Questions: Vascular Smooth Muscle and Autoregulation

Question 1

A patient's systemic blood pressure rises sharply. According to the myogenic mechanism of cerebral autoregulation, what happens to brain arteriolar diameter?

• A. The arterioles dilate to accommodate the higher pressure and maintain constant flow velocity
• B. The arterioles constrict because elevated pressure stretches the vessel wall, depolarizing smooth muscle and triggering calcium-mediated contraction
• C. The arterioles are unaffected because the brain's oxygen demand drives flow, not perfusion pressure
• D. The arterioles first dilate, then constrict after endothelial NO release is suppressed by high shear stress

Answer: (B)

The myogenic mechanism (Bayliss effect) is a direct, intrinsic response of vascular smooth muscle to stretch. When blood pressure rises, vessel wall tension increases, mechanically depolarizing the smooth muscle cell membrane and opening voltage-gated calcium channels. The resulting calcium influx activates the calmodulin–MLCK pathway, causing contraction and arteriolar narrowing. This is the opposite of what naive intuition suggests (more pressure → more flow), but it protects downstream capillaries from pressure overload and helps maintain constant cerebral blood flow.

Question 2

During vigorous exercise, blood pressure rises AND skeletal muscle metabolic activity increases dramatically. In the exercising muscle, blood flow:

• A. Decreases, because the myogenic response to elevated blood pressure causes arteriolar constriction in the muscle
• B. Increases substantially, because metabolic vasodilators produced by active muscle (adenosine, CO₂, lactate, K⁺, low PO₂) override myogenic constriction and dilate local arterioles
• C. Stays constant, because autoregulation maintains fixed flow regardless of metabolic state
• D. Increases only if cardiac output rises proportionally to supply more blood to working muscle

Answer: (B)

Metabolic autoregulation overrides myogenic tone when tissue demand is high. Exercising muscle produces adenosine, CO₂, lactic acid, K⁺, and reduced oxygen tension — all of which act locally on arteriolar smooth muscle to cause relaxation and vasodilation. This metabolic signal is proportional to the degree of activity: more active muscle produces more vasodilators and receives more blood flow. The result is a tight match between local metabolic demand and local blood supply, independent of systemic blood pressure changes.

Question 3

Vascular smooth muscle contraction uses the same molecular trigger as skeletal muscle: calcium binds troponin, which unblocks myosin-binding sites on actin filaments.

• T. True
• F. False

Answer: False

Vascular smooth muscle lacks troponin. Instead, calcium binds calmodulin, and the calcium–calmodulin complex activates myosin light chain kinase (MLCK), which phosphorylates myosin to enable cross-bridge cycling. Relaxation requires a phosphatase to remove the phosphate from myosin. This mechanism is slower and produces sustained tonic contraction appropriate for regulating vessel diameter, in contrast to the rapid, discrete twitches of skeletal muscle mediated by the troponin–tropomyosin system.

Question 4

Nitric oxide (NO) produced by endothelial cells causes vasodilation by ultimately reducing intracellular calcium in vascular smooth muscle through a cGMP-mediated signaling pathway.

• T. True
• F. False

Answer: True

NO diffuses from endothelial cells into underlying smooth muscle and activates guanylyl cyclase, raising cGMP. Elevated cGMP activates protein kinase G (PKG), which phosphorylates targets that reduce intracellular calcium — including inhibition of calcium entry channels and stimulation of the myosin phosphatase that dephosphorylates (inactivates) myosin. The net result is smooth muscle relaxation and vasodilation. Drugs like nitroglycerin work by releasing NO through this same pathway.

Question 5

Explain the myogenic mechanism of autoregulation (the Bayliss effect) and why it seems paradoxical: why does a rise in blood pressure cause arterioles to constrict rather than to dilate and allow more flow?

Model answer: The Bayliss effect is a direct mechanical response of smooth muscle to vessel wall stretch. When perfusion pressure rises, the arteriolar wall is stretched more. This mechanical stretch directly depolarizes the smooth muscle cell membrane — likely through mechanosensitive ion channels — opening voltage-gated calcium channels and triggering contraction via calmodulin and MLCK. The vessel narrows, increasing its resistance, and blood flow is maintained nearly constant despite the higher pressure. This seems paradoxical because in most physical systems, higher pressure produces more flow. But the myogenic response is a protective reflex: it prevents fragile downstream capillaries from being exposed to dangerous pressure levels and maintains steady tissue perfusion across a wide range of systemic pressures.

The key insight is that resistance, not pressure, determines flow in a regulated system. By constricting when pressure rises, the arteriole increases its resistance proportionally, keeping flow (= pressure / resistance) approximately constant. This active regulation is the definition of autoregulation.