A diazo-2 study of relaxation mechanisms in frog and barnacle muscle fibres: effects of pH, MgADP, and inorganic phosphate

The aim of this study was to compare the effects of increased concentration s of MgADP, inorganic phosphate (P-i) and H+ ([MgADP], [Pi] and [H+], respe ctively) on the rate of relaxation in two different muscle types: skinned m uscle fibres from the frog Rana temporaria and myofibrillar bundles from th e giant Pacific acorn barnacle Balanus nubilus. Relaxation transients are p roduced by the photolysis of diazo-2 and are well fitted with a double expo nential curve, giving two rate constants: k(1) [5.6 +/- 0.1 s(-1) for barna cle, n = 30; 26.3 +/- 0.7 s(-1) for frog, n = 14 (mean +/- SEM)] and k(2) [ 0.6 +/- 0.1 s(-1) in barnacle, n = 30; 10.4 +/- 1.0 s(-1) in frog, n = 14 ( mean +/- SEM)], at 10 degrees C. Decreasing the pH by 0.5 pH units did not significantly affect k(1) for barnacle relaxation [5.6 +/- 0.1 s(-1) (mean +/- SEM), n = 15] compared to the decrease in k(1) of 40% seen in frog. Use of the Ca2+-sensitive fluorescent label acrylodan on barnacle wild-type tr oponin C demonstrated that decreasing the pH from 7.0 to 6.6 only alters th e pCa(50) value by 0.23 in the cuvette, while stopped-flow experiments with acrylodan revealed no significant change in k(off) from the labelled prote in [322 +/- 32 s(-1) at pH 7.0 and 381 +/- 24 s(-1) (mean +/- SEM) at pH 6. 6]. Increasing [MgADP] by 20 mu M (50 mu M added ADP) from control values o f 50 mu M in frog decreased k(1) to 12.3 +/- 0.4 s(-1) (mean +/- SEM, n = 8 ), and at 400 mu M MgADP, k(1) = 9.6 +/- 0.1 s(-1) (mean +/- SEM, n = 12). In barnacle, 500 mu M MgADP had a much smaller effect on k(1) (4.0 +/- 0.9 s(-1), mean +/- SEM, n = 8). Increasing the free [P-i] from the contaminant level of 0.36 mM to 1.9 mM slowed k(1) by approximate to 15% in barnacle [ 4.8 +/- 0.8 s(-1), mean +/- SEM, n = 7], compared to a approximate to 30% r eduction seen in frog. We conclude that the differences between barnacle an d frog seen here are most probably due to different isomers of the contract ile proteins, and that events underlying the crossbridge cycle are the same or similar. We interpret our results according to a model of crossbridge t ransitions during relaxation.