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 sarcoplasmic reticulum (SR) function) caused a 20% reduction in peak tension at 20°C, but not 15°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 modes of Ca 2+ delivery to the myofilaments. Ca 2+ flux through the sarcolemmal (SL) L-type Ca 2+-channels is most likely the predominant pathway for Ca 2+ activation of the myofilaments, although reverse mode Na +/Ca 2+ exchange could potentially contribute to a significant extent. High levels of adrenergic stimulation overwhelmed the negative inotropy caused by ryanodine, returning tension to pre-ryanodine levels, further suggesting that the mackerel ventricle can maintain contractile function without Ca 2+ contribution from the SR. These results are discussed within the context of what is known about SR Ca 2+ utilization in rainbow trout and tuna hearts.