atrial-natriuretic-factor and kelatorphan

1. The present study examined the effect of exogenous atrial natriuretric peptide (ANP), alone or in presence of inhibitors of the two major mechanisms for clearing ANP, metabolism by neutral endopeptidase-24.11 (NEP) and internalization by C-ANP receptors, on arteriolar responses using intravital microscopy on the rat cremaster muscle after intravenous or topical administration of the peptide. 2. Topical application of ANP (3 x 10(-10) to 3 x 10(-8) M) produced a gradual increase in arteriolar diameter. NEP inhibitors, thiorphan (30 mg kg-1, i.v.), kelatorphan (10 mg kg-1, i.v.) and retrothiorphan (25 mg kg-1, i.v.) alone, did not significantly affect vascular tone but caused significant potentiation of the arteriolar responses to topically applied ANP. 3. When given as an i.v. bolus, ANP dilates skeletal arterioles at a high dose (20 micrograms kg-1). At a lower dose (10 micrograms kg-2), ANP alone or with retrothiorphan or the C-ANP receptor ligand C-ANP (4-23) did not produce any arteriolar responses, while after the combined administration of the two inhibitors, an increase in arteriolar diameter was induced. 4. These results indicate that low doses of topically applied ANP dilate rat cremaster arterioles and that the vasodilator responses can be potentiated by NEP inhibition. When given as an i.v. bolus, a high dose of ANP can also dilate skeletal arterioles. However at a lower dose the rapid metabolism of the peptide prevents it from producing its action.

Topics: Abdominal Muscles; Adenosine; Administration, Topical; Animals; Arterioles; Atrial Natriuretic Factor; Blood Pressure; Dipeptides; Injections, Intravenous; Male; Neprilysin; Nitroprusside; Peptide Fragments; Protease Inhibitors; Rats; Rats, Sprague-Dawley; Thiorphan; Vasodilator Agents

Atrial natriuretic peptides (ANPs) are degraded rapidly by renal brush border membranes in vitro. Here, we report that thiorphan, a specific inhibitor of endopeptidase 24.11, afforded almost complete protection against inactivation of ANPs by a renal brush border membrane preparation. The diastereoisomers of [3-(N-hydroxy)carboxamido-2-benzylpropanoyl]-L-alanine (HCBA) are potent inhibitors of endopeptidase 24.11 and were also tested for their abilities to inhibit ANP-(103-126) degradation. The (S,S)-diastereoisomer was more effective than the (R,S)-diastereoisomer (kelatorphan), but both were less potent than thiorphan. To determine if endopeptidase inhibitors could decrease ANP metabolism in in vivo, thiorphan and (S,S)-HCBA were given to rats with or without a continuous infusion of ANP-(103-126). Both inhibitors induced rapid increases in plasma ANP concentration in rats administered exogenous ANP-(103-126), but had no effect on endogenous ANP levels. Thus, specific inhibitors of endopeptidase 24.11 decrease the degradation of ANPs in vitro, and are effective in reducing the metabolism of ANP-(103-126) in vivo.

Topics: Animals; Atrial Natriuretic Factor; Dipeptides; Kidney; Male; Microvilli; Neprilysin; Protease Inhibitors; Rabbits; Rats; Thiorphan