The overwhelming majority of atrial natriuretic factor (ANF) receptors in kidney and vascular tissues do not mediate any of the known functional effects of the hormone. To test whether these receptors (C-ANF receptors) function as clearance receptors for circulating ANF-(1-28), we determined the effects of C-ANF-(4-23) [des[Gln18Ser19Gly20Leu21Gly22]rANF-(3-23)-NH2], a specific ligand of C-ANF receptors, on the pharmacokinetics and hydrolysis of 125I-labeled ANF-(1-28) in anesthetized rats. Radioactivity in plasma was characterized by trichloroacetic acid solubility and high-pressure liquid chromatography. C-ANF-(4-23) (1 and 10 micrograms.min-1.kg body wt-1) led to marked dose-dependent increases in initial plasma concentration of administered 125I-ANF-(1-28) and decreases in its volume of distribution at steady state (Vss), metabolic clearance rate (MCR), and appearance of hydrolytic products ([125I]monoiodotyrosine and free 125I) in plasma (Pm). At the highest dose, C-ANF-(4-23) decreased Vss from 97 +/- 12 to 36 +/- 2 ml/100 g body wt, MCR from 50 +/- 4 to 12 +/- 1 ml.min-1.100 g body wt-1, and Pm from 54 +/- 8 to 11 +/- 2% of initial plasma 125I-ANF-(1-28). The data demonstrate that C-ANF receptors are mainly responsible for the very large volume of distribution and fast MCR of ANF in the rat. In this manner, C-ANF receptors are likely to play an important role in the homeostasis of circulating ANF.

A novel small linear C-atrial natriuretic factor receptor ligand [C-ANF-(11-15)] and phosphoramidon (PHO) were used to determine the effects of C-ANF receptor blockade alone, or in combination with inhibition of neutral endopeptidase (NEP), on the pharmacokinetics and metabolism of ANF in the rat. C-ANF-(11-15) infusion decreased apparent volume of distribution (Vss) and metabolic clearance rate (MCR) of administered 125I-ANF-(1-28) to one-third of their control values, whereas PHO alone was without effect on these parameters. In combination with C-ANF-(11-15), however, PHO further decreased MCR of 125I-ANF-(1-28) and increased plasma half time by more than threefold. High-performance liquid chromatography analysis revealed that C-ANF-(11-15) inhibited the delayed appearance of free 125I and [125I]monoiodotyrosine but had no effect on the small proportion of NEP metabolites in plasma. The combination of C-ANF-(11-15) and PHO further delayed the appearance of small metabolites, abolished the appearance of NEP metabolites, and markedly prolonged the permanence of intact 125I-ANF-(1-28) in plasma. The results demonstrate that C-ANF receptor blockade by C-ANF-(11-15) impairs clearance and metabolism of ANF, an effect which is synergistically potentiated by concomitant inhibition of NEP. C-ANF-(11-15) alone or in combination with NEP inhibitors may be a potentially useful therapeutic tool in the treatment of cardiovascular and renal diseases.

The expression and regulation of atrial natriuretic peptide (ANP) receptor subtypes were examined in rat renal glomeruli and papillae. In glomeruli, approximately two-thirds of ANP binding sites represented guanylate cyclase-uncoupled ANP clearance receptors (ANPc) with a molecular mass of 64 kDa under reducing conditions. The remainder of glomerular ANP binding sites represented guanylate cyclase-coupled ANP receptors (ANPGC) with a molecular mass of 130 kDa. In rat papillae, only the 130-kDa ANPGC was expressed. In rats adapted to a low-salt diet, saline loading or acute ANP infusion resulted in a decrease in ANPC density, a difference that was not detected when glomeruli were first acidwashed to remove endogenous ANP, indicating that apparent regulation of ANPC reflected prior occupancy by endogenous ANP. Densities of glomerular ANPC and ANPGC were similar in spontaneously hypertensive rats (SHR) compared with those of the Wistar-Kyoto (WKY) controls. However, elimination of prior receptor occupancy revealed a significantly greater expression of glomerular ANPC in SHR compared with WKY rats, without significant differences in the density of the glomerular or papillary ANPGC subtype. The failure of the ANPGC subtype to be regulated may account for our previously reported findings that dietary salt intake does not affect glomerular ANP-stimulated guanosine-3',5'-cyclic monophosphate accumulation despite apparent regulation of ANP receptor density. Whether the increased expression of the ANPC subtype in SHR represents a primary defect or results from induction of ANP clearance receptor expression remains to be determined.

