Idealized representation of TGF. Movement from point A to point B along the solid TGF curve signifies activation of TGF, which occurs within a few seconds of an increase in macula densa chloride. Because the TGF curve has negative slope, SNGFR and macula densa chloride cannot change in the same direction unless the TGF curve also changes. The dashed curve represents a shift, or resetting, of the TGF curve. Rightward resetting has been observed during prolonged activation of TGF. Since prolonged activation of TGF also suppresses renin, it is proposed that the rightward resetting is mediated in part by macula densa–dependent suppression of renin release. Cheng et al. (1) argue that the macula densa control of renin involves a pathway from low chloride to p38 MAP kinase to expression of COX-2.