Autoradiographic localization of specific atrial natriuretic peptide binding sites on immunocytochemically identified cells in cultures from rat and guinea-pig hearts

Lack of evidence for functional natriuretic peptide receptors in the heart of the cane toad, Bufo marinus

Several studies have shown that the heart of species from each vertebrate class contains natriuretic peptide binding sites which suggests that ANP released from the heart may act in a paracrine/autocrine fashion. The present study used a set of techniques to study cell surface receptors in order to investigate the presence and nature of NPRs in the heart of the cane toad, Bufo marinus. Autoradiographical studies of both atria and ventricle showed no variation between total and non-specific binding, indicating a lack of NP binding sites in these tissues. This was confirmed with in vitro binding studies in which increasing concentrations of ANP did not compete for any specific binding. Increasing concentrations of ANP did not increase cGMP generation and physiological experiments showed that both ANP and CNP had no effect on the force or rate of contraction of a sino-atrial preparation. Molecular expression studies, however, showed that mRNA for NPRs was expressed in the heart, in spite of the lack of evidence for NPR on the cell surface. Overall, this study showed that no functional NPRs are present in the heart, and provided evidence that the heart is not a target organ for NP action in B. marinus.

Gene expression of natriuretic peptide receptors in myocardial cells

Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are cardiac hormones that serve to unload the heart through their effects on the kidney and vasculature. Whether the heart itself represents a site of action for these peptides is currently the subject of debate. Although functional studies indicate that ANP has some effects on isolated myocytes, several studies have been unable to detect binding of the hormone to these cells. The present study demonstrates that the genes for all three natriuretic peptide receptor (NPR) subtypes, NPR-A, NPR-B, and NPR-C, are expressed in the rat heart. For microlocalization of the receptor mRNAs in myocytes and nonmyocytic cells, a combination of cell isolation and reverse transcription-polymerase chain reaction (RT-PCR) was used. Cardiac myocytes were isolated by enzymatic dissociation of rat ventricular tissue, purified by density gradient centrifugation, and collected as single cells under microscopic control. Analysis by RT-PCR revealed the presence of transcripts for NPR-A as well as NPR-B and NPR-C. However, cGMP generation in purified myocytes was stimulated only by ANP and BNP, which specifically bind to NPR-A, whereas C-type natriuretic peptide (CNP, an NPR-B agonist) was ineffective. Therefore, rat ventricular myocytes appear to produce predominantly NPR-A. The expression of NPR-B may be low or even absent. The mRNAs for all three NPRs were also found in cultures of fibroblasts from the rat heart. In contrast to the myocytes, large increases in cGMP were observed in response not only to ANP but also to CNP.

Endocardial localization and characterization of natriuretic peptide binding sites in human fetal and adult heart

Specific, high affinity binding sites for 125I-human-alpha-atrial natriuretic peptide-(1-28)) (125I-hANP-(1-28)) were identified in human fetal and adult heart and the binding characterized using quantitative in vitro autoradiography. Binding sites were localized to atrial and ventricular endocardium, aorta, pulmonary arteries and epicardial mesothelium. Kinetic studies indicated a Kd value of 32 pM for ventricular endocardial 125I-hANP-(1-28) binding. The binding was completely inhibited by an excess (1 microM) of unlabelled hANP-(1-28), human brain natriuretic peptide-(1-32) (hBNP-(1-32)) and by the 'clearance receptor' specific ring-deleted analogue, C-ANP-(4-23). Competitive inhibition studies indicated a relative inhibitory potency for hBNP-(1-32) and C-ANP-(4-23) of 6% and 3% respectively. The data suggest that a distinct natriuretic peptide receptor subtype is expressed in the endocardium and in addition to a possible clearance function, may represent a site for feedback regulation and peptide interaction.

Atrial and brain natriuretic peptides share binding sites on cultured cells from the rat trachea

We examined the distribution of binding sites for alpha-atrial natriuretic peptide (125I-ANP1-28) and the recently discovered porcine brain natriuretic peptide (125I-pBNP) on immunocytochemically identified cells in dissociated culture preparations of the rat trachea. Specific binding sites for both 125I-ANP1-28 and 125I-pBNP were evenly distributed over distinct subpopulations of smooth muscle myosin-like immunoreactive muscle cells, fibronectin-like immunoreactive fibroblasts and S-100-like immunoreactive glial cells. Neither keratin-like immunoreactive epithelial cells nor protein gene product 9.5-like immunoreactive paratracheal neurones expressed natriuretic peptide binding sites, although autoradiographically labelled glial cells were seen in close association with both neuronal cell bodies and neurites. The binding of each radiolabelled peptide was abolished by the inclusion of either excess (1 microM) unlabelled rat ANP or excess unlabelled porcine BNP, suggesting that ANP and BNP share binding sites in the trachea. Furthermore, the ring-deleted analogue, Des-[Gln18, Ser19, Gly20, Leu21, Gly22]-ANF4-23-NH2, strongly competed for specific 125I-ANP1-28 and 125I-pBNP binding sites in the tracheal culture; this suggests that virtually all binding sites were of the "clearance" (ANP-C or ANF-R2) receptor subtype.