Nitric oxide effects depend on different mechanisms in different regions of the rat heart

The important role of nitric oxide (NO) in regulating cardiac functions has been investigated in prior research. However, NO-induced signaling mechanisms in the different regions of the heart have not been explored until now. In this study, the mechanism of NO effects on the spontaneously beating right atrium and left papillary muscle isolated from the rat heart was examined. The NO donor diethylamine NONOate (DEA/NO) (0.1–100 lM) depressed the resting and developed tensions, as well as the sinus rate, of the right atrium. The effect of DEA/NO on contractions of the right atrium was blocked by the soluble guanylate cyclase (sGC) inhibitor, ODQ (1H-[1,2,4] oxadiazolo[4,3-a]quinoxalin-1-one) (10 lM). The ATPsensitive potassium channel (KATP) blocker glyburide (3 lM) reversed DEA/NO-induced decreases in the resting tension. The suppressor effect of DEA/NO on the sinus rate was inhibited only by the superoxide radical scavenger superoxide dismutase (25 U/ml). Neither the cGMPdependent protein kinase (PKG) inhibitor KT5823 (0.1 lM) nor the cAMP-dependent protein kinase (PKA) inhibitor KT5720 (1 lM) changed DEA/NO responses in the right atrium. While the resting tension of the right atrium was decreased by the NO precursor L-arginine (1–100 lM), it was increased by the nitric oxide synthase inhibitor L-NMMA (0.1–100 lM). The sinus rate was not affected by L-arginine or L-NMMA. The left papillary muscle contraction was not influenced by any of these NO-related agents. These results show that high concentration NO-induced depression of the contraction of the right atrium is due to sGC and KATP channel activation, but suppression of the sinus rate depends on redox regulation. Our results may have important implications for the regiondependent functional disability of cardiac myocytes, as well as the regulation of heart performance in high NOinduced pathological conditions