NO donors potentiate the β‐adrenergic stimulation of I Ca,L and the muscarinic activation of I K,ACh in rat cardiac myocytes

The effects of nitric oxide (NO) donors on the L‐type Ca 2+ current ( I Ca,L ) and the muscarinic activated K + current ( I K,ACh ) were studied in isolated rat cardiac myocytes. The nitrosothiol S‐nitroso‐ N ‐acetyl‐ d,d ‐penicillamine (SNAP, 1 p m ‐1 μ m ) strongly potentiated the stimulation of the I Ca,L elicited by subthreshold concentrations of isoprenaline (Iso, 0.1–0.5 n m ) in ventricular myocytes. The effect of SNAP was mimicked by 2‐( N,N ‐diethylamino)‐diazenolate‐2‐oxide (DEANO, 1 p m ‐1 n m ), a NONOate that spontaneously releases NO in a pH‐controlled manner, and was blunted by 2‐(4‐carboxyphenyl)‐4,4,5,5‐tetramethylimidazoline‐1‐oxyl‐3‐oxide (100 μ m ), a NO trap. 1H‐[1,2,4]Oxadiazolo[4,3‐a]quinoxaline‐1‐one (10 μ m ), a guanylyl cyclase inhibitor, did not alter the effect of SNAP. SNAP (1 p m ‐1 μ m ) did not modify the effect of L858051 (0.1–0.3 μ m ), a forskolin analogue that activates adenylyl cyclase, on I Ca,L and did not enhance the basal I Ca,L in the presence of rolipram (1 μ m ), a phosphodiesterase type 4 inhibitor. Superfusion with Rp‐CPT‐cAMPS (500 μ m ), or internal dialysis with cAMP‐dependent protein kinase (cA‐PK) inhibitory peptide (PKI; 20 μ m ), inhibitors of the cA‐PK, blunted the effect of SNAP (1 n m and 1 μ m ) on the Iso‐stimulated (1‐100 p m ) I Ca,L . SNAP (1 n m and 1 μ m ) potentiated the threshold stimulation of I Ca,L elicited by internal GTP‐γS (10 μ m ), a non‐hydrolysable analogue of GTP. SNAP (1 p m ‐1 μ m ) and DEANO (1 μ m ) potentiated the stimulation of I K,ACh elicited by low concentrations of ACh (1–2 n m ) in rat atrial myocytes. The threshold stimulation of I K,ACh elicited by internal 5′‐guanylylimidodiphosphate (10 μ m ) was also potentiated by NO donors. SNAP (1 μ m ) did not modify I K,ACh reconstituted in human embryonic kidney 293 cells, in the absence or in the presence of ACh (1 or 10 n m ). Taken together, these data suggest that NO is a cGMP‐independent modulator of G‐protein‐coupled muscarinic and β‐adrenergic receptor actions on cardiac ion channels. Although this action of NO seemed to occur at the level of G proteins, it appeared to require a component distinct from receptors, G proteins or their effectors.