Uroguanylin and guanylin regulate transport of mouse cortical collecting duct independent of guanylate cyclase C.

Electrolyte and water homeostasis mostly depend on differentially regulated intestinal and renal transport. Guanylin and uroguanylin were proposed as first hormones linking intestinal with renal electrolyte and water transport, which is disturbed in pathophysiology. Guanylate cyclase C is the intestinal receptor for these peptides, but in guanylate cyclase C-deficient mice renal effects are retained. Unlike for the intestine the sites of renal actions and cellular mechanisms of guanylin peptides are still unclear.

After first data on proximal tubular effects in this study their effects are examined in detail in mouse cortical collecting duct (CCD). Effects of guanylin peptides on principal cells of isolated mouse CCD were studied by slow whole-cell patch-clamp analysis, reverse transcription-polymerase chain reaction (RT-PCR), and microfluorimetric measurements of intracellular Ca²⁺.

Guanylin peptides depolarized or hyperpolarized principal cells. Whereas 8-Br-cyclic guanosine monophosphate (8-Br-cGMP) hyperpolarized, 8-Br-cyclic adenosine monophosphate (8-Br-cAMP) depolarized principal cells. All effects of guanylin peptides were inhibited by Ba²⁺. Hyperpolarizations were blocked by clotrimazole or protein kinase G (PKG) inhibition, suggesting an involvement of basolateral Ca²⁺- and cGMP-dependent K⁺ channels. Effects remained in CCD isolated from guanylate cyclase C-deficient mice. Depolarizations were inhibited by arachidonic acid or inhibition of phospholipase A₂ (PLA₂), but not by protein kinase A (PKA) inhibition.

These results suggest the existence of two signaling pathways for guanylin peptides in principal cells of mouse CCD. One pathway is cGMP- and PKG-dependent but not mediated by guanylate cyclase C, the second involves PLA₂ and arachidonic acid. The first pathway most likely leads to an activation of the basolateral K⁺-conductance while the latter probably results in decreased activity of ROMK channels in the luminal membrane.