Calcium dependence of the mechanical response evoked by okadaic acid in smooth muscle.

The effects of okadaic acid (OA), obtained from a culture of the marine dinoflagellate Prorocentrum lima were studied on isolated strips of rat myometrium. The contractile response evoked by OA at 5, 10, and 20 microM in normal physiological solution was unaffected in the presence of tetrodotoxin (10 microM), indomethacin (3 microM), or a cocktail of antagonists which blocked muscarinic, adrenergic, histaminergic, serotonergic, and opioid receptors. Similarly, the response to OA was unaffected in the presence of nifedipine at a concentration (1 microM) which completely or highly blocked the response to KCl (60 mM), oxytocin (1 microM), or acetylcholine (100 microM). In a Ca(2+)-free 1 mM EGTA-containing solution, the response to 10 and 20 microM OA was slightly but significantly reduced whereas the response to 5 microM OA was abolished. However, a response similar to that evoked in Ca(2+)-containing solution was observed when 5 microM OA was added to the bath in the presence of 1 microM oxytocin or 160 microM vanadate in a Ca(2+)-depleted solution with 1 mM EGTA. These data suggest that the response of rat myometrium to OA (> or = 5 microM) is not mediated through activation of membrane receptors or neurotransmitter release nor by cyclo-oxygenase products. The response to OA (10 and 20 microM) is highly resistant to the absence of calcium in the medium and does not seem to involve calcium entry through dihydropyridine-sensitive Ca2+ channels.(ABSTRACT TRUNCATED AT 250 WORDS)