Cardiac ventricular myocytes exhibit a protein kinase A-dependent Cl- current (I-CI.PKa) mediated by the cystic fibrosis transmembrane conductance regulator (CFTR). There is conflicting evidence regarding the ability of the divalent cation nickel (Ni2+), which has been used widely in vitro in the study of other cardiac ionic conductances, to inhibit I-CI.PKa. Here the action of Ni2+ on I-CI.PKa activated by beta-adrenergic stimulation has been elucidated. Whole-cell patch-clamp recordings were made from rabbit isolated ventricular myocytes. Externally applied Ni2+ blocked I-CI.PKa activated by mu M isoprenaline with a log IC50 (M) of -4.107 +/- 0.075 (IC50 = 78.1 mu M) at +100 mV and -4.322 +/- 0.107 (IC50 = 47.6 mu M) at -100 mV. Thus, the block of I-CI.PKa by Ni2+ was not strongly voltage dependent. Ni2+ applied internally via the patch-pipette was ineffective at inhibiting isoprenaline-activated I-CI.PKa, but in the same experiments the current was suppressed by external Ni2+ application, indicative of an external site of Ni2+ action. In the presence of 1 mu M atenolol isoprenaline was ineffective at activating I-CI.PKa, but in the presence of the beta 2-adrenoceptor inhibitor ICI 118,551 isoprenaline still activated Ni2+-sensitive I-CI.PKa. Collectively, these data demonstrate that Ni2+ ions produce marked inhibition of beta 1-adrenoceptor activated ventricular I-CI.PKa at submillimolar [Ni2+]: an action that is likely to involve an interaction between Ni2+. and beta 1-adrenoceptors. The concentration-dependence for I-CI.PKa inhibition seen here indicates the potential for confounding effects on Icom to occur even at comparatively low Ni2+ concentrations, when Ni2+ is used to study other cardiac ionic currents under conditions of p-adrenergic agonism. (C) 2013 Elsevier Inc. All rights reserved.