Roles of Ca²⁺ and Crossbridge Kinetics in Determining the Maximum Rates of Ca²⁺ Activation and Relaxation in Rat and Guinea Pig Skinned Trabeculae

From the Department of Pharmacology, United Medical and Dental Schools, St. Thomas's Hospital, London, UK.

Correspondence to Dr J.C. Kentish, Department of Pharmacology, United Medical and Dental Schools, St. Thomas's Hospital, London SE1 7EH, UK. E-mail j.kentish{at}umds.ac.uk

Abstract—We examined the influences of Ca²⁺ and crossbridge kinetics on the maximum rate of force development during Ca²⁺ activation of cardiac myofibrils and on the maximum rate of relaxation. Flash photolysis of diazo-2 or nitrophenyl-EGTA was used to produce a sudden decrease or increase, respectively, in [Ca²⁺] within Triton-skinned trabeculae from rat and guinea pig hearts (22°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 rate is limited by crossbridge kinetics rather than by k_off. The maximum rates of rapid activation by Ca²⁺ after nitrophenyl-EGTA photolysis (k_act) and of force redevelopment after forcible crossbridge dissociation (k_tr) were similar and were 5-fold faster in rat (k_act=14.4±0.9 s^-1, k_tr=13.0±0.6 s^-1) than in guinea pig (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⁺ dependence of the rates of force development is consistent with current models for the Ca²⁺ activation of the crossbridge cycle, but these models do not explain the similarity in the maximal rates of activation and relaxation within a given species.

Key Words: crossbridge • photolysis • Ca²⁺ activation • cardiac myofibril • relaxation

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