RESUMEN
Incubation of BMG-1 cells with thallium chloride (201Tl) in the range of diagnostic dose did not show a smooth uptake curve and appeared to have an unsuspected deviation in initial phase. In the present study this unexpected phenomenon was explored, using commonly used radionuclides (viz., 201Tl and 131I). Comparison was made with technetium-99m pertechnetate (99mTcO4-) and technetium-99m labeled methoxyisobutylisonitrile (99mTc-MIBI) that are known to show conventional 2 phase graph representing inflow and outflow segments. Serial in vitro, ex-vivo and in vivo gamma scintigraphy as well as NMR spectroscopy experiments were conducted to corroborate the results. BMG-1 cells demonstrated a four-phase uptake pattern with 201Tl as compared to a conventional biphasic pattern with 99mTc-MIBI. Flow cytometry data however did not reveal any 201Tl induced cell injury. Further, mice tissue extracts injected with 201Tl also showed a transient depression in its uptake. Scintigraphy experiments in rabbits administered with diagnostic dose of 201Tl and 131I confirmed the in vitro and ex vivo findings. Further, proton NMR spectroscopy showed decrease in the level of choline at 3 h and 24 h in 201Tl treated animals as compared to control. Phosphoethanolamine peak firstly decreased at 3 h but reached normal level at 24 h time point. No significant change was observed in the level of betaine. This transient reduction in internalization of 201Tl and 131I may represent a hitherto unknown acute effect of low dose radiation, i.e., transient depression in Na+-K+ ATPase pump activity without any apparent evidence of cell damage, representing a transient cell membrane dysfunction. The phenomenon may present a mechanistical explanation of ‘thyroid stunning’ at cellular level and suggest that it may be more universal in nature than suspected till now.
RESUMEN
Oscillatory contractile activity is an inherent property of blood vessels. Various cellular mechanisms have been proposed to contribute to oscillatory activity. Mouse small mesenteric arteries display a unique low frequency contractile oscillatory activity (1 cycle every 10-12 min) upon phenylephrine stimulation. Our objective was to identify mechanisms involved in this peculiar oscillatory activity. First-order mesenteric arteries were mounted in tissue baths for isometric force measurement. The oscillatory activity was observed only in vessels with endothelium, but it was not blocked by L-NAME (100 µM) or indomethacin (10 µM), ruling out the participation of nitric oxide and prostacyclin, respectively, in this phenomenon. Oscillatory activity was not observed in vessels contracted with K+ (90 mM) or after stimulation with phenylephrine plus 10 mM K+. Ouabain (1 to 10 µM, an Na+/K+-ATPase inhibitor), but not K+ channel antagonists [tetraethylammonium (100 µM, a nonselective K+ channel blocker), Tram-34 (10 µM, blocker of intermediate conductance K+ channels) or UCL-1684 (0.1 µM, a small conductance K+ channel blocker)], inhibited the oscillatory activity. The contractile activity was also abolished when experiments were performed at 20°C or in K+-free medium. Taken together, these results demonstrate that Na+/K+-ATPase is a potential source of these oscillations. The presence of α-1 and α-2 Na+/K+-ATPase isoforms was confirmed in murine mesenteric arteries by Western blot. Chronic infusion of mice with ouabain did not abolish oscillatory contraction, but up-regulated vascular Na+/K+-ATPase expression and increased blood pressure. Together, these observations suggest that the Na+/K+ pump plays a major role in the oscillatory activity of murine small mesenteric arteries.