Paracrine Regulation of Blood Flow

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Paracrine regulators, as described in chapter 11, are molecules produced by one tissue that help to regulate another tissue of the same organ. Blood vessels are particularly subject to paracrine regulation. Specifically, the endothelium of the tunica interna produces a number of paracrine regulators that cause the smooth muscle of the tunica media to either relax or contract.

The endothelium produces several molecules that promote smooth muscle relaxation, including nitric oxide, bradykinin, and prostacyclin (chapter 11). The endothelium-derived relaxation factor that earlier research had shown to be required for the vasodilation response to nerve stimulation appears to be nitric oxide.

The endothelium of arterioles contains an enzyme, endothe-lial nitric oxide synthase (eNOS), which produces nitric oxide (NO) from L-arginine. The NO diffuses into the smooth muscle cells of the tunica media of arterioles and activates the enzyme guanylate cyclase, which converts GTP into cyclic GMP (cGMP) and pyrophosphate (PPi). The cGMP serves as a second messenger that, through a variety of mechanisms, acts to lower the cytoplasmic Ca2+ concentration. This leads to smooth muscle relaxation and thus vasodilation (see chapter 20, fig. 20.23). In many arterioles, a baseline level of NO production helps regulate the resting "tone" (degree of vasoconstriction/vasodilation) of the arterioles. The production of NO can be increased by ACh released from parasympathetic axons, which acts via the Ca2+-calmodulin system (chapter 11) to stimulate nitric oxide synthase in the endothelial cells of blood vessels.

Cocaine inhibits the reuptake of norepinephrine into the adrenergic axons, resulting in enhanced sympathetic-induced vasoconstriction. Chest pain, as a result of myocardial ischemia produced in this way, is a common cocaine-related problem. The nicotine from cigarette smoke acts synergistically with cocaine to induce vasoconstriction.

Table 14.4

Extrinsic Control of Vascular Resistance and Blood Flow

Extrinsic Agent



Sympathetic nerves



Vasoconstriction is the dominant effect of sympathetic nerve stimulation on the vascular system, and it occurs throughout the body.



There is some activity in arterioles in skeletal muscles and in coronary vessels, but effects are masked by

dominant alpha-receptor-mediated constriction.



Effects are localized to arterioles in skeletal muscles and are produced only during defense (fight-or-flight) reactions.

Parasympathetic nerves


Effects are restricted primarily to the gastrointestinal tract, external genitalia, and salivary glands and

have little effect on total peripheral resistance.

Angiotensin II


A powerful vasoconstrictor produced as a result of secretion of renin from the kidneys, it may function to help maintain adequate filtration pressure in the kidneys when systemic blood flow and pressure are reduced.

ADH (vasopressin)


Although the effects of this hormone on vascular resistance and blood pressure in anesthetized animals

are well documented, the importance of these effects in conscious humans is controversial.



Histamine promotes localized vasodilation during inflammation and allergic reactions.



Bradykinins are polypeptides secreted by sweat glands and by the endothelium of blood vessels; they promote local vasodilation.


Vasodilation or vasoconstriction

Prostaglandins are cyclic fatty acids that can be produced by most tissues, including blood vessel walls. Prostaglandin I2 is a vasodilator, whereas thromboxane A2 is a vasoconstrictor. The physiological significance of these effects is presently controversial.

In addition to serving as a paracrine regulator locally within a blood vessel, nitric oxide can also be carried by red blood cells because it binds to the sulfur atoms of the cysteines in hemoglobin. Red blood cells carry nitric oxide from areas where the oxygen concentration is high (nitric oxide is produced in the lungs) to areas where it is low. The lower the oxygen concentration, the more nitric oxide will be released from the red blood cells to cause vasodilation. The vasodilation then produces increased blood flow and delivery of oxygen to the tissue. This effect benefits ischemic tissues. Indeed, vasodilator drugs given to treat angina pectoris—such as nitroglycerin—promote vasodilation indirectly through their conversion to nitric oxide.

The endothelium also produces paracrine regulators that promote vasoconstriction. Notable among these is the polypeptide endothelin-1. This paracrine regulator stimulates vasoconstriction of arterioles, thus raising the total peripheral resistance. In normal physiology, this action may work together with those regulators that promote vasodilation to help regulate blood pressure.

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  • prospero
    What is the vasoconstriction paracrine released by the endothelium?
    8 years ago
  • harry
    Which of these is a paracrine regulator that stimulates vasoconstriction?
    8 years ago
  • imelda
    Which changes in paracrines cause vasodilation What is the result?
    8 years ago
  • Bruno
    Which of the following paracine regulator cause vaocobstriction?
    7 years ago
  • ambrosino
    What changes in the list of paracrines causes vasodilation and what is the result?
    7 years ago
  • algiso
    Which of the following paracrine regulators causes vasoconstriction of the blood vessels?
    7 years ago
  • max
    Is histamine a paracrine vasodilator?
    5 years ago
  • sointu
    Do paracrines cause vasoconstriction?
    5 years ago
  • muhammed
    What paracrine signal causes vasoconstriction?
    4 years ago
  • jeffrey
    Is paracrine regulations of blood flow extrinsic or intrinsic?
    4 years ago
  • rezene
    Which changes in paracrines cause vasodilation What is the result quizlet?
    4 years ago
  • julia
    Do prostaglandinds cause vasoconstriction or vasodilation?
    3 years ago
  • maik
    What is a paracarine refulation?
    3 years ago
  • maura
    Which paracrine does not cause vasodilation?
    3 years ago
  • Jolly
    Which changes in paracrines causes vasodilatin?
    3 years ago
  • Nile
    What do paracrines do to heart?
    1 year ago

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