C-TYPE NATRIURETIC PEPTIDE RECEPTORS AND SIGNALING IN RECTAL GLAND OF SQUALUS-ACANTHIAS

Recent evidence suggests that the newly described natriuretic peptide, C-type natriuretic peptide (CNP), may be the circulating form of natriuretic peptide in the shark. In the shark CNP has a major site of action in the rectal gland, which augments chloride secretion in response to stimulation by volume loading or CNP infusion. We therefore examined the shark rectal gland for natriuretic peptide receptors and determined the presence of guanylate cyclase-linked receptors and non-guanylate cyclase-linked receptors for CNP in this tissue. CNP binds with uniform high affinity (dissociation constant of 78 +/- 11 pM) to receptors of high density (receptor density of 61 +/- 0.7 fmol/mg protein) in plasma membranes prepared from the rectal gland. By use of rat atrial natriuretic peptide (rANP) as a competing ligand, two classes of receptors become apparent in this population, both of which have similar affinity for CNP, but different affinities for rANP. The low-molecular-weight natriuretic peptide receptor-specific peptide, des-[Gln116 Ser117, Gly118, Leu119,Gly120]rANP-(102-121), binds to 50% of the receptors in the rectal gland, but fails to bind to the remaining 50% even at micromolar concentrations. Porcine brain natriuretic peptide (pBNP) binds with uniformly diminished affinity to all receptors, whereas the unrelated peptide, porcine vasoactive intestinal peptide, does not bind these receptors. The importance of the integrity of the ring structure of CNP is underlined by the significant loss of affinity when the peptide ring is opened. Guanylate cyclase measurements demonstrate that, whereas CNP is a potent stimulator of enzyme activity (27.79 +/- 3.19 pmol.mg protein-1.min-1, 10(-7) M CNP; 8.56 +/- 0.93 pmol.mg protein-1.min-1, basal), both rANP and pBNP have little or no effect. Modifications to the CNP structure result in diminished cyclase activating potential of the peptide, and the broken ring structure peptide [CNP-(7-21)] has no activity. We conclude that the shark rectal gland expresses two populations of receptors for natriuretic peptides, one of which is linked to guanylate cyclase and the second of which resembles the low-molecular-weight receptor described for ANP.