Lecture 5: Hormonal Control and signaling

Define the relationship between cardiac output, total peripheral resistance, and blood pressure

Blood pressure (BP) is determined by the equation BP = CO × TPR. Cardiac output (CO) depends on heart rate (HR) and stroke volume (SV). Total peripheral resistance (TPR), also called systemic vascular resistance, is mainly determined by vessel radius, but viscosity and vessel length can also contribute. Small changes in vessel radius have large effects on resistance (r⁴ relationship).

Explain how vascular resistance relates to vessel radius

According to the Poiseuille–Hagen equation, resistance is inversely proportional to the fourth power of the radius. Thus, a small decrease in radius greatly increases resistance and blood pressure. Arterioles, with adjustable smooth muscle tone, are the primary regulators of vascular resistance.

Describe the structural characteristics of vascular smooth muscle

Vascular smooth muscle cells (VSMCs) contain caveolae (surface invaginations) and poorly developed sarcoplasmic reticulum. They have actin and myosin anchored by dense bodies (not sarcomeres) and lack troponin. Contractions are slow, sustained, and normally maintain resting tone under partial contraction.

Explain the mechanisms of smooth muscle contraction

Contraction begins when intracellular Ca²⁺ rises (via voltage-gated L-type Ca²⁺ channels or release from the SR). Ca²⁺ binds to calmodulin, forming a Ca²⁺–calmodulin complex that activates myosin light chain kinase (MLCK). MLCK phosphorylates myosin light chain (MLC20) at Ser19, enabling myosin–actin cross-bridging and contraction.

Describe ligand-mediated smooth muscle constriction

Ligand binding to Gq/11-coupled GPCRs (e.g., Ang II–AT₁R, α₁-AR) activates phospholipase C (PLC) → generates IP₃ → releases Ca²⁺ from the SR. Ca²⁺ then activates the Ca²⁺–calmodulin–MLCK pathway. Some ligands also activate L-type Ca²⁺ channels directly, increasing cytosolic Ca²⁺ and contraction.

Explain mechanical (myogenic) smooth muscle constriction

Stretching of vascular smooth muscle activates mechanosensitive ion channels, leading to Ca²⁺ influx and contraction — the myogenic response. This maintains constant blood flow despite changes in pressure, especially in resistance arteries and arterioles.

Identify the key molecular target in smooth muscle contraction

The regulatory myosin light chain MLC20 controls contraction. Phosphorylation of Ser19 by MLCK activates myosin ATPase, promoting contraction. MLC20 phosphorylation can also occur through Ca²⁺-independent pathways (e.g., RhoA/ROCK signaling), enhancing contraction.

Explain how smooth muscle relaxation occurs

Relaxation requires removal of phosphate groups from MLC20 by myosin light chain phosphatase (MLCP). MLCP activity is regulated by phosphorylation of its subunit MYPT1; phosphorylation inhibits MLCP, maintaining contraction. Thus, activation of MLCP is essential for vasodilation.

List and describe the main mechanisms of smooth muscle relaxation

1. Decrease Ca²⁺ entry (block L-type Ca²⁺ channels or hyperpolarize membrane).

2. Increase MLCP activity (via cAMP or cGMP signaling).

3. Direct vasodilator signaling (via NO, β-AR, or prostacyclin).

Relaxation is an active process requiring signaling to oppose Ca²⁺-dependent contraction.

Describe how cAMP signaling induces vasorelaxation

Adenylyl cyclase converts ATP to cAMP after Gs-coupled receptor activation (e.g., β₂-AR, prostacyclin-IP receptor). cAMP activates protein kinase A (PKA), which phosphorylates multiple targets:

• Inhibits RhoA (reduces MLCP inhibition)

• Activates K⁺ channels (hyperpolarization)

• Reduces Ca²⁺ entry and increases Ca²⁺ reuptake

→ Net effect: decreased cytosolic Ca²⁺ and increased MLCP → vasorelaxation.

Explain the role of phosphodiesterases (PDEs) in cyclic nucleotide signaling

PDEs degrade cAMP and cGMP, controlling spatial and temporal signaling. PDE inhibitors (e.g., theophylline, sildenafil) prolong vasodilation by preventing breakdown of cAMP or cGMP.

Describe the discovery and role of nitric oxide (NO) in vasorelaxation

NO was identified as endothelium-derived relaxing factor (EDRF) and recognized with the 1998 Nobel Prize. NO is synthesized from L-arginine by nitric oxide synthases (NOS) — mainly eNOS in endothelium, nNOS in neurons, and iNOS during inflammation. NO diffuses into smooth muscle and activates soluble guanylyl cyclase (sGC) → increases cGMP → activates protein kinase G (PKG) → vasorelaxation.

Differentiate between eNOS, nNOS, and iNOS

• eNOS: constitutively active, Ca²⁺/calmodulin-dependent; found in endothelium.

• nNOS: constitutive, Ca²⁺-dependent; found in neurons and other tissues.

• iNOS: inducible, Ca²⁺-independent; expressed during inflammation, producing high NO levels.

Explain the role of cGMP and PKG in smooth muscle relaxation

NO-activated sGC converts GTP → cGMP. cGMP activates PKG, which phosphorylates targets similar to PKA: inhibits Ca²⁺ influx, enhances Ca²⁺ reuptake, opens K⁺ channels, and stimulates MLCP. This results in relaxation of vascular smooth muscle.

Describe the role of the endothelium in vascular tone

The endothelium forms a monolayer lining blood vessels and releases vasoactive substances that regulate smooth muscle tone.

Vasodilators: NO, prostacyclin, acetylcholine (M₃), bradykinin (B₂).

Vasoconstrictors: Ang II (AT₁R), norepinephrine (α₁), endothelin (ETA).

Flow, hormones, and drugs can all modulate endothelial release.

Differentiate between endothelial and smooth muscle endothelin signaling

Endothelin binds ETA receptors (Gq) on smooth muscle → constriction.

On endothelium, ETB receptors (Gq) cause NO release → vasodilation.

Thus, endothelin has both constrictive and dilatory roles depending on receptor localization.

Describe autonomic regulation of vascular tone

Sympathetic nerves release norepinephrine (NE), which binds α₁-AR (Gq) for vasoconstriction. Epinephrine (Epi) from the adrenal medulla acts on β₂-AR (Gs) to cause vasodilation. Most blood vessels lack parasympathetic innervation; however, acetylcholine on endothelial M₃ (Gq) receptors stimulates NO release and vasodilation.

Explain how smooth muscle contraction and relaxation are pharmacological targets

Multiple drug classes act on these pathways:

• Ca²⁺ channel blockers: inhibit contraction (↓Ca²⁺ influx).

• β₂-agonists: increase cAMP → relaxation.

• NO donors (nitrates): activate sGC → cGMP → relaxation.

• α₁-antagonists: block Gq signaling → vasodilation.

• PDE inhibitors: prolong cAMP/cGMP → enhanced relaxation.

Summarize integrated signaling controlling vascular tone

Vascular tone results from balance between Ca²⁺-dependent constriction (via MLCK activation) and cAMP/cGMP-mediated relaxation (via MLCP activation). Endothelial factors (NO, prostacyclin, endothelin) and autonomic inputs (NE, Epi, ACh) continuously adjust tone to regulate blood pressure and perfusion.