global changes in $P_{{\mathrm{O}}_2}$ that affected both the aparenchymal arteriole and all other vessels and cell types in the cheek pouch. The diameter of the arteriole was measured by intravital video microscopy. B, results from a typical experiment in which either local increases in $P_{{\mathrm{O}}_2}$ were produced using the fluid‐filled pipette (Local) or global increases in $P_{{\mathrm{O}}_2}$ were produced by changing the $P_{{\mathrm{O}}_2}$ of the entire superfusate (Global) (replotted data are from Fig. 3 in Jackson, 1987). Local increases in $P_{{\mathrm{O}}_2}$ that effectively changed the $P_{{\mathrm{O}}_2}$ across the wall of the arteriole had no significant effect on arteriolar diameter, whereas a global increase in $P_{{\mathrm{O}}_2}$ produced sustained vasoconstriction. These data suggest that components of the arteriolar wall (endothelial cells, smooth muscle cells or perivascular nerves) are not the sensor cells responsible for arteriolar O₂ reactivity in the hamster cheek pouch. See Jackson (1987) for details.

Figure 4. Local increases in PO2 have no effect on arteriolar tone in occluded arterioles

A, a preparation similar to that depicted in Fig. 3, but with the inclusion of an occluding pipette that was pressed down on the arteriole to eliminate blood flow through the aparenchymal segment. B, summary data from these experiments (data are means ± SEM, n = 5). Local changes in $P_{{\mathrm{O}}_2}$ across the arteriolar wall produced by the fluid‐filled pipette (Local) were ineffective in producing arteriolar constriction. However, raising the $P_{{\mathrm{O}}_2}$ of the superfusate over the entire preparation (Global) produced consistent, sustained arteriolar constriction. These data, along with those shown in Fig. 3, suggest that components of the arteriolar wall (endothelial cells, smooth muscle cells or perivascular nerves) are not the sensor cells responsible for arteriolar O₂ reactivity in the hamster cheek pouch. Data are replotted from Fig. 4 B in Jackson (1987); see this reference for more details.

Figure 5. Perfusion of arterioles in situ with solutions equilibrated with high PO2 has no effect on arteriolar tone

A and B are reproduced from Jackson (1987). A, schematic diagram of a hamster cheek pouch arteriole in which a sharpened fluid‐filled pipette has been inserted through the wall of an arteriole allowing perfusion of the arteriole with PSS equilibrated with varied $P_{{\mathrm{O}}_2}$. The entire cheek pouch preparation was superfused with PSS to allow global changes in $P_{{\mathrm{O}}_2}$. B, summary data (means ± SEM). Perfusion of the arterioles with solutions equilibrated with high or low $P_{{\mathrm{O}}_2}$ had no significant effect on arteriolar diameter. Only when the global $P_{{\mathrm{O}}_2}$ was elevated via the superfusate did the arterioles constrict (compare low $P_{{\mathrm{O}}_2}$ superfusate points with high $P_{{\mathrm{O}}_2}$ superfusate points). These data suggest that components of the arteriolar wall (endothelial cells, smooth muscle cells or perivascular nerves) are not the sensor cells responsible for arteriolar O₂ reactivity in the hamster cheek pouch. See Jackson (1987) for details.

Figure 6. Covering aparenchymal segments with glass and occluding them to eliminate blood flow does not eliminate arteriolar O₂ reactivity

A, schematic diagram of an aparenchymal segment in which the parenchyma has been removed from a long segment of hamster cheek pouch arteriole as described by Jackson & Duling (1983). Elevation of the $P_{{\mathrm{O}}_2}$ of the solution flowing over the preparation from 12 to 150 mmHg resulted in arteriolar constriction as depicted in D. As shown in B, subsequent covering of the aparenchymal segment with a piece of glass coverslip (sealed in place with silicone grease), to eliminate contact of the arteriole with the superfusate, had no effect on O₂‐induced constriction as shown in D. To eliminate blood flow through the covered aparenchymal segments, an occluding pipette was used as shown in C. Despite the lack of access to the superfusate and flowing blood, these covered and occluded aparenchymal segments retained significant O₂ reactivity as shown in D. These data suggest that the constriction induced by elevated $P_{{\mathrm{O}}_2}$ can be conducted along the arteriolar wall. See Jackson & Duling (1983) for details.

