Life Lines by Dr. Dolittle

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Does size matter?


Image of a female cane toad by Benjamint444 via Wikimedia Commons 

When it comes to how blood vessels function, the answer is yes. A new study published in the American Journal of Physiology – Regulatory, Integrative and Comparative Physiology examined this question for the first time in toads (Rhinella marina).

Regulation of blood vessel diameter, and hence blood pressure, is a complex process involving a variety of factors that are secreted by cells in the vascular wall in addition to factors found in the circulation. Some factors cause blood vessels to constrict and thus raise blood pressure whereas others cause blood vessels to relax (dilate), which lowers blood pressure. For mammals, nitric oxide is an important dilator that is released by cells that form the inner lining of blood vessels. In smaller blood vessels, this relaxation response is more commonly caused by so-called endothelial-derived hyperpolarizing factors that work through potassium channels to promote relaxation. Whether or not amphibians have a similar difference in the predominant factors that regulate blood vessels of each size was largely unknown.

Researchers compared the function of iliac arteries, which are about 1 mm in inner diameter, and smaller sciatic arteries (about 400 micrometers in inner diameter). What they found was that vasodilation of the larger iliac arteries did not rely on nitric oxide release from the cells that form the inner lining of the blood vessels, unlike vasodilation in mammals. Rather, these larger blood vessels rely on nitric oxide that is released by the surrounding nerve endings. In contrast to the larger arteries, smaller blood vessels were found to depend more on endothelial-derived factors as opposed to the surrounding nerve endings.


MS Cameron, JA Donald. Different vasodilator mechanisms in intermediate- and small-sized arteries from the hindlimb vasculature of the toad Rhinella marina. American Journal of Physiology – Regulatory, Integrative and Comparative Physiology. 317(3):R379-R385, 201

Categories: Comparative Physiology, Diet and Exercise

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