Stimulation of proximal convoluted tubule phosphate transport by epidermal growth factor: Signal transduction

The present study investigated the signal-transduction pathway responsible for the epidermal growth factor (EGF) stimulation of phosphate transport (J(Phos)) in the rabbit proximal convoluted tubule (PCT). Genistein, 10^-4 M, bath and lumen, an inhibitor of EGF receptor tyrosine kinase activity, blocked the EGF effect on J(Phos), consistent with a role for tyrosine kinase in the signal-transduction pathway. Both staurosporine (5 x 10^-8 s M) and calphostin C (10 s M), inhibitors of protein kinase C, blocked the EGF stimulation of J(Phos), indicating that protein kinase C is involved in EGF signaling. Intracellular calcium (Ca(i)/²⁺) concentrations were measured in perfused tubules using fura PE3 to determine whether changes in Ca(i)²⁺ were also part of the signaling pathway. After addition of 3 nM EGF, there was no change in Ca(i)²⁺, suggesting that stimulation of protein kinase C is not from phosphatidylinositol hydrolysis by phospholipase C-γ. To determine whether phospholipase A₂ (PLA₂) is involved, the inhibitor mepacrine was used. Mepacrine (5 x 10^-5 M) had no direct effect on PCT transport but blocked the stimulatory effect of EGF on J(Phos). PLA₂ activity, assessed as free arachidonic acid release from proximal tubules in suspension, increased by 18.8% with 3 nM EGF. Thus the stimulation of J(Phos) by EGF is mediated via a signal-transduction pathway involving tyrosine kinase, protein kinase C, and PLA₂.