Figure 7. The site and mechanism of action of O₂ in the hamster cheek pouch

Schematic diagram depicting a mast cell (the proposed sensor site in the cheek pouch), a smooth muscle cell replete with receptors for cysteinyl leukotrienes (CysLTRs) and ion channels involved in arteriolar O₂ reactivity in the cheek pouch. Elevated $P_{{\mathrm{O}}_2}$ is sensed by the 5‐lipoxygenase (5‐LO) in the nuclear membrane of periarteriolar mast cells that decorate arterioles in this tissue (see Fig. 1). This results in conversion of arachidonic acid to cysteinyl leukotrienes (CysLTs) such as LTC₄, LTD₄ and LTE₄ through a process that involves presentation of the arachidonic acid to the 5‐LO by the 5‐LO‐activating protein (FLAP). This process can be inhibited by drugs such as MK 866, that blocks interaction of FLAP with the 5‐LO, or SC 43251, U 60257, nordihydroguaiaretic acid (NDGA), eicosatetraynoic acid (ETYA) or phenidone, inhibitors of the 5‐LO. The CysLTs then bind to and activate CysLTRs on vascular smooth muscle cells to induce vasoconstriction. CysLTR antagonists such as SKF 102922 or FPL 55712 can inhibit this step in the process. Activation of CysLTRs results in activation of L‐type Ca²⁺ channels (Ca_L), Ca²⁺ influx, an increase in intracellular Ca²⁺ and vasoconstriction, which can be antagonized by Ca_L blockers such as diltiazem or nifedipine. The increase in Ca²⁺ activates Ca²⁺‐activated Cl⁻ channels (Cl_Ca). The resulting efflux of Cl⁻ through these channels causes membrane depolarization, further activating Ca_L and amplifying the initial signal. Blockers of Cl_Ca channels such as niflumic acid or DIDs can inhibit this step in the process. The increase in intracellular Ca²⁺ and the membrane depolarization due to activation of Ca_L and Cl_Ca activates large conductance, Ca²⁺‐activated K⁺ channels (BK_Ca). The efflux of K⁺ through BK_Ca channels blunts the depolarizing effects of activation of Ca_L and Cl_Ca providing a degree of negative feedback, and limiting membrane depolarization. This step in the process can be inhibited by iberiotoxin or tetraethylammonium ions (TEA). Oxygen‐induced smooth muscle depolarization can be conducted along the vessel wall by gap junctions (GJ) supporting the conduction of O₂‐induced vasoconstriction that has been observed experimentally.

Figure 8. The site and mechanism of action of O₂ in cremaster muscle

Schematic diagram of a striated muscle fibre (the proposed sensor cell in this tissue) and a smooth muscle cell replete with ion channels that may be involved in arteriolar O₂ reactivity in this tissue. Elevated $P_{{\mathrm{O}}_2}$ is sensed by cytochrome P450_4A/4F ω‐hydroxylase (CYP4A/4F) located in the endoplasmic reticulum of striated muscle fibres, resulting in conversion of arachidonic acid into 20‐HETE, a process that can be inhibited by 17‐ODYA or DDMS. 20‐HETE then acts on smooth muscle cells to induce Ca²⁺ influx through L‐type Ca²⁺ channels (Ca_L), an increase in intracellular Ca²⁺ and vasoconstriction. As indicated by the ‘?’ next to the arrow connecting 20‐HETE and the smooth muscle cell, the precise receptor for 20‐HETE that is responsible for O₂ reactivity is unclear because 20‐HETE has been proposed to close large conductance, Ca²⁺‐activated K⁺ channels (BK_Ca), which would lead to membrane depolarization activating Ca_L. In contrast, other studies suggest that BK_Ca serve a negative feedback role as they do in the cheek pouch. As in the cheek pouch it is proposed that O₂‐induced depolarization of smooth muscle cells can be conducted along the vessel wall through gap junctions (GJ), consistent with the observed conducted vasoconstriction that has been observed experimentally. 6(Z),15(Z)‐20‐HEDE, 20‐hydroxy‐6Z,15Z‐eicosadienoic acid.