Figure 112

Endothelial-derived factors. In addition to serving as a diffusion barrier, the endothelial cells lining the vasculature participate actively in the regulation of vascular function. They do so by responding to various circulating hormones and physical stimuli and releasing paracrine agents that alter vascular smooth muscle tone and influence tubular transport function. (Examples are shown.) Angiotensin-converting enzyme (ACE) is present on endothelial cells and converts angiotensin I to angiotensin II. Nitric oxide is formed by nitric oxide synthase, which cleaves nitric oxide from L-arginine. Nitric oxide diffuses from the endothelial cells to activate soluble guanylate cyclase and increases cyclic GMP (cGMP) levels in vascular smooth muscle cells, thus causing vasodilation. Agents that can stimulate nitric oxide are shown. The relative amounts of the various factors released by endothelial cells depend on the physiologic circumstances and pathophysiologic status. Thus, endothelial cells can exert vasodilator or vasoconstrictor effects. At least one major influence participating in the normal regulation of vascular tone is nitric oxide. EDCF—endothelial derived constrictor factor; EDHF—endothelial derived hyper-polarizing factor; PGF2a—prostaglandin F2a; PGI2—prostaglandin I2; TXA2— thromboxane A2. (Adapted from Navar et al. [16].)

t Renal arterial pressure n 3

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