Na−Ca exchange in taenia coli of the guinea-pig

Summary

The effects of external Ca, of H⁺ and of different Na-substitutes on the⁴⁵Ca-efflux from smooth muscle cells of the guinea-pig taenia coli have been studied.

• 1. The efflux of⁴⁵Ca proceeds over a period of 200 min at a progressively slower rate. In the presence of 2.5 mM Ca the rate coefficient is initially higher, but later on lower than in Ca-free solution. Adding calcium to the Ca-free washing fluid causes a sudden but transient increase of the efflux rate.

• 2. A change of the pH from 7.4 to 5 increases, while a change from 7.4 to 10 decreases the⁴⁵Ca-efflux.

• 3. Substitution of sucrose for Na decreases the rate coefficient by a factor of 0.5 in Ca-free solution and by a factor of 0.25 in a 2.5 mM Ca-solution.

• 4. Neither the⁴⁵Ca-content nor the rate of⁴⁵Ca efflux are significantly affected by complete replacement of internal K by Na.

• 5. Substitution of Na by Li, K, Tris or choline gives variable results. Li is more effective than Na in stimulating the Ca-efflux, while K, Tris and choline cause a decrease of this efflux.

• 6. Membrane leakiness induced by prolonged exposure to DNP and IAA or membrane destruction caused by high temperature did not eliminate the effect of Na-substitutes on the Ca-efflux.

• 7. These experiments throw serious doubts on the interpretation of the available evidence in favour of a Ca-extrusion system in smooth muscle depending on an inwardly directed Na-gradient. The extensive binding of tracer calcium to extracellular structures is probably an important factor in complicating the interpretation of efflux curves.