ABSTRACT
The mechanisms of UTP-induced tension in human and rat skinned fibers were investigated using isometric tension recordings, electrophysiological techniques and biochemical methods. In fast-type fibers from rat extensor digitorum longus (EDL) the UTP-induced tension: a) required previous loading of CA2+ into the sarcoplasmic reticulum (SR); b) was inhibited by previous exposure to caffeine; c) was abolished by functional disruption of the SR; d) was not affected by blockade of the SR Ca2+-release channels by ruthenium red or heparin; e) was prevented by spermidine. These data point to the SR as the target of UTP action and suggest a pathway of UTP-induced Ca2+-release independent of the ryanodine- or the IP3-sensitive Ca2+-release channels. Accordingly, UTP failed to stimulate the electrophysiological activity of ryanodine-sensitive channels, incorporated into lipid bilayers. We suggest that UTP-induced Ca2+-release might occur via the channel form of the SR Ca2+-ATPase. The UTP-induced tension in human slow-type fibers was not affected by the SR Ca2+ content or by disruption of the SR, but was accompanied by changes in the tension-pCa relationship, namely increase in maximum Ca2+-activated tension, and in apparent Ca2+-affinity of troponin. The UTP-induced tension in slow-type fibers from rat soleus was partially inhibited by Ca2+-depletion from, or by disruption of the SR, and was accompanied by changes in tension/pCa relationship, similar to those observed in human fibers. Both in skinned fibers and in isolated SR vesicles, UTP was less effective than ATP as a substrate for the SR Ca2+-ATPase. This effect might contribute to UTP-induced tension.