Medline ® Abstracts for References 30,31

Rhabdomyolysis and myoglobinuria occur commonly in men who sustain environmental heat injury during intensive physical training in hot climates. These also occur in patients with potassium depletion. Since physical training in hot climates may be accompanied by serious losses of body potassium, the possibility was considered that performance of strenuous exercise when potassium deficient might enhance susceptibility to rhabdomyolysis. Potassium is released from contracting skeletal muscle fibers and its rising concentration in interstitial fluid is thought to dilate arterioles thereby mediating the normal rise of muscle blood flow during exercise. If potassium release from deficient muscle were subnormal, exercise would not be accompanied by sufficient muscle blood flow and rhabdomyolysis could occur by ischemia. This hypothesis was examined by comparing the effect of electrically stimulated exercise on muscle blood flow, potassium release, and histology of the intact gracilis muscle preparation in normal and potassium-depleted dogs. In normal dogs, muscle blood flow and potassium release rose sharply during exercise. In contrast, muscle blood flow and potassium release were markedly subnormal in depleted dogs despite brisk muscle contractions. Although minor histologic changes were sometimes observed in nonexercised potassium-depleted muscle, frank rhabdomyolysis occurred in each potassium-depleted animal after exercise. These findings support the hypothesis that ischemia may be the mechanism of rhabdomyolysis with exercise in potassium depletion.

In arteries, muscarinic agonists such as acetylcholine release an unidentified, endothelium-derived hyperpolarizing factor (EDHF) which is neither prostacyclin nor nitric oxide. Here we show that EDHF-induced hyperpolarization of smooth muscle and relaxation of small resistance arteries are inhibited by ouabain plus Ba2+; ouabain is a blocker of Na+/K+ ATPase and Ba2+ blocks inwardly rectifying K+ channels. Small increases in the amount of extracellular K+ mimic these effects of EDHF in a ouabain- and Ba2+-sensitive, but endothelium-independent, manner. Acetylcholine hyperpolarizes endothelial cells and increases the K+ concentration in the myoendothelial space; these effects are abolished by charbdotoxin plus apamin. Hyperpolarization of smooth muscle by EDHF is also abolished by this toxin combination, but these toxins do not affect the hyperpolarizaiton of smooth muscle by added K+. These data show that EDHF is K+ that effluxes through charybdotoxin- and apamin-sensitive K+ channels on endothelial cells. The resulting increase in myoendothelial K+ concentration hyperpolarizes and relaxes adjacent smooth-muscle cells by activating Ba2+-sensitive K+ channels and Na+/K+ ATPase. These results show that fluctuations in K+ levels originating within the blood vessel itself are important in regulating mammalian blood pressure and flow.