The physiological and pharmacological properties of contraction and the ultrastructure of buccal mass retractor muscle (I4) and gill-pinnule closure muscle (GPCM) in Aplysia kurodai were studied to learn more about the sources of activator Ca2 in molluscan smooth muscle. Acetylcholine (ACh) and high K -induced contractions were reduced by lowering the external Ca2 concentration, and eliminated by the removal of extracellular Ca2 . Nifedipine appreciably reduced ACh- and high K -induced contractions, while amiloride decreased only ACh-induced contractions and had no significant effect on high K -induced contractions. When nifedipine and amiloride were applied together, either type of contraction was still appreciable. Serotonin (5-HT) could potentiate subsequent ACh- and high K -induced contractions in I4; potentiated tension was significantly reduced by nifedipine and amiloride, whereas 5-HT inhibited ACh- and high K -induced contractions in GPCM. The potentiating effects of 5-HT may be mediated by the activation of the Ca2 -channel to increase the influx from extracellular Ca2 . Caffeine caused contractions in Ca2 -free solution in both muscles. Electron microscopy revealed sarcolemmal vesicles underneath the plasma membrane in both muscle fibers. Electron microscopical cytochemistry demonstrated that pyroantimonate precipitates were localized in the sarcolemmal vesicles and in the inner surface of plasma membranes in the resting fibers. Present results indicate that the contractions of I4 and GPCM fibers are caused not only by Ca2 -influx but also by Ca2 release from the intracellular storage sites, such as the sarcolem-mal vesicles and the inner surface of plasma membranes.
You have requested a machine translation of selected content from our databases. This functionality is provided solely for your convenience and is in no way intended to replace human translation. Neither BioOne nor the owners and publishers of the content make, and they explicitly disclaim, any express or implied representations or warranties of any kind, including, without limitation, representations and warranties as to the functionality of the translation feature or the accuracy or completeness of the translations.
Translations are not retained in our system. Your use of this feature and the translations is subject to all use restrictions contained in the Terms and Conditions of Use of the BioOne website.