|Year : 2004 | Volume
| Issue : 2 | Page : 84-86
Comparison of the inhibitory effect of ATP-dependent K+ channel blockers glibenclamide and chlorpropamide, on diazoxide-induced relaxation of rat ileum
A Babaei , J Mohiti
Departments of Pharmacology, Shahid Sadoughi University of Medical Science, Yazd, Iran
Departments of Pharmacology, Shahid Sadoughi University of Medical Science, Yazd, Iran
Objective: This study was conducted to compare the potential inhibitory effect of ATP-dependent K+ channel blockers, glibenclamide and chlorpropamide on diazoxide-induced relaxation of isolated rat ileum.
Material and Methods: The study was performed on rat ileum connected to an isotonic transducer and the contractions were recorded on one-channel MD2 recorder. In this study the relaxant effect of diazoxide on KCl-induced precontracted rat ileum, and its inhibition by different concentrations of glibenclamide and chlorpropamide were studied.
Results: Diazoxide (10-5-10-3M) produced dose-dependent relaxation. EC50 for diazoxide-induced relaxation was 8 x 10-5M. Neither glibenclamide nor chlorpropamide had any effect on the resting tension of the rat ileum preparation. Both glibenclamide and chlorpropamide reversed diazoxide-induced relaxation. The pA2 values for glibenclamide and chlorpropamide reversal of diazoxide induced- relaxation were 6.3 and 5 respectively.
Conclusion: The results confirm that the inhibitory effect of glibenclamide on diazoxide-induced relaxation in rat ileum is higher than that of chlorpropamide.
|How to cite this article:|
Babaei A, Mohiti J. Comparison of the inhibitory effect of ATP-dependent K+ channel blockers glibenclamide and chlorpropamide, on diazoxide-induced relaxation of rat ileum. Indian J Pharmacol 2004;36:84-6
|How to cite this URL:|
Babaei A, Mohiti J. Comparison of the inhibitory effect of ATP-dependent K+ channel blockers glibenclamide and chlorpropamide, on diazoxide-induced relaxation of rat ileum. Indian J Pharmacol [serial online] 2004 [cited 2019 Dec 6];36:84-6. Available from: http://www.ijp-online.com/text.asp?2004/36/2/84/6765
| » Introduction|| |
Recently, increasing attention has been paid to cell membrane K+ channels as a site of action for a number of drugs. ATP-dependent K+ channels have been described in diverse cell types, including cardiomyocytes, pancreatic beta cells, neurons, smooth muscle cells and skeletal muscle cells, where they play important physiological and pathophysiological roles. Blockers of K+ channels are able to promote insulin secretion from pancreatic beta cells and suppress dysrhythmias of the heart. K+ channel openers cause relaxation of a variety of smooth muscles. Some antihypertensive agents such as minoxidil and diazoxide have been used clinically for years. They produce their action by opening the K+ channels.
This study was undertaken to compare the action of two competitive antagonists of ATP-dependent K+ channels, namely glibenclamide and chlorpropamide against the relaxant effect of diazoxide, a K+ channel opener, on rat ileum.
| » Material and Methods|| |
Experimental animals and procedures
Segments of terminal ileum (1-1.5 cm long) were removed from male Wistar rats weighing 250-300 g. The tissue was suspended in 50 ml organ bath containing Tyrode's solution (mM): NaCl 137, KCl 2.7, CaCl2 1.5, MgCl2 1, NaHCO3 12, NaH2PO4 0.4, and glucose 5.5 (Merck, Germany), which was bubbled with carbogen (5% CO2 and 95% O2) at 370C. The tissue was connected longitudinally by surgical thread to an isotonic transducer and the contractions were recorded on one-channel MD2 recorder. The tissues were allowed to equilibrate for 30 min and the bathing fluid was exchanged by overflow every 10 min.
At the end of the equilibration period, the tissue was contracted by 20 mM KCl and the responses were allowed to reach a plateau. This was obtained within 10-15 min after the addition of KCl. Diazoxide (Sigma) was then added in a cumulative manner (10-5-10-3M). Sufficient time was allowed before the next dose and the tissue was washed every 5 min till baseline was restored. Subsequently, the tissue was contracted once again with 20 mM KCl, and at plateau, glibenclamide (10-7-10-5M) or chlorpropamide (3 ×10-6-10-4M) or an equivalent volume of vehicle (dimethyl sulfoxide - Merck, Germany) was added and allowed to equilibrate for 10 min before the addition of diazoxide.
Treatment of data and statistics
The relaxant effects of diazoxide are expressed as percent of inhibition of tension produced by 20 mM KCl. For determination of EC50, logarithm of (response/ [max response- response] was plotted against the logarithm of the concentration of diazoxide (agonist). The pA2 values for glibenclamide and chlorpropamide were determined by the Schild plot, in which the logarithm of (dose ratio-1) is plotted as a function of the negative logarithm of the antagonist concentration (pAx). The intercept on the abscissa is -log K1 or pA2. The term of dose ratio is applied to the expression [C']/[C], which is agonist concentration in the presence and absence of antagonist. Results are means ± SEM. One-way analysis of variance (ANOVA) followed by post hoc Dunnett's multiple comparisons test was used for comparison of the effect of different concentrations of glibenclamide and chlorpropamide. Differences between means were considered to be significant at P<0.05.
| » Results|| |
As shown in [Figure - 1], the cumulative application of diazoxide (10-5-10-3M) inhibited contractions induced by 20 mM KCl in a concentration-dependent manner. The EC50 for diazoxide-induced relaxation in this tissue was determined to be 8×10-5 M. Neither diazoxide nor glibenclamide had any effect on the resting tension of the rat ileum.
