Some characteristics of Ca2+- regulated force production in EGTA-treated muscles from rat heart.
McClellan and Winegrad (1980, J. Gen. Physiol., 75:283-295) have reported that in rat ventricular muscles that have reportedly been made "hyperpermeable" to small ions such as Ca2+, CaEGTA2-, and MgATP2- by a soak in EGTA, the maximum Ca2+-regulated force can be permanently increased by a short exposure to positively inotropic drugs, such as epinephrine or cAMP plus theophylline, in the presence of the detergent Triton X-100. The experiments reported here were begun as an attempt to repeat and extend this important observation. However, no evidence could be found for a potentiation of force that was not merely produced by Triton alone. In addition, the thickest muscles used (250-440 microns diameter) exhibited very low values for force per unit cross-sectional area, which suggested that either Ca2+ reached only a fraction of the myofibrils or the myofibrils were in a state of low contractility. The results of further experiments that were designed to test the permeability characteristics of these EGTA-treated muscles indicated that the movement of certain ions into these preparations was restricted, even in thin muscles (80-200 microns diameter). The rate of development of Ca2+-regulated force was slow (t1/2 approximately equal to 1-3 min), but was greatly accelerated after the muscles had been superfused with Triton X-100 (t1/2 approximately equal to 10-20 s). Removal of creatine phosphate (CP) in the presence of MgATP produced a partial rigor contracture in the EGTA-treated muscles. The results were consistent with the suggestion that the EGTA-treated muscles were permeable to some extent to Ca2+ and HCP2- ions but not to CaEGTA2- and MgATP2-. Thus, it would seem unlikely that the [Ca2+], [MgATP2-], and [Mg2+] in the immediate vicinity of the myofibrils in these preparations can be adequately controlled by the solution bathing the muscles.
D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002463:Cell Membrane Permeability / Q000187:drug effects; D004533:Egtazic Acid / Q000494:pharmacology*; D005026:Ethylene Glycols / Q000494:pharmacology*; D005260:Female; D006321:Heart / Q000187:drug effects / Q000502:physiology*; D006352:Heart Ventricles; D066298:In Vitro Techniques; D009200:Myocardial Contraction / Q000187:drug effects*; D009206:Myocardium / Q000378:metabolism; D017830:Octoxynol; D011092:Polyethylene Glycols / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains
D005026:Ethylene Glycols; D011092:Polyethylene Glycols; D004533:Egtazic Acid; D017830:Octoxynol; D002118:Calcium