Defective ANF-R2/ANP-C receptor-mediated signalling in hypertension

Altered neural and vascular mechanisms in hypertension

Essential hypertension is a multifactorial disorder which belongs to the main risk factors responsible for renal and cardiovascular complications. This review is focused on the experimental research of neural and vascular mechanisms involved in the high blood pressure control. The attention is paid to the abnormalities in the regulation of sympathetic nervous system activity and adrenoceptor alterations as well as the changes of membrane and intracellular processes in the vascular smooth muscle cells of spontaneously hypertensive rats. These abnormalities lead to increased vascular tone arising from altered regulation of calcium influx through L-VDCC channels, which has a crucial role for excitation-contraction coupling, as well as for so-called "calcium sensitization" mediated by the RhoA/Rho-kinase pathway. Regulation of both pathways is dependent on the complex interplay of various vasodilator and vasoconstrictor stimuli. Two major antagonistic players in the regulation of blood pressure, i.e. sympathetic nervous system (by stimulation of adrenoceptors coupled to stimulatory and inhibitory G proteins) and nitric oxide (by cGMP signaling pathway), elicit their actions via the control of calcium influx through L-VDCC. However, L-type calcium current can also be regulated by the changes in membrane potential elicited by the activation of potassium channels, the impaired function of which was detected in hypertensive animals. The dominant role of enhanced calcium influx in the pathogenesis of high blood pressure of genetically hypertensive animals is confirmed not only by therapeutic efficacy of calcium antagonists but especially by the absence of hypertension in animals in which L-type calcium current was diminished by pertussis toxin-induced inactivation of inhibitory G proteins. Although there is considerable information on the complex neural and vascular alterations in rats with established hypertension, the detailed description of their appearance during the induction of hypertension is still missing.

Enhanced expression of Gialpha protein and adenylyl cyclase signaling in aortas from 1 kidney 1 clip hypertensive rats

We have previously shown the augmented levels of Gialpha-2 and Gialpha-3 proteins (isoforms of inhibitory guanine nucleotide regulatory protein (G-protein)), and not of Gsalpha, in the hearts and aortas of spontaneously and experimentally induced hypertensive rats. The increased expression of Gialpha and blood pressure was restored toward WKY levels by captopril treatment, suggesting a role for angiotensin (Ang) II in the enhanced expression of Gialpha protein and blood pressure. This study was undertaken to investigate whether 1 kidney 1 clip (1K-1C) hypertensive rats that exhibit enhanced levels of Ang II also express enhanced levels of Gialpha proteins. Aortas from 1K-1C hypertensive rats were used. The expression of G-proteins was determined at protein levels with immunoblotting techniques, using specific antibodies for different isoforms of G-proteins. The levels of Gialpha-2 and Gialpha-3 proteins were significantly higher in aortas from 1K-1C hypertensive rats than in control rats; Gsalpha levels were unchanged. The inhibitory effect of low concentrations of guanosine 5'-[gamma-thio]triphosphate (GTPgammaS) on forskolin (FSK)-stimulated adenylyl cyclase (AC) activity was significantly enhanced in aortas from 1K-1C hypertensive rats; the inhibitory effect of C-ANP(4-23), which specifically interacts with the atrial natriuretic peptide (ANP)-C receptor, and Ang II on AC was attenuated. GTPgammaS, isoproterenol, glucagon, NaF, and FSK stimulated the AC activity in aortas from control and hypertensive rats to varying degrees; however, the stimulations were significantly lower in hypertensive rats than in control rats. These data suggest that aortas from 1K-1C hypertensive rats exhibit enhanced expression of Gialpha proteins and associated functions.

Natriuretic peptides increase cAMP production in human thyrocytes via the natriuretic peptide clearance receptor (NPR-C)

The relationship between natriuretic peptides and adenylyl cyclase/cAMP signal transduction has generally been shown to be an inhibitory one, mediated via the NPR-C receptor coupled to adenylyl cyclase by inhibitory G proteins (Gi). In the present studies, we have investigated the modulation of cAMP by natriuretic peptides in a long-term culture of human thyroid cells. Competition of [125I] rat ANF binding to human thyrocytes (HTU-5) by rat ANF (99-126) and by the NPR-C-specific analog C-ANF (4-23) indicated that greater than 97% of the ANF binding sites on HTU-5 cells are of the NPR-C type. However, rather than inhibiting intracellular cAMP in these cells, ANF increased maximal cAMP to 200-300% of control value. The ANF-induced increase in cAMP was duplicated by C-ANF (4-23). Basal cAMP content was reduced, and the response to ANF was abolished when the cells were grown in low (0.5%) serum without the addition of pituitary and hypothalamic extracts. CNP-22 also increased cAMP above control in HTU-5 cells identically to ANF. Neither ANF nor C-ANF (4-23) had any effect on cAMP in a culture of rat aortic smooth muscle cells. These results provide the first evidence for a positive effect of natriuretic peptides on cAMP mediated through the NPR-C, suggesting the possibility of an alternative mode of signaling by this receptor subtype.

