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Regulation of Na⁺-K⁺-2Cl^- cotransport in turkey red cells: the role of oxygen tension and protein phosphorylation

M. C. Muzyamba, A. R. Cossins * and J. S. Gibson

Na⁺-K⁺-2Cl^- cotransport (NKCC) was studied in turkey red cells using Na⁺ dependence or bumetanide sensitivity of ⁸⁶Rb⁺ influx to monitor activity of the transporter.

Deoxygenation was the major physiological stimulus for NKCC activity: oxygen tensions (P_O2) over the physiological range modulated the transporter, with a P_O2 for half-maximal activation of about 41 mmHg (n = 3). In air, activity of NKCC was also stimulated by shrinkage and isoproteronol (isoprenaline, 5 µM). By contrast, in deoxygenated cells, although the transporter activity was markedly elevated, it was no longer sensitive to volume or -adrenergic stimulation.

Calyculin A, a protein phosphatase inhibitor, stimulated cotransport with a lag of about 5 min. N-Ethylmaleimide (NEM) inhibited cotransport and also blocked the stimulatory effect of calyculin A if administered before calyculin A. Stimulation by calyculin A and deoxygenation were not additive. Staurosporine (2 µM) inhibited deoxygenated-stimulated K⁺ influxes, but not those stimulated by calyculin A. NEM added during calyculin A stimulation, i.e. during the 5 min lag, caused transport activity to be clamped at levels intermediate between maximal (calyculin A alone) and control. Cells treated with calyculin A alone or with calyculin A followed by NEM were no longer sensitive to volume, isoproteronol or P_O2.

The results have characterized the interaction between deoxygenation and other stimuli of NKCC activity. They have also shown that it is possible to manipulate the transporter in a reciprocal way to that shown previously for K⁺-Cl^- cotransport.

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