Effects of cyclopiazonic acid and ryanodine on cytosolic calcium and contraction in vascular smooth muscle

Abstract

1 In smooth muscle, both Ca2+ release from the sarcoplasmic reticulum (SR) and Ca2+ influx across the plasma membrane are responsible for the increase in the cytosolic Ca2+ level ([Ca2+]j). To understand further the role of SR on smooth muscle contraction, the effects of an inhibitor of the SR Ca2+ pump, cyclopiazonic acid (CPA 10 gM), an inhibitor of the Ca2+-induced Ca2+ release, ryanodine, (10 gM), and an activator of the Ca2'-induced Ca2+ release, caffeine (20 mM), on [Ca2+]i and contractile force were examined in the ferret portal vein loaded with a photoprotein, aequorin. 2 CPA induced a small increase in the aequorin signal reaching a maximum at 7 min. Several minutes after the increase in the aequorin signal, muscle tension increased reaching a maximum at 21.5 min. In contrast, ryanodine changed neither the aequorin signal nor contraction. In the presence of ryanodine, caffeine induced a sustained increase in the aequorin signal and transient contraction. After washing ryanodine and caffeine, the aequorin signal and muscle tone returned to their respective control levels. After treatment with ryanodine and caffeine, the second addition of caffeine was almost ineffective whereas CPA still increased the aequorin signal and muscle tension.3 In the presence of external Ca2 , noradrenaline (NA, 10 gM) induced a transient increase followed by a sustained increase in the aequorin signal and sustained contraction. In contrast, KCl (70 mM) induced sustained increases in the aequorin signal and sustained contraction. In Ca2+-free solution, NA induced a small transient increase in the aequorin signal and a small transient contraction. These changes were inhibited in the presence of CPA or on pretreatment of the muscle with ryanodine and caffeine. These results suggest that CPA or ryanodine and caffeine depleted Ca2+ in SR. High K+ was ineffective in the absence of external Ca2 . 4 In the presence of external Ca2+ and CPA, NA and high K+ induced larger aequorin signals than in the absence of CPA, whereas the magnitude and shape of the contractions did not change. In contrast, pretreatment with ryanodine and caffeine did not have such an effect. In the muscle pretreated with ryanodine and caffeine, CPA changed the responses to high K+ and NA in a similar manner to that in the muscle without the pretreatment with ryanodine and caffeine.5 Dissociation of contraction from [Ca2+]i as measured with aequorin suggests that NA and high K+ increase Ca2+ in two compartments: a compartment containing contractile elements (contractile compartment) and another compartment unrelated to contractile elements (non-contractile compartment). Because CPA augmented the stimulant-induced increase in aequorin signal without changing contraction, the non-contractile compartment may be located near the SR and the CPA-sensitive SR Ca2+ pump may regulate the Ca2+ level in this compartment. However, because CPA changed neither the magnitude nor shape of the contractions in the presence of external Ca2, the SR Ca2+ pump may have little effect on...

Inhibitory Effect of Forskolin on Myosin Phosphorylation-Dependent and Independent Contractions in Bovine Tracheal Smooth Muscle.

“…In canine trachea, it has been demonstrated that isoproterenol inhibits CCh-stimulated [Ca2+] i, support ing the first possibility (Gunst and Bandyopadhyay, 1989;Ozaki et al, 1990a). However, it was also demonstrated that FK and isoproterenol inhibit high K+-induced contraction without decreasing [Ca2+] i in tracheal smooth muscle (Ozaki et al, 1990a;Tang et al, 1992), as is the case with vascular smooth muscle (Abe and Karaki, 1989;Ushio-Fukai et al, 1993). In bovine tracheal smooth muscle stimulated with carbachol (CCh), it has been reported that isoproter enol decreased MLC phosphorylation and force of contraction in parallel (Tang et al, 1992;Stull et al, 1990).…”

Doxorubicin-induced vasomotion and [Ca2+]i elevation in vascular smooth muscle cells from C57BL/6 mice

“…The Ca 2+ released from SR seems to be mainly contributed to initial phasic contraction, while the Ca 2+ that enters through voltage-dependent Ca 2+ channels and/or Ca 2+ -permeable non-selective cation channels on the cell membrane contributes to later tonic contraction [17][18][19] . In the present study, 100 µmol/L doxorubicin was shown to induce both phasic and tonic contractions in the denuded vessels.…”

“…The data from previous studies indicate that the effects of ryanodine on vessel contraction and transient [Ca 2+ ] i are determined by at least three factors, ie, the type of vessel, the basal tension of the vessel, and the concentration of ryanodine. Significant ryanodineinduced vessel contraction and/or increase in transient [Ca 2+ ] i have been observed in dog cerebral arteries with basal tone at 1-100 µmol/L [20] , in fetal rabbit distal (but not proximal) pulmonary artery smooth muscle cells at 50 µmol/L [21] , and in rat femoral arteries at 30 nmol/L−30 µmol/L [22] ; the absence of an effect of ryanodine on vessel contraction and/or transient [Ca 2+ ] i has been shown in the ferret portal vein at a dose of 10 µmol/L [17] , in the aorta of C57BL/6 mice at 10 µmol/L [10] , and in rat aortas at 10 µmol/L [23] . Therefore, one should be very cautious to compare the effects of ryanodine on the vasomotion and/or transient [Ca 2+ ] i of cells and to explain the interaction between ryanodine and other inhibitors of RyRs in different vascular smooth muscles.…”

Rat arterial smooth muscle devoid of ryanodine receptor function: effects on cellular Ca2+ handling

“…Elevation of basal calcium level, increased net calcium in¯ux, and slowed cellular extrusion of calcium ions are some of the main ®ndings in muscle treated with Ry, SR Ca 2+ pump inhibitors, or caeine, which at high doses also acts to release Ca 2+ via Ry receptors (Chen & van Breemen, 1992;Abe et al, 1996).…”