Activation of distinct cAMP-dependent and cGMP-dependent pathways by nitric oxide in cardiac myocytes

Abstract—Nitric oxide (NO) donors were recently shown to produce biphasic contractile effects in cardiac tissue, with augmentation at low NO levels and depression at high NO levels. We examined the subcellular mechanisms involved in the opposing effects of NO on cardiac contraction and investigated whether NO modulates contraction exclusively via guanylyl cyclase (GC) activation or whether some contribution occurs via cGMP/PKG-independent mechanisms, in indo 1–loaded adult cardiac myocytes. Whereas a high concentration of the NO donor S-nitroso-N-acetylpenicillamine (SNAP, 100 mmol/L) significantly attenuated contraction amplitude by 24.464.5 % (without changing the Ca21 transient or total cAMP), a low concentration of SNAP (1 mmol/L) significantly increased contraction amplitude (38610%), Ca21 transient (26610%), and cAMP levels (from 6.2 to 8.5 pmol/mg of protein). The negative contractile response of 100 mmol/L SNAP was completely abolished in the presence of the specific blocker of PKG KT 5823 (1 mmol/L); the positive contractile response of 1 mmol/L SNAP persisted, despite the presence of the selective inhibitor of GC 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 10 mmol/L) alone, but was completely abolished in the presence of ODQ plus the specific inhibitory cAMP analog Rp-8-CPT-cAMPS (100 mmol/L), as well as by the NO scavenger oxyhemoglobin. Parallel experiments in cell suspensions showed significant increases in adenylyl cyclase (AC) activity at low concentrations (0.1 to 1 mmol/L) of SNAP (AC, 18 % to 20 % above basal activity). We conclude that NO can regulate both AC and GC in cardiac myocytes. High levels of NO induce large increases in cGMP and a negativ