Na+ channel and Na+/K+ ATPase involvement in norepinephrine- and veratridine-stimulated metabolism in perfused rat hind limb

In the constant flow perfused rat hind limb, norepinephrine (NE) evoked increases in oxygen uptake (V̇o2) and lactate efflux (LE) were inhibited by the cardiac glycoside ouabain (1 mM), without interrupting the NE-mediated vasoconstriction. The membrane labilizer veratridine, previously shown to increase V̇o2 and LE, without increasing perfusion pressure, was also shown to be inhibited by the cardiac glycoside ouabain, as well as by the ouabain analogues digitoxin and digoxin. The stimulatory actions of veratridine on V̇o2 were inhibitable by low doses of the specific sodium channel blocker tetrodotoxin (TTX), while NE effects were unaffected, suggesting that NE may be acting via a TTX-insensitive sodium channel. It is concluded that agents such as NE (a vasoconstrictor) or veratridine (a membrane labilizer), which stimulate V̇o2 in the perfused rat hind limb, do so by increasing Na+ influx. The observed increases in oxygen consumption and LE are due to Na+- K+ ATPase activity to pump Na+ out of the cell at the expense of ATP turnover. Energy dissipation due to Na+ cycling may be a form of facultative thermogenesis attributable to NE that can be stimulated by membrane labilizers such as veratridine in the constant flow perfused rat hind limb.