Spreading the signal for vasodilatation: implications for skeletal muscle blood flow control and the effects of ageing

Abstract

Blood flow control requires coordinated contraction and relaxation of smooth muscle cells (SMCs) along and among the arterioles and feed arteries that comprise vascular resistance networks. Whereas smooth muscle contraction of resistance vessels is enhanced by noradrenaline release along perivascular sympathetic nerves, the endothelium is integral to coordinating smooth muscle relaxation. Beyond producing nitric oxide in response to agonists and shear stress, endothelial cells (ECs) provide an effective conduit for conducting hyperpolarization along vessel branches and into surrounding SMCs through myoendothelial coupling. In turn, bidirectional signalling from SMCs into ECs enables the endothelium to moderate adrenergic vasoconstriction in response to sympathetic nerve activity. This review focuses on the endothelium as the cellular pathway that coordinates spreading vasodilatation. We discuss the nature and regulation of cell-to-cell coupling through gap junctions, bidirectional signalling between ECs and SMCs, and how oxidative stress during ageing may influence respective signalling pathways. Our recent findings illustrate the role of small (SK(Ca)) and intermediate (IK(Ca)) Ca(2+) activated K(+) channels as modulators of electrical conduction along the endothelium. Gaps in current understanding indicate the need to determine mechanisms that regulate intracellular Ca(2+) homeostasis and ion channel activation in the resistance vasculature with advancing age.

Figure 1. Endothelial cell signalling pathways for conveying relaxation of smooth muscle

Bottom, endothelial function. Stimulation of M₃ receptors (M₃R) (bottom of illustration) produces inositol trisphosphate (IP₃) which in turn acts on IP₃ receptors (IP₃R) to release Ca²⁺ from the endoplasmic reticulum (ER) into the cytosol. These internal Ca²⁺ stores are replenished via uptake of Ca²⁺ from the cytosol into the ER through sarcoplasmic/endoplasmic calcium ATPase (SERCA) pumps to sustain signalling. The increase in cytosolic Ca²⁺ activates small and intermediate Ca²⁺ activated K⁺ channels (SK_Ca and IK_Ca) to initiate hyperpolarization (negative sign) to be transmitted to smooth muscle cells through myoendothelial gap junctions. An alternative source of endothelial Ca²⁺ is influx from extracellular fluid through transient receptor potential (TRP) channels; hyperpolarization increases the electrical gradient for Ca²⁺ influx. Middle, response of smooth muscle to endothelial signalling. Hyperpolarization inhibits voltage-gated Ca²⁺ channels (VGCC) to prevent Ca²⁺ entry. Additionally, increased endothelial Ca²⁺ stimulates production of nitric oxide (NO), which diffuses to smooth muscle and increases the open probability of voltage-gated K⁺ channels (K_V) via cGMP-dependent protein kinase I (cGKI) to hyperpolarize and inhibit VGCC. Top, myoendothelial feedback. Activation of α₁-adrenoreceptors (α₁R) on smooth muscle by noradrenaline (NA) released from sympathetic nerves (or the selective α₁R agonist phenylephrine, PE) results in IP₃ production to elicit Ca²⁺ release through IP₃Rs in the sarcoplasmic reticulum (SR) and evoke contraction. When elevated in smooth muscle, IP₃ and Ca²⁺ diffuse through myoendothelial gap junctions into endothelial cells to activate SK_Ca/IK_Ca and/or NO production, providing negative feedback to smooth muscle contraction.

Figure 2. Conducted hyperpolarization along the endothelium and the effect of sympathetic nerve activity

A, the spread of hyperpolarization. Hyperpolarization (circled negative symbols) of endothelial cells initiated by activation of small and intermediate Ca²⁺ activated K⁺ channels (SK_Ca and IK_Ca) spreads to neighbouring cells through gap junctions. The efficacy of spreading hyperpolarization between cells depends upon both gap junction patency and ‘leakiness’ of plasma membranes (open SK_Ca/IK_Ca during steady-state conditions). Under resting conditions only a few SK_Ca/IK_Ca are open and gap junction patency is high, enabling effective conduction of hyperpolarization from cell to cell along the endothelium to govern spreading vasodilatation. B, proposed effect of increased sympathetic nerve activity. As manifest during ageing, enhanced sympathetic nerve activity (↑SNA) leads to greater activation of SK_Ca/IK_Ca (see Fig. 1). The greater loss of charge (i.e. current dissipation) from each cell impairs the spread of hyperpolarization to neighbouring cells and thereby attenuates spreading vasodilatation.

Figure 3. Endothelial signalling and potential actions of reactive oxygen species

Increases in endothelial Ca²⁺ convey relaxation of smooth muscle via activation of small and intermediate conductance Ca²⁺-activated K⁺ channels (SK_Ca and IK_Ca) and nitric oxide (NO). A primary reactive oxygen species signalling molecule is hydrogen peroxide (H₂O₂) derived from mitochondria (Mito). Targets of reactive oxygen species (indicated by red jagged outlines) include endothelial and myoendothelial gap junctions, and the Ca²⁺ handling proteins which include transient receptor potential (TRP) channels, sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA), inositol trisphosphate receptors (IP₃R), SK_Ca/IK_Ca and endothelial nitric oxide synthase (eNOS). Each of these sites may be affected during ageing as a consequence of increased oxidative stress.