4.5 Vascular E/C Coupling and Control of Vasculature Flashcards
Describe EC coupling
Excitation-contraction (ec) coupling is the mechanism that links activation of the cell to contraction of the cell. The activation can be membrane depolarisation (for skeletal or cardiac muscle), activation of receptor operated ion channels (for smooth muscle), or activation of secondary messenger systems (for smooth muscle). All of these then cause cellular contraction which mostly occurs through raising of intracellular calcium levels.
What is the impact of vascular smooth muscle contraction
Vascular smooth muscle is continuously active, but can be contracted more and can be relaxed more. It’s level of contraction determines the vascular dimensions, this affects the vascular flow and blood pressure
What is blood pressure and how is it determined.
Blood pressure is the force exerted by circulating blood on the walls of blood vessels.
Blood pressure is determined by the cardiac output (the amount of blood pumped by the heart in a minute) multiplied by the total peripheral resistance (the resistance offered by blood vessels to blood flow).
What is blood flow
Blood flow is the volume of blood moving through a vessel in a unit of time, so an increase in vessel diameter will increase blood flow.
What happens to blood pressure and blood flow during vasoconstriction.
During vasoconstriction, the smooth muscle contracts, narrowing the vessel lumen. This increases total peripheral resistance causing a rise in blood pressure. This also decreases the vessel diameter causing a decrease in blood flow.
What happens to blood pressure and blood flow during vasodilation.
During vasodilation, the smooth muscle relaxes, widening the vessel lumen. This decreases total peripheral resistance, causing a fall in blood pressure. This also increases the vessel diameter causing an increase in blood flow.
What are some chemical factors that influence smooth muscle contraction
metabolism, paracrine effects and local hormones
What are some physical factors that influence smooth muscle contraction
blood pressure and flow
What is hyperaemia
Increased blood flow
What is autoregulation
Autoregulation is the ability of vessels to maintain a stable blood flow despite changes in blood pressure. This is particularly important in the brain and heart circulation.
Describe some different principles of blood flow regulation
Re-active hyperaemia:
When blood supply has been blocked off for a while and metabolites build up, these metabolites then cause vascular dilation in order to ‘repay’ the nutrient debt to the tissues
Active hyperaemia:
During exercise, more metabolites build up, causing vasodilation
Myogenic tone:
Myogenic tone is where when pressure increases it causes the smooth muscle to stretch. This triggers contraction to prevent excessive blood flow. *Contraction is known as smooth muscle tone. This allows for autoregulation.
Flow-dependent vasodilation:
When the blood flow increases, the smooth muscle relaxes. Via nitric oxide released from endothelium
Describe the two types of smooth muscle contraction and where they are used.
The two types of smooth muscle contraction are isotonic (actin and myosin interactions causing shortened filaments to narrow vessel) and isometric (maintaining tension against blood pressure without narrowing the vessel).
Isotonic contractions are important in smaller resistance arteries, these regulate blood flow and control blood pressure by adjusting vessel diameter.
Isometric contractions are important in larger conducting arteries (e.g aorta), these are known as Windkessel vessels. They have elastic properties allowing them to stretch and recoil to maintain flow and pressure. Dense body and intermediate filament structures allow for the isometric contraction.
Describe the variety of factors that link smooth muscle excitation to smooth muscle contraction
There are many mechanisms from outside of the cell with a broad spectrum of receptor and channel types (e.g receptor operated channels, calcium channels for contraction, potassium channels for relaxation, receptors with secondary messengers, transmembrane receptors etc) which respond to different stimuli such as hormones, metabolites and physical forces. All these regulators can induce both positive and negative regulation of contraction.
Describe the basic E-C coupling process
As intracellular calcium levels increase, 4 calcium ions bind to calmodulin and form a calcium calmodulin complex in order to activate the calmodulin. The calmodulin can then activate myosin light chain kinase (MLCK). The MLCK can then phosphorylate mysoin using an ATP molecule and the actin and myosin can then interact to generate force (also using an ATP molecule).
What are some of the EC coupling regulators that are also involved in flow regulation
Adenosine - a metabolite that binds to cell surface receptors to induce re-active hyperaemia aswell as inducing muscle relaxation
Calcium channels - cause contraction and thus influence myogenic tone, affecting flow
Nitric Oxide - is released from the endothelium to cause flow dependent vasodilation (causing muscle relaxation and increased flow)
What are the 2 types of ways intracellular calcium concentration is increased
Most of the calcium regulation comes from calcium ions entering into the cell but some calcium ions are also released from stores inside the cell by secondary messengers like IP3
How can vascular diseases alter smooth muscle EC coupling
The EC coupling process can become deranged in some vascular diseases, for example, calcium entering smooth muscle may be altered in atherosclerosis
