MODULATION OF L-TYPE Ca2+ CURRENTS AND INTRACELLULAR CALCIUM BY AGMATINE IN RAT CARDIOMYOCYTES

It is shown that agmatine inhibits L-type Ca2+ currents in isolated cardiomyocytes of rats in a dose-dependent manner The inhibitory analysis indicates that imidazoline receptors of type I (I(1)Rs) rather than alpha(2)-adrenoceptors (alpha(2)-ARs) are implicated in mediating the effects of agmatine. Agmatine affects the dynamics of intracellular Ca2+ concentration changes in spontaneously active cardiomyocytes. The averaged intracellular Ca2+ concentration ([Ca2+](in)) varied biphasically, depending on the agmatine dose: at 1-500 mu M, agmatine decreased [Ca2+](in); at 500 mu M-2 mM, [Ca2+](in) remained unchanged, and at concentrations above 2 mM agmatine caused an increase of [Ca2+](in). The effects of low agmatine concentrations were inhibited by 7NI, an inhibitor of NO synthases (NOS), as well as by the inhibitors of the sarcoplasmic reticulum Ca2+-ATPase (SERCA) thapsigargin and cyclopiazonic acid. In contrast, ODQ, a blocker of NO-sensitive guanylate cyclase, and the antagonist of I(1)Rs efaroxan were ineffective. At low concentrations agmatine did not affect the increase in [Ca2+](in) induced by stimulating doses of ryanodine (40 nM). In addition, agmatine at low doses was found to markedly stimulate NO production. When efaroxan (10 mu M) or ryanodine (200 mu M) were added to the bath to inhibit I(1)Rs and ryanodine receptors (RyRs), respectively, [Ca2+](in) became much less sensitive to millimolar agmatine. In contrast to low concentrations (100 mu M), high agmatine doses (10-15 mM) did not stimulate the NO synthesis but were effective as NOS inducer in cells pretreated with efaroxan. The selective I1R agonist rilmenidine increased [Ca2+](in) in a dose-dependent manner. The effect of rilmenidine was similar to that of agmatine at high doses and was abolished by RyRs inhibition. Our findings indicate that in spontaneously active cardiomyocytes agmatine at low concentrations decreases [Ca2+](in), does not stimulate I(1)Rs but most likely enhances NO synthase followed by an increase in SERCA activity due to the direct nitrosylation of SERCA and/or phospholamban. The effects of high agmatine doses are apparently mediated by I(1)Rs and involve RyRs.