Abstract
1. 1. Ouabain-sensitive 86Rb + uptake by tissue preparations has been used as an estimate of Na + pump activity. This uptake, however, may be a measure of the Na + influx rate, rather than capacity of the Na + pump, since intracellular Na + concentration is a determinant of the active Na +/Rb + exchange reaction under certain conditions. This aspect was examined by studying the effect of altered Na + influx rate on ouabain-sensitive 86Rb + uptake in atrial preparations of guinea pig hearts. 2. 2. Electrical stimulation markedly enhanced ouabain-sensitive 86Rb + uptake without affecting nonspecific, ouabain-insensitive uptake. Paired-pulse stimulation studies indicate that the stimulation-induced enhancement of 86Rb + uptake is due to membrane depolarizations, and hence related to the rate of Na + influx. 3. 3. Alterations in the extracellular Ca 2+ concentration failed to affect the 86Rb + uptake indicating that the force of contraction does not influence 86Rb + uptake. 4. 4. Reduced Na + influx by low extracellular Na + concentration decreased 86Rb + uptake, and an increased Na + influx by a Na +-specific ionophore, monensin, enhanced 86Rb + uptake in quiescent atria. 5. 5. Grayanotoxins, agents that increase transmembrane Na + influx, and high concentrations of monensin appear to have inhibitory effects on ouabain-sensitive 86Rb + uptake in electrically stimulated and in quiescent atria. 6. 6. Electrical stimulation or monensin enhanced ouabain binding to (Na + + K +)-ATPase and also increased the potency of ouabain to inhibit 86Rb + uptake indicating that the intracellular Na + available to the Na + pump is increased under these conditions. 7. 7. The ouabain-sensitive 86Rb + uptake in electrically stimulated atria was less sensitive to alterations in the extracellular Na + concentration, temperature and monensin than that in quiescent atria. 8. 8. These results indicate that the rate of Na + influx is the primary determinant of oubain-sensitive 86Rb + uptake in isolated atria. Electrical stimulation most effectively increases the Na + available to the the Na + pump system. The ouabain-sensitive 86Rb + uptake by atrial preparations under electrical stimulation at a relatively high frequency seems to represent the maximal capacity of the Na + pump in this tissue.
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