title: Length-dependent activation by Ba2+ and Sr2+ of skinned cardiac and skeletal muscle of the rabbit.
abstract: Over a wide range of sarcomere lengths, force activation by Ca2+, Ba2+, and Sr2+ was studied in papillary muscle and in fast skeletal fibers of the gracilis muscle of the rabbit, both skinned by means of freeze drying. The length-tension relations of Ba2+ activation differ significantly from those of Sr2+ and Ca2+ activation with respect to both the value and the position of the maximum. At (almost) full activation, force induced in gracilis muscle by Ba2+ was 50% of the developed force induced by Ca2+. The position of the Sr2+ sensitivity curve for papillary muscle preparations is independent of sarcomere length, in contrast to the position of the Ca2+ sensitivity curves. The binding of Sr2+ to the papillary preparation proves to be very stable as observed from the long-lasting relaxation after activation. Immersion of the papillary preparation in the relaxation fluid after activation with Ba2+ results in a tension transient: a rise in tension followed by a decrease was observed. The maximal value of the tension transient was up to twice the steady tension, dependent on Ba2+ concentration. The steady-state tension was approximately 50% of the Ca2(+)-induced tension. Ba2+ sensitivity curves are not sigmoidal but show a maximum. Above [Ba2+] greater than 10(-5) to 10(-4) M (dependent on sarcomere length) tension decreased. These observations suggest that two counteracting processes govern Ba2+ contraction in papillary muscle preparations, namely activation and inhibition.
authors.affiliation: Department of Medical Physiology, University of Utrecht, The Netherlands.
authors.forename: R L
authors.initials: RL
authors.lastname: Gründeman
authors.forename: E L
authors.initials: EL
authors.lastname: de Beer
authors.forename: C
authors.initials: C
authors.lastname: van den Berg
authors.forename: K J
authors.initials: KJ
authors.lastname: van Buuren
authors.forename: P
authors.initials: P
authors.lastname: Schiereck
mesh_terms: D000818:Animals; D001464:Barium / Q000494:pharmacology*; D002118:Calcium / Q000494:pharmacology; D066298:In Vitro Techniques; D009119:Muscle Contraction / Q000187:drug effects*; D009132:Muscles / Q000187:drug effects / Q000502:physiology*; D010210:Papillary Muscles / Q000187:drug effects / Q000502:physiology*; D011817:Rabbits; D012518:Sarcomeres / Q000187:drug effects / Q000502:physiology; D013314:Stress, Mechanical; D013324:Strontium / Q000494:pharmacology*
chemical_list: D001464:Barium; D002118:Calcium; D013324:Strontium