Disruption of Pressure-Induced Ca2+ Spark Vasoregulation of Resistance Arteries, Rather Than Endothelial Dysfunction, Underlies Obesity-Related Hypertension

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Obesity-related hypertension is one of the world’s leading causes of death and yet little is understood as to how it develops. As a result, effective targeted therapies are lacking and pharmacological treatment is unfocused. To investigate underlying microvascular mechanisms, we studied small artery dysfunction in a high fat–fed mouse model of obesity. Pressure-induced constriction and responses to endothelial and vascular smooth muscle agonists were studied using myography; the corresponding intracellular Ca²⁺ signaling pathways were examined using confocal microscopy. Principally, we observed that the enhanced basal tone of mesenteric resistance arteries was due to failure of intraluminal pressure-induced Ca²⁺ spark activation of the large conductance Ca²⁺ activated K⁺ potassium channel (BK) within vascular smooth muscle cells. Specifically, the uncoupling site of this mechanotransduction pathway was at the sarcoplasmic reticulum, distal to intraluminal pressure-induced oxidation of Protein Kinase G. In contrast, the vasodilatory function of the endothelium and the underlying endothelial IP-3 and TRPV4 (vanilloid 4 transient receptor potential ion channel) Ca²⁺ signaling pathways were not affected by the high-fat diet or the elevated blood pressure. There were no structural alterations of the arterial wall. Our work emphasizes the importance of the intricate cellular pathway by which intraluminal pressure maintains Ca²⁺ spark vasoregulation in the origin of obesity-related hypertension and suggests previously unsuspected avenues for pharmacological intervention.

"A Step and a Ceiling": mechanical properties of Ca2+ spark vasoregulation in resistance arteries by pressure-induced oxidative activation of PKG

We investigated the biomechanical relationship between intraluminal pressure within small mesenteric resistance arteries, oxidant activation of PKG, Ca2+ sparks, and BK channel vasoregulation. Mesenteric resistance arteries from wild type (WT) and genetically modified mice with PKG resistance to oxidative activation were studied using wire and pressure myography. Ca2+ sparks and Ca2+ transients within vascular smooth muscle cells of intact arteries were characterized using high-speed confocal microscopy of intact arteries. Arteries were studied under conditions of varying intraluminal pressure and oxidation. Intraluminal pressure specifically, rather than the generic stretch of the artery, was necessary to activate the oxidative pathway. We demonstrated a graded step activation profile for the generation of Ca2+ sparks and also a functional "ceiling" for this pressure --sensitive oxidative pathway. During steady state pressure - induced constriction, any additional Ca2+ sensitive-K+ channel functional availability was independent of oxidant activated PKG. There was an increase in the amplitude, but not the Area under the Curve (AUC) of the caffeine-induced Ca2+ transient in pressurized arteries from mice with oxidant-resistant PKG compared with wild type. Overall, we surmise that intraluminal pressure within resistance arteries controls Ca2+ spark vasoregulation through a tightly controlled pathway with a graded onset switch. The pathway, underpinned by oxidant activation of PKG, cannot be further boosted by additional pressure or oxidation once active. We propose that these restrictive characteristics of pressure-induced Ca2+ spark vasoregulation confer stability for the artery in order to provide a constant flow independent of additional pressure fluctuations or exogenous oxidants.

Pressure-induced oxidative activation of PKG enables vasoregulation by Ca2+ sparks and BK channels

Activation of Ca²⁺-sensitive, large-conductance potassium (BK) channels in vascular smooth muscle cells (VSMCs) by local, ryanodine receptor-mediated Ca²⁺ signals (Ca²⁺ sparks) acts as a brake on pressure-induced (myogenic) vasoconstriction-a fundamental mechanism that regulates blood flow in small resistance arteries. We report that physiological intraluminal pressure within resistance arteries activated cGMP-dependent protein kinase (PKG) in VSMCs through oxidant-induced formation of an intermolecular disulfide bond between cysteine residues. Oxidant-activated PKG was required to trigger Ca²⁺ sparks, BK channel activity, and vasodilation in response to pressure. VSMCs from arteries from mice expressing a form of PKG that could not be activated by oxidants showed reduced Ca²⁺ spark frequency, and arterial preparations from these mice had decreased pressure-induced activation of BK channels. Thus, the absence of oxidative activation of PKG disabled the BK channel-mediated negative feedback regulation of vasoconstriction. Our results support the concept of a negative feedback control mechanism that regulates arterial diameter through mechanosensitive production of oxidants to activate PKG and enhance Ca²⁺ sparks.