Forskolin Changes the Relationship between Cytosolic Ca2+ and Contraction in Guinea Pig Ileum

Koon Hee Han; Gap Jin Cheon; Dong Soo Yeon; Seong Chun Kwon

doi:10.4196/kjpp.2009.13.3.189

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Han, Cheon, Yeon, and Kwon: Forskolin Changes the Relationship between Cytosolic Ca2+ and Contraction in Guinea Pig Ileum

Original Article

Korean J Physiol Pharmacol 2009;13(3):189-194.

Published online: 17 June 2009

DOI: https://doi.org/10.4196/kjpp.2009.13.3.189

Forskolin Changes the Relationship between Cytosolic Ca²⁺ and Contraction in Guinea Pig Ileum

Koon Hee Han^1,², Gap Jin Cheon^1,², Dong Soo Yeon², Seong Chun Kwon²

¹Department of Internal Medicine, Gangnung Asan Hospital, Gangnung 210-711, Korea

²Department of Physiology, College of Medicine, Kwandong University, Gangneung 210-751, Korea

Corresponding to: Seong Chun Kwon, Department of Physiology, College of Medicine, Kwandong University, 522, Naegok-dong, Gangnung 210-751, Korea. (Tel) 82-33-649-7463, (Fax) 82-33-641-1074, (E-mail) skwon2028@kd.ac.kr

Received 28 April 200915 May 2009 Accepted 2 June 2009

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

This study was designed to clarify the mechanism of the inhibitory effect of forskolin on contraction, cytosolic Ca²⁺ level ([Ca²⁺]_i), and Ca²⁺ sensitivity in guinea pig ileum. Forskolin (0.1 nM~10 μM) inhibited high K⁺ (25 mM and 40 mM)- or histamine (3 μM)-evoked contractions in a concentration-dependent manner. Histamine-evoked contractions were more sensitive to forskolin than high K⁺-evoked contractions. Spontaneous changes in [Ca²⁺]_i and contractions were inhibited by forskolin (1 μM) without changing the resting [Ca²⁺]_i. Forskoln (10 μ M) inhibited muscle tension more strongly than [Ca²⁺]_i stimulated by high K⁺, and thus shifted the [Ca²⁺]_i-tension relationship to the lower-right. In histamine-stimulated contractions, forskolin (1 μ M) inhibited both [Ca²⁺]_i and muscle tension without changing the [Ca²⁺]_i-tension relationship. In α-toxin-permeabilized tissues, forskolin (10 μM) inhibited the 0.3 μ M Ca²⁺-evoked contractions in the presence of 0.1 mM GTP, but showed no effect on the Ca²⁺-tension relationship. We conclude that forskolin inhibits smooth muscle contractions by the following two mechanisms: a decrease in Ca²⁺ sensitivity of contractile elements in high K⁺-stimulated muscle and a decrease in [Ca²⁺]_i in histamine-stimulated muscle.

Keywords: Fosrkolin, Cytosolic Ca²⁺, Ca²⁺ sensitivity, Guinea pig ileum

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Fig. 1.

Concentration-response relationship for the inhibitory effect of forskolin on contractions induced by 25 mM KCl, 40 mM KCl, and 3 μM histamine. Forskolin (0.1 nM~10 μM) was cumulatively added after the contractions reached a steady state.

Fig. 2.

The effect of forskolin (1 μM) on [Ca²⁺]_i (upper trace) and muscle tension (lower trace) in spontaneously active ileum. 100% represents the steady state [Ca²⁺]_i in the presence of 40 mM KCl. Forskolin (1 μM) inhibited rhythmic increases in [Ca²⁺]_i and tension without changing the basal [Ca²⁺]_i. EGTA (4 mM) decreased basal [Ca²⁺]_i below the resting level with no further decrease in muscle tension.

Fig. 3.

Effects of forskolin on 40 mM KCl (A)- and 3 μM histamine (B)-stimulated [Ca²⁺]_i (upper trace) and muscle tension (lower trace). 100% represents the 40 mM KCl- or histamine-induced increases in [Ca²⁺]_i before addition of forskolin. In panel A, after 40 mM KCl-stimulated [Ca²⁺]_i and muscle tension reached a steady state level, 10 μM forskolin and verapamil were sequentially added. Forskolin partially inhibited contraction without changing [Ca²⁺]_i. Verapamil (10 μM), on the other hand, inhibited [Ca²⁺]_i and tension to the resting level. In panel B, when the [Ca²⁺]_i and muscle tension induced by histamine (3 μM) reached a steady state level, 1 μM forskolin and 10 μM verapamil were sequentially added. Forskolin (1 μM) significantly inhibited [Ca²⁺]_i and contractions. Verapamil (10 μM) inhibited [Ca²⁺]_i and tension to the resting level.

Fig. 4.

Effect of forskolin (10 μM, open circles) on the relationship between [Ca²⁺]i (abscissa) and muscle tension (ordinate) in the presence various concentrations of KCl (10, 15, 20, 30, and 40 mM) or histamine (0.03, 0.1, 0.3, 1, and 3 μM). 100% represents 40 mM KCl-induced increases in [Ca²⁺]i and muscle tension measured before cumulative addition of the stimuli. Each point represents the mean of 7~10 experiments, and the SR mean is shown by vertical and horizontal bars.

Fig. 5.

(A) Effect of 10 μM forskolin on the contraction evoked by Ca²⁺ in α-toxin-permeabilized ileum. Ca²⁺ was cumulatively applied. The experiments were repeated after application of 10 μM forskolin. As a control, 10 μM Ca²⁺ was applied. (B) The pCa²⁺-tension relationship observed before (•) and after application of 10 μM forskolin (○). The amplitude of 10 μM Ca²⁺ was taken as 100%. Each point represents the mean of 6 experiments and the SE mean is shown by a vertical bar. (C) Effects of 10 μM forskolin treated with 0.1 mM GTP. After permebilizing the tissues, 0.3 μM Ca²⁺ was applied. After the contraction induced by 0.3 μM Ca²⁺ had reached a steady level, 0.1 mM GTP and forskolin were sequentially applied.

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