Effects of sodium gradient manipulation upon cellular calcium 45Ca fluxes and cellular sodium in the guinea‐pig taenia coli

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Abstract
Sucrose and choline were utilized as NaCl substitutes in order to investigate Na-Ca interactions in the smooth muscle of the guinea pig tenia coli. Progressive substitution of NaCl by sucrose caused a progressive increase in cellular exchangeable Ca. This uptake, which amounted to .apprx. 300 .mu.mole Ca/kg tissue upon total Na replacement, reached a plateau within 20 min. Complete substitution of NaCl by choline chloride caused cellular Ca to increase rapidly to an initial peak and then decrease to a stable plateau which was also .apprx. 300 .mu.mole/kg above control. Replacement of NaCl by sucrose or choline chloride caused a transient increase in the Ca influx rate, which was measured using a 3 min pulse labeling with 45Ca. This increase was more pronounced in choline chloride. NaCl substitution by sucrose or choline chloride caused a decrease in the 45Ca efflux rate. Two exponential components of transmembrane 45Ca efflux were found in control and Na-free media. Treatment of tissues with 3 .times. 10-5 M-ouabain did not significantly affect the cellular Ca content after 80 min, at which time the Na and K gradients were largely dissipated. Removal of medium K caused a slower dissipation of the Na and K gradients. This treatment decreased cellular Ca, did not affect the Ca influx rate and increased the 45Ca efflux rate. Tissues were incubated in depolarizing media containing 10-4 M-ouabain in order to remove the Na gradient. Subsequent measurement of cellular Na indicated the absence of a significant fraction of bound Na. The ratio [Na]o/[Na]i had a value of 6.3 in control medium and decreased as [Na]o was progressively lowered by sucrose substitution, reaching a value of < 1 in a medium containing 5 mM-Na. A Na-Ca exchange carrier does not play an important role in regulation of tension in this muscle. The Ca gradient is not solely dependent on the Na gradient in guinea pig tenia coli.