Effects of osmolality and solutes on performance of shark heart mitochondria

Abstract

AbstractDogfish shark heart mitochondria isolated from Squalus acanthias were exposed to increasing concentrations of mannitol/sucrose, trimethylamine oxide (TMAO), betaine, and urea. For all but urea, respiratory control ratios (RCR) and rates of oxygen uptake (V̇O2) in the presence of malate/glutamate were low in basal medium (220 mOsm), increased with solute concerntration to about 420–870 mOsm, and declined in the highest concentrations (1,150 mOsm). Increasing urea concentration up to about 1,150 mOsm had relatively little effect on RCR or V̇O2. The results were not changed significantly by prolonged incubation in these media or by the addition of 0.3 M urea to either mannitol/sucrose or TMAO media. We conclude that, like mannitol/sucrose, betaine, and probably TMAO permeate shark heart mitochondria slowly or not at all, but that urea readily permeates the mitochondria and is thus osmotically inactive. The highest V̇O2 values were obtained in media at 580 mOsm, hypoosmotic to shark blood (897 mOsm) and to local seawater (879 mOsm) but approximately isoosmotic to blood without the osmotic contribution of the permeant urea. We conclude that V̇O2 is superior to RCR as a comparative measure of mitochondrial activity.