Therapy with nitrates: increasing evidence of vascular toxicity

Carbon monoxide and the eye: Implications for glaucoma therapy

In the late 1990s, the scientific community witnessed a very peculiar phenomenon: the transformation of nitric oxide (NO) from a noxious gas into a key chemical messenger. The importance of NO in biology and medicine was highlighted in 1998 when the Nobel Prize was awarded in Physiology and Medicine to Robert Furchgott, Louis Ignarro and Ferid Murad for their pioneering work on the role of NO in the nervous, cardiovascular and immune systems. In this same time period, carbon monoxide (CO), another gas usually associated with environmental pollution, air poisoning and suicidal behavior, was also undergoing a similar change in image, although not as closely followed. It had been known for several decades that the human body generated CO upon the decomposition of hemoglobin, which was determined by the discovery that heme oxygenase (HO) is the enzymatic source of CO. However, CO's role as an endogenous neurotransmitter was established only in the early 1990s. Since then, many biological activities of CO have been demonstrated in studies using different tools, such as the pharmacological induction of HO by hemin, the direct administration of CO or the use of pro-drugs that generate CO. This review focuses on CO as a fine modulator of intraocular pressure and on its potential implications in glaucoma.

Coadministration of carvedilol attenuates nitrate tolerance by preventing cytochrome p450 depletion

BACKGROUND

Long-term administration of nitroglycerin (NTG) causes tolerance secondary to increased vascular formation of reactive oxygen species. Carvedilol, which has potent antioxidant activity in addition to functioning as an adrenergic blocker, prevents nitrate tolerance by a still to be elucidated mechanism. The present study investigated how carvedilol attenuates nitrate tolerance, particularly with reference to cytochrome P450 (CYP), an enzyme involved in the development of tolerance.

METHODS AND RESULTS

Male Wistar rats were subjected to 48-h continuous infusion of NTG alone (0.5 mg/h) or NTG with concomitant carvedilol (20 or 100 microg/h), and then compared with vehicle-treated rats (4 groups; n=6 in each group). Following the continuous administration, nitrate tolerance, assessed by bolus NTG injections, was hemodynamically prevented by coadministration of carvedilol. Levels of CYP1A1/1A2, superoxide production, and phosphorylated vasodilator-stimulated phosphoprotein at serine 239 (P-VASP) were examined in the aortic wall and heart tissue. When NTG alone was continuously administered, vascular superoxide was produced, there was a decrease in the cardiac CYP1A1/1A2 level, and depletion of P-VASP. However, each of these changes induced by continuous NTG administration was significantly attenuated by coadministration of carvedilol and the extent of attenuation was more pronounced at the higher dose (100 microg/h).

CONCLUSIONS

Coadministration of carvedilol attenuates nitrate tolerance through maintenance of NO/cGMP pathway activity by preventing free radical generation and CYP depletion.

Novel therapies for cyclic GMP control of vascular smooth muscle growth

Cyclic GMP, guanosine 3',5'-cyclic monophosphate, is a critical and multifunctional second-messenger molecule that mediates diverse physiological and pathophysiological functions in cardiac and vascular tissues. Synthesized through nitric oxide, carbon monoxide, and/or natriuretic peptide-mediated guanylate cyclase stimulation and guanosine triphosphate dephosphorylation, cyclic GMP is capable of stimulating a cascade of serine/threonine kinase events, including signaling through cyclic GMP- and/or cyclic AMP-dependent protein kinases, eliciting protein kinase-independent actions such as modulation of ion channels or transporters, or undergoing hydrolytic degradation through actions of cyclic GMP-regulated phosphodiesterases. Substrates, enzymes, cofactors, and associated variables in this multifaceted system have historically been targets of vital pharmacotherapies with perhaps most common the use of vascular smooth muscle-targeting organonitrates in cardiac patients and phosphodiesterase inhibitors in individuals with erectile dysfunction. Accumulating basic science and clinical evidence, however, suggests that cyclic GMP signaling is compromised under conditions of disease or elevated physiological stresses. Moreover, nitric oxide can stimulate an array of cytotoxic effects and nitric oxide-based therapies can be limited by diminished bioactivity and the development of tachyphylaxis or tolerance after prolonged use. Consequently, an emerging area for clinical drug development and therapeutic drug evaluation for conditions of cardiovascular adversity has focused on identification of cyclic GMP signaling pathways that act under oxidized or nitric oxide-unresponsive conditions and/or that operate irrespective of nitric oxide-induced complications. The aim of this therapeutic review is to describe novel, nitric oxide-alternate avenues for cyclic GMP signaling in vascular smooth muscle growth with particular emphasis on pharmacotherapeutics of recently characterized cyclic GMP-specific approaches.