Thesis Format

Integrated Article

Functional Bias of Contractile Mechanisms in Resistance Arteries

Author

Nadia Haghbin, Western UniversityFollow

Degree

Doctor of Philosophy

Program

Physiology and Pharmacology

Supervisor

Dr. Donald G. Welsh

Abstract

Constrictor stimuli regulate arterial tone, the degree of tension in the arterial walls, through two distinct coupling mechanisms: electromechanical (dependent on membrane potential) and pharmacomechanical (independent of membrane potential). Electromechanical coupling involves the opening of voltage-dependent L-type Ca²⁺ channels leading to muscle contraction, whereas pharmacomechanical coupling relies on intracellular signaling pathways activated by G-protein-coupled receptor (GPCR) agonists. The goal of this study is to investigate whether the contributions of each mechanism are fixed and proportionate, or variable and functionally biased. To achieve this goal, we formulated three specific objectives. First, we determined whether constrictor agonists elicit functionally biased responses in mouse cerebral and mesenteric arteries. We examined whether G_q/11 and G_q/11/G_12/13 coupled receptor agonists (phenylephrine and U46619) elicit responses biased towards electro- or pharmacomechanical coupling, depending on concentration, the manner of agent application and the intrinsic properties of receptors and tissue. Our results showed that both agonists induced concentration-dependent constrictions, initially via electromechanical coupling, with pharmacomechanical coupling becoming prominent at higher concentrations, indicating functional bias. Phenylephrine-induced pharmacomechanical coupling was linked to protein kinase C (PKC) activity, while U46619 involved both PKC and Rho-kinase. Further, a functional assessment complemented by computational modeling revealed a complete switch to pharmacomechanical coupling when stimulation was restricted to a small portion of artery. Second, we identified the PKC isoforms and targets involved in pharmacomechanical coupling. Our investigation revealed that PKCδ, rather than PKCα, mediated the pharmacomechanical response to U46619. This was evidenced by PKCδ's translocation to the membrane upon agonist application and its regulation through CPI-17, HSP-27, and caldesmon. Lastly, we explored the role of PKC and pharmacomechanical coupling in arterial constriction in cerebral arteries and in-vivo. We explored the role of PKC and pharmacomechanical coupling in cerebral artery constriction and observed their effects in-vivo. Using two photon (2P) fluorescence imaging, we monitored diameter variations along penetrating arterioles in live brain tissue. Inhibition of PKCδ, but not pan-PKC or L-type Ca²⁺ channels, significantly reduced these diameter variations, confirming the role of PKCδ in focal constrictions in-vivo. The complexity of excitation-contraction coupling in vascular tissue, encompassing both electromechanical and pharmacomechanical components, underscores the functional bias in vasomotor responses. These insights enhance our understanding of hemodynamic regulation and its potential dysregulation in diseases such as hypertension and arterial vasospasm, potentially opening new avenues for targeted therapeutic interventions.

Summary for Lay Audience

The cardiovascular system, which includes the heart and blood vessels, is essential for delivering blood and nutrients throughout the body. Resistance arteries are small blood vessels that help regulate blood pressure by narrowing or widening. This study explores the detailed processes that control how these arteries change their tightness, focusing on two main methods: electromechanical and pharmacomechanical coupling. Electromechanical coupling involves quick changes in electrical charge across cell membranes, leading to smooth muscle contraction. In contrast, pharmacomechanical coupling depends on internal chemical signals that function independently of electrical changes. Our research aimed to determine whether these methods always work in the same way or if they can change based on different conditions. We found that both mechanisms help arteries constrict, but their importance shifts depending on the situation. For instance, a chemical trigger, like classic G protein coupled receptor agonists, initially causes contraction through electromechanical coupling. However, as the chemical concentration increases or becomes more localized, the artery mainly uses pharmacomechanical coupling. We further examined the specific pathways and proteins involved, focusing on protein kinase C subtype δ, which we found is crucial for pharmacomechanical coupling, especially in the brain’s small arteries. Observations of live brain tissue showed that blocking protein kinase C δ significantly reduced changes in artery diameter. Understanding these processes is vital because they are key to regulating blood flow and oxygen delivery. Disruptions can lead to serious conditions such as high blood pressure and cerebral artery spasms. By highlighting the interaction between electromechanical and pharmacomechanical coupling in resistance arteries, our research suggests new possibilities for potential treatments.

Recommended Citation

Haghbin, Nadia, "Functional Bias of Contractile Mechanisms in Resistance Arteries" (2024). Electronic Thesis and Dissertation Repository. 10480.
https://ir.lib.uwo.ca/etd/10480