Cellular uptake mechanisms of (TcN)-Tc-99m-NOET in cardiomyocytes from newborn rats - Calcium channel interaction

Background-Bis[N-ethoxy,N-ethyl(dithiocarbamato)]nitrido Tc (V) (TcN-NOET) is a new technetium complex proposed as a tracer of myocardial perfusion. H owever, its cellular uptake mechanisms are unknown, although membrane local ization on rat heart preparations and preferential binding to polymorphonuc lear neutrophils (PMNs) have been reported. Because of the central role of calcium in PMN actions, a relationship was hypothesized between this ion fl ux and TcN-NOET cellular uptake. Methods and Results-The mechanisms of cellular uptake of TcN-NOET were inve stigated in newborn rat cardiomyocytes by study of the effect of calcium ch annel modulators on tracer binding. Nifedipine had no effect on tracer upta ke at 1 minute. However, verapamil 0.1 mu mol/L and diltiazem 0.5 mu mol/L induced a 40% decrease in uptake. Conversely, Bay K 8644 0.25 mu mol/L incr eased TcN-NOET uptake by 73%. Alterations in other membrane ion transports failed to modify tracer uptake, indicating the specificity of the relations hip between TcN-NOET uptake and calcium channels. Kinetic studies indicated that cellular net accumulation of the tracer was slow (t(1/2)=28.5 minutes ) and retention was prolonged (84% of initial activity retained after 120 m inutes of washout). The energy dependence of TcN-NOET uptake was investigat ed after 60 minutes of metabolic inhibition by iodoacetic acid plus rotenon e, The ATP decrease was not associated with reduction in tracer uptake at 1 minute (114.9+/-21.9% of control, P=NS). Conclusions-The decrease in uptake observed with verapamil and diltiazem, t he increase with Bay K 8644, and the lack of effect with nifedipine suggest that TcN-NOET binds to L-type calcium channels in the open configuration, without entering cardiomyocytes. The kinetics of TcN-NOET accumulation and retention are slow, and the mechanism for cellular uptake is not energy-dep endent. From st clinical point of view, the effect of concurrent treatment by calcium inhibitors on myocardial binding of TcN-NOET should be taken int o account.