The effect of ryanodine on isometric tension development in isolated ventricular trabeculae from Pacific mackerel (Scomber japonicus)

An isometric muscle preparation was used to study the inhibitory effect of ryanodine on contractile function in isolated ventricular trabeculae of the Pacific mackerel (Scomber japonicus). Ryanodine (an inhibitor of sarcoplas mic reticulum (SR) function) caused a 20% reduction in peak tension at 20 d egrees C, but not 15 degrees C, over the range of frequencies (0.2-3.0 Hz) tested. This indicates that in the absence of a functional SR, the mackerel ventricle can maintain most of its contractile strength utilizing other mo des of Ca2+ delivery to the myofilaments. Ca2+ flux through the sarcolemmal (SL) L-type Ca2+ -channels is most likely the predominant pathway for Ca2 activation of the myofilaments, although reverse mode Na+/Ca2+ exchange co uld potentially contribute to a significant extent. High levels of adrenerg ic stimulation overwhelmed the negative inotropy caused by ryanodine, retur ning tension to pre-ryanodine levels, further suggesting that the mackerel ventricle can maintain contractile function without Ca2+ contribution from the SR. These results are discussed within the context of what is known abo ut SR Ca2+ utilization in rainbow trout and tuna hearts. (C) 2000 Elsevier Science Inc. All rights reserved.