If basal phosphorylation is a mechanism of control of TP receptor function, then phosphatases would be necessary to effect dephosphorylation. Phosphatases that dephos-phorylate TP receptors appear to exist, since Kinsella, et al (66) observed that inhibition of PKC or PKA resulted in increased ligand binding to TP receptors. This study did not specifically demonstrate a decrease in TP receptor phosphorylation, but as noted previously (See IV. TP RECEPTOR FUNCTION, E. TP RECEPTOR PHOSPHORYLATION, 2. ROLE OF PHOSPHORYLATION IN TP RECEPTOR ACTIVATION above), stimulation of both PKC and PKA resulted in decreased [3H]-SQ29548 binding that was associated with increased receptor phosphorylation (66). The phosphatases responsible for dephosphorylation of TP receptors are unknown. Several novel serine/threonine phosphatases have recently been identified (168). Protein phosphatase 5 (PP5), a member of the PPP family, has interesting properties that suggest it as a candidate for TP receptor dephosphorylation. PP5 interacts with glucocorticoid receptors and with atrial natriuretic peptide (ANP)/guanylate cyclase receptors that are phosphorylated on many serine and threonine residues in the absence of ligand. Following agonist binding, ANP receptors are desensitized by dephosphorylation that may be mediated by PP5 (168). Another characteristic of PP5 that suggests a possible association with TP receptors is its activation by polyunsaturated fatty acids such as arachidonic acid

(169). A candidate for a tyrosine phosphatase that may mediate TP receptor dephosphorylation is SHP-1. SHP-1 has been shown to down-regulate phosphorylation and mitogenesis in hematopoietic cells and to be associated with the somatostatin GPCR

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