Comparison of 45Ca binding and mobilization in cartilage and smooth muscle of guinea pig trachea

In guinea pig trachea, separation of smooth muscle from cartilage permits measurement of 45Ca fluxes in more homogeneous preparations. The role of cartilage Ca++ as a source for smooth muscle Ca++ mobilization can be assessed by similar experiments with the two dissociated preparations. Equilibration of 45Ca into cartilage occurred over 2 hr; plotting of 45Ca uptake on Scatchard coordinates yields high and low affinity Ca++ uptake components similar to those in tracheal and other smooth muscles. Washout of 45Ca from cartilage demonstrated two components with the more rapid including the extracellular (14C-sucrose) compartment. Washout of higher affinity 45Ca (after incubation in 0.03 mM Ca++) was increased in cartilage by Sr++, added Ca++, La+++ and EGTA. Comparison with tracheal smooth muscle 45Ca washouts indicates that those Ca++ components previously identified as of intracellular origin were not present in cartilage. Exposure to 60 mM (added) K+ during 45Ca washout increased 45Ca efflux in cartilage but not in smooth muscle. This release of 45Ca by K+ may indicate that Ca++ can be mobilized from cartilage sites by K+ to support smooth muscle contractile responses. Thus, cartilage Ca++ could be an important source of Ca++ for excitation-contraction coupling in guinea pig tracheal smooth muscle