Membrane hyperpolarization inhibits agonist-induced synthesis of inositol 1,4,5-trisphosphate in rabbit mesenteric artery.
1. Effects of membrane hyperpolarization induced by pinacidil on Ca2+ mobilization induced by noradrenaline (NA) were investigated by measuring intracellular Ca2+ concentration ([Ca2+]i), isometric tension, membrane potential and production of inositol 1,4,5-trisphosphate (IP3) in smooth muscle cells of the rabbit mesenteric artery. 2. Pinacidil (0.1-10 microM) concentration dependently hyperpolarized the smooth muscle membrane with a reduction in membrane resistance. Glibenclamide (1 microM) blocked the membrane hyperpolarization induced by 1 microM-pinacidil. NA (10 microM) depolarized the smooth muscle membrane with associated oscillations. Pinacidil (1 microM) inhibited this response and glibenclamide (1 microM) prevented the action of pinacidil on both the NA-induced events. 3. In thin smooth muscle strips, 10 microM-NA produced a large phasic and a subsequent small tonic increase in [Ca2+]i with associated oscillations. These changes in [Ca2+]i seemed to be coincident with phasic, tonic and oscillatory contractions, respectively. Pinacidil (0.1-1 microM) inhibited the increases in [Ca2+]i and in tension induced by NA, but not by 128 mM-K+. Glibenclamide inhibited these actions of pinacidil. Pinacidil (1 microM) also inhibited the contraction induced by 10 microM-NA in strips treated with A23187 (which functionally removes cellular Ca2+ storage sites), suggesting that membrane hyperpolarization inhibits Ca2+ influxes activated by NA. 4. In Ca2(+)-free solution containing 2 mM-EGTA, NA (10 microM) transiently increased [Ca2+]i, tension and synthesis of IP3. Pinacidil (over 0.1 microM) inhibited the increases in [Ca2+]i, tension and synthesis of IP3 induced by 10 microM-NA in Ca2(+)-free solution containing 5.9 mM-K+, but not in a similar solution containing 40 or 128 mM-K+. Glibenclamide (1 microM) inhibited these actions of pinacidil. These inhibitory actions of pinacidil were still observed in solutions containing low Na+ or low Cl-. These results suggest that pinacidil inhibits NA-induced Ca2+ release from storage sites through an inhibition of IP3 synthesis resulting from its membrane hyperpolarizing action. 5. In beta-escin-treated skinned strips, NA (10 microM) or IP3 (20 microM) increased Ca2+ in Ca2(+)-free solution containing 50 microM-EGTA and 3 microM-guanosine triphosphate (GTP) after brief application of 0.3 microM-Ca2+, suggesting Ca2+ is released from intracellular storage sites. Heparin (500 micrograms/ml, an inhibitor of the IP3 receptor), but not pinacidil (1 microM) or glibenclamide (1 microM), inhibited the Ca2+ release from storage sites induced by NA or IP3. These results suggest that membrane hyperpolarization is essential for the inhibitory action of pinacidil on the NA-induced Ca2(+)-releasing mechanism.(ABSTRACT TRUNCATED AT 400 WORDS)
Itoh
T
T
Department of Pharmacology, Faculty of Medicine, Kyushu University, Fukuoka, Japan.
Seki
N
N
Suzuki
S
S
Ito
S
S
Kajikuri
J
J
Kuriyama
H
H
D000818:Animals; D002118:Calcium / Q000378:metabolism; D006146:Guanidines / Q000494:pharmacology; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate / Q000096:biosynthesis*; D007537:Isometric Contraction / Q000187:drug effects; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D008638:Mesenteric Arteries / Q000187:drug effects / Q000378:metabolism; D009131:Muscle, Smooth, Vascular / Q000187:drug effects / Q000378:metabolism*; D009638:Norepinephrine / Q000494:pharmacology; D019269:Phosphatidylinositol 4,5-Diphosphate; D010716:Phosphatidylinositols / Q000378:metabolism; D020110:Pinacidil; D011188:Potassium / Q000494:pharmacology; D011817:Rabbits; D014665:Vasodilator Agents / Q000494:pharmacology
D006146:Guanidines; D019269:Phosphatidylinositol 4,5-Diphosphate; D010716:Phosphatidylinositols; D014665:Vasodilator Agents; D020110:Pinacidil; D015544:Inositol 1,4,5-Trisphosphate; D011188:Potassium; D002118:Calcium; D009638:Norepinephrine