Distribution and exchange of calcium in connective tissue and smooth muscle

Abstract

Ca exchange of the frog's stomach muscle was rapid and much larger than in skeletal muscles. More than half of the fiber Ca exchanged and was lost in Ca-free solutions in 10 min. To evaluate extracellular Ca in these experiments exchange and binding of Ca in tendon and fascia were studied. The amount of extracellular Ca was estimated from the loss of Ca in isosmotic sucrose solution. In high Ca solutions the Ca concentration in muscle increased several times, but remained much lower than the Ca concentration in the medium. A large increase in Ca_i appears to be associated with a loss of Na and K. The question of how the unequal distribution of Ca is maintained is discussed. Because of the rapid exchange and because of the large difference in concentration of Ca ions the energy requirement of a Ca pump would be prohibitively high. The distribution of Ca can be understood as an equilibrium if Ca enters the fibers bound to a diffusible carrier and if bound Ca competes with monovalent ions for binding sites.