[The role of cations from the external medium in regulating the intracellular Ca++ ion concentration in ureteral smooth muscle cells].

Intracellular concentration of Ca++ ions was determined by the isotope analysis method for the guinea-pig ureter smooth muscle cells in solutions containing Ca++, Na+ and Li+. It was shown that the Ca++ concentration in muscles incubated in Na(+)-free solution containing 2.5 to 15 mmol/l Ca++, reached the maximal value during the period in which spikes were recorded. In the muscles enriched by calcium, intracellular Ca++ concentration was reduced by over 2.5 times in solutions containing Na+ and Ca++ ions (120 mmol/l and 10 mmol/l, resp.,) as compared with Na(+)-free medium. The reducing of calcium content was mainly obvious during first 5 min. Na+ ions were not specific for calcium removal from cells, Li+ ions being able to substitute them in this process. The findings suggest a possible participation of Na(+)-Ca(++)-exchange mechanism in the maintenance of the calcium homeostasis in the ureter smooth muscle cells.