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Renal atrial natriuretic factor receptors in hamster cardiomyopathy
S Mukaddam-Daher1, M Jankowski, T V Dam
1Laboratory of Cardiovascular Biochemistry, Centre de Recherche Hótel-Dieu de Montréal, Université de Montréal, Quebec, Canada.
Insights
Hamsters with heart failure show reduced sodium and water excretion despite high atrial natriuretic factor (ANF) levels. This study found that renal ANF receptors are not altered, indicating they do not cause this reduced response.
Area of Science:
- Cardiovascular Physiology
- Renal Physiology
- Molecular Endocrinology
Background:
- Cardiomyopathy (CMO) in hamsters, an experimental model of congestive heart failure, is characterized by sodium and water retention.
- This retention persists despite elevated plasma atrial natriuretic factor (ANF), suggesting hyporesponsiveness to ANF.
- The role of renal ANF receptor subtypes in this hyporesponsiveness remains unclear.
Purpose of the Study:
- To characterize renal ANF receptor subtypes (GC-A and GC-B) in normal and cardiomyopathic hamsters.
- To investigate whether alterations in these receptors contribute to renal resistance to ANF in cardiomyopathy.
Main Methods:
- Quantitative PCR was used to detect GC-A and GC-B receptor mRNA in different kidney regions.
- Competitive binding studies, autoradiography, and affinity cross-linking assessed receptor presence and function.
- ANF-stimulated guanosine 3',5'-cyclic monophosphate (cGMP) production was measured.
Main Results:
- GC-A mRNA levels were increased in the outer and inner medulla of cardiomyopathic hamsters compared to controls.
- Functional GC-B receptors were absent in the kidney glomeruli and inner medulla of both groups.
- Renal ANF receptor density and affinity were not altered by cardiomyopathy, and ANF-stimulated cGMP production was similar in normal and CMO hamsters.
Conclusions:
- Renal guanylyl cyclase-A (GC-A) and guanylyl cyclase B (GC-B) receptor subtypes are present in the hamster kidney.
- Despite increased GC-A mRNA in the medulla of cardiomyopathic hamsters, renal ANF receptors do not appear to be altered.
- These findings demonstrate that renal ANF receptors do not contribute to the attenuated renal responses to ANF observed in hamster cardiomyopathy.
Abstract:
Hamsters with cardiomyopathy (CMO), an experimental model of congestive heart failure, display stimulated renin-angiotensin-aldosterone and enhanced sympathetic nervous activity, all factors that lead to sodium retention, volume expansion and subsequent elevation of plasma atrial natriuretic factor (ANF) by the cardiac atria. However, sodium and water retention persist in CMO, indicating hyporesponsiveness to endogenous ANF. These studies were undertaken to fully characterize renal ANF receptor subtypes in normal hamsters and to evaluate whether alterations in renal ANF receptors may contribute to renal resistance to ANF in cardiomyopathy. Transcripts of the guanylyl cyclase-A (GC-A) and guanylyl cyclase B (GC-B) receptors were detected by quantitative polymerase chain reaction (PCR) in renal cortex, and outer and inner medullas. Compared to normal controls, the cardiomyopathic hamster's GC-A mRNA was similar in cortex but significantly increased in outer and inner medulla. Levels of GC-B mRNA were not altered by the disease. On the other hand, competitive binding studies, autoradiography, and affinity cross-linking demonstrated the absence of functional GC-B receptors in the kidney glomeruli and inner medulla. Also, C-type natriuretic peptide (CNP), the natural ligand for the GC-B receptors, failed to stimulate glomerular production of its second messenger cGMP. In CMO, sodium and water excretion were significantly reduced despite elevated plasma ANF (50.5 +/- 11.1 vs. 309.4 +/- 32.6 pg/ml, P < 0.001). Competitive binding studies of renal glomerular ANF receptors revealed no change in total receptor density, Bmax (369.6 +/- 27.4 vs. 282.8 +/- 26.2 fmol/mg protein), nor in dissociation constant, Kd (647.4 +/- 79.4 vs. 648.5 +/- 22.9 pM). Also, ANF-C receptor density (254.3 +/- 24.8 vs. 233.8 +/- 23.5 fmol/mg protein), nor affinity were affected by heart failure. Inner medullary receptors were exclusively of the GC-A subtype with Bmax (153.2 +/- 26.4 vs. 134.5 +/- 21.2 fmol/mg protein) and Kd (395.7 +/- 148.0 vs. 285.8 +/- 45.0 pM) not altered by cardiomyopathy. The increase in ANF-stimulated glomerular cGMP production was similar in normal and CMO hamsters (94- vs. 75-fold). These results demonstrate that renal ANF receptors do not contribute to the attenuated renal responses to ANF in hamster cardiomyopathy.