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) 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 F2alpha (PGF2alpha). 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 x 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 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 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 microg/ml for 6 h), which activates the stimulatory G protein of adenylyl cyclase (Gs), significantly suppressed PGF2alpha-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 Gs may partly account for the cyclic AMP-independent relaxant mechanism.
Yamaki
F
F
Department of Pharmacology, Toho University School of Pharmaceutical Sciences, Funabashi-City, Chiba, Japan.
Kaga
M
M
Horinouchi
T
T
Tanaka
H
H
Koike
K
K
Shigenobu
K
K
Toro
L
L
Tanaka
Y
Y
D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology; D000225:Adenine / Q000031:analogs & derivatives* / Q000494:pharmacology; D000067956:Adenylyl Cyclase Inhibitors; D000818:Animals; D001013:Aorta, Thoracic / Q000187:drug effects* / Q000502:physiology; D002772:Cholera Toxin / Q000494:pharmacology; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000037:antagonists & inhibitors / Q000096:biosynthesis / Q000502:physiology*; D004305:Dose-Response Relationship, Drug; D011464:Epoprostenol / Q000031:analogs & derivatives* / Q000494:pharmacology*; D005260:Female; D019205:GTP-Binding Protein alpha Subunits, Gs / Q000378:metabolism; D006168:Guinea Pigs; D051036:Large-Conductance Calcium-Activated Potassium Channels; D008297:Male; D026902:Potassium Channel Blockers / Q000494:pharmacology; D024681:Potassium Channels, Calcium-Activated / Q000037:antagonists & inhibitors / Q000378:metabolism / Q000502:physiology*; D044006:Receptors, Epoprostenol; D011982:Receptors, Prostaglandin / Q000819:agonists / Q000502:physiology*; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D014664:Vasodilation / Q000187:drug effects* / Q000502:physiology; D014665:Vasodilator Agents / Q000494:pharmacology*
D000067956:Adenylyl Cyclase Inhibitors; D051036:Large-Conductance Calcium-Activated Potassium Channels; D026902:Potassium Channel Blockers; D024681:Potassium Channels, Calcium-Activated; D044006:Receptors, Epoprostenol; D011982:Receptors, Prostaglandin; D014665:Vasodilator Agents; C017759:9-(tetrahydro-2-furyl)-adenine; D005576:Colforsin; D015124:8-Bromo Cyclic Adenosine Monophosphate; C048081:beraprost; D002772:Cholera Toxin; D011464:Epoprostenol; D000242:Cyclic AMP; D019205:GTP-Binding Protein alpha Subunits, Gs; D000225:Adenine