Effects of cyclopiazonic acid and thapsigargin on electromechanical activities and intracellular Ca²⁺ in smooth muscle of carotid artery of hypertensive rats

Abstract

1 The effects of cyclopiazonic acid (CPA) and thapsigargin (TG), both of which are known to inhibit sarcoplasmic reticular Ca2+-ATPase, on the mechanical activities, intracellular Ca2+ level and electrical activities of smooth muscle of the carotid artery of stroke-prone spontaneously hypertensive rats (SHRSP) and Wistar Kyoto rats (WKY) were compared. 2 Both CPA and TG induced elevation of tension of the smooth muscle, which was composed of a phasic and a tonic component. The level of tension attained, especially the tonic component, was greater in the preparation from SHRSP. 3 The elevation of tension was associated with an increased intracellular Ca21 level. Both the elevation of tension and the increase in intracellular Ca2+ were diminished by the removal of extracellular Ca2+ or by the application of verapamil. 4 The resting membrane potential of the preparations from SHRSP were depolarized to a greater extent than those from WKY. CPA depolarized the smooth muscle from both SHRSP and WKY, and the final level was also more depolarized in the preparation from SHRSP. 5 These results indicate that the elevation of tension induced by these drugs is mainly due to increased Ca2+ influx through voltage-dependent Ca21 channels, and the difference in the action between the preparation from SHRSP and that from WKY can be explained mainly by the changes in the channels. 6 Thus, differences in the action of these drugs on the tension of smooth muscle between preparations from WKY and SHRSP can mainly be explained by the difference in the membrane potential which is related to the difference in voltage-dependent Ca2+ influx.

Effects of estradiol on phenylephrine contractility associated with intracellular calcium release in rat aorta

“…The activation of ␣ 1 -adrenoceptors causes phospholipase C-mediated hydrolysis of the membrane phospholipid, phosphatidylinositol 4,5-bisphosphate, yielding two second messengers, inositol 1,4,5-trisphosphate (IP 3 ) and diacylglycerol (6,21,23). IP 3 binds to receptors located on the sarcoplasmic reticulum and releases stored Ca 2ϩ (11,23), which is responsible for the transient contractile response to phenylephrine that was observed in rat aortic rings incubated in Ca 2ϩ -free solution.…”

Effects of Flufenamic Acid on Smooth Muscle of the Carotid Artery Isolated from Spontaneously Hypertensive Rats.

“…The membrane potential was recorded intracellularly using a glass capillary microelectrode (tip resistance, 40-80 MΩ) filled with 3M KCl, as previously described (Sekiguchi et al, 1996). The reference electrode was an Ag-AgCl plate.…”

“…We have reported that the endotheliumCorrespondence to: K. Shimamura, Department of Clinical Pharmacology, Faculty of Pharmaceutical Sciences, Health Science University of Hokkaido, Kanazawa 1757, Ishikari-Tobetsu, Hokkaido 061-0293, Japan Phone: +81-1332-3-1211 ext.3160 Fax: +81-1332-3-1669 e-mail: shimamu@hoku-iryo-u.ac.jp removed carotid artery of stroke-prone spontaneously hypertensive rats (SHRSP) contracts spontaneously with an increase in intracellular free Ca 2+ concentration (Sekiguchi et al, 1996).…”

Ca2+ Buffering Function of the Sarcoplasmic Reticulum Is Increased in the Carotid Artery from Spontaneously Hypertensive Rats

“…We conclude that the SHR carotid artery has an increased total capacity of SR for purpose of Ca 2 The results that the contractile responses to CPA and thapsigargin were inhibited by nifedipine may suggest that Ltype Ca 2 channels are directly activated by these agents. Although such a direct action has never been reported, Sekiguchi et al (17) reported that CPA depolarized the smooth muscle membrane of the carotid arteries, probably due to an inhibition of Ca 2 -activated K channels via the depletion of SR Ca 2 and/or an activation of nonspecific cation channels. However, in the present study, the following findings provided no support for the idea that CPA has a membrane depolarizing effect: 1) after a 5-min incubation with 45 Ca, CPA decreased the basal Ca 2 influx; and 2) SR Ca 2 was not depleted during the addition of CPA.…”

“…We have previously reported that, even in the resting state, the carotid artery of SHR shows greater levels of Ca 2 influx and [Ca 2 ]i than the carotid artery of normotensive WistarKyoto rats (WKY) (15,16). To compensate for the increased level of [Ca 2 ]i in this artery, Ca 2 that enters the cells is likely to be taken up into the SR, because the addition of CPA or thapsigargin caused a larger contraction in SHR than in WKY (17). This further suggests that the Ca 2 uptake function of the SR during rest in the carotid artery is increased in SHR compared with WKY.…”