Regulation of potential-dependent L-type Ca²⁺ currents by agmatine. Imidazoline receptors in isolated cardiomyocytes

The main goal of the present work was to study the mechanisms of regulation of voltage-gated L-type Ca²⁺ currents by agmatine in isolated cardiomyocytes and to determine whether agmatine is involved in mediating the "arginine paradox". It was shown that agmatine at concentrations from 200 μM to 15 mM inhibited L-type Ca²⁺ currents in isolated cardiomyocytes in a dose-dependent manner. Selective antagonists of α₂-adrenoceptors(α₂-ARs) yohimbine and rauwolscine did not interfere with the effect of agmatine. In contrast,efaroxan and idazoxan,which are known to antagonize both α₂-ARs and type 1 imidazoline receptors(I₁Rs),decreased the efficiency of agmatine almost twofold. The inhibitor of NO-synthase 7NI insignificantly influenced the suppressive action of agmatine on L-type Ca²⁺ currents,whereas the inhibitor of protein kinase C calphostin C markedly reduced the effects of agmatine. Arginine did not affect L-type Ca²⁺ currents in the presence of agmatine and vice versa. These data suggest that agmatine is not involved in mediating the "arginine paradox",and that its effects are not due to activation of endothelial NO-synthase(eNOS) followed by cGMP-dependent inhibition of the L-type Ca²⁺ current. Most likely,agmatine acts via I₁Rs coupled to a signaling pathway involving activation of protein kinase C. Consistently,we showed that single cardiomyocytes express nischarin gene,which is considered in the modern literature as the major candidate for the gene encoding I₁Rs. To our knowledge,this is the first demonstration of the I₁Rs expression at the level of individual cells,including cardiomyocytes.