Endoplasmic reticulum contribution to the relaxant effect of cGMP- and cAMP-elevating agents in feline aorta

The contribution of endoplasmic reticulum (ER) and phosphorylation of phosp holamban (PLB) to the relaxant effect of cGMP- and cAMP-elevating agents wa s studied in feline aorta. Sodium nitroprusside (NP, 100 mu M) completely r elaxed contracture induced by 10 mu M norepinephrine. This NP-induced relax ation was partially prevented by tetraethylammonium, suggesting that a frac tion of NP-induced relaxation was mediated by activation of K+ channels. In the absence and presence of tetraethylammonium, the relaxant effect of NP was associated with a significant increase in Ser(16) phosphorylation of PL B immunodetected by phosphorylation site-specific antibodies. The relaxant effect of NP on aortic strips precontracted with 80 mM KCl was significantl y reduced by 1 mu M thapsigargin. This decrease, which represents the ER co ntribution to the relaxant effect of NP, reached 23 +/- 9% at 100 mu M NP a nd was closely associated with a dose-dependent increase in Ser(16) phospho rylation (128 +/- 49% over control at 100 mu M NP). Effects of NP were asso ciated with a significant increase in activity of protein kinase G and were mimicked by 8-bromo-cGMP. Forskolin produced a dose-dependent relaxant eff ect on KCl-induced contracture, which reached 64 +/- 8% at 50 mu M and was associated with an increase in phosphorylation of Ser(16) residue of PLB (8 8 +/- 18% over control). Thapsigargin reduced this relaxant effect by 38 +/ - 9%. 8-Bromo-cAMP mimicked effects of forskolin. The ER-mediated relaxant effect and the increase in Ser(16) phosphorylation produced by forskolin we re partially blocked by the protein kinase A inhibitor H-89 (5 mu M). The r esults indicate that ER partially contributes to the relaxant effect of NP and forskolin in feline aorta. This effect may be mediated by the associate d increase in Ser(16) phosphorylation of PLB.