Thrombin inhibits atrial natriuretic peptide receptor activity in cultured bovine endothelial cells

Vasopressin-dependent inhibition of the C-type natriuretic peptide receptor, NPR-B/GC-B, requires elevated intracellular calcium concentrations

Natriuretic peptides bind their cognate cell surface guanylyl cyclase receptors and elevate intracellular cGMP concentrations. In vascular smooth muscle cells, this results in the activation of the type I cGMP-dependent protein kinase and vasorelaxation. In contrast, pressor hormones like arginine-vasopressin, angiotensin II, and endothelin bind serpentine receptors that interact with G(q) and activate phospholipase Cbeta. The products of this enzyme, diacylglycerol and inositol trisphosphate, activate the conventional and novel forms of protein kinase C (PKC) and elevate intracellular calcium concentrations, respectively. The latter response results in vasoconstriction, which opposes the actions of natriuretic peptides. Previous reports have shown that pressor hormones inhibit natriuretic peptide receptors NPR-A or NPR-B in a variety of different cell types. Although the mechanism for this inhibition remains unknown, it has been universally accepted that PKC is an obligatory component of this pathway primarily because pharmacologic activators of PKC mimic the inhibitory effects of these hormones. Here, we show that in A10 vascular smooth muscle cells, neither chronic PKC down-regulation nor specific PKC inhibitors block the AVP-dependent desensitization of NPR-B even though both processes block PKC-dependent desensitization. In contrast, the cell-permeable calcium chelator, BAPTA-AM (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, tetraacetoxymethyl ester), abrogates the AVP-dependent desensitization of NPR-B, and ionomycin, a calcium ionophore, mimics the AVP effect. These data show that the inositol trisphosphate/calcium arm of the phospholipase C pathway mediates the desensitization of a natriuretic peptide receptor in A10 cells. In addition, we report that CNP attenuates AVP-dependent elevations in intracellular calcium concentrations. Together, these data reveal a dominant role for intracellular calcium in the reciprocal regulation of these two important vasoactive signaling systems.

Regulation of ANP clearance receptors by EGF in mesangial cells from NOD mice

Mesangial cells from nonobese diabetic (NOD) mice (D-NOD) that develop diabetes at 2-4 mo express an increased density of atrial natriuretic peptide (ANP) clearance receptors [natriuretic peptide C receptor (NPR-C)] and produce less GMP in response to ANP than their nondiabetic counterparts (ND-NOD). Our purpose was to investigate how both phenotypic characteristics were regulated. Epidermal growth factor (EGF) and heparin-binding (HB)-EGF, but not platelet-derived growth factor or insulin-like growth factor I, inhibited (125)I-ANP binding to ND-NOD and D-NOD mesangial cells, particularly in the latter. NPR-C density decreased with no change in the apparent dissociation constant, and there was also a decrease in NPR-C mRNA expression. The EGF effect depended on activation of its receptor tyrosine kinase but not on that of protein kinase C, mitogen-activated protein kinases, or phosphoinositide-3 kinase. Activation of activator protein-1 (AP-1) was necessary, as shown by the inhibitory effect of curcumin and the results of the gel-shift assay. The cGMP response to physiological concentrations of ANP was greater in EGF-treated D-NOD cells. These studies suggest that EGF potentiates the ANP glomerular effects in diabetes by inhibition of its degradation by mesangial NPR-C via a mechanism involving AP-1.