Question

2. What is a substrate (futile) cycle? Describe an example of a pathology that develops from a substrate cycle.

Answer 1

A futile cycle is also known as a substrate cycle, occurs when two metabolic pathways run simultaneously in opposite directions and have no overall effect other than to dissipate energy in the form of heat.

example of apparently futile cycling is the case of regulation of peripheral vascular resistance. Resistance vessels in the microcirculation are under the influence of two systems with opposite effects on vascular resistance. One is the “vasoconstrictor system,” which includes the sympathetic vasoconstrictor system and endothelial vasoconstrictor molecules. The second is the “vasodilator system,” which includes endothelial vasodilator molecules, local vasodilator metabolites, and a sympathetic vasodilator system. Sympathectomy has been shown to increase blood flow to resting skeletal muscle . Also, there is a basal level of endothelial vasodilator molecules and local vasodilator metabolites in the blood stream . Thus, it is clear that in the baseline state, in which blood pressure and vascular resistance are normal, there is “activity”. in both vasodilator and vasoconstrictor systems. The occurrence of cycling makes possible great changes in tissue blood flow.