Potentiation of slow component of delayed rectifier K+ current by cGMP via two distinct mechanisms:: inhibition of phosphodiesterase 3 and activation of protein kinase G

1 Regulation of the slowly activating component of delayed rectifier K+ current (I-Ks) by intracellular guanosine 3'5' cyclic monophosphate (cGMP) was investigated in guinea-pig sino-atrial (SA) node cells using the whole-cell patch-clamp method. 2 When a cell was dialyzed with pipette solution containing 100 muM cGMP, I-Ks started to gradually increase and reached a maximum increase of a factor of 2.37+/-0.39 (n = 4) about 10-15 min after rupture of patch membrane. Atrial natriuretic peptide (ANR 100 nM) also potentiated I-Ks, consistent with intracellular cGMP-induced enhancement of I-Ks. 3 Bath application of a selective blocker of the cGMP-inhibited phosphodiesterase (PDE3) milrinone (100 muM) enhanced I-Ks by a factor of 1.50+/-0.09 (n=4) but failed to further enhance IK, after a maximum stimulation by intracellular cGMP (100 muM), suggesting that blockade of PDE3 activity is involved in the enhancement of I-Ks. A potent but nonspecific P DE inhibitor 3-isobutyl-1-methylxanthine (IBMX, 100 muM) further increased I-Ks stimulated by 100 muM milrinone, indicating that PDE subtypes other than PDE3 are also involved in the regulation of basal I-Ks in guinea-pig SA node cells. 4 Bath application of 100 muM 8-bromoguanosine 3'5' cyclic monophosphate (8-Br-cGMP) increased I-Ks by a Factor of 1.48+/-0.11 (n=5) and this stimulatory effect was totally abolished by cGMP-dependent protein kinase (PKG) inhibitor KT-5823 (500 nM), suggesting that the activation of PKG also mediates cGMP-induced potentiation of I-Ks. 5 These results strongly suggest that intracellular cGMP potentiates I-Ks not only by blocking PDE3 but also by activating PKG in guinea-pig SA node cells.