Na+, K+-ATPase is ubiquitously expressed in the plasma membrane of all animal cells where it serves as the principal regulator of intracellular ion homeostasis. Na+, K+-ATPase is responsible for generating and maintaining transmembrane ionic gradients that are of vital importance for cellular function and subservient activities such as volume regulation, pH maintenance, and generation of action potentials and secondary active transport. The diversity of Na+, K+-ATPase subunit isoforms and their complex spatial and temporal patterns of cellular expression suggest that Na+, K+-ATPase isozymes perform specialized physiological functions. Recent studies have shown that the alpha subunit isoforms possess considerably different kinetic properties and modes of regulation and the beta subunit isoforms modulate the activity, expression and plasma membrane targeting of Na+, K+-ATPase isozymes. This review focuses on recent developments in Na+, K+-ATPase research, and in particular reports of expression of isoforms in various tissues and experiments aimed at elucidating the intrinsic structural features of isoforms important for Na+, K+-ATPase function.

4. Cells incubated in ouabain* for 24 hr bind an additional amount of ouabain when exposed to 2 x 10-7 M ouabain* in K-free Krebs.5. There is a close relationship between the % of the total ouabain bound in 24 hr and the % inhibition of the Na efflux suggesting that this ouabain is bound to the Na pumps.6 LESLEY J. BOARDMAN AND OTHERS 9. These results are compatible with the hypothesis that partial blocking of Na pumps leads to the production of more pumping sites by the cell.

SUMMARY1. HeLa cells were grown in normal and altered growth solutions; the ion contents, volumes, K sensitive ouabain binding, the Na-K-ATPase and the Na and K transport measured.2.

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1. A new perfused preparation of frog's ventricle is described, whose main advantage is that there are short diffusion distances between the cells and the washing fluid. 2. This preparation responds within a few seconds to alterations in sodium, potassium or calcium concentration, or alteration in the osmotic pressure of the bathing fluid. The speed of these alterations is consistent with the diffusional distances involved measured histologically. 3. On depolarizing the preparation with Ringer + excess potassium, tension development starts at about ‐25 mV and is still increasing at ‐8 mV (the lowest voltage studied). As in earlier experimetns in frog ventricle (Niedergerke, 1956b) and in skeletal muscle (Hodgkin & Horowicz, 1960) the relation between potential and tension is very steep. 4. Sodium free contractures (Lüttgau & Niedergerke, 1958) are very rapid in onset with this preparation. To maintain the tension developed, a rapid rate of washing is necessary for 1–2 min. After this, slow washing is sufficient. It is proposed that this is due to the leaching out of cellular sodium. 5. In fresh preparations the twitch and maximum K contracture are of similar size when stimulated at 30/min in 1–2 m M‐Ca. With the onset of hypodynamia the twitch tension falls, but the K contracture remains unaltered. In hypodynamic ventricles the sensitivity of the twitch to the ratio [Ca]/[Na]2 declines, whereas that of the K contractures remains unaltered. Hypodynamia therefore does not affect the contractile elements themselves. 6. During the staircase phenomenon the K contracture and twitch size alter in a similar manner, as previously described by Niedergerke. The sodium free contracture, however, remains unaltered. This tends to favour the hypothesis that the staircase phenomenon is due to effects at the cell membrane rather than in the cell interior. 7. Maximum K contractures occur at a [Ca]/[Na]2 ratio of about 0·7 × 10−4 m M−1, a figure similar to that obtained in earlier experiments (Lüttgau & Niedergerke, 1958). 8. The length—tension curves of K contractures are similar to those previously described for heart muscle, using single twitches to generate tension. 9. Application of K free solutions produced no rapid contractures in this preparation.