MaxiK channel-mediated relaxation of guinea-pig aorta following stimulation of IP receptor with beraprost via cyclic AMP-dependent and -independent mechanisms

The present study was aimed to elucidate the cellular pathway(s) controllin g vascular relaxation triggered by stimulation of prostaglandin I-2 (PGI(2) , IP) receptor with a stable PGI(2) analog, beraprost. Beraprost caused a c oncentration-dependent relaxation in de-endothelialized guinea-pig aorta co ntracted with prostaglandin F-2 alpha (PGF(2 alpha)). Beraprost-induced rel axation was almost abolished in high-KCI-contracted tissue, indicating a ma jor role of K+ conductances. In contrast to other PGI(2) analogs (e.g. cica prost and iloprost), beraprost-induced relaxation was practically abolished by a selective voltage and Ca2+-activated K+ (MaxiK, BK) channel blocker I beriotoxin (10(-7) M) or by tetraethylammonium (2 X 10(-3) M). The relaxati on induced by beraprost was not significantly affected by other K+ channel blockers glibenclamide (10(-6) M) or Ba2+ (10(-5) M), but was slightly atte nuated by 4-aminopyridine (10(-4) M). Beraprost increased intracellular cyc lic AMP levels, suggesting a role for cyclic AMP-dependent pathways. A sele ctive inhibitor of cyclic AMP-specific phosphodiesterase, RO-20-1724 (10(-4 ) M), significantly potentiated beraprost-induced relaxation. Iberiotoxin ( 10(-7) M) completely counteracted this potentiation. Moreover, tension decr ement due to forskolin (3 x 10(-7) M) or 8-bromo-cyclic AMP (10(-2) M) was thoroughly restored by Iberiotoxin (10(-7) M), confirming a role for a cycl ic AMP-dependent mechanism. However, SQ 22,536 (10(-4) M), an adenylyl cycl ase inhibitor, did not affect beraprost-induced relaxation though it almost totally inhibited the elevation of cyclic AMP contents induced by berapros t, suggesting the existence of an additional mechanism that is cyclic AMP-i ndependent. Moreover, cholera toxin (CTX, 1 mug/ml for 6 h), which activate s the stimulatory G protein of adenylyl cyclase (G(s)), significantly suppr essed PGF(2 alpha)-induced contraction both in the absence and presence of SQ 22,536 (10(-4) M). Iberiotoxin (10(-7) M) was also capable of restoring the relaxation induced by CTX. These findings suggest that MaxiK channel pl ays a primary role in mediating smooth muscle relaxation following stimulat ion of IP receptor with beraprost in guinea-pig aorta. Both cyclic AMP-depe ndent and -independent pathways contribute to the MaxiK channel-mediated re laxation following IP receptor stimulation in this vascular tissue. Direct regulation of MaxiK channels by G(s) may partly account for the cyclic AMP- independent relaxant mechanism.