MECHANOCHEMICAL TRANSDUCTION IN MUSCLE

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The overall goal of these studies is to describe quantitatively the various energy requiring reactions in mammalian smooth and skeletal muscle, and hopefully to identify some of the factors which control the rate of energy usage by the different processes. Skeletal muscle energetics experiments will be done to determine whether or not the labile portion of the energy usage due to prestimulation can be apportioned into a force dependent and/or a force independent process. Other work planned on skeletal muscle will deal with measurement of the crossbridge cycling rate (from Vmax determinations) at different times during an isometric tetanus, and will determine the effect, if any, of prestimulation on the time dependent changes in Vmax in a single tetanus. The rationale for the experiments is to merge the energetics data with an independent measurement of crossbridge cycling rate so that a coherent description of the factors giving rise to the total high energy phosphate usage can be obtained under different mechanical conditions. Included in these experiments will be tests of Vmax of the intact muscle in different metabolic states. Our previous work has shown that there is no consistent relationship between the degree of myosin light chain phosphorylation and the average rate of high energy phosphate usage in the rabbit taenia coli. We are presently extending these studies to include measurements of maximum shortening velocity under all of the conditions in which we have energetics and myosin light chain phosphorylation data. These experiments will critically test the hypothesis that myosin light chain phosphorylation controls crossbridge cycling rate, but not force output of mammalian smooth muscle. Further experiments which have just been initiated concerning the regulation of smooth muscle contraction involve measurement of mechanics and myosin light chain phosphorylation during isometric tetani in the ferret portal vein. This tissue was chosen since it has recently been used to measure calcium transients with the Ca+2-sensitive protein aequorin. It has been found that the calcium transients are quite different when the tissue is activated with phenylephrine compared to potassium depolarization. A complete mechanical and light chain phosphorylation study in this tissue should clarify whether free cytoplasmic calcium concentration plays a role in regulation of intact smooth muscle independently of myosin light chain phosphorylation.