CALCIUM MOVEMENTS IN EXCITATION-CONTRACTION COUPLING

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The transduction of action potential to muscle contraction (EC coupling) is an example of fast communication between cell membrane events and metabolic state. As in many systems, the central messenger of this coupling is calcium, which in muscle is released from the Sarcoplasmic Reticulum (SR) to activate contractile proteins. Our goal is to understand quantitatively these Ca movements. The release channels of the SR are controlled by changes in potential at the plasma and T-tubular membrane in an ill-understood way, involving a voltage sensor in the T-tubular membrane and an unknown mechanism of communication between voltage sensor and release channel (T-SR coupling). We can measure the main variables of the EC coupling steps: charge movement currents associated to the voltage senor, the rapid change in myoplasmic [Ca] and Ca flux through the release channel. The transverse tubular membrane of muscle has a ('slow') CA channel, of no known functional role, in large numbers. Our results suggest that the slow Ca channel is intimately related to charge movement and Ca release. We plan to study charge movement, seeking a unified model of its many described componennts to clarify its relationship to CA release. We will compare in detail the pharmacology of the slow Ca channel and the EC coupling events to test the hypothesis that the slow Ca channel is involved in EC coupling. If this is the case a role in T-SR coupling will be sought for the Ca channel and the current carried by the channel. We will study the slow Ca channel reconsituted in artificial bilayer membranes to characterize its properties in better defined ionic conditions. We will compare the merits of three proposed mechanism of T-SR coupling: a mechanical link, calcium, and IP3 as a transmitter. This will be done by introducing changes in the intracellular medium that selectively interfere with the proposed mechanisms, including a fast Ca buffer and a blocker of IP3 metabolism. Release of Ca and its removal by proteins and the SR pump are the main fluxes involved in Ca regulation. Progress in the study of one flux generates and requires more knowledge about the other. The relationship between CA removal fluxes and an intrinsic optical signal generated by the muscle will be studied. The Ca binding protein pravalbumin will be introduced into muscle fibers to test its role in the intrinsic signal and in the process of removal that eventually closes the cycle of calcium movements.