Addition of glibenclamide (10-7-10-5M) or chlorpropamide (3 x 10-6-10-4M) to KCl-contracted ileum produced no change in tension. However, both shifted the dose-response curve of diazoxide rightward in a concentration-dependent manner [Figure - 1] and [Figure:2]. The pA2 values for glibenclamide and chlorpropamide reversal of diazoxide relaxation were 6.3 and 5 respectively.
| » Discussion|| |
The K+ channel opener diazoxide inhibits KCl-induced contractions at 20 mM in the rat ileum. The effect of diazoxide was antagonized by glibenclamide (pA2 = 6.3) and chlorpropamide (pA2 = 5).
Our results are in agreement with a previous report indicating dose-dependent inhibition of rat ileal smooth muscle by K+ channel activator lemakalim and the presence of glibenclamide-sensitive K+ channel in this tissue. On the other hand, Davies and co-workers reported that diazoxide had no effect on contractions elicited by KCl or electrical stimulation in rat intestinal smooth muscle, in spite of the relaxant effect of nicorandil, another K+ channel modulating agent. This may be because of significant differences in the pharmacological and electrophysiological profiles of ATP-dependent K+ channels in various smooth muscles. For example, the sensitivity of ATP-dependent K+ channels to compounds such as diazoxide and lemakalim varies and differs between preparations.,
Displacement experiment in the presence of various sulfonylureas also showed greater potency of glibenclamide in comparison with chlorpropamide and tolbutamide in guinea pig isolated small intestine. Studies of K+ channel modulating agents on mouse intestinal smooth muscle also showed greater effectiveness of glibenclamide in comparison with tolbutamide (another first generation sulfonylurea) against pinacidil-induced relaxation.
The pA2 value obtained for glibenclamide (6.3) from the concentration-response curves of diazoxide was different from the pA2 obtained for diazoxide and glibenclamide on vascular smooth muscle (7.2). This may be due to the involvement of different subtype of K+ channels in vascular and gastrointestinal smooth muscles.
This study showed the relaxant effect of diazoxide against KCl (20 mM)-induced contraction in rat ileum and the greater potency of glibenclamide in relation to chlorpropamide in inhibiting diazoxide-induced relaxation of rat ileum.
| » Acknowledgments|| |
We are grateful to Chemidaru Company (Iran) for providing chlorpropamide and glibenclamide.
| » References|| |
|1.||Quayle JM, Nelson MT, Standen NB. ATP-sensitive and inwardly rectifying potassium channels in smooth muscle. Physiol Rev 1997;77:1165-232. [PUBMED] |
|2.||Cook NS. The pharmacology of potassium channels and their therapeutic potential. Trends Pharmacol Sci 1988;9:21-8. [PUBMED] |
|3.||Shieh CC, Coghlan M, Sullivan JP, Gopalakrishnan M. Potassium channels: Molecular defects, diseases and therapeutic opportunities. Pharmacol Rev 2000;52:557-94. [PUBMED] [FULLTEXT]|
|4.||Pratt WB, Taylor P. Principles of drug action. The basis of pharmacology. 3rd ed. Churchill Livingstone. USA: Molecular basis of pharmacologic selectivity. 1990; p. 1-103. |
|5.||Franck H, Puschmann A, Schusdziarra V, Allescher HD. Functional evidence for a glibenclamide-sensitive K+ channel in rat ileal smooth muscle. Eur J Pharmacol 1994;271:379-86. [PUBMED] |
|6.||Davies MP, McCurrie JR, Wood D. Comparative effects of K+ channel modulating agents on contractions of rat intestinal smooth muscle. Eur J Pharmacol 1996;297:249-56. [PUBMED] [FULLTEXT]|
|7.||Koh SD, Bradley KK, Rae MG, Keef KD, Horowitz B, Sanders KM. Basal activation of ATP-sensitive potassium channels in murine colonic smooth muscle cell. Biophys J 1998;75:1793-800. [PUBMED] [FULLTEXT]|
|8.||Standen NB, Quayle JM, Davies NW, Brayden JE, Huang Y, Nelson MT. Hyperpolarising vasodilators activate ATP-sensitive K+ channels in arterial smooth muscle. Science 1989;245:177-80. [PUBMED] |
|9.||Kajioka S, Kitamura K, Kuriyama H. Guanosine diphosphate activates an adenosine- 5'-triphosphate-sensitive K+ channel in the rabbit portal vein. J Physiol 1991;444:397-418. [PUBMED] |
|10.||Zini S, Ben-Ari, Y, Ashford ML. Characterization of sulfonylurea receptors and the action of K+ channel openers on cholinergic neurotransmission in guinea pig isolated small intestine. J Pharmacol Exp Ther 1991;259:566-73. |
|11.||Yeung CK, McCurrie JR, Wood D. Characterization of the effect of potassium modulating agents on mouse intestinal smooth muscle. J Pharm Pharmacol 2002;54:425-33. [PUBMED] [FULLTEXT]|
|12.||Winquist RJ, Heaney LA, Wallace AA, Baskin EP, Stein RB, Garcia ML, et al. Glyburide blocks the relaxation response to BRL 34915 (cromakalim), minoxidil sulfate and diazoxide in vascular smooth muscle. J Pharmacol Exp Ther 1989;248:149-56. [PUBMED] |