Modulating NO-GC Pathway in Pulmonary Arterial Hypertension

The pathogenesis of complex diseases such as pulmonary arterial hypertension (PAH) is entirely rooted in changes in the expression of some vasoactive factors. These play a significant role in the onset and progression of the disease. Indeed, PAH has been associated with pathophysiologic alterations in vascular function. These are often dictated by increased oxidative stress and impaired modulation of the nitric oxide (NO) pathway. NO reduces the uncontrolled proliferation of vascular smooth muscle cells that leads to occlusion of vessels and an increase in pulmonary vascular resistances, which is the mainstay of PAH development. To date, two classes of NO-pathway modulating drugs are approved for the treatment of PAH: the phosphodiesterase-5 inhibitors (PD5i), sildenafil and tadalafil, and the soluble guanylate cyclase activator (sGC), riociguat. Both drugs provide considerable improvement in exercise capacity and pulmonary hemodynamics. PD5i are the recommended drugs for first-line PAH treatment, whereas sGCs are also the only drug approved for the treatment of resistant or inoperable chronic thromboembolic pulmonary hypertension. In this review, we will focus on the current information regarding the nitric oxide pathway and its modulation in PAH.

Effect of digoxin on the somatotroph responsiveness to growth hormone-releasing hormone (GHRH) alone or combined with arginine in normal young volunteers

BACKGROUND

The activity of the GH/IGF-I axis, known to play a major role in myocardial structure and function, has been reported to be altered in patients with chronic heart failure.

AIM AND DESIGN OF THE STUDY

In order to evaluate the possibility that clinically used cardioactive drugs may exert neuroendocrine influences on somatotroph secretion, we studied the effects of pretreatment with enalapril (20 mg/day orally for 3 days), furosemide (20 mg i.v. as a bolus at -5 minutes) or digoxin (0.25 mg orally 4x/day for 3 days) on the GH response to growth hormone-releasing hormone (GHRH) (1.0 microg/ kg i.v. as a bolus at 0 minutes) in 12 healthy male adults (age [mean +/- SEM] 30.2 +/- 1.4 years; BMI 22.7 +/- 0.7 kg/ m2). In a subgroup of 8 subjects the same study was performed testing the GH response to GHRH + arginine (ARG; 0.5 g/kg i.v. from 0 to + 30 minutes). RESULTS The GH response to GHRH (1,304.1 +/- 248-5 microg/l/h) was not modified by enalapril (1,368.7 +/- 171.2 microg/l/h) or by furosemide (1,269.3 +/- 185.2 microg/l/h) but was significantly blunted by digoxin (613.6 +/- 73.2 microg/l/h, P < 0.05). On the other hand digoxin, enalapril and furosemide did not modify the GH response to GHRH +ARG.

CONCLUSIONS

Digoxin, but not enalapril or furosemide, inhibits the GH response to GHRH in normal subjects. The blunting effect of digoxin on the GHRH-induced GH response is counteracted by arginine. These findings show that digoxin possesses an inhibitory effect on somatotroph secretion that may be mediated at the hypothalamic level.

Patients with dilated cardiomyopathy show reduction of the somatotroph responsiveness to GHRH both alone and combined with arginine

OBJECTIVE

Altered function of the GH/IGF-I axis in patients with dilated cardiomyopathy (DCM) has been reported. In fact, DCM patients show reduction of IGF-I levels, which could reflect slight peripheral GH resistance or, alternatively, reduced somatotroph secretion. Spontaneous GH secretion has been reported to be altered by some but not by other authors, whereas the GH response to GHRH, but not that to GH-releasing peptides, seems reduced in DCM patients. On the other hand, it is well known that the GH response to GHRH in humans is markedly potentiated by arginine (ARG), which probably acts via inhibition of hypothalamic somatostatin release; in fact the GHRH+ARG test is known as one of the most reliable to evaluate the maximal secretory capacity of somatotroph cells.

METHODS

In order to further clarify the somatotroph function in DCM, in well-nourished patients with DCM (34 male, 4 female; age (mean+/-s.e. m.) 57.8+/-1.1 years; body mass index (BMI) 24.6+/-0.6kg/m(2); left ventricular ejection fraction 23.2+/-1.6%; New York Heart Association classification I/1, II/17, III/18, IV/2) we studied the GH response to GHRH (1.0 microgram/kg i.v.) alone or combined with ARG (0.5g/kg i.v.). The results in DCM patients were compared with those in age-matched control subjects (CS) (39 male, 7 female; age 58.9+/-1.0 years; BMI 23.2+/-0.3kg/m(2)).

RESULTS

Mean IGF-I levels in DCM patients were lower than in CS (144.3+/-6.9 vs 175.1+/-8. 4 microgram/l, P<0.05) whereas basal GH levels were similar in both groups (1.7+/-0.3 vs 1.7+/-0.3 microgram/l). The GH response to GHRH in DCM patients was lower (P<0.05) than that in CS (GH peak 6.5+/-1.2 vs 10.7+/-2.1 microgram/l). In both groups the GH response to GHRH+ARG was higher (P<0.001) than that to GHRH alone. However, the GH response to GHRH+ARG in DCM patients remained clearly lower (P<0.01) than that in CS (18.3+/-3.2 vs 34.1+/-4.6 microgram/l). The GH response to GHRH alone and combined with ARG was not associated with the severity of the disease.

CONCLUSION

DCM patients show blunted GH responses to GHRH both alone and combined with ARG. Evidence that ARG does not restore the GH response to GHRH in DCM patients makes it unlikely that the somatotroph hyporesponsiveness to the neurohormone reflects hyperactivity of hypothalamic somatostatinergic neurons.