Modulatory effects of taurine on jejunal contractility

Abstract

Taurine (2-aminoethanesulfonic acid) is widely distributed in animal tissues and has diverse pharmacological effects. However, the role of taurine in modulating smooth muscle contractility is still controversial. We propose that taurine (5-80 mM) can exert bidirectional modulation on the contractility of isolated rat jejunal segments. Different low and high contractile states were induced in isolated jejunal segments of rats to observe the effects of taurine and the associated mechanisms. Taurine induced stimulatory effects on the contractility of isolated rat jejunal segments at 3 different low contractile states, and inhibitory effects at 3 different high contractile states. Bidirectional modulation was not observed in the presence of verapamil or tetrodotoxin, suggesting that taurine-induced bidirectional modulation is Ca(2+) dependent and requires the presence of the enteric nervous system. The stimulatory effects of taurine on the contractility of isolated jejunal segments was blocked by atropine but not by diphenhydramine or by cimetidine, suggesting that muscarinic-linked activation was involved in the stimulatory effects when isolated jejunal segments were in a low contractile state. The inhibitory effects of taurine on the contractility of isolated jejunal segments were blocked by propranolol and L-NG-nitroarginine but not by phentolamine, suggesting that adrenergic β receptors and a nitric oxide relaxing mechanism were involved when isolated jejunal segments were in high contractile states. No bidirectional effects of taurine on myosin phosphorylation were observed. The contractile states of jejunal segments determine taurine-induced stimulatory or inhibitory effects, which are associated with muscarinic receptors and adrenergic β receptors, and a nitric oxide associated relaxing mechanism.

Figure 1. Effects of taurine on the contractile amplitude of isolated jejunal segments. Representative traces and statistical analysis (n=6) of taurine-induced effects on the contractile amplitude of isolated jejunal segments in the normal contractile state (NCS, control). The contractile amplitude in NCS is set to 100%; the effects of taurine on the contractile amplitude are the relative values compared with NCS. *P<0.05 vs the contractile amplitude in NCS before taurine administration (one-way ANOVA).

Figure 2. Taurine-induced bidirectional modulations on the contractile amplitude of isolated jejunal segments. A, Representative traces and statistical analysis (n=6) of the inhibitory effects of taurine on the contractile amplitude of isolated jejunal segments in 3 high contractile states (HCS). B, Representative traces and statistical analysis (n=6) of the stimulatory effects of taurine on the contractile amplitude of isolated jejunal segments in 3 low contractile states (LCS). The contractile amplitude in the normal contractile state (NCS, control) is set to 100%; the contractile amplitude in HCS and LCS are the relative values compared with NCS. CBC: carbachol; Adr: adrenaline. *P<0.05 vs the contractile amplitude in NCS; ^#P<0.05 vs the contractile amplitude in LCS or HCS before taurine administration (one-way ANOVA).

Figure 3. Effects of taurine on the contractile amplitude of isolated jejunal segments pretreated with tetrodotoxin (TTX). Representative traces and statistical analysis (n=6) of taurine-induced effects on the contractile amplitude of isolated jejunal segments in the normal contractile state (NCS, control), high contractile state (HCS) induced by high Ca²⁺ (5.0 mM) Krebs buffer and low contractile state (LCS) induced by low Ca²⁺ (1.25 mM) Krebs buffer pretreated with TTX (0.3 μM). The contractile amplitude in NCS is set to 100%; other data are the relative values compared with NCS. CS: contractile state.

Figure 4. Effects of taurine on the contractile amplitude of isolated jejunal segments pretreated with receptor antagonists. A, Representative traces and statistical analysis (n=6) of taurine-induced effects on the contractile amplitude of isolated jejunal segments pretreated with 1 μM atropine, 10 μM diphenhydramine, and 10 μM cimetidine in low contractile states (LCS) induced by low Ca²⁺ (1.25 mM) Krebs buffer. B, Representative traces and statistical analysis (n=6) of taurine-induced effects on the contractile amplitude of isolated jejunal segments pretreated with 1 μM phentolamine, 1 μM propranolol, and 300 μM L-NNA in high contractile states (HCS) induced by high Ca²⁺ (5.0 mM) Krebs buffer. The contractile amplitude in the normal contractile state (NCS, control) is set to 100%; other data are the relative values compared with NCS. *P<0.05 vs the contractile amplitude in NCS; ^#P<0.05 vs the contractile amplitude in LCS or HCS before taurine administration (one-way ANOVA).

Figure 5. Effects of taurine on the contractile amplitude of isolated jejunal segments pretreated with verapamil. Representative traces and statistical analysis (n=6) of taurine-induced effects on the contractile amplitude of isolated jejunal segments pretreated with 1 μM verapamil in the normal contractile state (NCS, control), CS induced by high Ca²⁺ (5.0 mM) Krebs buffer and CS induced by low Ca²⁺ (1.25 mM) Krebs buffer. The mean contractile amplitude without verapamil treatment in NCS is set to 100%; other data are the relative values compared with NCS. CS: contractile state.

Figure 6. Effects of taurine on phosphorylated myosin. A, Effects of taurine on high-extent phosphorylated myosin, 0.02 μM myosin light chain kinase (MLCK), and 4.0 μM myosin purified from chicken gizzard smooth muscle used in the assay. B, Effects of taurine on low-extent phosphorylated myosin, 2.0 μM MLCK and 4.0 μM myosin purified from chicken gizzard smooth muscle used in the assay. Lanes 0-6 represent unphosphorylated myosin (without MLCK and taurine), high- or low-extent phosphorylated control (without taurine), high- or low-extent phosphorylated myosin with 5, 10, 20, 40, and 80 mM taurine, respectively. C, Extent of myosin phosphorylation, which was analyzed using the Gelpro software. The extent of phosphorylated myosin was calculated as percentage of phosphorylated regulatory myosin light chain of 20 kDa (MLC₂₀) in total MLC₂₀. Mono-phosphorylation was calculated as 100% phosphorylation. *P<0.05 vs high- or low-extent phosphorylated control (without taurine) (one-way ANOVA) (lane 1). MLC₂₀: unphosphorylated MLC₂₀ (20 kDa regulatory myosin light chain); p-MLC₂₀: mono-phosphorylated MLC₂₀; MLC₁₇: 17 kDa myosin essential light chains.