Endothelial nitric oxide synthase decreases -adrenergic responsiveness via inhibition of the L-type Ca current

Wang H, Kohr MJ, Wheeler DG, Ziolo MT. Endothelial nitric oxide synthase decreases -adrenergic responsiveness via inhibition of the L-type Ca current. Am J Physiol Heart Circ Physiol 294: H1473–H1480, 2008. First published January 18, 2008; doi:10.1152/ajpheart.01249.2007.—Signaling via endothelial nitric oxide synthase (NOS3) limits the heart’s response to -adrenergic ( AR) stimulation, which may be protective against arrhythmias. However, mechanistic data are limited. Therefore, we performed simultaneous measurements of action potential (AP, using patch clamp), Ca transients (fluo 4), and myocyte shortening (edge detection). L-type Ca current (ICa) was directly measured by the whole cell ruptured patchclamp technique. Myocytes were isolated from wild-type (WT) and NOS3 knockout (NOS3 / ) mice. NOS3 / myocytes exhibited a larger incidence of -AR (isoproterenol, 1 M)-induced early afterdepolarizations (EADs) and spontaneous activity (defined as aftercontractions). We also examined ICa, a major trigger for EADs. NOS3 / myocytes had a significantly larger -AR-stimulated increase in ICa compared with WT myocytes. In addition, NOS3 / myocytes had a larger response to -AR stimulation compared with WT myocytes in Ca transient amplitude, shortening amplitude, and AP duration (APD). We observed similar effects with specific NOS3 inhibition [L-N-(1-iminoethyl)-ornithine (L-NIO), 10 M] in WT myocytes as with NOS3 knockout. Specifically, L-NIO further increased isoproterenol-stimulated EADs and aftercontractions. L-NIO also further increased the isoproterenol-stimulated ICa, Ca transient amplitude, shortening amplitude, and APD (all P 0.05 vs isoproterenol alone). L-NIO had no effect in NOS3 / myocytes. These results indicate that NOS3 signaling inhibits the -AR response by reducing ICa and protects against arrhythmias. This mechanism may play an important role in heart failure, where arrhythmias are increased and NOS3 expression is decreased.