Effects of homopantothenic acid on intestinal smooth muscles.

The effects of homopantothenic acid (HOPA) on intestinal smooth muscles were investigated and compared with those of gamma-aminobutyric acid (GABA). HOPA contracted the rat ileum and produced a biphasic response (relaxation followed by contraction) in the guinea-pig ileum and taenia coli. Half-maximal effects were obtained with 0.3-0.9 nM HOPA and maximal responses with 2-4 nM in the rat and the guinea-pig ileum. In the guinea-pig taenia, half-maximal effects were obtained with 0.2-0.25 microM HOPA and maximal responses with 2-4 microM HOPA. Tachyphylaxis occurred with higher concentrations. All these responses were not significantly affected by tetrodotoxin (TTX), atropine, diphenhydramine or metiamide. The relaxation elicited by HOPA was not reduced by simultaneous application of phentolamine and propranolol. Unlike HOPA, GABA produced a biphasic response (a relaxation followed by a contraction) in the guinea-pig ileum, which was almost abolished by TTX. These results suggest that HOPA mainly acts on intestinal smooth muscle directly, whereas GABA does indirectly through nerves.