Figure 8. Proposed model for Na⁺-K⁺-2Cl⁻ cotransport regulation in ferret erythrocytes.

The cotransporter (COT) is phosphorylated, perhaps at multiple sites (to COT-P) by a kinase or kinases, which are lumped together as kinase K1, and dephosphorylated by a phosphatase or phosphatases, P1. Increased levels of COT-P are associated with increased rates of transport (Lytle, 1997). The transport rate is also affected by interactions between the cotransporter and the cytoskeleton (Klein & O'Neill, 1995; Matthews et al. 1998). The activity of P1 is inhibited by phosphorylation catalysed by kinase K2, an enzyme that is inhibited (directly or indirectly) by staurosporine, PP1 and possibly genistein (Flatman & Creanor, 1999b). P1 is dephosphorylated by phosphatase P2, an enzyme that is inhibited by calyculin A. Reduction of [Mg²⁺]_i to very low levels inhibits all kinases and possibly phosphatase P1 (Mildvan, 1987; Flatman, 1988; Flatman & Creanor, 1999b). Genistein may also directly inhibit the cotransporter (Flatman & Creanor, 1999b). Our observations suggest that arsenite stimulates cotransport by acting through a single common pathway in this scheme - the activation of kinase K1, an effect which may be indirect, and may occur at several steps.