Effects of nitric oxide on large‐conductance Ca²⁺‐activated K⁺ currents in human cardiac fibroblasts through PKA and PKG‐related pathways

Summary The human cardiac fibroblast ( HCF ) is the most abundant cell type in the myocardium, and HCF s play critical roles in maintaining normal cardiac function. However, unlike cardiomyocytes, the electrophysiology of HCF s is not well established. In the cardiovascular system, Ca 2+ ‐activated K + ( KC a) channels have distinct physiological and pathological functions, and nitric oxide ( NO ) plays a key role. In this study, we investigated the potential effects of NO on KC a channels in HCF s. We recorded strong oscillating, well‐maintained outward K + currents without marked inactivation throughout the test pulse period and detected outward rectification in the I‐V curve; these are all characteristics that are typical of KC a currents. These currents were blocked with iberiotoxin ( IBTX , a BKC a blocker) but not with TRAM ‐34 (an IKC a blocker). The amplitudes of the currents were increased with SNAP (an NO donor), and these increases were inhibited with IBTX . The SNAP ‐stimulating effect on the BKC a currents was blocked by pretreatment with KT 5823 (a protein kinase G [ PKG ] inhibitor) or 1 H‐[1,‐2, ‐4] oxadiazolo‐[4,‐3‐a] quinoxalin‐1‐one ( ODQ ; a soluble guanylate cyclase inhibitor). Additionally, 8‐bromo‐cyclic guanosine 3’,5’‐monophosphate (8‐Br‐ cGMP ) stimulated the BKC a currents, and pretreatment with KT 5720 (a protein kinase A [ PKA ] inhibitor) and SQ 22536 (an adenylyl cyclase inhibitor) blocked the NO ‐stimulating effect on the BKC a currents. Furthermore, 8‐bromo‐cyclic adenosine 3’,5’‐monophosphate (8‐Br‐ cAMP ) activated the BKC a currents. These data suggest that BKC a current is the main subtype of the KC a current in HCF s and that NO enhances these currents through the PKG and PKA pathways.