ROLES OF CA2-PIG SKINNED TRABECULAE( AND CROSSBRIDGE KINETICS IN DETERMINING THE MAXIMUM RATES OF CA2+ ACTIVATION AND RELAXATION IN RAT ANDGUINEA)

We examined the influences of Ca2+ and crossbridge kinetics on the max imum rate of force development during Ca2+ activation of cardiac myofi brils and on the maximum rate of relaxation. Flash photolysis of diazo -2 or nitrophenyl-EGTA was used to produce a sudden decrease or increa se, respectively, in [Ca2+] within Triton-skinned trabeculae from rat and guinea pig hearts (22 degrees C). Trabeculae from both species had similar Ca2+ sensitivities, suggesting that the rate of dissociation of Ca2+ from troponin C (k(off)) is similar in the 2 species. However, the rate of relaxation after diazo-2 photolysis was 5 times faster in the rat (16.1+/-0.9 s(-1), mean+/-SEM, n=11) than in the guinea pig ( 2.99+/-0.26 s(-1), n=7). This indicates that the maximum relaxation ra te is limited by crossbridge kinetics rather than by k(off). The maxim um rates of rapid activation by Ca2+ after nitrophenyl-EGTA photolysis (k(act)) and of force redevelopment after forcible crossbridge dissoc iation (k(tr)) were similar and were approximate to 5-fold faster in r at (k(act)=14.4+/-0.9 s(-1), k(tr)=13.0+/-0.6 s(-1)) than in guinea pi g (k(act)=2.57+/-0.14 s(-1), k(tr)=2.69+/-0.30 s(-1)) trabeculae. This too may be mainly due to species differences in crossbridge kinetics. Both k(act) and k(tr) increased as [Ca2+] increased. This Ca2+ depend ence of the rates of force development is consistent with current mode ls for the Ca2+ activation of the crossbridge cycle, but these models do not explain the similarity in the maximal rates of activation and r elaxation within a given species.