Contractile activation and the effects of 2,3-butanedione monoxime (BDM) in skinned cardiac preparations from normal and dystrophic mice (129/ReJ)

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Abstract
(1) Small cardiac myofibrillar preparations were obtained from the right ventricle of normal (129/ReJ) and dystrophic (129/ReJ dy/dy) mice and were chemically skinned in a relaxing solution by exposure to Triton X-100 (3% v/v). (2) The isometric force produced in these skinned cardiac preparations at different sarcomere lengths was measured in solutions of different [Ca2+] and ionic strength. The effect of the negative inotropic drug 2,3-butanedione monoxime (BDM), which is known to act at the myofibrillar level was also investigated. (3) The murine cardiac preparation from normal animals was found to develop 50% maximal force at a pCa (=−log10[Ca2+]) of 5.59±0.08 and 5.94±0.03 (mean ±SD) under physiological (ionic equivalents concentration, I=154 mM; pH 7.10; [Mg2+] 1 mM) and low ionic strength (I=94 mM; pH 7.10; [Mg2+] 1 mM) conditions respectively. The isometric force curves were significantly shallower at low ionic strength (Hill coefficient, 1.8±0.1) than at physiological ionic strength (Hill coefficient, 2.6±0.3) and the sarcomere length effect on the force-pCa relation was markedly reduced at lower ionic strength. (4) Increasing BDM concentrations in solutions up to 100 mM reduced the maximum Ca2+-activated force of cardiac preparations from normal mice to less than 6% of the control values in a dose dependent fashion. BDM also rendered the cardiac preparations less sensitive to Ca2+ by a factor of up to 1.5 in a process which showed saturation at BDM concentrations higher than 15 mM. (5) Cardiac preparations from dystrophic animals compared with those from normal mice were significantly more sensitive to Ca2+ under physiological conditions, were more sensitive to the action of BDM at concentrations higher than 15 mM, changed sensitivity to Ca2+ less following a change in sarcomere length and in general were less affected by a decrease in ionic concentration. (6) The results indicate that dystrophy in mice affects the characteristics of both the contractile and regulatory systems of cardiac muscle and that BDM directly affects the Ca2+-activated contractile response possibly by binding to saturable sites on the myofilaments.