Membrane guanylate cyclase C (GC-C) is the receptor for guanylin, uroguanylin, and heat-stable enterotoxin (STa) in the intestine. GC-C-deficient mice show resistance to STa in intestine but saluretic and diuretic effects of uroguanylin and STa are not disturbed. Here we describe the cellular effects of these peptides using immortalized human kidney epithelial (IHKE-1) cells with properties of the proximal tubule, analyzed with the slow-whole-cell patch clamp technique. Uroguanylin (10 or 100 nM) either hyperpolarized or depolarized membrane voltages (V m ). Guanylin and STa (both 10 or 100 nM), as well as 8-Br-cGMP (100 M), depolarized V m . All peptide effects were absent in the presence of 1 mM Ba 2؉ . Uroguanylin and guanylin changed V m pH dependently. Pertussis toxin (1 g/ml, 24 h) inhibited hyperpolarizations caused by uroguanylin. Depolarizations caused by guanylin and uroguanylin were blocked by the tyrosine kinase inhibitor, genistein (10 M). All three peptides increased cellular cGMP. mRNA for GC-C was detected in IHKE-1 cells and in isolated human proximal tubules. In IHKE-1 cells GC-C was also detected by immunostaining. These findings suggest that GC-C is probably the receptor for guanylin and STa. For uroguanylin two distinct signaling pathways exist in IHKE-1 cells, one involves GC-C and cGMP as second messenger, the other is cGMP-independent and connected to a pertussis toxin-sensitive G protein.

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The effect of bromosulfophthalein (BSP) on potential-stimulated p-aminohippurate (PAH) uptake as well as on Na⁺-D-glucose and Na⁺-L-alanine cotransport was investigated in rat renal brush-border membrane vesicles. All three transport mechanisms were inhibited by BSP in the following order: PAH > D-glucose > L-alanine. Half-maximal inhibition of initial uptakes was observed at approximately 10,75 and 250 µM BSP, respectively. The inhibition of initial Na⁺-D-glucose uptake could not be attributed to increased cation permeability, since equilibrium exchange was equally affected by BSP. The nonspecific short-term inhibition of unrelated transport systems by BSP suggests a more general effect, such as a change in membrane fluidity. In addition, long-term incubation with 0.5 mM BSP resulted in a loss of vesicle integrity.

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