[Paper Review] Modelling the Role of Nitric Oxide in Cerebral Autoregulation

Mark Catherall|arXiv (Cornell University)|Jan 1, 2014

Traumatic Brain Injury and Neurovascular Disturbances11 references3 citations

TL;DR

This study develops a novel mathematical model integrating nitric oxide (NO) signaling with the myogenic response in cerebral arterioles to investigate their roles in cerebral autoregulation. The model demonstrates that while NO is critical for setting the baseline vessel radius, it plays a negligible role in dynamic responses to blood pressure changes, with calcium dynamics in vascular smooth muscle cells being the primary determinant of system stability and vasomotion.

ABSTRACT

Malfunction of the system which regulates the bloodflow in the brain is a major cause of stroke and dementia, costing many lives and many billions of pounds each year in the UK alone. This regulatory system, known as <em>cerebral autoregulation</em>, has been the subject of much experimental and mathematical investigation yet our understanding of it is still quite limited. One area in which our understanding is particularly lacking is that of the role of nitric oxide, understood to be a potent vasodilator. The interactions of nitric oxide with the better understood myogenic response remain un-modelled and poorly understood. In this thesis we present a novel model of the arteriolar control mechanism, comprising a mixture of well-established and new models of individual processes, brought together for the first time. We show that this model is capable of reproducing experimentally observed behaviour very closely and go on to investigate its stability in the context of the vasculature of the whole brain. In conclusion we find that nitric oxide, although it plays a central role in determining equilibrium vessel radius, is unimportant to the dynamics of the system and its responses to variation in arterial blood pressure. We also find that the stability of the system is very sensitive to the dynamics of Ca²⁺ within the muscle cell, and that self-sustaining Ca<em>2+</em> waves are not necessary to cause whole-vessel radius oscillations consistent with vasomotion.

Motivation & Objective

• To resolve the unresolved role of nitric oxide (NO) in cerebral autoregulation, particularly its interaction with the myogenic response.
• To develop a comprehensive mathematical model integrating NO signaling, calcium dynamics, and mechanical feedback in arterioles.
• To test whether NO is essential for dynamic regulation or merely sets the equilibrium vessel radius.
• To investigate the mechanisms underlying spontaneous vasomotion and the necessity of coordinated calcium waves.
• To evaluate the stability and dynamic behavior of the autoregulatory system under varying hemodynamic conditions.

Proposed method

• Developed a multi-component model combining established and novel submodels: NO production, cGMP signaling, 4-state calcium kinetics, and mechanical vessel wall dynamics.
• Integrated the arteriolar model into a whole-brain hemodynamic framework to assess systemic stability and flow regulation.
• Used steady-state and dynamic simulations to validate model behavior against experimental data on blood flow velocity and pressure responses.
• Applied a novel sensitivity analysis method to assess the impact of parameter variations on system dynamics, particularly focusing on calcium time constants.
• Conducted stability analysis using eigenvalue and frequency-domain methods to identify conditions for limit-cycle oscillations.
• Explored the role of intracranial compliance and flow inertia in enabling spontaneous vasomotion without requiring pre-existing calcium waves.

Experimental results

Research questions

• RQ1 What is the relative contribution of nitric oxide (NO) versus calcium dynamics to the dynamic regulation of cerebral blood flow?
• RQ2 Is coordinated whole-cell calcium wave propagation necessary to initiate vasomotion in cerebral arterioles?
• RQ3 How does the inclusion of NO signaling affect the stability and response characteristics of the autoregulatory system?
• RQ4 Can the model reproduce experimentally observed ABP-CBFV correlations under eNOS blockade and phenylephrine stimulation?
• RQ5 What are the key system parameters that determine the onset of spontaneous vasomotion in the absence of pre-existing calcium waves?

Key findings

• Nitric oxide (NO) plays a dominant role in determining the equilibrium radius of cerebral arterioles but has negligible influence on the dynamic response to changes in arterial blood pressure.
• The stability and dynamic behavior of the autoregulatory system are highly sensitive to the time constant of intracellular calcium (Ca2+) dynamics, not to NO signaling.
• Spontaneous vasomotion can emerge from feedback between mechanical and biochemical components of the myogenic response without requiring self-sustaining calcium waves.
• The model reproduces experimentally observed ABP-CBFV correlations under both eNOS blockade and phenylephrine stimulation, explaining the lack of difference between these conditions.
• The 4-state kinetic model of calcium-dependent myosin phosphorylation shows that cGMP (downstream of NO) only modulates contractility in the mid-range of calcium concentrations, becoming ineffective at high [Ca2+].
• The model suggests that eNOS blockade and phenylephrine stimulation may achieve similar hemodynamic outcomes by shifting the system to equivalent operating points, despite different underlying mechanisms.