Abstract
Two substances identified as ((3β, 4α, 16β, 21β, 22α)-21-tigloxy-16, 22, 23, 28-tetrahydroxyolean-12-en-3-yl-β-D-glucopyranosiduronic acid) (GA1) and ((3β, 4α, 16β, 21β, 22α)-21-(2-methylbutyroxy)-16, 22, 23, 28-tetrahydroxyolean-12-en-3-yl-β-D-glucopyranosiduronic acid) (GA2) identified among the gymnemic acids are triterpene glycosides extracted from Gymnema sylvestre leaves. We examined the effects of GA1 or GA2 on high K+-induced contraction in the guinea-pig longitudinal muscle. A sustained muscle contraction induced by hyperosmotically added 65.4mM KCl (H-65K+) was suppressed by GA1 or GA2 (7.7×10-5M). Simultaneous measurements of reduced pyridine nucleotide (PNred) or oxidized flavin protein (FPox) by the fluorescence technique and of contractile force revealed that GA1 and GA2 reduced the increase of PNred fluorescence and contractile force induced by H-65K+, whereas FPox fluorescence induced by it further increased. Reduced muscle contraction induced by GA1 or GA2 was restored by 5.5mM pyruvate. Simultaneous measurements of intracellular Ca2+ [Ca2+] i level and contractile force indicated that [Ca2+] i level, which increased by H-65K+, hardly changed with GA1 and GA2. In summary, both GA1 and GA2, which are among the gymnemic acids, suppressed high K+-induced contraction in the guinea-pig ileal longitudinal muscle. The difference between these two gymnemic acids was not significant. The inhibitory effect of GA1 and GA2 on smooth muscle were assumed to be a result of inhibiting glucose uptake, which is an energy source of the muscle, whereas the inhibitory mechanisms were probably not mediated by Ca2+.
