Regulation of Vascular Smooth Muscle Tone

The Action of IP3 and DAG

The two second messengers work in tandem to elevate calcium levels:

1. IP3 travels to the sarcoplasmic reticulum (the cell's internal calcium store) and triggers the release of stored $Ca^{2+}$ into the cytoplasm.
2. DAG activates protein kinase C, which facilitates the influx of additional $Ca^{2+}$ from the extracellular space through voltage-gated channels in the sarcolemma.

The resulting surge in intracellular calcium causes the smooth muscle to contract, narrowing the vessel lumen and increasing vascular resistance.

Vasodilation: The Nitric Oxide cGMP Pathway

To counteract constriction and promote blood flow, the body utilizes pathways that lower intracellular calcium. One of the most important is the Nitric Oxide (NO) cGMP pathway.

Nitric oxide is a potent smooth muscle relaxant. Its production is stimulated by various factors, including chemical mediators like acetylcholine and bradykinin, or physical forces such as shear stress (the frictional force of blood flowing against the vessel wall).

The Biochemical Cascade

The process follows a specific enzymatic sequence:

1. Nitric Oxide Synthetase (NOS), an enzyme in the endothelium, converts the amino acid L-arginine into Nitric Oxide (NO).
2. The NO molecule diffuses from the endothelial cells into the adjacent smooth muscle cells.
3. Once inside the smooth muscle, NO activates the enzyme guanylate cyclase.
4. Guanylate cyclase converts GTP into cyclic GMP (cGMP), a second messenger that signals the cell to reduce its intracellular calcium levels.
5. As calcium levels drop, the muscle relaxes, resulting in vasodilation.

To ensure the signal is not permanent, the enzyme phosphodiesterase (PDE) eventually deactivates cGMP, converting it back to GMP and terminating the relaxation signal.

Vasodilation: The G-protein cAMP Pathway

A third major pathway involves the G-protein cAMP pathway. It is important to note a key physiological distinction: while increased cAMP in cardiac myocytes generally increases contractility, in vascular smooth muscle, increased cAMP leads to relaxation.

For example, when norepinephrine binds to Beta-2 receptors on a vascular smooth muscle cell, it activates Gs proteins. This activation stimulates adenyl cyclase to increase the production of cyclic AMP (cAMP), which in turn activates Protein Kinase A (PKA).

How Protein Kinase A (PKA) Promotes Relaxation

PKA acts as a multi-functional regulator that lowers intracellular calcium through several distinct mechanisms:

• Inhibiting voltage-gated $Ca^{2+}$ channels: This prevents calcium from entering the cell from the outside.
• Inhibiting $Ca^{2+}$ release: It blocks the release of calcium from the sarcoplasmic reticulum.
• Reducing myofilament sensitivity: It makes the contractile proteins less responsive to whatever calcium is still present.
• Facilitating $Ca^{2+}$ reuptake: It helps the SERCA2 pump move calcium back into the sarcoplasmic reticulum more efficiently, clearing it from the cytoplasm.

Through these combined actions, the cAMP/PKA pathway effectively drives the vessel toward a dilated state.