Several recent studies have addressed the role of mitoK_Ca channels in cardiac tissue protection against ischemia. However, most of these studies have relied upon the specificity of a particular chemical, NS1619, as an opener of mitoK_Ca channels. In the present work, we investigated a series of unspecific effects that this drug has on isolated rat heart mitochondria. In state IV, mitochondria using malate plus glutamate as energetic substrates presented a respiratory rate increase of 92 ± 3 % when 20 μM NS1619 was present. This respiratory rate increase, indicating that NS1619 promotes mitochondrial uncoupling, was not changed by substituting Na⁺ for K⁺ as the main cation in the incubation medium, suggesting that it is caused by a non-specific permeabilization of the inner mitochondrial membrane to cations. Indeed, we measured mitochondrial osmotic swelling as an indicative of cation transport across the inner membrane and found that swelling in K⁺-rich buffers was increased by NS1619 at concentrations ranging from 3 to 50 μM. This swelling was also observed when K⁺ was replaced by tetraethylammonium, again indicating a non-selective permeabilization induced by this drug. NS1619-induced swelling was not reversed by mitoK_Ca channel blocker paxilline or by inhibitors of permeability transition  and of uncoupling proteins. Additionally, NS1619 caused flavoprotein reduction, but oxidation of NADH, suggesting that the main effects of the drug are uncoupling and complex II inhibition. In fact, maximal respiratory rates were reduced 45 ± 7 % or 68.3 ± 2.0 % by 50 μM NS1619, with malate plus glutamate or succinate, respectively, as substrates. Thus, NS1619 promotes respiratory inhibition predominantly targeted to complex II, but not restricted to it. Our results indicate that the most prominent effects of NS1619 upon mitochondria do not appear to relate to mitoK_Ca channel opening. Indeed, mitochondrial volume and membrane potential measurements in the presence of calcium did not uncover any evidence of mitoK_Ca channel activity, challenging the existence and physiological relevance of these channels.

Supported by FAPESP and CNPq.