Relaxation rate of intact striated muscle fibres after flash photolysis of a caged calcium chelator (diazo-2).
Single twitch fibres from lumbrical muscles of Xenopus 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 microM). 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 degrees 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.
Lännergren
J
J
Department of Physiology II, Karolinska Institute, Stockholm, Sweden.
Arner
A
A
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002614:Chelating Agents / Q000494:pharmacology* / Q000528:radiation effects; D003979:Diazonium Compounds; D004121:Dimethyl Sulfoxide / Q000494:pharmacology; D007537:Isometric Contraction; D009126:Muscle Relaxation / Q000187:drug effects*; D010642:Phenoxyacetates; D010782:Photolysis; D014982:Xenopus laevis
D002614:Chelating Agents; D003979:Diazonium Compounds; D010642:Phenoxyacetates; C063685:diazo-2; D002118:Calcium; D004121:Dimethyl Sulfoxide