AJP - Cell Physiology, Vol 260, Issue 3 C609-C617, Copyright © 1991 by American Physiological Society
Length-dependent activation by Ba2+ and Sr2+ of skinned cardiac and skeletal muscle of the rabbit
R. L. Grundeman, E. L. de Beer, C. van den Berg, K. J. van Buuren and P. Schiereck
Department of Medical Physiology, University of Utrecht, The Netherlands.
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.
