Relaxation rate of intact striated muscle fibres after flash photolysis of a caged calcium chelator (diazo-2)

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Abstract
Single twitch fibres from lumbrical muscles ofXenopus have been loaded with the photolysable calcium-chelator diazo-2 by incubation in Ringer solution containing the membrane permeable acetoxymethyl ester (AM) form of diazo-2. Incubation caused a progressive slowing of tetanus rise and relaxation which is ascribed to calcium-buffering by unphotolyzed diazo-2 (Kd=2.2 μM). After incubation, exposure to a brief UV flash caused a three to four fold increase in the rate of tension fall. A flash given 16–18 ms after the last tetanic stimulus (at 22–24°C) resulted in 10% increase in relaxation rate compared with the control before incubation. A much bigger effect was observed when a flash was given half-way into the slow phase, where an 1.8–1.9-fold increase in relaxation rate, above the preincubation slope, was observed. It is concluded that rapid lowering of [Ca]i, and hence more rapid removal of Ca2+ from troponin, speeds up relaxation, indicating that calcium translocation is the major determinant of the rate of tension fall during the isometric phase of relaxation.

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