PKC isoforms in rat medullary thick ascending limb: Selective activation of the δ-isoform by PGE₂

In the medullary thick ascending limb (MTAL) of the rat kidney, prostaglandin E₂ (PGE₂) reverses inhibition of HCO₃/^-absorption by arginine vasopressin (AVP). This effect of PGE₂ is blocked by chelerythrine or staurosporine and mimicked by pherbol ester, suggesting a critical role for protein kinase C (PKC). The present study was designed to examine directly regulation of PKC isoforms by PGE₂ in the inner stripe of the outer medulla and in microdissected MTALs. Immunoblots with isoform-specific anti- PKC antibodies detected α-, βII-, δ-, ε-, and ζ-isoforms in both inner stripe and MTAL. The βI- and γ-isoforms were not detected. Translocation and activation of PKC were assessed by immunoblot analysis and by direct measurement of enzyme activity using an immune complex kinase assay. In inner stripe tissue incubated with 10^-10 M AVP, PGE₂ (10^-6 M for 20 min) induced translocation of PKC-δ from the cytosolic fraction to the membrane fraction. This translocation was associated with an 85% increase in PKC-δ activity in the membrane fraction and a 70% decrease in PKC-δ activity in the cytosolic fraction. PGE₂ had no effect on the subcellular distribution or the activities of the other isoforms. Activation of PKC-δ was confirmed directly in microdissected MTALs, in which PGE₂ caused a near complete loss of PKC-δ from the cytosolic fraction. PGE₂ did not induce translocation of PKC-δ in the absence of AVP. These results demonstrate that 1) the MTAL expresses Ca²⁺ dependent (α, βII) and Ca²⁺-independent (δ, ε, ζ) PKC isoforms; 2) PGE₂ causes selective activation of PKC-δ, which likely mediates the action of PGE₂ to reverse AVP inhibition of HCO₃/^- absorption; and 3) PGE₂ activation of PKC-δ requires the presence of AVP, which may explain the fact that PGE₂ influences HCO₃/^- transport only when AVP is present.