Peptide-regulated guanylate cyclase pathways in rat colon: In situ localization of GCA, GCC, and guanylin mRNA

Guanylate cyclases play a role in both physiological and pathological secretion in the mammalian intestine. Agents that raise guanosine 3',5'- cyclic monophosphate (cGMP) levels, such as atrial natriuretic peptide (ANP), guanylin (an endogenous intestinal peptide), or Escherichia coli heat-stable enterotoxin type a (STa; a bacterial toxin), enhance electrolyte secretion and the accumulation of luminal fluid. Although secretion in all parts of intestine is sensitive to changes in cGMP metabolism, an increasing body of evidence suggests that these responses are particularly important in proximal colon. To date, three peptide-sensitive membrane-bound guanylate cyclases [types A, B, and C (GCA, GCB, and GCC, respectively)] have been cloned from mammalian tissues. GCA responds to ANP, GCB to C-type natriuretic peptide, and GCC to guanylin and STa. Expression of these receptor/cyclase genes has not previously been investigated at the cellular level in the colon. Nucleotide probes specific for GCA, GCB, GCC, and guanylin were generated by polymerase chain reaction. These probes were used to evaluate colonic cyclase and guanylin mRNA expression in the rat. GCB mRNA is not detectable in this tissue either by in situ hybridization or by Northern blot analysis. In contrast, GCA, GCC, and guanylin mRNAs are all conspicuously expressed. With the in situ hybridization technique, GCA mRNA expression is seen in epithelial cells throughout colonic crypts, and also, although at a slightly lower level, in cells of the surface epithelium. Guanylin mRNA expression is limited to a different (but probably overlapping) subset of epithelial cells: expression is strong in surface epithelium and in cells within the upper 20% of the crypts but is undetectable in cells at the bases of the crypts. Given the distinct physiological functions of different types of cells in the colon, this sort of anatomic information is important for understanding the role that cGMP plays in gastrointestinal secretion.