Calcium Flux and Contractility in Guinea Pig Atria

The calcium in guinea pig atria can be divided into three components by kinetic studies with Ca⁴⁵: (a) a rapidly exchangeable fraction with a half-time of 4.5 minutes; (b) a slowly exchangeable fraction with a half-time of 86 (or 168) minutes; and (c) an inexchangeable fraction. In Krebs-Henseleit solution containing 2.5 mM calcium, the calcium content of the tissue at rest remains constant, the flux being about 0.02 µµmol/cm²-second. An increase or a decrease in extracellular calcium concentration by 1.25 mM causes a proportionate change in influx. A large increase in Ca⁴⁵ entry, equivalent to as much as 0.55 µµ/mol/cm² accompanies a contraction. When the strength of contraction is varied by stimulating at different frequencies or in solutions containing calcium at different concentrations, the increment of Ca⁴⁵ uptake per beat changes proportionally with the strength of the beat. Total atrial calcium is not increased by stimulation; however, the increase in outflux of Ca⁴⁵ during contraction that this constant tissue calcium implies could not be demonstrated under the experimental conditions employed. The observations are discussed in the light of the possible role of calcium transfer in excitation-contraction coupling.