Effect of external ions on 45Ca efflux from cat intestinal muscle

The surface-bound Ca of isolated circular smooth muscle of cat small intestine can be removed by substitution of LiCl for NaCl in Krebs solution. This substitution removed surface-bound Ca (45Ca) and allowed us to study transmembrane 45Ca efflux. Neither the resting membrane potential nor contractility changed when Li was substituted for Na. Li removed the same extracellular 45Ca store as did ethylene glycol-bis-(beta-aminoethylether)-N,N'-tetraacetic acid. The resting transmembrane 45Ca efflux was inhibited by La3+ and was unchanged in Li, tris(hydroxymethyl)aminomethane, arginine, and sucrose Krebs solution. The extra 45Ca efflux observed upon electrical stimulation was no greater in Na-Krebs than Li-Krebs, but during response to acetylcholine the extra 45Ca efflux was greater in Na-Krebs than Li-Krebs. We conclude that the surface-bound Ca is sensitive to external Na and that the transmembrane Ca efflux is not completely dependent on external Na.