Mesangial cells from diabetic NOD mice constitutively express increased density of atrial natriuretic peptide C receptors

BACKGROUND

Experimental evidence shows that natriuretic peptides (NPs) play a pathophysiological role in the glomerular hemodynamic abnormalities that occur in diabetes mellitus.

METHODS

In this study, the cGMP response to NPs and the different subtypes of NP receptors were examined in mesangial cells derived from a genetic model of diabetes, the nonobese diabetic (NOD) mouse. Multiple mesangial cell lines were derived from diabetic (D-NOD) and nondiabetic (ND-NOD) adult mice and were studied at different passages.

RESULTS

cGMP accumulation after stimulation by atrial NP (ANP) or C-type NP (CNP) was markedly inhibited in D-NOD cells irrespective of the glucose concentration (6 or 20 mM) in the culture medium. In contrast, NP receptor density measured from [125I]-ANP saturation binding curves was 7.5 times greater in D-NOD than in ND-NOD cells. No change in KD (200 pM in both cell lines) was observed. Competitive inhibition studies showed that 4-23 C-ANP, which is specific of clearance receptors (NPR-C), displaced 90% of the maximum fraction bound, suggesting the predominance of NPR-C in both cell lines. Further identification was obtained from RNase protection assay and reverse transcription-polymerase chain reaction, which also demonstrated the higher expression of NPR-C mRNA in D-NOD cells. In contrast, NPR-A mRNA was not modified. Increased expression of NPR-C in D-NOD cells was associated with an increase of ANP internalization rate at 37 degrees C, indicating that these receptors were functional.

CONCLUSIONS

These studies demonstrate that the constitutive overexpression of NPR-C in D-NOD mesangial cells is associated with a decreased response of cGMP to ANP or CNP treatment. This could be due to the lesser availability of the peptides for binding to NPR-A or NPR-B or to an inhibitory effect on NP-dependent guanylate cyclase activity via the activation of NPR-C.

ANP enhances bradycardic reflexes in normotensive but not spontaneously hypertensive rats

Baroreflex control of heart rate in spontaneously hypertensive rats (SHR) is defective, largely because of a poor vagal contribution to the reflex. We have demonstrated previously that atrial natriuretic peptide (ANP) enhances reflex bradycardia in normotensive rats through an action on nonarterial vagal afferent pathways. In the present study, we investigated whether ANP could reverse the baroreflex abnormality in SHR. Heart rate reflexes were activated by three different methods in conscious, instrumented SHR and Wistar-Kyoto rats (WKY) in the presence of intravenous infusions of vehicle (saline) or rat ANP (150 ng/kg per minute). Heart rate responses were measured by (1) the steady-state changes in blood pressure after alternating slow infusions (over approximately 15 to 30 seconds) of a pressor (methoxamine) and depressor (nitroprusside) drug (stimulating predominantly arterial baroreceptors), (2) the ramp method of rapid infusion of methoxamine (over < 10 seconds; stimulating arterial and cardiopulmonary baroreceptors), and (3) the von Bezold-Jarisch method of activating chemically sensitive cardiac receptors through serotonin injections. ANP enhanced the heart rate range of the arterial baroreflex (steady-state method) by 13 +/- 3% in WKY but had no significant effect on the sensitivity or any other parameter of the steady-state baroreflex. When a very rapid rise in blood pressure was elicited by the ramp method in WKY, ANP significantly enhanced baroreflex bradycardia (sensitivity increased by 29 +/- 9%, P < .05). ANP also enhanced the bradycardia of the von Bezold-Jarisch reflex (by 33 +/- 16%, P < .05) in WKY. By contrast, ANP did not influence baroreceptor or chemoreceptor heart rate reflex responses in SHR. We conclude that in normotensive rats, ANP facilitates cardiopulmonary bradycardic reflexes. The lack of effect of ANP in SHR may be related to an underlying structural or genetic alteration in their cardiac sensors, perhaps associated with cardiac hypertrophy, that prevents the ANP-induced activation of cardiac sensory afferents, resulting in cardioinhibition.