Korean J Physiol Pharmacol.  2009 Jun;13(3):189-194. 10.4196/kjpp.2009.13.3.189.

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

• Han KH
• Cheon GJ
• Yeon DS
• Kwon SC

• KMID: 2071670
• DOI: http://doi.org/10.4196/kjpp.2009.13.3.189

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 micrometer) inhibited high K+ (25 mM and 40 mM)- or histamine (3 micrometer)-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 micrometer) without changing the resting [Ca2+]i. Forskoln (10 micrometer) 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 micrometer) inhibited both [Ca2+]i and muscle tension without changing the [Ca2+]i-tension relationship. In alpha-toxin-permeabilized tissues, forskolin (10 micrometer) inhibited the 0.3 micrometer 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.

Keyword

Fosrkolin; Cytosolic Ca2+; Ca2+ sensitivity; Guinea pig ileum

MeSH Terms

Animals
Contracts
Cytosol
Forskolin
Guanosine Triphosphate
Guinea
Guinea Pigs
Histamine
Ileum
Muscle Tonus
Muscle, Smooth
Muscles
Forskolin
Guanosine Triphosphate
Histamine

Figure

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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.

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  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.

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  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.

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  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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