Cation activation of the pig kidney sodium pump: transmembrane allosteric effects of sodium

Electrogenic sodium-calcium exchange in cultured embryonic chick heart cells

1. The membrane potential (Em) of cultured chick embryonic heart cells depolarized to -36 mV after inhibition of the Na+-K+ pump by 0.1 mM-ouabain in a [K+]o of 24 mM: this was accompanied by a rise in Na+ content of approximately 65% in 3 min. Lowering [Na+]o to 27 mM then caused a fall in Na+ content, a rise in Ca2+ content and a small hyperpolarization of approximately 5 mV. The fall in Na+ content indicated a movement of Na+ which was in the opposite direction to the Na+ electrochemical gradient (a countergradient movement). 2. In the presence of 10 mM-Cs+ or 1 mM-Ba2+ the hyperpolarization was approximately 10 or approximately 30 mV, respectively. A 30 mV hyperpolarization took Em negative to the reversal potentials for K+, and Cl- as measured by ion-selective micro-electrodes. 3. The decay of the intracellular Na+ activity alpha iNa, in an [Na+]o of 27 mM followed a simple exponential time course (time constant, 36 s). The initial rate depended on the value to which [Na+]o was lowered in a manner suggesting a simple competitive inhibition of the exchange by external Na+. 4. The low-[Na+]o hyperpolarization was unaffected by amiloride (0.1 or 1 mM) or verapamil (20 microM). Both La3+ (1 mM) and Mn2+ (20 mM) blocked the hyperpolarization sufficiently to prevent Em hyperpolarizing negative to the reversal potentials for K+, Na+ and Cl-. 5. Re-establishing [Na+]o caused a rise in Na+ content and a countergradient drop in Ca2+ content. The effects of verapamil (20 microM), amiloride (0.1 and 1 mM), dichlorobenzamil (0.1 mM), quinidine (1 mM), Mn2+ (20 mM) and La3+ (1 mM) were tested on the movements of Na+ and Ca2+ both during exposure to an [Na+]o of 27 mM and on re-establishing [Na+]o. The only consistent and substantial effects were the attenuation by La3+ and Mn2+ and Ca2+ movements during exposure to an [Na+]o of 27 mM. However, neither La3+ nor Mn2+ affected the movements of Na+ and Ca2+ on re-establishing [Na+]o. 6. We conclude that cultured embryonic chick heart cells contain a Na+-Ca2+ exchange evidenced by the ability to cause movements of Na+ and Ca2+ which are counter to their respective electrochemical gradient and which are accompanied by downhill movements of the counter ion.(ABSTRACT TRUNCATED AT 400 WORDS)

(Na+ + K+)-ATPase in artificial lipid vesicles: influence of the concentration of mono- and divalent cations on the pumping rate

(Na+ + K+)-ATPase from kidney outer medulla was incorporated into artificial dioleoylphosphatidylcholine vesicles. Transport activity was induced by adding ATP to the external medium. A voltage-sensitive dye was used to detect the ATP-driven potassium extrusion in the presence of valinomycin. The observed substrate-protein interactions of the reconstituted (Na+ + K+)-ATPase largely agree with that from native tissues. An agreement between ATP hydrolysis and transport activity is given for concentration dependences of sodium, potassium, magnesium and calcium ions. The only significant deviations were observed in the influence of pH. Protons were found to have different influence on transport, enzymatic activity and phosphorylation of the enzyme. The transport studies showed a twofold interaction of protons with the protein: competition with sodium at the cytoplasmic ion binding sites, a non competitive inhibition of transport which is not correlated with protein phosphorylation.

(Na+ + K+)-ATPase in artificial lipid vesicles: influence of lipid structure on pumping rate

(Na+ + K+)-ATPase from kidney outer medulla was incorporated into tightly-sealed, single-shelled lipid vesicles by a detergent-dialysis procedure. The rate of ATP-driven potassium extrusion from vesicles formed from different phosphatidylcholines (PC) was measured optically, using a voltage-sensitive dye in the presence of valinomycin. High transport rates were observed for di(18:1)PC, di(20:1)PC and di(22:1)PC, whereas vesicles formed from di(14:1)PC and di(16:1)PC were virtually inactive. The variation of pumping activity with lipid structure mainly results from differences in the amount of enzyme incorporated with the correct orientation into the vesicle membrane, and to a lesser extent from lipid-dependent variations of the intrinsic turnover rate of the enzyme. The activation energy of ion transport decreases in the order di(16:1)PC, di(18:1)PC, di(20:1)PC approximately equal to di(22:1)PC.