Thallium uptake by canine iliac arteries

Free radicals uncouple the sodium pump in pig coronary artery

Free radicals may impair vital functions of several types of tissues including coronary artery smooth muscle. Because the Na+ pump plays a key role in maintaining coronary tone, the effects of superoxide and peroxide on this protein were examined. Ouabain-sensitive Rb+ uptake by denuded coronary artery rings was used in lieu of K+ transport by this pump. It was inhibited by exposing the rings for 90 min either to peroxide [50% inhibitory concentration (IC50) = 0.56 +/- 0.18 mM] or to superoxide generated by xanthine oxidase (XO; 0.3 mM xanthine and xanthine oxidase, IC50 = 0.08 +/- 02 mU/ml). The effect of peroxide was not overcome by superoxide dismutase and that of superoxide was not prevented by catalase. K(+)-activated ouabain-sensitive hydrolysis of p-nitrophenyl phosphate in the plasma membrane-enriched fraction isolated from the coronary artery smooth muscle was monitored as the hydrolytic activity of the Na+ pump. It was inhibited by exposing the membranes only to very high concentrations of peroxide (IC50 = 9.85 +/- 3.5 mM) or XO (IC50 = 5 +/- 2 mU/ml). The exposure to 2.5 mM H2O2 or 0.5 mU/ml XO reduced the Na(+)-dependent acylphosphate levels only by 41 +/- 3 and 30 +/- 4%, respectively even though either inhibited the Rb+ uptake by > 80%. Thus superoxide and peroxide uncoupled the hydrolytic activity of the Na+ pump from Rb+ uptake. We speculate that such an uncoupling in ischemia and reperfusion would result in dual damage: ion imbalance and continuous hydrolysis of ATP in the cells that are already starved.

Ouabain-insensitive, halide-sensitive Tl+ uptake by canine iliac arteries

Ouabain-insensitive Tl+ uptake by canine iliac arteries was studied. A significant fraction was halide-sensitive, with Br- substituting well for Cl-. The halide-sensitive component was inhibited by diuretics (MK196, bumetanide), PCMBS, low temperatures and external cations K+, Rb+. External but not internal Na+ was necessary for the uptake process. The process was not sensitive to disulphonic stilbenes. The halide-sensitive uptake appears to represent the operation of a (Na+/K+/Cl-)-cotransport process in arteries.

The sodium pump in opossum vascular smooth muscle

Ouabain-sensitive Rb+ uptake and [3H]ouabain binding were used to measure rates of Na+ pumping and the number of pump sites, respectively, in thoracic aortae from opossums. From the number of Rb+ ions pumped per site per minute, estimates of pump turnover have been made. Values obtained are comparable to those of other species (see Table 1).