HALOPERIDOL DIFFERENTIATES SMOOTH-MUSCLE CONTRACTIONS INDUCED BY RELEASE OF INTRACELLULARLY STORED CA AND BY INFLUX OF EXTRACELLULAR CA

1. The effects of haloperidol on smooth muscle contraction induced by carbachol, histamine, high K or caffeine in the presence or absence of extracellular Ca were investigated. 2. In the presence of extracellular Ca, the maximal contraction induced by carbachol was reduced by haloperidol, while that by histamine or high K was much less affected. 3. In Ca-free solution, contraction induced by histamine was extremely reduced by haloperidol, while that by carbachol was not affected. 4. These results suggest that haloperidol selectively inhibited signal transduction processes from activation of muscarinic acetylcholine receptors to influx of extracellular Ca and from activation of histamine H-1-receptors to release of intracellularly stored Ca. 5. Caffeine-induced contraction in Ca-free solution was markedly potentiated by haloperidol, although haloperidol did not elicit contraction in Ca-free solution by itself. 6. These results suggest that haloperidol increased the sensitivity of Ca-induced Ca release channels to caffeine.