The capillary endothelium is a single layer of cells lining the inner surface of blood vessels and plays an essential role in vascular homeostasis by synthesizing and releasing several biologically active substances that regulate vascular tone, blood flow, platelet aggregation, and inflammation.

Substances Commonly Secreted by Endothelial Cells

1. Prostacyclin (PGI2):

   • Secreted by endothelial cells and acts as a potent vasodilator and inhibitor of platelet aggregation.

   • Helps maintain blood fluidity and prevents thrombosis.pubmed.ncbi.nlm.nih+1

2. Endothelin:

   • A powerful vasoconstrictor peptide produced by endothelial cells.

   • Regulates vascular tone and blood pressure, opposing prostacyclin and nitric oxide actions.pubmed.ncbi.nlm.nih

3. Nitric Oxide (NO):

   • Synthesized by endothelial nitric oxide synthase (eNOS).

   • Causes smooth muscle relaxation leading to vasodilation and inhibits platelet aggregation.clevelandclinic+1

The Exception: Guanosine

• Guanosine is a nucleoside, a breakdown product of guanine nucleotides.

• It is NOT a secreted substance of endothelial cells.

• While guanosine and its derivatives may have biological roles, they are not recognized as active secretory products regulating vascular functions by the endothelium.

Summary Table

Conclusion

The substance that is NOT secreted by capillary endothelium is:

(2) Guanosine

Additional Information

Endothelial cells balance the production of vasodilators and vasoconstrictors to fine-tune vascular tone and maintain cardiovascular health. Understanding these secretory functions is central to vascular biology and pathology.