The Ca2+-independent portion of transient outward K+current (Ito) exhibits a transmural gradient in ventricle. To investigate control mechanisms for this gradient, we studied canine epicardial and endocardial ventricular myocytes with use of the whole-cell patch-clamp technique.Itowas larger in amplitude, had a more negative voltage threshold for activation, and had a more negative midpoint of inactivation in epicardium. Recovery from inactivation was >10-fold slower in endocardium. Incubation of epicardial myocytes with angiotensin II for 2 to 52 hours alteredItoto resemble unincubated endocardium and reduced the amplitude of the phase 1 notch of the action potential. In contrast, incubation of endocardial myocytes with losartan for 2 to 52 hours alteredItoto resemble unincubated epicardium and induced a phase 1 notch in the action potential. With RNase protection assays, we determined that incubations with angiotensin II or losartan did not alter mRNA levels for either Kv4.3 or Kv1.4; thus, a change in the &agr; subunit forItois unlikely to be responsible. To test whether posttranslational modification produced the effects of angiotensin II, we coexpressed Kv4.3 and the angiotensin II type 1a receptor inXenopusoocytes. Incubation with angiotensin II increased the time constant for recovery from inactivation of the expressed current by 2-fold with an incubation time constant of 3.7 hours. No effect on activation or inactivation voltage dependence was observed. These results demonstrate that the properties ofItoin endocardium and epicardium are plastic and likely under the tonic-differing influence of the renin-angiotensin system.