Calcium Dependence of the Mechanical Response Evoked by Okadaic Acid in Smooth Muscle

Abstract

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 µM in normal physiological solution was unaffected in the presence of tetrodotoxin (10 µM), indomethacin (3 µM), 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 µM) which completely or highly blocked the response to KCl (60 mM), oxytocin (1 µM), or ace-tylcholine (100 µM). In a Ca²⁺-free 1 mM EGTA-containing solution, the response to 10 and 20 µM OA was slightly but significantly reduced whereas the response to 5 µM. OA was abolished. However, a response similar to that evoked in Ca²⁺-containing solution was observed when 5 µM OA was added to the bath in the presence of 1 µM oxytocin or 160 µM vanadate in a Ca²⁺-depleted solution with 1 mM EGTA. These data suggest that the response of rat myometrium to OA (≥ 5 µM) is not mediated through activation of membrane receptors or neurotransmitter release nor by cyclooxygenase products. The response to OA (10 and 20 µM) is highly resistant to the absence of calcium in the medium and does not seem to involve calcium entry through dihydropyridine-sensitive Ca²⁺ channels. However, our results also show that the response evoked by OA is not completely independent of extracellular calcium and/or calcium accessible to EGTA and suggest that a minimal activation of a Ca²⁺-dependent mechanism is necessary for OA to be effective.

Key words

Marine dinoflagellates - Prorocentrum lima - okadaic acid - protein phosphatases - calcium - rat myometrium