Atrial natriuretic peptide: binding and cyclic GMP response in cultured vascular smooth muscle cells from spontaneously hypertensive rats

Atrial natriuretic peptide (ANP) is vasodilatory and natriuretic, but whereas increased plasma ANP levels occur in spontaneously hypertensive rats, their elevated vascular resistance suggests inappropriate target tissue responsiveness to ANP. This study examines ANP-receptor binding properties (at 25 degrees C and 4 degrees C) in cultured vascular aortic smooth muscle cells from spontaneously hypertensive (SHR) and control Wistar-Kyoto (WKY) rats. [I125]-human ANP saturation (0.0625-12.0 nmol) profiles were analyzed using nonlinear regression (LIGAND). Vascular smooth muscle cells from WKY possessed both high affinity (KD1 0.3 nmol; R1 33 fmol/10(5) cells) and low affinity (KD2 15 nmol; R2 400 fmol/10(5) cells) binding sites for ANP. In contrast, for smooth muscle cells from SHR, two receptor forms could not be resolved using identical analytical protocols. Parameter estimates at 25 degrees C and 4 degrees C were not different for either SHR or WKY. The number of receptors for SHR (Bmax approximately 100 fmol/10(5) cells) was lower than the total number of receptors for WKY (high plus low affinity approximately 430 fmol/10(5) cells). The intermediary KD value (approximately 1.0 nmol) for ANP binding in SHR suggests an ANP-receptor interconversion from high affinity to low affinity in smooth muscle cells from SHR. Competition-binding experiments also revealed a decreased affinity for ANP in SHR-derived smooth muscle cells. The cyclic GMP response (intracellular accumulation and extracellular levels) was decreased in SHR smooth muscle cells compared to WKY, although this difference was evident only after prolonged (one hour) stimulation with ANP. Our data indicate a reduced sustained vascular responsiveness to ANP in hypertension.