Twenty-six healthy female pigs weighing 19.5-33 kg were used in three separate experiments. The animals were fed individually twice a day. Trimethoprim/sulphadiazine (TMP/SDZ) formulation was added to feed in the amount of 6 mg/kg bw (TMP) and 30 mg/kg bw (SDZ). TMP and SDZ concentrations in blood plasma, muscles, liver and kidneys were measured. Pharmacokinetic parameters show that the absorption of TMP from the alimentary tract in pigs is faster than the absorption of SDZ, and the elimination of TMP is slower than that of SDZ. The absorption half-lives were 0.96 (TMP) and 2.24 h (SDZ), whereas elimination half-lives were 5.49 (TMP) and 4.19 h (SDZ). The observed TMP:SDZ ratios in blood plasma after multiple dose administration ranged from 1:11.4 to 1:23.2. One day after administration of the last dose of TMP/SDZ the plasma concentration ratio was 1:15.5, but in muscles, liver and kidneys it was much lower: 1:0.79, 1:0.14 and 1:1.53 respectively. The absolute TMP and SDZ tissue concentrations 1 day after the last multiple dose administration were very low (maximum TMP: 0.29 micrograms/g in liver; maximum SDZ: 0.23 micrograms/g in kidneys). Neither drug was detected in any tissue 8 days after the last administration of TMP/SDZ. Based on our results, it was concluded that there is no support for the TMP:SDZ pharmaceutical ratio 1:5 in oral formulations of these compounds for pigs. The administration oral TMP/SDZ formulations once a day may result in the absolute tissue concentrations of these drugs being too low for antibacterial activity. The withdrawal period for such an oral TMP/SDZ formulation for pigs (according to accepted guidelines in Europe for MRL of TMP < 0.05 mg/kg of tissue) should not be less than 5 days.

C-type natriuretic peptide (CNP) is a newly described 22-amino acid peptide of endothelial cell origin, which has selective cardiovascular actions and is structurally related to atrial natriuretic peptide (ANP). Recent in vitro studies have demonstrated that an important regulatory pathway for the clearance of natriuretic peptides involves binding to a common clearance receptor [natriuretic peptide C receptor (NPR-C)]. Although CNP has also been identified as a ligand for NPR-C in binding assays, no studies have defined the in vivo interaction of CNP with NPR-C. CNP (10 ng.kg-1.min-1) followed by C-ANP-(4-23), a specific ligand for NPR-C blockade, was infused intravenously in two groups (both n = 7) of anesthetized dogs at two different doses (0.1 or 1.0 micrograms.kg-1.min-1) to permit calculation of total metabolic clearance rate (TMCR). C-ANP-(4-23) increased circulating CNP and reduced TMCR in both groups. Pulmonary metabolic clearance rate was negative at baseline, suggesting a net secretion of CNP across the lung, which was increased during CNP infusion and was abolished with NPR-C blockade. Renal and femoral metabolic clearance rates were positive at baseline and increased with CNP infusion. A decrease in cardiac output and cardiac filling pressures in response to CNP administration was potentiated by NPR-C blockade. We conclude that 1) circulating CNP achieved by CNP infusion is regulated by NPR-C in vivo, 2) the pulmonary circulation is a possible site of CNP secretion, 3) the renal and peripheral circulations are sites of CNP clearance, and 4) NPR-C blockade potentiates the selective cardiovascular actions of CNP.