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

Abstract.

The present study was aimed to elucidate the cellular pathway(s) controlling vascular relaxation triggered by stimulation of prostaglandin I₂ (PGI₂, IP) receptor with a stable PGI₂ analog, beraprost. Beraprost caused a concentration-dependent relaxation in de-endothelialized guinea-pig aorta contracted with prostaglandin F_{2 α} (PGF_{2 α}). Beraprost-induced relaxation was almost abolished in high-KCl-contracted tissue, indicating a major role of K⁺ conductances. In contrast to other PGI₂ analogs (e.g. cicaprost and iloprost), beraprost-induced relaxation was practically abolished by a selective voltage and Ca²⁺-activated K⁺ (MaxiK, BK) channel blocker Iberiotoxin (10^–7 M) or by tetraethylammonium (2×10^–3 M). The relaxation induced by beraprost was not significantly affected by other K⁺ channel blockers glibenclamide (10^–6 M) or Ba²⁺ (10^–5 M), but was slightly attenuated by 4-aminopyridine (10^–4 M). Beraprost increased intracellular cyclic AMP levels, suggesting a role for cyclic AMP-dependent pathways. A selective 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 decrement due to forskolin (3×10^–7 M) or 8-bromo-cyclic AMP (10^–2 M) was thoroughly restored by Iberiotoxin (10^–7 M), confirming a role for a cyclic AMP-dependent mechanism. However, SQ 22,536 (10^–4 M), an adenylyl cyclase inhibitor, did not affect beraprost-induced relaxation though it almost totally inhibited the elevation of cyclic AMP contents induced by beraprost, suggesting the existence of an additional mechanism that is cyclic AMP-independent. Moreover, cholera toxin (CTX, 1 µg/ml for 6 h), which activates the stimulatory G protein of adenylyl cyclase (G_s), significantly suppressed PGF_{2 α}-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 plays a primary role in mediating smooth muscle relaxation following stimulation of IP receptor with beraprost in guinea-pig aorta. Both cyclic AMP-dependent and -independent pathways contribute to the MaxiK channel-mediated relaxation 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.