The Influence of Intracellular Sodium Concentration on Calcium Influx in Isolated Guinea-Pig Auricles / Der Einfluß der intracellulären Natriumkonzentration auf den Calcium-influx in isolierte Meerschweinchenvorhöfe
Ca efflux from isolated cardiac preparations has been described by Reuter and Seitz  proposing a carrier-mediated transport system. In this system Na- and Ca-ions compete for the carrier at the outer surface of the membrane. If the carrier is similarly activated at the inner surface of the membrane an increase in internal Na concentration ([Na] i ) should induce more carrier molecules to move from inside to outside. This, however, should also provide more carrier to be loaded at the outside and therefore an increase in Ca influx should occur. This hypothesis was checked by measuring Ca influx in isolated guinea-pig auricles with different [Na] i . [Na] i was increased by electrical stimulation (300/min) or by cooling (5°C) in K- and Ca-poor solution and reduced in K-rich solution. Na- and K-contents of the preparations were measured by flame photometry, Ca-contents by a fluorometric method. Ca influx was measured by loading the resting auricles in 45Ca-containing Tyrode’s solution or Na-poor solution (NaCl replaced by choline-Cl) for 10 min. Afterwards the preparations were soaked in Na- and Ca-free solution for 25 min in order to wash the extracellular space free of calcium without changing [Ca] i . The Table shows that Ca influx from both solutions increased appreciably with increasing [Na] i . Furthermore, 45Ca influx from Na-poor solution corresponded to an about equal increase in [Ca] i , while [Ca] i did not change much in preparations loaded in Tyrode’s solution.