Original Paper
Cellular Physiology and Biochemistr Biochemistryy
Cell Physiol Biochem 2002;12:119-126
Accepted: December 18, 2001
Effect of Zinc Ions on Caffeine-Induced Contracture in Vascular Smooth Muscle and Skeletal Muscle of Rat Xiao-Yang Cheng, Ke-Ying Chen, Xiao-Hui Zhang and Pei-Hong Zhu Unit of Cell Signal Transduction, Key Laboratory of Neuroscience, Shanghai Institutes for Biological Sciences and of Physiology, Chinese Academy of Sciences, Shanghai
Key Words Caffeine contracture • Zinc ions • Smooth muscle • Skeletal muscle Abstract The effect of zinc ions on caffeine-induced contracture in vascular smooth muscle and skeletal muscle of rat was studied. In aortic strips, caffeine contracture was depressed by Zn2+ in a dose-dependent manner. Moreover, the extent of the depression of caffeine contracture in the Zn 2+ loaded smooth muscle increased with repetitive caffeine exposures. For instance, in the preparations perfused with a medium containing 100 µM [Zn2+] for 15 or 30 min, the first caffeine contractures were similarly depressed to about 60% of the control. However, the subsequent caffeine exposure at 15 min interval could not evoke any contracture. In this study this feature is referred to as activation dependence of the Zn2+ effect. In small bundles of soleus muscle 2∼100 µM [Zn2+] similarly caused a depression of caffeine contracture, and the activation dependence also was evident. However, an evident potentiation of caffeine contracture was seen in the preparations loaded with lower [Zn2+] such as 0.5 µM, indicating that the effect of Zn2+ on caffeine
Fax +41 61 306 12 34 E-Mail
[email protected] www.karger.com
© 2002 S. Karger AG, Basel 1015-8987/02/0123-0119$17.50/0 Accessible online at: www.karger.com/journals/net
contracture of skeletal muscle was somewhat different from that seen in smooth muscle. By observing how the depression effect depends on the intervals between caffeine exposures as well as on caffeine concentrations, it is indicated that the activation dependence of the Zn2+ effect, at least in skeletal muscle, may not be explained by depletion of intracellular Ca 2+ stores alone. The possible mechanism for that caffeine contracture of smooth muscle and skeletal muscle was differentially affected by Zn2+ ions is discussed. Copyright © 2002 S. Karger AG, Basel
Introduction Zinc ions (Zn2+), as the second most abundant trace element in the body, function as a co-factor or structural component of many enzymes or non-enzymatic proteins and play many roles in regulating various cellular functions [1]. Several effects of Zn2+ on the function of both smooth muscle and skeletal muscle have been described: inhibition of Ca-ATPase of the sarcoplasmic
Pei-Hong Zhu Unit of Cell Signal Transduction, Key Lab of Neuroscience, CAS 320 Yue-Yang Road, Shanghai 200031 (China) Tel. 86-21-64370080, ext.147, Fax 86-21-64332445 E-Mail
[email protected]
119
reticulum (SR) [2], depression of Ca2+ release from the SR [3], alteration of excitation-contraction coupling [4], induction of delayed contracture [5] etc. More recently, we found that ryanodine binding to the SR vesicles of skeletal muscle and cardiac muscle was differentially affected by Zn2+ [6, 7]. The binding to the SR of skeletal muscle was biphasically modulated by Zn2+. The binding was increased by [Zn2+] of less than 1 µM, and a peak binding occurred at about 0.3 µM [Zn2+]. The decrease of the binding became obvious with [Zn2+] of more than 1 µM [6]. In contrast, the effect of Zn2+ in cardiac muscle was monophasic [7]. Ryanodine binding to the SR vesicles of cardiac muscle could not be changed by low [Zn2+], while a significant decrease of the binding occurred with raising [Zn2+] to 0.5 µM. Ryanodine binding assay has been used to investigate the effect of various ligands on the function of ryanodine receptor/ Ca 2+ release channels (RyRs) [8]. Three isoforms of RyRs have been identified in the cells of mammalian animals: RyR1, RyR2 and RyR3 [8]. The main isoform of RyRs in skeletal muscle cells is RyR1, while the predominant isoform in cardiac muscle belongs to RyR2. It is known that all of three isoforms of RyRs are differentially expressed in vascular smooth muscle cells [9], but the main isoform is RyR2 [10]. The different effect of Zn2+ on ryanodine binding to the SR vesicles of skeletal and cardiac muscle suggests that the effect of Zn2+ on RyRs is isoform-specific. Although a close correlation between ryanodine binding and the functional state of RyRs has been established for many ligands [8], it is still of importance to illustrate that the effect of Zn2+ on the function of RyRs is isoform-specific. For this purpose, several approaches can be taken. It may be a good choice to work with different isoforms of RyRs reconstituted in bilayers. Similar information can be derived from examining the effect of Zn2+ on Ca2+ release from the SR vesicles. However, to have more understanding of the Zn2+ effect in intact cells, caffeine contractures were used in the present study, although a few shortcomings, especially unknown intracellular free Zn2+ concentration ([Zn2+]i) and the presence of several potential sites of action of Zn 2+ in the intact cells, evidently are inherent in this method. It is known that the main effect of caffeine on calcium mobilization in muscle cells is to activate RyRs and release Ca2+ from intracellular Ca2+ stores [11]. Considering the factor of
the possible change of intracellular Ca2+ stores induced by Zn2+, the information about the effect of Zn2+ on RyR may be inferred from the change of caffeine contracture.
120
Cheng/Chen/Zhang/Zhu
Materials and Methods Preparation and solution The experiments were performed on thoracic aorta and soleus muscle of Sprague-Dawley rat (200-250 g). The animals were anesthetized with sodium pentobarbital, and then the muscles were dissected and taken out quickly. The aorta and soleus muscles were placed in Tyrode’s and Locke’s solution, respectively. Fat and connective tissues surrounding the aorta were removed carefully, and the aorta was cut into strips (about 2 mm in width × 20 mm in length). To make denuded aortic strips, the endothelium was removed by gently inserting a cotton thread through the lumen of the aorta before cutting into strips. The removal of the endothelium was verified by the lack of relaxation response to 10-4 M ACh. Small muscle bundles, each containing about 20∼30 fibers, were prepared from soleus muscle under a dissecting microscope. The composition of Tyrode’s solution was (in mM): NaCl 123.7, KCl 2.7, CaCl2 2.5, MgCl2 1.0, Tris-HCl 25, glucose 5.5. Locke’s solution consisted of (in mM): NaCl 150, KCl 5, CaCl2 2, MgCl2 1, HEPES 4, glucose 11.All these solutions were titrated to pH 7.4 with HCl or NaOH. Assessment of contractile properties The aortic strips were suspended in a perfusion chamber with one terminal fixed by a clamp and the other one connected to the lever of a transducer. To be stabilized, the preparation was subject to a load of 1.0 g and perfused with Tyrode’s solution bubbled with 100% O2 at a rate of 2 ml/min at 37°C for one hour. Then, Tyrode’s solution containing 10 mM caffeine was given to induce a contracture as the control. After the contracture reached the peak, the strip was washed with Tyrode’s solution for about 5 min, and then perfused with the solution containing different concentrations of ZnCl2 for 15 or 30 min. Afterwards, the Zn2+ loaded preparation was exposed to 10 mM caffeine to obtain the first caffeine contracture. Then, at an intervals of 15 or 30 min subsequent caffeine contractures were induced (see Results for detail). The caffeine contracture in the presence of Zn2+ is represented as a percentage of the peak of the control. Small bundles of soleus muscle were put in a chamber, and perfused with Locke’s solution at 25°C. After being adjusted to an optimal length [12], the preparation was exposed to caffeine until the contracture reached the peak. Probably due to diffusion delay, caffeine contracture of skeletal muscle preparation often showed a biphasic raising phase. Therefore, longer caffeine exposed usually was needed. Otherwise, the protocol basically was similar to that in smooth muscle. The data were expressed as means ± S.D. Student’s t-test was used for comparisons between groups.
Results Effect of Zn2+ on caffeine-induced contracture in aortic strips A) Dose dependence of the effect of Zn2+. In the absence of Zn2+ caffeine contracture was not obviously changed by repetitive exposures to 10 mM caffeine, at an interval of either 15 min (Fig.1A and Fig.3) or 30 min (Fig.1B and Fig.3). However, as shown previously [3], caffeine contracture significantly depressed by Zn2+ (Fig.1C and D). With repetitive caffeine exposures, caffeine contracture gradually decreased. At last, due to unknown mechanism, only a prolonged reduction of the tension could be induced by caffeine in Zn2+ loaded preparations. A new finding of this study is that, in response to caffeine exposure, a small transient decrease of the tension (TDT) was seen in Zn2+ loaded preparations before the increase of the tension (Fig.1C and D). In contrast, the TDT was lacking in the control (Fig.1A and B). Thus, it is unlikely that the TDT is an artefact, e. g. from solution change. Because the amplitude of the TDT was too small and the size of the preparations was not the same, the statistics could not be performed on the TDT. The effect of Zn 2+ was dependent on Zn 2+ concentration ([Zn 2+ ]). Figure 2 represents the summarized results of the [Zn2+] dependence. In this experiment the interval between caffeine exposures was 15 min. It is evident that the first caffeine contracure did not change in the preparations, which had been perfused with 0.5∼50 µM [Zn2+] for 15 min. But, when [Zn2+] increased to 100 µM, the depression of the first caffeine contracture was significant. B) Time dependence of the effect of Zn2+. It has been shown previously that, an increased depression of caffeine contracture of Zn2+ loaded preparations occurred with repetitive caffeine exposures [3]. This phenomenon was validated and called as activation dependence of the Zn2+ effect in the present study (Fig.2). It can be seen that, the first caffeine contracture did not clearly change in the preparations perfused with 50 µM [Zn2+] for 15 min, but the second one was reduced to 20.3±13.6 % of the control (Mean±S. D., n=8; p
